For vertical intervals in cartography, see Contour line.
|
|
This article does not cite any sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2009) (Learn how and when to remove this template message) |
In a raster graphics display, the vertical blanking interval (VBI), also known as the vertical interval or VBLANK, is the time between the end of the final line of a frame or field and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the VBI, the incoming data stream is not displayed on the screen. In raster cathode ray tube displays, the beam is blanked to avoid displaying the retrace line; see raster scan for details. The signal source, such as a television broadcast, does not supply image information during the blanking period.
The VBI was originally needed because of the inductive inertia of the magnetic coils which deflect the electron beam vertically in a CRT; the magnetic field, and hence the position being drawn, cannot change instantly. Additionally, the speed of older circuits was limited. For horizontal deflection, there is also a pause between successive lines, to allow the beam to return from right to left, called the horizontal retrace or horizontal blanking interval. Modern CRT circuitry does not require such a long blanking interval, and thin panel displays require none, but the standards were established when the delay was needed (and to allow the continued use of older equipment). Blanking of a CRT may not be perfect due to equipment faults or brightness set very high; in this case a white retrace line shows on the screen, from bottom right to top left.
In analog television systems the vertical blanking interval can be used for datacasting (to carry digital data), since nothing sent during the VBI is displayed on the screen; various test signals, time codes, closed captioning, teletext, CGMS-A copy-protection indicators, and various data encoded by the XDS protocol (e.g., the content ratings for V-chip use) and other digital data can be sent during this time period. In U.S. analog broadcast television, line 19 was reserved for a ghost-cancelling signal; line 21 was reserved for captioning data. The obsolete Teletext service contemplated the use of line 22 for data transmission.
The pause between sending video data is used in real time computer graphics to perform various operations on the back buffer before copying it to the front buffer instead of just switching both pointers, or to provide a time reference for when switching such pointers is safe.
In video game systems the vertical blanking pulses are extensively used to time the plotting of new graphics/removal of old ones in order to avoid screen tearing, as they occur at an accurately known frequency, and many systems up to the 16-bit era featured games and other graphical programs where drawing was conducted during the blanking interval for this reason. Cases where synchronising game code this way was more necessary than preferable made early video game systems such as the Atari 2600 difficult to program.
Special raster techniques on the Atari 2600, Nintendo Entertainment System, and other consoles allowed extending this interval at the cost of some blank scanlines at the top or bottom of the screen, which may or may not end up in the overscan area. The use of double buffering in modern graphics hardware has rendered these techniques obsolete.
Most consumer VCRs use the known black level of the vertical blanking pulse to set their recording levels. The Macrovision copy protection scheme inserts pulses in the VBI, where the recorder expects a constant level, to disrupt recording to videotapes.
See also
- Vertical blank interrupt
- Datacasting
- Horizontal blanking interval
- Nominal analogue blanking
- Raster scan
|
Broadcast video formats
|
|
|
Television
|
|
|
Analog
|
|
525 lines
|
- System M
- NTSC
- NTSC-J
- PAL-M
|
|
|
625 lines
|
- PAL
- System B
- System D
- System G
- System H
- System I
- System K
- PAL-N
- PALplus
- SECAM
- System B
- System D
- System G
- System K
- System L (SECAM-L)
|
|
|
Audio
|
- BTSC (MTS)
- EIAJ
- NICAM
- SAP
- Sound-in-Syncs
- Zweikanalton (A2/IGR)
|
|
|
Hidden signals
|
- Captioning
- CGMS-A
- EPG
- GCR
- PDC
- Teletext
- VBI
- VEIL
- VIT
- VITC
- WSS
- XDS
|
|
|
Historical
|
- Pre-1940
- Mechanical television
- 180-line
- 405-line
- 441-line
- 819-line
- MAC
- MUSE
|
|
|
|
Digital
|
|
Interlaced
|
|
|
|
Progressive
|
|
|
|
MPEG-2 standards
|
- ATSC
- DVB
- ISDB
- DTMB
- DVB 3D-TV
|
|
|
MPEG-4 AVC standards
|
- ATSC A/72
- DMB
- DTMB
- DVB
- SBTVD
- 1seg
|
|
|
HEVC standards
|
|
|
|
Audio
|
- AC-3 (5.1)
- DTS
- MPEG-1 Audio Layer II
- MPEG Multichannel
- PCM
- LPCM
- AAC
- HE-AAC
|
|
|
Hidden signals
|
- AFD
- Broadcast flag
- Captioning
- CPCM
- EPG
- Teletext
|
|
|
|
|
|
Technical issues
|
|
- 14:9 compromise
- Broadcast-safe
- Digital cinema (DCI)
- Display motion blur
- Moving image formats
- MPEG transport stream
- Reverse Standards Conversion
- Standards conversion
- Television transmitter
- Video on demand
- Video processing
- Widescreen signaling
- Templates (Analogue TV Topics)
|
|
|
Analog television broadcasting topics
|
|
| Systems |
- 180-line
- 405-line ( System A )
- 441-line
- 525-line ( System J , System M )
- 625-line ( System B , System C , System D , System G , System H , System I , System K , System L , System N )
- 819-line ( System E , System F )
|
|
| Color systems |
- NTSC
- PAL
- PAL-M
- PAL-S
- PALplus
- SECAM
|
|
| Video |
- Back porch and front porch
- Black level
- Blanking level
- Chrominance
- Chrominance subcarrier
- Colorburst
- Color killer
- Color TV
- Composite video
- Frame (video)
- Horizontal scan rate
- Horizontal blanking interval
- Luma
- Nominal analogue blanking
- Overscan
- Raster scan
- Safe area
- Television lines
- Vertical blanking interval
- White clipper
|
|
| Sound |
- Multichannel television sound
- NICAM
- Sound-in-Syncs
- Zweikanalton
|
|
| Modulation |
- Frequency modulation
- Quadrature amplitude modulation
- Vestigial sideband modulation (VSB)
|
|
| Transmission |
- Amplifiers
- Antenna (radio)
- Broadcast transmitter/Transmitter station
- Cavity amplifier
- Differential gain
- Differential phase
- Diplexer
- Dipole antenna
- Dummy load
- Frequency mixer
- Intercarrier method
- Intermediate frequency
- Output power of an analog TV transmitter
- Pre-emphasis
- Residual carrier
- Split sound system
- Superheterodyne transmitter
- Television receive-only
- Direct-broadcast satellite television
- Television transmitter
- Terrestrial television
- Transposer
|
|
| Frequencies & Bands |
- Frequency offset
- Microwave transmission
- Television channel frequencies
- UHF
- VHF
|
|
| Propagation |
- Beam tilt
- Distortion
- Earth bulge
- Field strength in free space
- Knife-edge effect
- Noise (electronics)
- Null fill
- Path loss
- Radiation pattern
- Skew
- Television interference
|
|
| Testing |
- Distortionmeter
- Field strength meter
- Vectorscope
- VIT signals
- Zero reference pulse
|
|
| Artifacts |
- Dot crawl
- Ghosting
- Hanover bars
- Sparklies
|
