The process of reducing the bandwidth or data rate required to transport a video or audio stream. The old analog broadcast standards, PAL, NTSC and SECAM are, in fact, compression systems which greatly reduce the information that was present in the original RGB sources.
Digital compression systems analyze their sources, such as pictures and sound, to find and remove redundancy both within the signal. Some of the techniques were primarily developed for digital data transmission but have been adopted as a means of reducing broadcast transmission bandwidths and storage requirements on disks and VTRs.
A number of compression systems are in regular use for moving images. These include ETSI, JPEG, Motion JPEG, JPEG 2000, DV, MPEG-1, MPEG-2, MPEG-4, AVC, H.264, Windows Media, Real and many more. Where different techniques are used in the same workflow, quality problems can occur and picture quality/ audio can suffer more than if the same method is used throughout.
The MPEG-2 family of compression schemes, which was originally designed for program transmission, has been adapted for studio use in Betacam SX and IMX recorders.
While there is much debate, and new technologies continue to be developed, it remains true that the best compressed results are produced from the highest quality source pictures. Poor quality inputs do not compress well. Noise, which may be interpreted as important picture detail, is the enemy of compression.
The need for ever more efficient compression systems is partly driven by the bigger TV picture formats, HD images are about five times the area of SD. More recently UHDTV-1 (4K) and UHDTV-2 (8K), respectively are 4 and 16 times the picture area of 1920 x 1080 HD. New HEVC / H.265 coding is expected to be twice as efficient as MPEG-4, which is already twice as efficient as MPEG-2. This efficiency combined with the twice-as-efficient DVB-T2 and DVB-S2, together mean that 32 SD channels can now broadcast in the space that used to carry one anaolg TV channel. It also means that the channel could carry one 4K TV channel.