Page 1:    Introduction (this page)

Page 2:    AVI files and associated compression algorithms

Page 3:    MPEG-1

Page 4:    MPEG-2

Page 5:    MPEG-4

Page 6:    DV-AVI

Page 7:    Microsoft WMV, WMA, and ASF

Page 8:    Real Media

Page 9:    Transmission and Recording Standards: DV (tape and FireWire), Cable, and Satellite

Page 10:  Notes on various Video and Audio Coding Standards

Appendix: My letter to Widescreen Review about MPEG-4



Digitally encoded video material (including associated audio tracks) may be stored on tape, disc, memory card (such as a SmartMedia card), or on a PC’s hard-drive.  It may also be transmitted over a communications link, such as a satellite TV downlink, a cable TV system, or a FireWire link between two devices.  When stored on random-access media (disc, card, or hard-drive), digital video material is generally organized into files.  On a tape, or on a continuous feed over a communications link, there is no file structure but the material is generally organized into frames to allow the receiving end to find a starting point for interpreting the bits.  (This continuous-stream situation should not be confused with the use of a communications link to copy a file of digital video material between one PC and another, where a file-transfer protocol copies the file as a complete unit across the link, using a file-transfer protocol.)

Uncompressed “raw” digitalized video and audio signals are created from the analog signals generated by the camera and microphones in a TV studio or camcorder, or generated by a film-to-video convertor.  Before these signals are stored or transmitted they are often processed through a compression algorithm to reduce the total number of bits to be stored/transmitted.  But in a few special cases the “raw” uncompressed digital streams are maintained.  Uncompressed video is used in the creation of high-quality digital “masters” for digitally-shot movies or TV shows, and in the creation of digital masters from traditionally-shot films. 

Various compression algorithms are in widespread use, including DV, MPEG-1, MPEG-2, and MPEG-4.  These vary in terms of the efficiency with which they compress the video information.  MPEG-2 (used on DVDs and most satellite and cable TV systems) is more efficient than the older MPEG-1 (used on VCDs), and MPEG-4 is more efficient than MPEG-2.  DV is, by design, the least efficient algorithm.  It was designed so that it reduced the bit-rate just enough to make storage on tape and transmission over FireWire easier, while maintaining high-quality discrete images for each video frame.  The resulting material is typically edited before a more aggressive compression algorithm is applied.

When stored as a file, digital video material (including associated audio material) generally has the following characteristics:

§         The video and audio components are interleaved in some way, so that any section of the file can be easily played without making the job of accurately aligning the timing of the extracted video and audio components needlessly difficult.

§         The file has a name using the standard computer format “name.extension”.

§         The file contains header information that identifies the compression algorithm (if any) that has been used in preparing the information for the file.  (Note that, in some cases, a file extension such as “.mpg” or “.vob” may also indicate which algorithm has been used for compression.  However, extensions like “.avi” are not unique to one algorithm.  Where the extension and the information in the file header are in conflict, most video players will ignore the extension and follow the header information.)


AVI files and associated compression algorithms


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MPEG-4     [Also: See My letter to Widescreen Review about MPEG-4]


Microsoft WMV, WMA, and ASF

Real Media

Transmission and Recording Standards: DV (tape and FireWire), Cable, and Satellite

Notes on various Video and Audio Coding Standards