出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/09/18 00:21:59」(JST)
| 「ステレオ」のその他の用法については「ステレオ (曖昧さ回避)」をご覧ください。 |
ステレオ (stereo) は、音響工学ではステレオフォニック (stereophonic) の略語であり、左右2つのスピーカーで音声を再生する方式のことである。広義には、ステレオフォニック再生のための音声信号を集音、録音、伝送、通信、放送、加工する技術全般、またはステレオフォニック再生のための音響再生装置（ステレオセット）を指す。
ステレオフォニック再生は、典型的には、聴取者の水平方向前方左右30度の位置に一対のスピーカーを配して2チャンネルの音声を再生する。それに対し、前方正面の1つのスピーカーから1チャンネルの音声を再生する方式をモノフォニックと呼ぶ。なお、一般にモノラルと混同されるが、これは後述の通り別の再生方式である。また、1チャンネルの音声をステレオフォニック用の2つのスピーカーから同時に再生して聴取する方式はダイオティックと呼ばれ、モノフォニックとは厳密には区別される。
ステレオフォニック再生はモノフォニック再生に比較して、音像定位や音場感が加わり、再生音の臨場感が増す効果がある。2つのスピーカと聴取者頭部が一辺3メートルの正三角形に位置する配置が最も望ましいとされている。この時の聴取者の位置のことをリスニング・ポイントまたはスイート・スポットと呼ぶ。
録音については、左右1対のマイクロフォンで集音してそのまま2チャンネルの音声とする方式と、個々の楽器や歌手に個別のマイクをあてがい、オーディオミキサーで2チャンネルの音声にまとめる方式とがある。現在の殆どのコンパクトディスクはステレオフォニック再生用として収録されているが、前者の方式で録音されたものはクラシック音楽などの一部であり、大半は後者の方式で録音されている。
ステレオフォニック再生で臨場感が増す理由として、人間が元々左右の耳に入る音の位相差および音量差などを利用して音源の方向を把握している点が挙げられる。これを2つのマイクロフォンでシミュレーションする方式として2チャンネル音声伝送は考案されたが、当初はステレオフォニックではなく、バイノーラルと呼ばれる方式であった。これは、2つのマイクロフォンを両耳の位置に備えた擬似頭部を用いて集音した2チャンネル音声を左右の耳にあてがった1対のイヤーフォンで聴取するもので、この再生方式をバイノーラルと呼ぶ。因みに、左右どちらか一方の耳で1つのイヤーフォンで聴取する再生方式をモノラル（モノーラル）と呼ぶ。
バイノーラル再生の効果はパリ博覧会にて複数の電話を用いることで偶然に発見されたとされているが、真偽は不明である。現在に至るまで、バイノーラル録音はドイツで研究が盛んであり、HATS（ヘッド・アンド・トルソー・シミュレータ : ダミーヘッドに肩部や胴体も加えたシステム）も性能を高め、サラウンド以上の臨場感が得られるケースも出てきている。
()内は英語。
音源が普通のステレオ録音の場合、通常のステレオ装置ではステレオフォニック再生、ヘッドフォンステレオではバイフォニック再生を行っていることになる。モノ録音のCDを普通のステレオで再生すると、厳密にはモノフォニックにはならない（わずかながら左右のチャンネルの特性が異なること、スピーカーの個体差および設置条件により再生が全く同じ音を発しないから）。
また、クラシック音楽など自然な音響再生を好む聴衆向けに、原理通り2個のマイクロフォン（ペアマイクまたはステレオマイクを用いる）を用いて収録する場合もわずかながら存在する（ワンポイントステレオ録音などと呼ばれる）が、殆どの商業録音ではマルチマイク録音が行われる。これは各楽器、パート別に多数のマイクロフォンを配置し、マルチトラックレコーダを用いて多チャンネル録音を行い、後で各チャンネルの音楽的バランスをとりつつ2チャンネル（あるいは伝送媒体の規定するチャンネル数）にトラックダウンする方法である。音像の位置はパンポットを用いた左右の音量差のみで決めることが普通である。その点はワンポイントステレオマイク（→マイクロフォン#その他の分類）一本で記録した音も同じであるが、実際に音場を形成したわけではないので、ある意味擬似的なステレオフォニックである。
単一のスピーカーとは限らないが、ステレオ音源に対比して単一の信号音源を出力する方式である。スピーカが単一又は複数にかかわらない。ステレオ録音の音源の再生では、左右の和信号の再生をしないと片方だけの信号になるため和信号の再生が一般的である。左右の和信号を得るのはワンポイントステレオマイクによる録音やマルチマイクによる録音では問題がないが、間隔をあけたペアマイクによる収録では位相差に伴う櫛形フィルタ効果が問題になる。
名称の「モノラル」あるいは「モノーラル」（英:monaural）は、「バイノーラル」（英:Binaural）に対するレトロニムである。
3つ以上のスピーカーを使って音を出すサラウンドサウンドは、通常はスピーカーのうち2つが前方左右に配されており、ステレオの一種である。DVD など新しい記録メディアでは、多チャンネル記録によってサラウンド効果を出せるよう、最初から規格化されている。
1881年、クレマン・アデールは世界初の2チャンネル音響システムをパリで公開した。オペラ座の舞台からパリ電気博覧会会場に設置した部屋まで一連の電話通信装置を接続し、オペラ座での公演の音声を生中継で転送して聞かせるものだった。サイエンティフィック・アメリカン誌はこの模様を次のようにレポートしている。
この2チャンネル電話方式はフランスで Théâtrophone の名で1890年から1932年まで、イギリスでは Electrophone の名で1895年から1925年まで販売された。どちらもホテルやカフェに硬貨投入式の受話器を設置するか、個人宅で受信設備を導入してサービス提供を受ける形式だった[2]。
1930年代、EMIのアラン・ブラムレイン (Alan Blumlein) がステレオ録音、ステレオ映画、さらにはサラウンドの特許を取得した[3]。1931年、EMIのブラムレインらは2チャンネルのステレオ録音方式を開発し、1933年に特許を取得した。これは25年後ステレオレコードの主流となる方式と同じで、レコードの溝の2つの壁を直角に交わらせて左右の壁に2つのチャンネルを録音するもので、1933年にEMIが開発した。ベル研究所のハーヴェイ・フレッチャーもステレオ録音・再生方式を研究していた。フレッチャーの試した方式の1つに "wall of sound"（音の壁）と呼ばれるものがある。これは、オーケストラの前に多数のマイクロフォンをずらっと並べて録音し、別のリスニングルームでそれぞれの録音を同じ形に配置したスピーカーで再生するものである。最大80個のマイクロフォンを使ったことがある。1932年3月には、フィラデルフィアの Academy of Music が2つのマイクロフォンで1つのレコード盤に2つの溝を刻む方式（バイノーラル盤に近い）も試されている。演奏はフィラデルフィア管弦楽団で指揮はレオポルド・ストコフスキーである。このときの録音で現存している最古のものは、1932年3月12日に録音されたスクリャービンの『プロメテ - 火の詩』である[4]。
1933年4月27日、ベル研究所が3チャンネルのステレオ音声のデモンストレーションを行った。これは、フィラデルフィアでフィラデルフィア管弦楽団が演奏した音声をワシントンD.C.の Constitution Hall に生中継し、指揮者のレオポルド・ストコフスキーが Constitution Hall で音声のミキシングを制御するというデモンストレーションだった。同年開催されたシカゴ万国博覧会でも、ベル研究所がバイノーラル音声のデモンストレーションを行っている[5]。このときは、2つの信号を振幅変調でそれぞれ別の周波数で放送した[6]。
1940年4月9日と10日にベル研究所がカーネギー・ホールで行ったデモンストレーションは、3台の大型スピーカーシステムを使用した。映画用フィルムに3つのサウンドトラックの形で録音することで3チャンネルの同期を達成している。ダイナミックレンジが制限されているため、音量圧縮技法が使われ、第4のトラックを音量伸長の調節に使用した。1970年代のドルビーノイズリダクションシステムも基本は似たような技法だが、はるかに洗練されている。音量圧縮と伸長は完全自動ではなく、手動で音質を改善させることを意図して設計してあった。すなわち、全体の音量と各トラックの相対音量を芸術的感性で調節可能だった。音響技術に興味を持っていたレオポルド・ストコフスキーの指揮でフィラデルフィア管弦楽団の演奏を録音した。ストコフスキー自身も録音の「改善」に参加した。
スピーカーは合計で1500ワットの音声出力であり、100デシベルの音声レベルを生成した。デモンストレーションの模様を報じた記事によれば、観客は「魅了され、時には大いに恐れた」という[7]。デモンストレーションを聴いたセルゲイ・ラフマニノフは、「驚くべきものだった」と述べたが、同時に「余りにも音量が大きすぎて、なんとなく音楽的ではなかった」とも述べている。また、「『展覧会の絵』は、あまりにも『改善』され、あまりにもストコフスキー的だったため、よくよく聴くまで曲名がわからなかった」とも述べている。
1937年、ニューヨークのベル研究所で、同研究所と Electrical Research Products, Inc. が開発した2チャンネルステレオ音声つき映画のデモンストレーションが行われた[8]。レオポルド・ストコフスキーの指揮でフィラデルフィアの Academy of Music が開発した9トラック音響システムに録音し、ユニバーサル・ピクチャーズが1937年に撮影した映画『オーケストラの少女』のサウンドトラックに使用したものである[9][10]。9トラックで録音したものをミキシングして2トラックにしている。1938年、メトロ・ゴールドウィン・メイヤーが映画のサウンドトラックとして3トラック方式を採用し、すぐに4トラックに改良した。1トラックは俳優の台詞用、2トラックは音楽用、1トラックは音響効果用である。この方式はステレオ音声にすることを目的としたものではなく、4トラックをミキシングして1つの光学式サウンドトラックを作りやすくするためのものだった[要出典]。この方式を採用した最初の映画は1938年の『初恋合戦』で、ジュディ・ガーランドの歌 "It Never Rains But What It Pours" がバイノーラル録音されている（最終的にはモノラルになっている）。
ステレオ音声を映画館に導入した最初の映画はウォルト・ディズニーの『ファンタジア』で、1940年11月の公開である。このために特別な音響方式 Fantasound を開発している。Fantasound では4つの光学式サウンドトラックを持つ別のフィルムを使っている。そのうち3トラックが音声トラックで、残る1つは映画館のアンプの音量を制御するための情報が格納されていた[11]。興行的には失敗したが、主要都市で音響設備の整った映画館だけで2か月間公開した後、サウンドトラックをモノラルにミキシングし直して、拡大公開を行った。1940年代初めごろ、アルフレッド・ニューマンが20世紀フォックスの多チャンネル録音が可能な録音ステージの建設を指揮した。この時代に録音された多チャンネルのサウンドトラックはいくつか現存しており、DVDのリリース時に使われた。例えば、『わが谷は緑なりき』、『アンナとシャム王』、『銀嶺セレナーデ』、『地球の静止する日』などがある。
