Nominal analogue blanking is the outermost part of the

overscan Overscan is a behaviour in certain television sets, in which part of the input picture is shown outside of the visible bounds of the screen. It exists because cathode-ray tube (CRT) television sets from the 1930s through to the early 2000s were h ...

of a

standard definition Standard-definition television (SDTV, SD, often shortened to standard definition) is a television system which uses a resolution that is not considered to be either high or enhanced definition. "Standard" refers to it being the prevailing sp ...

digital television Digital television (DTV) is the transmission of television signals using digital encoding, in contrast to the earlier analog television technology which used analog signals. At the time of its development it was considered an innovative advanc ...

image. It consists of a gap of black (or nearly black) pixels at the left and right sides, which correspond to the end and start of the

horizontal blank Horizontal blanking interval refers to a part of the process of displaying images on a computer monitor or television screen via raster scanning. CRT screens display images by moving beams of electrons very quickly across the screen. Once the bea ...

ing interval: the

front porch A porch (from Old French ''porche'', from Latin ''porticus'' "colonnade", from ''porta'' "passage") is a room or gallery located in front of an entrance of a building. A porch is placed in front of the facade of a building it commands, and form ...

at the right side (the end of a line, before the

sync pulse Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, phase and frequency of an analog s ...

), and the back porch at the left side (the start of a line, after the sync pulse and before drawing the next line). Digital television ordinarily contains 720 pixels per line, but only 702 (PAL) to 704 (NTSC) of them contain picture content. The location is variable, since analogue equipment may shift the picture sideways in an unexpected amount or direction. The exact width is determined by taking the definition of the time required for an active line in PAL or NTSC, and multiplying it by the pixel clock of 13.5 MHz of Digital SDTV. PAL is exactly 52μs, so it will equate to exactly 702 pixels. Notably, screen shapes and aspect ratios were defined in an era of purely analogue broadcasting for TV. This means that any picture with nominal analogue blanking, whether it be 702, around 704, or less, will be — by definition — a 4:3 picture. Therefore, when cross-converting into a square-pixel environment (like MPEG-4 and its

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), this width must always scale to 768 (''PAL'') or 640 (''NTSC''). ''This has the outcome of causing a full picture of 720x576 or 720x480 to be wider than 4:3''. In fact, a purely digitally sourced SDTV image, with no analogue blanking, will be close to 788x576 or 655x480 once stretched to square pixels. Standard definition widescreen pictures were also defined in an analogue environment and must also be treated as such. This means that a purely digitally sourced widescreen SDTV image, with no analogue blanking, will be close to 1050x576 or 873x480.{{citation needed, date=October 2014 For details, see the technical specifications of overscan amounts.

References

* ITU-R BT.601: Studio encoding parameters of digital television for standard 4:3 and wide screen 16:9 aspect ratios * ITU-R BT.1700: Characteristics of composite video signals for conventional analogue television systems

References

See also