of the actual 4:3 or 16:9 image. For SMPTE 259M-C compliance, an SDTV broadcast image is scaled to 720 pixels wide for every 480 NTSC (or 576 PAL) lines of the image with the amount of non-proportional line scaling dependent on either the display or pixel aspect ratio. The display ratio for broadcast widescreen is commonly 16:9, the display ratio for a traditional or letterboxed broadcast is 4:3. An SDTV image outside the constraints of the SMPTE standards requires no non-proportional scaling with 640 pixels for every line of the image. The display and pixel aspect ratio is generally not required with the line height defining the aspect. For widescreen 16:9, 360 lines define a widescreen image and for traditional 4:3, 480 lines define an image. See also Digital Audio Broadcasting Moving Picture Experts Group ISDB-T International Rec. 601 (aka CCIR 601) Notes References External links Programmer's Guide to Video Systems ATSC Digital television Broadcast engineering Broadband History of television