磁気テープによる録音の発明により、高音質で多チャンネルを同期させて録音することが容易になり、また安価にできるようになった。1950年代初めには主要なスタジオが35mm磁気テープで録音するようになった。しかし実際に映画がステレオ音声で公開されるには、映画館側の整備が必要である。1952年9月20日に公開された『これがシネラマだ』 (This is Cinerama) は、ステレオが興行的に成り立つことを証明した。シネラマは現代のIMAXと比較しても遜色のないワイドスクリーン方式である。このため、シネラマを上映する映画館は構造的にいくつかの条件を満たす必要があった。シネラマのサウンドトラックは磁気テープ上の7トラックで構成され、5トラックはスクリーンの背後の5台のスピーカーを駆動し、2トラックがサラウンド用である。このシステムは磁気録音技術の先駆者 Hazard E. Reeves が開発した。シネラマを体験したことのある人によると、その音響は現代の標準と比較しても素晴らしいものだった。
1953年4月、ニューヨークでのみ『これがシネラマだ』が公開されているころ、多くの観客はヴィンセント・プライス主演の立体映画『肉の蝋人形』（ワーナー・ブラザーズ）でステレオ音声を体験していた。その音響方式は WarnerPhonic と呼ばれ、35mm磁気テープに左右と中央の3トラックを記録し、これを2台の映写機と同期させ、映画のフィルムにも光学式でサウンドトラックを設けている。光学式サウンドトラックの1つは他に問題が生じたときのバックアップとしてモノラル音声を記録している。WarnerPhonic を使った立体映画は他に2本だけ製作された（The Charge at Feather River と Island in the Sky）。これらの映画の磁気テープトラックは紛失している。他にも立体映画が製作されたが、その多くが3チャンネルの磁気テープによる音声を採用していた。例えば、It Came from Outer Space、I, the Jury、The Stranger Wore a Gun、Kiss Me, Kate などがある。
シネラマに触発され、映画業界はより単純で安価なワイドスクリーンシステムの開発に乗り出した。20世紀フォックスが開発したシネマスコープは4トラックの磁気サウンドトラックを採用していた。シネマスコープは標準的な35mmフィルムを採用していたため、既存の映画館で上映が可能だった。20世紀フォックスは、55mmフィルムを採用した CinemaScope 55 も開発した。こちらは6トラックのステレオを採用していた。しかし、こちらは新たな映写機を必要とするため失敗に終わり、2作品（『回転木馬』と『王様と私』）がこの方式で作られたが、結局35mmのシネマスコープで公開された。埋め合わせのため、『回転木馬』は6トラックのサウンドトラックで公開され、『王様と私』は1961年に70mmフィルムと6トラックのサウンドトラックで再公開された。
1954年以降も、ステレオ設備を導入できない映画館のため、ステレオ音声の映画をモノラルに変換したものが必要とされた[12]。ドルビーラボラトリーズのドルビーステレオが使われ始めた1975年ごろまで、このような状況が続いた。例えば、フランコ・ゼフィレッリの『ロミオとジュリエット』のサウンドトラックのアルバムはステレオになっていた。ステレオを使う映画は、『ウエスト・サイド物語』、『マイ・フェア・レディ』、Camelot[13]などのミュージカル映画、『クレオパトラ』などの叙事詩、サイモン&ガーファンクルの曲を多用した『卒業』のような音楽を重視した映画に限られていた。今では、映画は基本的に全てステレオで公開されている。
1940年代から1970年代にかけてのステレオの進歩は、複数チャンネルの録音・再生時の同期の困難さを技術的に克服していく過程であり、同時に新たな録音媒体と録音再生装置を市場に売り込んでいく過程だった。大まかに言って、ステレオシステムはアンプとスピーカーが2つずつ必要となり、モノフォニックシステムの2倍のコストがかかる。消費者がそれだけの金額を出すに値すると考えるかどうかは明らかではなかった。
1952年、エモリー・クック（1913年 - 2002年）はバイノーラル盤を作成する装置を開発した。レコード盤上に2本の溝が刻まれていて、両方に針を落とし、同時に再生する。それぞれのピックアップは別々のアンプとスピーカーに接続されていた。クックはレコード製造装置を売る目的でこれをニューヨークのオーディオフェアに出品した。しかし、すぐにバイノーラル盤のレコードを作って欲しいという注文が入るようになり、クックはレコード盤の商業生産に乗り出した。クックが製造したレコードは、鉄道の音から嵐の音まで様々なものがあった。なお、「バイノーラル盤」とバイノーラル録音には直接の関係はない。1953年のクックのカタログには、オーディオマニア向けの25種類のステレオレコードが掲載されている[14]。
1952年、1/4インチ磁気テープに2つの録音・再生ヘッドを使ってステレオ録音するデモンストレーションが行われた[15]。1953年、Remington Records は、ソーア・ジョンソン (Thor Johnson) 指揮のシンシナティ交響楽団の演奏などをステレオでテープ録音し始めた。同年RCAビクターもレオポルド・ストコフスキーやニューヨークのミュージシャンの演奏を試験的にステレオ・テープ録音している。1954年2月、RCAはシャルル・ミュンシュ指揮のボストン交響楽団の演奏によるベルリオーズの『ファウストの劫罰』をテープ録音し、それをきっかけとして同社はステレオ・テープ録音を常に行うようになった。直後、伝説の指揮者アルトゥーロ・トスカニーニ（1957年没）の最後の2回のコンサートが磁気テープにステレオで録音された。しかし、これら初期のステレオ録音がステレオのままでリリースされるのは1987年から2007年のことである。イギリスでは1954年中ごろ、デッカ・レコードがステレオでのテープ録音を始めた。1954年、ConcertapesやRCAビクターといった企業がステレオ録音済みオープンリールテープをモノラル録音の2倍の価格で発売した[16]。真面目なオーディオマニアがこれを購入し、ついに一般家庭にステレオ音響がもたらされた[17]。ステレオ録音は音楽業界では1957年までに広く採用されることになった。
1957年11月、世界初のステレオ盤レコードの大量生産を弱小レーベル Audio Fidelity Records が始めた。創業者で社長のシドニー・フレイはウェスタン・エレクトリックのWestrexブランドのステレオレコード製造装置を使い、大手レーベルに対抗しようとした[18][19]。A面は Dukes of Dixieland、B面は鉄道の音だった。12月16日フレイはビルボード誌に広告を掲載し、その中で業界関係者が会社のレターヘッド入り用箋で申し込んできたら、試聴用レコードを無料で進呈すると書いた[20][21]。
この動きは大いにメディアに取り上げられた[22]。フレイはさらに4タイトルのステレオ盤レコードをリリースした。ステレオ蓄音機の販売店は他に選択肢がないので Audio Fidelity Records のレコードを店頭で流した。ステレオ盤レコードの普及は、1958年にステレオ用カートリッジが250ドルから29.95ドルに値下げしたことで拍車がかかった[23]。1958年夏、Audio Fidelity の Marching Along with the Phenomenal Dukes of Dixieland, Volume 3 を筆頭としてステレオ盤レコードが一般の店頭に大量に並ぶようになった[24][25]。1968年までにレコード会社の多くはモノラル盤の製造をやめた[26][27][28]。
1925年12月、ノーサンプトンシャーのダヴェントリーにあるBBCの試験放送局 5XX が世界初のステレオラジオ放送を行った。内容はハミルトン・ハーティ指揮でマンチェスターで行われたコンサートである。右のチャンネルは長波の全国放送で、左のチャンネルは中波のローカル放送だった。BBCは1926年まで、ロンドンの 2LO とダヴェントリーの 5XX を使った実験を繰り返した。1958年にはロンドンでFMステレオの試験放送が開始され、毎週土曜日の朝にテレビの音声と中波（AM）ラジオで両方のチャンネルを提供した。BBCのFMステレオの本放送は1962年8月28日に始まった。
1952年5月22日、シカゴのAMラジオ放送局 WGN（FM局はWGNB）は1時間のステレオ試験放送を行った。1つのチャンネルをAM局で放送し、もう1つをFM局で放送する方式である[29]。ニューヨークのWQXRは1952年10月にステレオ放送を部分的に開始し、1954年には全ての音楽番組をステレオ放送にしている。こちらもAMとFMで左右のチャンネルを放送する方式である[30]。レンセラー工科大学は1952年11月、キャンパスにある2つのAM局を使って1週間のステレオ放送を行ったが、受信エリアはキャンパス内だけだった[31]。
ペンシルベニア州ピッツバーグのKDKA-FMは、1960年7月から8月にかけて、FMのみのステレオ放送システム6種類の比較試験を行った[32]。連邦通信委員会は1961年4月、ステレオFM放送の技術規格を発表し、1961年6月1日、アメリカで初のステレオFM放送が正式にライセンスされることになった[33]。最初のステレオFM放送を行ったのは、シカゴのWEFMとスケネクタディのWGFMである[34] 。
1952年12月11日、ニューヨークのメトロポリタン歌劇場で演じられたオペラ『カルメン』をケーブルテレビ方式で全米各地の劇場に放送し、それにRCAの開発したステレオ音響システムが使われた[35]。1958年から1959年には、ABCネットワークが75の受信エリアで音楽番組 The Lawrence Welk Show を何回か実験的にステレオで放送した。このときは一方のチャンネルをABCのAMラジオネットワークで放送した[36][37]。1958年10月21日、NBCも同じようにテレビとラジオのネットワークを使い、The George Gobel Show という番組の中で3分間だけステレオ放送を行ったことがある[38]。1959年1月30日、ABCは Walt Disney Presents の番組枠で The Peter Tchaikovsky Story のステレオ放送を実施した。これはディズニーのアニメ映画『眠れる森の美女』の場面を使ったもので、ABCはAM局とFM局を動員して左右の音声チャンネルを放送した。
1961年にFMステレオ放送が始まると、サイマル放送方式で音楽テレビ番組の音声をFMステレオで放送する例も出てきた[39]。1960年代から1970年代にそのような放送を行う場合、録画素材の音声をオープンリールテープにしてFM局に送り、手動で同期させていた。1980年代には放送衛星で番組を配信できるようになり、同期も自動化された。サイマル方式のステレオテレビ放送はFCCが MTS stereo を承認するまで行われていた。
