It is often desired to display a movie originally recorded on film by means of a television broadcast. In order to comply with television broadcast field rates, some sort of film-to-video conversion must be performed.
Typically, a movie is recorded and displayed at a frame rate of 24 frames per second. However, television broadcasts use a different rate, such as the 59.94 field per second rate of the NTSC standard where every two fields are interlaced and comprise one frame.
One method of converting film frame rates to television field rates is referred to as the "3:2 pulldown" scanning method. A first film frame is scanned three times, then a second film frame is scanned twice, the next frame three times, etc. Every six film frames take the same time as fifteen fields of the television signal. To accommodate the fact that the NTSC vertical scan period is slightly less than 60 fields per second, fields may be dropped to match the actual rate of the receiver.
Film to video conversion is now being combined with techniques for digitizing the video data. These techniques include digital compression techniques for providing less bandwidth (bits per second). One compression technique is embodied in a standard known as "MPEG", named for the Moving Picture Experts Group that began the effort to provide a standard. The MPEG standard attempts to strike a balance between the high compression associated with interframe coding and the random access capability associated with intraframe coding. To answer this challenge, the MPEG standard uses two interframe coding techniques, predictive and interpolative, and an intracoded technique. For coding video data, the MPEG encoding techniques are used to encode MPEG "pictures" from fields or frames of video data.