Nominal analogue blanking
Encyclopedia
Nominal analog blanking or nominal analogue blanking is the outermost part of the overscan
of a standard definition digital television
image. It consists of a variable and arbitrary gap of black (or nearly black) pixels at the left and right sides, which correspond to the end and start of the horizontal blank
ing interval: the front porch at the right side (the end of a line, before the sync pulse), 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, but only about 702 (PAL) to 704 (NTSC) of them contain picture signal. The edge may not be clean — some blurring can occur — and the location is arbitrary, since analogue equipment may typically shift the picture sideways in an unexpected amount or direction. Really old analogue equipment can cause the total width of 'active picture' to vary, usually downwards, perhaps to as little as 680.
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 13.5MHz 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 variants
), 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 854x480.
For details, see the technical specifications of overscan amounts.
Overscan
Overscan is extra image area around the four edges of a video image that may not be seen reliably by the viewer. It exists because television sets in the 1930s through 1970s were highly variable in how the video image was framed within the cathode ray tube .-Origins of overscan:Early televisions...
of a standard definition digital television
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
image. It consists of a variable and arbitrary 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 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 back and forth from the left to right side of the screen...
ing interval: the front porch at the right side (the end of a line, before the sync pulse), 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, but only about 702 (PAL) to 704 (NTSC) of them contain picture signal. The edge may not be clean — some blurring can occur — and the location is arbitrary, since analogue equipment may typically shift the picture sideways in an unexpected amount or direction. Really old analogue equipment can cause the total width of 'active picture' to vary, usually downwards, perhaps to as little as 680.
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 13.5MHz 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 variants
DivX
DivX is a brand name of products created by DivX, Inc. , including the DivX Codec which has become popular due to its ability to compress lengthy video segments into small sizes while maintaining relatively high visual quality.There are two DivX codecs; the regular MPEG-4 Part 2 DivX codec and the...
), 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 854x480.
For details, see the technical specifications of overscan amounts.
See also
- Safe areaSafe areaSafe area is a term used in television production to describe the areas of the television picture that can be seen on television screens.Older televisions can display less of the space outside of the safe area than ones made more recently...
- Overscan
- Horizontal blanking interval
- Vertical blanking intervalVertical blanking intervalThe vertical blanking interval , also known as the vertical interval or VBLANK, is the time difference between the last line of one frame or field of a raster display, and the beginning of the first line of the next frame. It is present in analog television, VGA, DVI and other signals. During the...