ケーブルテレビは MTS stereo が普及するまで複数の周波数を使ってステレオ番組を提供していた。ステレオケーブル放送をいち早く採用したのは The Movie Channel だが、ステレオ放送を最も多用したのはMTVである。
日本では、1978年にNHKと日本テレビ放送網が音声多重放送の実験放送を開始し[40]、同年10月より前述の2局に加えてTBSテレビ、フジテレビジョン、テレビ朝日及び一部地域を除く系列局が、また同年12月下旬に東京12チャンネル（現・テレビ東京）がステレオ放送の本放送を開始した[41]。1984年には、番組の12%、時間にして一週間のうち14時間から15時間が音声多重放送となっている。テレビ放送のステレオ化は、番組よりもコマーシャル放送のほうが先行し、1980年代末にはほぼ全てのコマーシャル放送がステレオになった。それを利用ししてタイマー録画においてコマーシャルを自動的にカットする機能が、1990年代初頭に販売されたビデオデッキに搭載された事がある。
西ドイツでは1984年、第2ドイツテレビがステレオ番組の放送を開始した[40]。
アメリカでは、1984年に MTS(Multichannel Television Sound) がFCCによって標準化された。NTSCフォーマットの音声搬送波に3つの音声チャンネルを追加で符号化する技法である。1984年6月26日、NBCが部分的にこれを使ったステレオ放送を開始した（ただし、ニューヨーク局のみ）[42]。定期的なステレオ放送は1985年になってからである。
Stereophonic sound or, more commonly, stereo, is a method of sound reproduction that creates an illusion of multi-directional audible perspective. This is usually achieved by using two or more independent audio channels through a configuration of two or more loudspeakers (or stereo headphones) in such a way as to create the impression of sound heard from various directions, as in natural hearing.[1] Thus the term "stereophonic" applies to so-called "quadraphonic" and "surround-sound" systems as well as the more common two-channel, two-speaker systems. It is often contrasted with monophonic, or "mono" sound, where audio is heard as coming from one position, often centered in the sound field (analogous to a visual field). In the 2000s, stereo sound is common in entertainment systems such as broadcast radio and TV, recorded music and the cinema.
The word stereophonic derives from the Greek "στερεός" (stereos), "firm, solid"[2] + "φωνή" (phōnē), "sound, tone, voice"[3] and it was coined in 1927 by Western Electric, by analogy with the word "stereoscopic".
Stereo sound systems can be divided into two forms: The first is "true" or "natural" stereo in which a live sound is captured, with any natural reverberation or ambience present, by an array of microphones. The signal is then reproduced over multiple loudspeakers to recreate, as closely as possible, the live sound.
Secondly "artificial" or "pan-pot" stereo, in which a single-channel (mono) sound is reproduced over multiple loudspeakers. By varying the relative amplitude of the signal sent to each speaker an artificial direction (relative to the listener) can be suggested. The control which is used to vary this relative amplitude of the signal is known as a "pan-pot" (panoramic potentiometer). By combining multiple "pan-potted" mono signals together, a complete, yet entirely artificial, sound field can be created.
In technical usage, true stereo means sound recording and sound reproduction that uses stereographic projection to encode the relative positions of objects and events recorded.
During two-channel stereo recording, two microphones are placed in strategically chosen locations relative to the sound source, with both recording simultaneously. The two recorded channels will be similar, but each will have distinct time-of-arrival and sound-pressure-level information. During playback, the listener's brain uses those subtle differences in timing and sound level to triangulate the positions of the recorded objects. Stereo recordings often cannot be played on monaural systems without a significant loss of fidelity. Since each microphone records each wavefront at a slightly different time, the wavefronts are out of phase; as a result, constructive and destructive interference can occur if both tracks are played back on the same speaker. This phenomenon is known as phase cancellation.
Clément Ader demonstrated the first two-channel audio system in Paris in 1881, with a series of telephone transmitters connected from the stage of the Paris Opera to a suite of rooms at the Paris Electrical Exhibition, where listeners could hear a live transmission of performances through receivers for each ear. Scientific American reported:
This two-channel telephonic process was commercialized in France from 1890 to 1932 as the Théâtrophone, and in England from 1895 to 1925 as the Electrophone. Both were services available by coin-operated receivers at hotels and cafés, or by subscription to private homes.[5]
Modern stereophonic technology was invented in the 1930s by British engineer Alan Blumlein at EMI, who patented stereo records, stereo films, and also surround sound.[6]
In early 1931, Blumlein and his wife were at a local cinema. The sound reproduction systems of the early "talkies" invariably only had a single set of speakers - which could lead to the somewhat disconcerting effect of the actor being on one side of the screen whilst his voice appeared to come from the other. Blumlein declared to his wife that he had found a way to make the sound follow the actor across the screen.
