Source Input Format

Source Input Format defined in MPEG-1, is a video format that was developed to allow the storage and transmission of digital video.

625/50 SIF format has a resolution of ' active pixels [or active pixels ] and a refresh rate of 25 frames per second.
525/59.94 SIF Format has a resolution of ' active pixels [or active pixels ] and a refresh rate of 29.97 frames per second.

When compared to the CCIR 601 specifications, which defines the appropriate parameters for digital encoding of TV signals, SIF can be seen as being reduced by half in all of height, width, frame-rate, and chrominance. SIF video is known as a constrained parameters bitstream.
On square-pixel displays SIF images should be rescaled so that the picture covers a 4:3 area, in order to avoid a "stretched" look. So the computer industry has defined "square-pixel SIF" to be 320 x 240 active pixels or 384 x 288 active pixels, with a refresh rate of whatever the computer is capable of supporting. To reach that the SIF content need to be "expanded" horizontally by 12:11 for PAL and "reduced" horizontally by 10:11 for NTSC.