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Nominal analogue blanking is the outermost part of theoverscan of astandard definitiondigital television 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 thehorizontal blanking interval: thefront porch at the right side (the end of a line, before thesync pulse), and theback 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 52 μs, so it will equate to exactly 702 pixels.[citation needed]
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 itsvariants), 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 to788 × 576 or655 × 480 once stretched to square pixels.[citation needed]
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 to1050 × 576 or873 × 480.[citation needed]
For details, seethe technical specifications of overscan amounts.