The genesis of these ideas is uncertain, but he explained them to Isaac Shoenberg in the late summer of 1931. His earliest notes on the subject are dated 25 September 1931, and his patent had the title "Improvements in and relating to Sound-transmission, Sound-recording and Sound-reproducing Systems". The application was dated 14 December 1931, and was accepted on 14 June 1933 as UK patent number 394,325.[7]
The patent covered many ideas in stereo, some of which are used today and some not. Some 70 claims include:
Blumlein began binaural experiments as early as 1933, and the first stereo discs were cut later the same year, twenty-five years before that method became the standard for stereo phonograph discs. These discs used the two walls of the groove at right angles in order to carry the two channels.
Much of the development work on this system for cinematic use did not reach completion until 1935. In Blumlein's short test films (most notably, "Trains At Hayes Station", which lasts 5 minutes 11 seconds, and, "The Walking & Talking Film"), his original intent of having the sound follow the actor was fully realised.[8]
In 1934, Blumlein recorded Mozart's Jupiter Symphony conducted by Sir Thomas Beecham at Abbey Road Studios in London using his vertical-lateral technique.[6]
In the United States, Harvey Fletcher of Bell Laboratories was also investigating techniques for stereophonic recording and reproduction. One of the techniques investigated was the "wall of sound", which used an enormous array of microphones hung in a line across the front of an orchestra. Up to 80 microphones were used, and each fed a corresponding loudspeaker, placed in an identical position, in a separate listening room. Several stereophonic test recordings, using two microphones connected to two styli cutting two separate grooves on the same wax disc, were made with Leopold Stokowski and the Philadelphia Orchestra at Philadelphia's Academy of Music in March 1932. The first (made on March 12, 1932), of Scriabin's Prometheus: Poem of Fire, is the earliest known surviving intentional stereo recording.[9]
Bell Laboratories gave a demonstration of three-channel stereophonic sound on April 27, 1933, with a live transmission of the Philadelphia Orchestra from Philadelphia to Constitution Hall in Washington, D.C. over multiple Class A telephone lines. Leopold Stokowski, normally the orchestra's conductor, was present in Constitution Hall to control the sound mix. Five years later, the same system would be expanded onto multi-channel film recording and used from the concert hall in Philadelphia to the recording labs at Bell Labs in New Jersey in order to record Walt Disney's Fantasia (1940) in what Disney called Fantasound.
Later that same year, Bell Labs also demonstrated binaural sound, at the Chicago World's Fair in 1933 using a dummy with microphones instead of ears, .[10] The two signals were sent out over separate AM station bands.[11]
Utilizing selections recorded by the Philadelphia Orchestra, under the direction of Leopold Stokowski, intended for but not used in Walt Disney's Fantasia, the Carnegie Hall demonstration by Bell Laboratories on April 9 and 10, 1940, used three huge speaker systems. Synchronization was achieved by making the recordings in the form of three motion picture soundtracks recorded on a single piece of film with a fourth track being used to regulate volume expansion.
This was necessary due to the limitations of dynamic range on optical motion picture film of the period, however the volume compression and expansion were not fully automatic, but were designed to allow manual studio "enhancement"; i.e., the artistic adjustment of overall volume and the relative volume of each track in relation to the others. Stokowski, who was always interested in sound reproduction technology personally participated in the "enhancement" of the sound at the demonstration.
The speakers produced sound levels of up to 100 decibels, and the demonstration held the audience "spellbound, and at times not a little terrified", according to one report.[12] Sergei Rachmaninoff, who was present at the demonstration, commented that it was "marvellous" but "somehow unmusical because of the loudness." "Take that Pictures at an Exhibition", he said. "I didn't know what it was until they got well into the piece. Too much 'enhancing', too much Stokowski."
In 1937, Bell Laboratories in New York City gave a demonstration of two-channel stereophonic motion pictures, developed by Bell Labs and Electrical Research Products, Inc.[13] Once again, conductor Leopold Stokowski was on hand to try out the new technology, recording onto a special proprietary nine-track sound system at the Academy of Music in Philadelphia, during the making of the movie One Hundred Men and a Girl for Universal Pictures in 1937, after which the tracks were mixed down to one for the final soundtrack.[14][15] A year later, MGM started using three tracks instead of one to record the musical selections of movie soundtracks, and very quickly upgraded to four. One track was used for dialogue, two for music, and one for sound effects. The purpose for this form of multitrack recording was to make mixing down to a single optical track easier and was not intended to be a recording for stereophonic purposes.[citation needed] The very first two-track recording MGM made (although released in mono) was "It Never Rains But What It Pours" by Judy Garland, recorded on June 21, 1938, for the movie Love Finds Andy Hardy.
Walt Disney began experimenting with multi-channel sound in the early 1930s as noted above.[16] The first commercial motion picture to be exhibited with stereophonic sound was Walt Disney's Fantasia, released in November 1940, for which a specialized sound process (Fantasound) was developed. As in the Carnegie Hall demonstrations six months earlier, Fantasound used a separate film containing four optical sound tracks. Three of the tracks were used to carry left, center and right audio, while the fourth track carried three tones which individually controlled the volume level of the other three.[17][18] The film was not a financial success, however, and after two months of road-show exhibition in selected cities, its soundtrack was remixed into mono sound for general release. It was not until its 1956 re-release that stereo sound was restored to the film. In the early 1940s, composer-conductor Alfred Newman directed the construction of a sound stage equipped for multichannel recording for 20th Century Fox studios. Several soundtracks from this era still exist in their multichannel elements, some of which have been released on DVD, including How Green Was My Valley, Anna and the King of Siam, The Day the Earth Stood Still and Sun Valley Serenade which, along with Orchestra Wives, feature the only stereophonic recordings of the Glenn Miller Orchestra as it was during its heyday of the Swing Era.
Motion picture theatres, however, are where the real introduction of stereophonic sound to the public occurred. Amid great fanfare, Stereo sound was officially proven commercially viable for the public on September 30, 1952 with the release of a Cinerama demonstration film by Lowell Thomas and Mike Todd entitled This is Cinerama. The format was a spectacular widescreen process featuring three separate 35mm motion picture films (plus a separate sound film) running in synchronization with one another at 26 fps, adding one picture panel each to the viewer's left and right at 45 degree angles, in addition to the usual front and center panel, creating a truly immersive panoramic visual experience, comparable in some ways to today's curved screen IMAX OMNI.
Similarly, the Cinerama audio soundtrack technology, developed by Hazard E. Reeves, a pioneer in magnetic recording, utilized seven discrete sound tracks on full-coat magnetic 35mm film, in order to envelop the theatregoer in an aural experience just as spectacular as that playing on the screen: five main channels behind the screen, two surround channels in the rear of the theater, plus a sync-track to interlock the 4 machines, which were specially outfitted with aircraft servo-motors made by Ampex.
The advent of multi-track magnetic tape and film recording made high fidelity synchronized multichannel recording more technically straightforward, though costly. By the early 1950s, all of the major studios were recording on 35mm magnetic film for mixing purposes, and many of these so-called individual angles still survive, allowing for soundtracks to be remixed into Stereo or even Surround.
In April 1953, while This is Cinerama was still playing only in New York City, most moviegoing audiences heard stereophonic sound for the first time with House of Wax, an early 3-D film starring Vincent Price and produced by Warner Bros. Unlike the 4-track mag release-print stereo films of the period which featured four thin strips of magnetic material running down the length of the film, inside and outside the sprocket holes, the sound system developed for House of Wax, dubbed WarnerPhonic, was a combination of a 35 mm fully coated magnetic film that contained the audio tracks for Left-Center-Right, interlocked with the two dual-strip Polaroid system projectors, one of which carried a mono optical surround track and one that carried a mono backup track, should anything go wrong.
Only two other films featured this strange hybrid WarnerPhonic sound: the 3-D production of The Charge at Feather River, and Island in the Sky. Unfortunately, as of 2012, the stereo magnetic tracks to both these films are considered lost forever. In addition, a large percentage of 3-D films carried variations on three-track magnetic sound: It Came from Outer Space; I, the Jury; The Stranger Wore a Gun; Inferno; Kiss Me, Kate; and many others.
Inspired by Cinerama, the movie industry moved quickly to create simpler and cheaper widescreen systems, the first of which, Todd-AO, was developed by Broadway promoter Michael Todd with financial backing from Rodgers and Hammerstein, to use a single 70mm film running at 30 frames per second with 6 magnetic sound tracks, for their screen presentation of "Oklahoma!". Major Hollywood studios immediately rushed to create their own unique formats, such as Warner Bros. Panavision, Paramount Pictures' VistaVision and Twentieth Century-Fox Film Corporation's CinemaScope, the latter of which used up to four separate magnetic sound tracks.
Because of the standard 35 mm-size film, CinemaScope and its stereophonic sound was capable of being retrofitted into existing theaters. CinemaScope 55 was created by the same company in order to use a larger form of the system (55 mm instead of 35 mm) to allow for greater image clarity onscreen, and was supposed to have had 6-track stereo instead of four. However, because the film needed a new, specially designed projector, the system proved impractical, and the two films made in the process, Carousel and The King and I, were released in 35 mm CinemaScope reduction prints. To compensate, the premiere engagement of Carousel used a six-track magnetic full-coat in an interlock, and a 1961 re-release of The King and I, featured the film "printed down" to 70 mm, used a six-track stereo soundtrack as well.
Eventually, 50 complete sets of combination 55/35 mm projectors and "penthouse" reproducers were completed and delivered by Century and Ampex, respectively, and 55 mm release print sounding equipment was delivered by Western Electric. Several samples of 55 mm sound prints can be found in the Sponable Collection at the Film and Television Archives at Columbia University. The subsequently abandoned 55/35 mm Century projector eventually became the Century JJ 70/35MM projector.
After this disappointing experience with their proprietary "wide gauge" system, Fox purchased the Todd-AO system and re-engineered it into a more modern 24 fps system with brand-new 65MM self-blimped production cameras (Mitchell BFC ... "Blimped Fox Camera") and brand-new 65MM MOS cameras (Mitchell FC ... "Fox Camera") and brand-new Super Baltar lenses in a wide variety of focal lengths, first employed on South Pacific. Essentially, although Todd-AO was also available to others, the format became Fox's premier origination and presentation apparatus, replacing CinemaScope 55. Current DVDs of the two CinemaScope 55 feature titles were transferred from the original 55mm negatives, often including the separate 35MM films as extras for comparison.
However, beginning in 1957, films recorded in stereo (except for those shown in Cinerama or Todd-AO) carried an alternate mono track for theatres not ready or willing to re-equip for stereo.[19] From then until about 1975, when Dolby Stereo was used for the first time in films, most motion pictures—even some from which stereophonic soundtrack albums were made, such as Zeffirelli's Romeo and Juliet—were still released in monaural sound,[20] stereo being reserved almost exclusively for expensive musicals such as West Side Story,[21] My Fair Lady,[22] or Camelot;[23] epics such as Ben-Hur[24] or Cleopatra.[25] Stereo was also reserved for dramas with a strong reliance on sound effects or music, such as The Graduate,[26] with its Simon and Garfunkel score.
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Today, virtually all films are released in stereophonic sound as the Westrex Stereo Variable-Area system developed in 1977 for Star Wars is no more expensive to manufacture than mono. The format employs the same Western Electric/Westrex/Nuoptix RA-1231 recorder, and coupled with QS quadraphonic matrixing technology licensed to Dolby Labs from Sansui, this SVA system can produce the same Left, Center, Right and Surround sound of the original CinemaScope system of 1953 by using a single standard width optical track. This important development finally brought stereo sound to so-called Flat (Non-Wide-Screen) films presented at the most common aspect ratio of 1.85:1 although a number of `flat' films are photographed and presented at a ratio of 1.66:1, common in Europe or 1.75:1 common in museums.
Producers often took advantage of the six magnetic soundtracks available for 70mm film release prints, and productions shot in either 65MM or to save money, in 35MM and then blown up to 70MM. In these instances, the 70MM prints would be mixed for stereo, while the 35MM reduction prints would be remixed for mono.
Some films shot in 35MM, such as Camelot, featured four-track stereophonic sound and were then "blown-up" to 70MM so that they could be shown on a giant screen with six-track stereophonic sound. Unfortunately however, many of these presentations were only pseudo stereo, utilizing a somewhat artificial six-track panning method. A process known somewhat derogatorily as the "Columbia Spread" was often used to synthesize Left Center and Right Center from a combination of Left and Center and Right and Center, respectively, or, for effects, the effect could be "panned" anywhere across the five stage speakers using a one-in/five-out pan pot.
Dolby Stereo was succeeded by Dolby Digital 5.1 in the cinema and more recently with the introduction of digital cinema, Dolby Surround 7.1 and Dolby Atmos in 2010 and 2012 respectively.
From 1940 to 1970, the progress of stereophonic sound was paced by the technical difficulties of recording and reproducing two or more channels in synchronization with one another and by the economic and marketing issues of introducing new audio media and equipment. A stereo system cost roughly twice as much as a monophonic system, since a stereo system had to be assembled by the user after purchasing two preamplifiers, two amplifiers, and two speaker systems in addition to purchasing a twin-tuner radio, upgrading their tape recorder to a stereo model and having their phonograph fitted with a stereo cartridge. In the early days it was not clear whether consumers would think the sound was so much better as to be worth twice the price.
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Edison had been recording in a hill-and-dale or vertically modulated format on his cylinders and discs since 1877, and Berliner had been recording in a side-to-side or lateral format since shortly thereafter. Each format developed on its own trajectory until the late 1920s when electric recording on disc, utilizing a microphone surpassed acoustic recording where the performer needed to shout or play very loudly into what basically amounted to a megaphone in reverse.
At that time, AM radio had been around for roughly a decade, and broadcasters were looking for both better materials from which to make phonograph records as well as a better format in which to record them to play over the narrow and thus inherently noisy radio channel. As radio had been playing the same shellac discs available to the public, it was found that, even though the playback system was now electric rather than acoustic, the surface noise on the disc would mask the music after just a few plays.
The development of acetate, bakelite, and vinyl, and the production of radio broadcast transcriptions helped to solve this. Once these considerably more quiet compounds were developed, it was discovered that the rubber-idler-wheel driven turntables of the period had a great deal of low-frequency rumble - but only in the lateral plane. So, even though with all other factors being equal, the lateral plane of recording on disc had the higher fidelity, it was decided to record vertically to produce higher-fidelity recordings on these new `silent-surface' materials, for two reasons, the increase in fidelity and the incompatibility with home phonographs which, with their lateral-only playback systems would only produce silence from a vertically modulated disc.
After 33-1/3 RPM recording had been perfected for the movies in 1927, the speed of radio program transcriptions was reduced to match, once again to inhibit playback of the discs on normal home consumer equipment. Even though the stylus size remained the same as consumer records at either 3 mils or 2.7 mils, the disc size was increased from 12-inches to the same 16-inches as those used in early talking pictures in order to prevent the practice even further. Now, not only could the records not be played on home equipment due to incompatible recording format and speed, they wouldn't even fit on the player either, which suited the copyright holders.
During the same period engineers got a bright idea. Split the signal into two parts, bass and treble, and record the treble on its own track near the edge of the disc in a lateral format so that there would be no high-frequency distortion, and then record the bass on its own track in a vertical fashion to get rid of the rumble. Unfortunately, vertical grooves take up more space than lateral grooves; so when the bass track was full, starting halfway through the disc and ending up at the center, the treble track had a large amount of unused space at the end. The alternative was to record it at a wider pitch i.e. lines-per-inch in order to match up with the bass track and keep both styli in the same place, limiting the playing time to slightly longer than a single even at 33-1/3 RPM on a 12-inch disc.
Another failed experiment in the late 1920s and early '30s involved recording the left channel on the left side of the disc (when held vertically with its edge facing the user) and recording the right channel on the right side of the disc. These were manufactured on twin film-company recording lathes which ran in perfect sync with one another with no variation, and were capable of not only outside-in as well as inside-out recordings (see Radio Programming Vinyl Sequence under Gramophone record) but also counter-clockwise as well as conventional clockwise recording by mounting the cutting head wrong-way-out with a special adapter. One master was recorded conventionally and the other was recorded counterclockwise, each master was run separately through the plating process, lined up to match, and subsequently mounted in a press. This recording method was later used to record counter-clockwise discs by Mattel for one of its answers to the GAF Talking View Master in the mid-60s.
The dual-sided stereo disc was then played vertically, first in a system that featured two tonearms on the same post facing one another, and later on in an offset system where one tonearm was placed conventionally and the other tonearm was placed opposite, i.e. not only on the other side of the mechanism, but facing the other way as well so that both tonearms could start at the edge and play to the center. But, even with playing the disc vertically in a rotating clamp, the same trouble was observed with keeping the two tonearms in their respective synchronous revolutions. The system was developed further however and adapted so that a single tonearm could play one side of a record or the other in jukeboxes of the late 1930s and early '40s.
Five years later, Bell Labs was experimenting with a two-channel Lateral-Vertical system, where the left channel was recorded laterally and the right channel was recorded vertically, still utilizing a standard 3-mil 78-RPM groove, over three times larger than the modern LP stylus of the late 20th Century. The trouble with that was, once again, all the low-frequency rumble was in the left channel and all the high-frequency distortion was in the right channel. Over a quarter of a century later, it was decided to tilt the recording head 45 degrees off to the right side so that both the low frequency rumble and high frequency distortion were shared equally by both channels, producing the 45/45 system we know today.
In 1952, Emory Cook (1913–2002), who already had become famous by designing new feedback disk-cutter heads to improve sound from tape to vinyl, took the two-channel high-fidelity system described above and developed a somewhat misnamed "binaural" record out of it, which consisted of the same two separate channels cut into two separate groups of grooves running next to each other as described above, i.e. one running from the edge of the disc to halfway through and the other starting at the halfway point and ending up towards the label, but he used two lateral grooves with a 500 Hz crossover in the inner track to try and compensate for the lower fidelity and high frequency distortion on the inner track.
Each groove needed its own monophonic needle and cartridge on its own branch of tonearm, and each needle was connected to a separate amplifier and speaker. This setup was intended to give a demonstration at a New York audio fair of Cook's cutter heads rather than to sell the record; but soon afterward, the demand for such recordings and the equipment to play it grew, and Cook Records began to produce such records commercially. Cook recorded a vast array of sounds, ranging from railroad sounds to thunderstorms.[note 1] By 1953, Cook had a catalog of about 25 stereo records for sale to audiophiles.[27]
The first stereo recordings using magnetic tape were made in Germany in the early 1940s using Magnetophon recorders. Around 300 recordings were made of various symphonies, most of which were seized by the Red Army at the end of World War II. The recordings were of relatively high fidelity, thanks to the discovery of AC bias. A 1944 recording of Anton Bruckner's Symphony No. 8 directed by Herbert von Karajan and the Orchester der Berliner Staatsoper and a 1944 or 1945 recording of Walter Gieseking playing Beethoven's Piano Concerto No. 5 (with flak fire audible in the background) are the only recordings still known to exist.
In the US, stereo magnetic tape recording was demonstrated on standard 1/4-inch tape for the first time in 1952, using two sets of recording and playback heads, upside-down and offset from one another.[28] A year later, Remington Records began recording a number of its sessions in stereo, including performances by Thor Johnson and the Cincinnati Symphony Orchestra.
Later that same year, more experimental stereo recordings were conducted with Leopold Stokowski and a group of New York studio musicians at RCA Victor Studios in New York City. In February 1954, the label also recorded a performance of Berlioz' masterpiece The Damnation of Faust by the Boston Symphony Orchestra under the direction of Charles Münch, the success of which led to the practice of regularly recording sessions in stereo.
Shortly afterwards, RCA Victor recorded the last two NBC Radio broadcast concerts by famed conductor Arturo Toscanini and the NBC Symphony Orchestra, on stereophonic magnetic tape, however they were never officially released, though they have long been available on pirated LPs and CDs. In the UK, Decca Records began recording sessions in stereo in mid-1954, and by that time even smaller labels in the U.S. such as Concertapes, Bel Canto and Westminster along with major labels such as RCA Victor began releasing stereophonic recordings on two-track prerecorded reel-to-reel magnetic tape, priced at twice or three times the cost of monaural recordings, which retailed for around $2.95 to $3.95 apiece for a standard monaural LP. Even two-track monaural tape which had to be flipped over halfway through and carried exactly the same information as the monaural LP - but without the crackles and pops - were being sold for $6.95.[29]
One has to understand that, in the United States, the average working man in 1954 might be taking home $50–$60 a week if he was lucky and paying $75–$100 a month in rent for his two-room apartment. Therefore, the price of a great deal of two-track stereo tape recordings of the period being upwards of $12.95-$18.95 apiece for a full-length album when the corresponding mono LP was only $3.95, would be prohibitive. In addition, the cost of the stereophonic recorder upon which to play them may have been equal to or greater than the cost of a new car.
However, audiophiles, with little or no regard for the cost, bought them and the players anyway, and stereophonic sound came to at least a select few living rooms of the mid-1950s.[30] Stereo recording became widespread in the music business by the 3rd quarter of 1957.
In November 1957, the small Audio Fidelity Records label released the first mass-produced stereophonic disc. Sidney Frey, founder and president, had Westrex engineers, owners of one of the two rival stereo disk-cutting systems, cut a disk for release before any of the major record labels could do so.[31][32] Side 1 featured the Dukes of Dixieland, and Side 2 featured railroad and other sound effects designed to engage and envelop the listener. This demonstration disc was introduced to the public on December 13, 1957 at the Times Auditorium in New York City.[33] Only 500 copies of this initial demonstration record were pressed and three days later, Frey advertised in Billboard Magazine that he would send a free copy to anyone in the industry who wrote to him on company letterhead.[34][35] The move generated such a great deal of publicity[36] that early stereo phonograph dealers were forced to demonstrate on Audio Fidelity Records.
Also in December 1957, Bel Canto Records, another small label, produced its own stereophonic demonstration disc on multicolored vinyl so that stereo dealers would have more than one choice for demonstration. With the supplied special turntables featuring a clear platter lighted from underneath to show off the color as well as the sound, the stunt worked even better for Bel Canto, whose roster of jazz, easy listening and lounge music, pressed onto their trademark Caribbean-blue vinyl sold well throughout 1958 and early into 1959.
When Audio Fidelity released its stereophonic demonstration disc, there was no affordable magnetic cartridge on the market capable of playing it. After the release of other demonstration discs and the respective libraries from which they were culled, the other spur to the popularity of stereo discs was the reduction in price of a stereo cartridge, for playing the disks, from $250 to $29.95 in June 1958.[37] The first four mass-produced stereophonic discs available to the buying public were released in March 1958—Johnny Puleo and his Harmonica Gang Volume 1 (AFSD 5830), Railroad – Sounds of a Vanishing Era (AFSD 5843), Lionel – Lionel Hampton and his Orchestra (AFSD 5849) and Marching Along with the Dukes of Dixieland Volume 3 (AFSD 5851). By the end of March, the company had four more stereo LPs available, interspersed with several Bel Canto releases.[38]
Although both monaural as well as stereo LP records were manufactured for the first ten years of stereo on disc, the major record labels stopped making monaural albums after 1968, relegating the format to 45 RPM singles, flexidiscs and radio promotional materials which continued on through the end of 1974.[39][40][41]
In December 1925, the BBC's experimental transmitting station, 5XX, in Daventry, Northamptonshire, made radio's first stereo broadcast—a concert from Manchester, conducted by Sir Hamilton Harty—with 5XX broadcasting the right channel nationally by long wave and local BBC stations broadcasting the left channel by medium wave. The BBC repeated the experiment in 1926, using 2LO in London and 5XX at Daventry. Following experimental FM stereo transmissions in the London area in 1958 and regular Saturday morning demonstration transmissions using TV sound and medium wave (AM) radio to provide the two channels, the first regular BBC transmissions using an FM stereo signal began on the BBC's Third Programme network on August 28, 1962.
Chicago AM radio station WGN (and its sister FM station, WGNB) collaborated on an hourlong stereophonic demonstration broadcast on May 22, 1952, with one audio channel broadcast by the AM station and the other audio channel by the FM station.[42] New York City's WQXR initiated its first stereophonic broadcasts in October 1952, and by 1954, was broadcasting all of its live musical programs in stereophonic sound, using its AM and FM stations for the two audio channels.[43] Rensselaer Polytechnic Institute began a weekly series of live stereophonic broadcasts in November 1952 by using two campus-based AM stations, although the listening area did not extend beyond the campus.[44]
Tests of six competing FM-only systems were conducted on KDKA-FM in Pittsburgh, Pennsylvania during July and August 1960.[45] The Federal Communications Commission announced stereophonic FM technical standards in April 1961, with licensed regular stereophonic FM radio broadcasting set to begin in the United States on June 1, 1961.[46] WEFM (in the Chicago area) and WGFM (in Schenectady, New York) were reported as the first stereo stations.[47]
A December 11, 1952 closed-circuit television performance of Carmen, from the Metropolitan Opera House in New York City to 31 theaters across the United States, included a stereophonic sound system developed by RCA.[48] The first several shows of the 1958–59 season of The Plymouth Show (AKA The Lawrence Welk Show) on the ABC (America) network were broadcast with stereophonic sound in 75 media markets, with one audio channel broadcast via television and the other over the ABC radio network.[49][50] By the same method, NBC Television and the NBC Radio Network offered stereo sound for two three-minute segments of The George Gobel Show on October 21, 1958.[51] On January 30, 1959, ABC's Walt Disney Presents made a stereo broadcast of The Peter Tchaikovsky Story—including scenes from Disney's latest animated feature, Sleeping Beauty—by using ABC-affiliated AM and FM stations for the left and right audio channels.[52]
With the advent of FM stereo in 1961, a small number of music-oriented TV shows were broadcast with stereo sound using a process called simulcasting, in which the audio portion of the show was carried over a local FM stereo station.[53] In the 1960s and 1970s, these shows were usually manually synchronized with a reel-to-reel tape delivered by mail to the FM station (unless the concert or music originated locally). In the 1980s, satellite delivery of both television and radio programs made this fairly tedious process of synchronization unnecessary. One of the last of these simulcast programs was Friday Night Videos on NBC, just before MTS stereo was approved by the FCC.
The BBC made extensive use of simulcasting between 1974 and around 1990. The first such transmission was in 1974, when the BBC broadcast a recording of Van Morrison's London Rainbow Concert simultaneously on BBC2 TV and Radio 2. After that it was used for many other music programmes, live and recorded, including the annual BBC Promenade concerts and the Eurovision Song Contest. The advent of NICAM stereo sound with TV rendered this unnecessary.
Cable TV systems delivered many stereo programs utilizing this method for many years until prices for MTS stereo modulators dropped. One of the first stereo cable stations was The Movie Channel, though the most popular cable TV station that drove up usage of stereo simulcasting was MTV.
Japanese television began multiplex (stereo) sound broadcasts in 1978,[54] and regular transmissions with stereo sound came in 1982.[55] By 1984, about 12% of the programming, or about 14 or 15 hours per station per week, made use of the multiplex technology. West Germany's second television network, ZDF, began offering stereo programs in 1984.[54]
In 1979, The New York Times reported, "What has prompted the [television] industry to embark on establishing high-fidelity [sound] standards now, according to engineering executives involved in the project, is chiefly the rapid march of the new television technologies, especially those that are challenging broadcast television, such as the video disk."[56]
Multichannel television sound, better known as MTS (often still as BTSC, for the Broadcast Television Systems Committee that created it), is the method of encoding three additional channels of audio into an NTSC-format audio carrier. It was adopted by the FCC as the United States standard for stereo television transmission in 1984. Sporadic network transmission of stereo audio began on NBC on July 26, 1984, with The Tonight Show Starring Johnny Carson—although at the time, only the network's New York City flagship station, WNBC, had stereo broadcast capability.[57] Regular stereo transmission of programs began in 1985.
This uses two parallel omnidirectional microphones some distance apart, capturing time-of-arrival stereo information as well as some level (amplitude) difference information—especially if employed in close proximity to the sound source(s). At a distance of about 60 cm (24 in), the time delay (time-of-arrival difference) for a signal reaching the first microphone and then the other one from the side is approximately 1.5 ms (1 to 2 ms). If you increase the distance between the microphones, you effectively decrease the pickup angle. At a 70 cm (28 in) distance, it is approximately equivalent to the pickup angle of the near-coincident ORTF setup.
This technique can produce phase issues when the stereo signal is mixed to mono.
Here, two directional microphones are at the same place, typically pointing at an angle between 90° and 135° to each other.[58] The stereo effect is achieved through differences in sound pressure level between two microphones. A difference in levels of 18 dB (16 to 20 dB) is needed for hearing the direction of a loudspeaker. Due to the lack of differences in time-of-arrival/phase ambiguities, the sonic characteristic of X-Y recordings has less sense of space and depth when compared to recordings employing an A-B setup. When two figure-eight microphones are used, facing ±45° with respect to the sound source, the X-Y setup is called a Blumlein Pair. The sonic image produced is realistic.
This coincident technique employs a bidirectional microphone facing sideways and another microphone at an angle of 90°, facing the sound source. The second microphone is generally a variety of cardioid, although Alan Blumlein described the usage of an omnidirectional transducer in his original patent.
The left and right channels are produced through a simple matrix: Left = Mid + Side; Right = Mid − Side (the polarity-reversed side signal). This configuration produces a completely mono-compatible signal and, if the Mid and Side signals are recorded (rather than the matrixed Left and Right), the stereo width can be manipulated after the recording has taken place. This makes it especially useful for film-based projects.
These techniques combine the principles of both A-B and X-Y (coincident pair) techniques. For example, the ORTF stereo technique of the Office de Radiodiffusion Télévision Française (Radio France) calls for a pair of cardioid microphones placed 17 cm apart at a total angle between microphones of 110°, which results in a stereophonic pickup angle of 96° (Stereo Recording Angle, or SRA).[59] In the NOS stereo technique of the Nederlandse Omroep Stichting (Dutch Broadcasting Organisation), the total angle between microphones is 90° and the distance is 30 cm, thus capturing time-of-arrival stereo information as well as level information. It is noteworthy that all spaced microphone arrays and all near-coincident techniques use a spacing of at least 17 cm or more. 17 cm roughly equals the human ear distance and therefore provides the same interaural time difference (ITD) or more, depending on the spacing between microphones. Although the recorded signals are generally intended for playback over stereo loudspeakers, reproduction over headphones can provide remarkably good results, depending on the microphone arrangement.
In the course of restoration or remastering of monophonic records, various techniques of "pseudo-stereo", "quasi-stereo", or "rechanneled stereo" have been used to create the impression that the sound was originally recorded in stereo. These techniques first involved hardware methods (see Duophonic) or, more recently, a combination of hardware and software. Multitrack Studio, from Bremmers Audio Design (The Netherlands),[60] uses special filters to achieve a pseudo-stereo effect: the "shelve" filter directs low frequencies to the left channel and high frequencies to the right channel, and the "comb" filter adds a small delay in signal timing between the two channels, a delay barely noticeable by ear,[note 2] but contributing to an effect of "widening" original "fattiness" of mono recording.[61][62]
The special pseudo-stereo circuit—invented by Kishii and Noro, from Japan—was patented in the United States in 2003,[63] with already previously issued patents for similar devices.[64] Artificial stereo techniques have been used to improve the listening experience of monophonic recordings or to make them more "saleable" in today's market, where people expect stereo. Some critics have expressed concern about the use of these methods.[65]
Engineers make a technical distinction between "binaural" and "stereophonic" recording. Of these, binaural recording is analogous to stereoscopic photography. In binaural recording, a pair of microphones is put inside a model of a human head that includes external ears and ear canals; each microphone is where the eardrum would be. The recording is then played back through headphones, so that each channel is presented independently, without mixing or crosstalk. Thus, each of the listener's eardrums is driven with a replica of the auditory signal it would have experienced at the recording location. The result is an accurate duplication of the auditory spatiality that would have been experienced by the listener had he or she been in the same place as the model head. Because of the inconvenience of wearing headphones, true binaural recordings have remained laboratory and audiophile curiosities. However "loudspeaker-binaural" listening is possible with Ambiophonics.
Numerous early two-track-stereo reel-to-reel tapes as well as several experimental stereo disc formats of the early 1950s branded themselves as binaural, however they were merely different incarnations of the above-described stereo or two-track mono recording methods (lead vocal or instrument isolated on one channel and orchestra on the other sans lead.)
Stereophonic sound attempts to create an illusion of location for various sound sources (voices, instruments, etc.) within the original recording. The recording engineer's goal is usually to create a stereo "image" with localization information. When a stereophonic recording is heard through loudspeaker systems (rather than headphones), each ear, of course, hears sound from both speakers. The audio engineer may, and often does, use more than two microphones (sometimes many more) and may mix them down to two tracks in ways that exaggerate the separation of the instruments, in order to compensate for the mixture that occurs when listening via speakers.
Descriptions of stereophonic sound tend to stress the ability to localize the position of each instrument in space, but this would only be true in a carefully engineered and installed system, where speaker placement and room acoustics are taken into account. In reality, many playback systems, such as all-in-one boombox units and the like, are incapable of recreating a realistic stereo image. Originally, in the late 1950s and 1960s, stereophonic sound was marketed as seeming "richer" or "fuller-sounding" than monophonic sound, but these sorts of claims were and are highly subjective, and again, dependent on the equipment used to reproduce the sound. In fact, poorly recorded or reproduced stereophonic sound can sound far worse than well done monophonic sound. When playing back stereo recordings, the best results are obtained by using two identical speakers, in front of and equidistant from the listener, with the listener located on a center line between the two speakers. In effect, an equilateral triangle is formed, with the angle between the two speakers around 60 degrees as seen from the listener's point of view.
Although Decca had recorded Ansermet's conducting of Antar in stereo May 1954 it took four years for the first stereo LPs to be sold.[66] In 1958, the first group of mass-produced stereo two-channel vinyl records was issued, by Audio Fidelity in the USA and Pye in Britain, using the Westrex "45/45" single-groove system. Whereas the stylus moves horizontally when reproducing a monophonic disk recording, on stereo records, the stylus moves vertically as well as horizontally. One could envision a system in which the left channel was recorded laterally, as on a monophonic recording, with the right channel information recorded with a "hill and dale" vertical motion; such systems were proposed but not adopted, due to their incompatibility with existing phono pickup designs (see below).
In the Westrex system, each channel drives the cutting head at a 45-degree angle to the vertical. During playback, the combined signal is sensed by a left-channel coil mounted diagonally opposite the inner side of the groove and a right-channel coil mounted diagonally opposite the outer side of the groove.[67] The Westrex system provided for the polarity of one channel to be inverted: this way large groove displacement would occur in the horizontal plane and not in the vertical one. The latter would require large up-and-down excursions and would encourage cartridge skipping during loud passages.
The combined stylus motion is, in terms of the vector, the sum and difference of the two stereo channels. Effectively, all horizontal stylus motion conveys the L+R sum signal, and vertical stylus motion carries the L−R difference signal. The advantages of the 45/45 system are that it has greater compatibility with monophonic recording and playback systems.
Even though a monophonic cartridge will technically reproduce an equal blend of the left and right channels, instead of reproducing only one channel, this was not recommended in the early days of stereo due to the larger stylus (1.0 mil vs 0.7 mil for stereo) coupled with the lack of vertical compliance of the mono cartridges available in the first ten years of stereo. These factors would result in the stylus ``digging into' the stereo vinyl and carving up the stereo portion of the groove, destroying it for subsequent playback on stereo cartridges. This is why one often notices the banner PLAY ONLY WITH STEREO CARTRIDGE AND STYLUS on stereo vinyl issued between 1958 and 1964.
Conversely, and with the benefit of no damage to any type of disc even from the beginning, a stereo cartridge reproduces the lateral grooves of monophonic recording equally through both channels, rather than through one channel. Also, it gives a more balanced sound, because the two channels have equal fidelity as opposed to providing one higher-fidelity laterally recorded channel and one lower-fidelity vertically recorded channel. Overall, this approach may give higher fidelity, because the "difference" signal is usually of low power, and is thus less affected by the intrinsic distortion of "hill and dale"-style recording.
Additionally, surface noise tends to be picked up in a greater capacity in the vertical channel, therefore a mono record played on a stereo system can be in worse shape than the same record in stereo and still be enjoyable. (See Gramophone record for more on lateral and vertical recording.)
This system was conceived by Alan Blumlein of EMI in 1931 and was patented in the U.K. the same year, but was not reduced to actual practice as was a requirement for patenting in the U.S. and elsewhere at that time. (Blumlein was killed in a plane crash while testing radar equipment during WW-II, and he, therefore, never reduced the system to actual practice through both a recording and a reproducing means.) EMI cut the first stereo test discs using the system in 1933, but it was not applied commercially until a quarter of a century later, and by another company (Westrex division of Litton Industries Inc, as a successor to Western Electric Company), and dubbed StereoDisk. Stereo sound provides a more natural listening experience, since the spatial location of the source of a sound is (at least in part) reproduced.
In the 1960s, it was common practice to generate stereo versions of music from monophonic master tapes, which were normally marked "electronically reprocessed" or "electronically enhanced" stereo on track listings. These were generated by a variety of processing techniques to try to separate out various elements; this left noticeable and unsatisfactory artifacts in the sound, typically sounding "phasey". However, as multichannel recording became increasingly available, it has become progressively easier to master or remaster more plausible stereo recordings out of the archived multitrack master tapes.
The Red Book CD specification includes two channels by default, and so a mono recording on CD either has one empty channel, or else the same signal being relayed to both channels simultaneously. However, noncommercial CDs in other formats such as White Book or Orange Book can feature up to four hours of stereo music on one CD for the purposes of extended programming in public spaces, such as malls.
These formats slightly reduce both the sampling frequency from 44.1 kHz as well as the bit-depth frequency from 16-bit and employ other proprietary technologies in order to increase the time on the disc and, as in the 16-inch transcriptions above - render them unplayable on the vast majority of consumer equipment.
In FM broadcasting, the Zenith-GE pilot-tone stereo system is used throughout the world.
Because of the limited audio quality of the majority of AM receivers, and also because AM stereo receivers are relatively scarce, relatively few AM stations employ stereo. Various modulation schemes are used for AM stereo, of which the best-known is Motorola's C-QUAM, the official method for most countries in the world that transmit in AM stereo. More AM stations are adopting digital HD Radio, which allows the transmission of stereo sound on AM stations. For Digital Audio Broadcasting, MP2 audio streams are used. DAB is one of the Digital Radio formats that is used to broadcast Digital Audio over terrestrial broadcast networks or satellite networks. DAB is extended to video, and the new format is called DMB.
In Sweden, Televerket invented a different stereo broadcasting system called the Compander System. It had a high level of channel separation and could even be used to broadcast two mono signals - for example for language studies (with two languages at the same time). But tuners and receivers with the pilot-tone system were sold so people in southern Sweden could listen to, for example, Danish radio. At last Sweden (the Televerket) decided to start broadcasting in stereo according to the pilot-tone system in 1977. But stereo radio was delayed in Sweden because of the two competing systems.[citation needed]
For analog TV (PAL and NTSC), various modulation schemes are used in different parts of the world to broadcast more than one sound channel. These are sometimes used to provide two mono sound channels that are in different languages, rather than stereo. Multichannel television sound is used mainly in the Americas. NICAM is widely used in Europe, except in Germany, where Zweikanalton is used. The EIAJ FM/FM subcarrier system is used in Japan. For Digital TV, MP2 audio streams are widely used within MPEG-2 program streams. Dolby Digital is the audio standard used for Digital TV in North America, with the capability for anywhere between 1 and 6 discrete channels.
In common usage, a "stereo" is a two-channel sound reproduction system, and a "stereo recording" is a two-channel recording. This is cause for much confusion, since five (or more)-channel home theater systems are not popularly described as "stereo".
Most two-channel recordings are stereo recordings only in this weaker sense. Pop music, in particular, is usually recorded using close miking techniques, which artificially separate signals into several tracks. The individual tracks (of which there may be hundreds) are then "mixed down" into a two-channel recording. The audio engineers determine where each track will be placed in the stereo "image", by using various techniques that may vary from very simple (such as "left-right" panning controls) to more sophisticated and extensively based on psychoacoustic research (such as channel equalization, compression and mid-side processing). The end product using this process often bears little or no resemblance to the actual physical and spatial relationship of the musicians at the time of the original performance; indeed, it is not uncommon for different tracks of the same song to be recorded at different times (and even in different studios) and then mixed into a final two-channel recording for commercial release.
Classical music recordings are a notable exception. They are more likely to be recorded without having tracks dubbed in later as in pop recordings, so that the actual physical and spatial relationship of the musicians at the time of the original performance can be preserved on the recording.
Balance can mean the amount of signal from each channel reproduced in a stereo audio recording. Typically, a balance control in its center position will have 0 dB of gain for both channels and will attenuate one channel as the control is turned, leaving the other channel at 0 dB.[68]
See also Panning.
Blumlein lodged the patent for 'binaural sound', in 1931, in a paper which patented stereo records, stereo films and also surround sound. He and his colleagues then made a series of experimental recordings and films to demonstrate the technology, and see if there was any commercial interest from the fledgling film and audio industry.
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| リンク元 | 「立体」「steric」「ステレオ」 |
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