- H.120
H.120 was the first digital
video encoding standard. It was developed byCOST 211 and published by theCCITT in 1984, with a revision in 1988. The video turned out not to be of adequate quality, there were few implementations, and there are no existingcodec s for the format, but it provided important knowledge leading directly to its practical successors, such asH.261 .tream format
H.120 streams ran at 1544 kbit/s for
NTSC and 2048 kbit/s forPAL . Version 1 (1984) featuredconditional replenishment , differentialpulse-code modulation ,scalar quantization , variable-length coding and a switch forquincunx sampling. Version 2 (1988) addedmotion compensation andbackground prediction. [ [http://www.itu.int/ITU-T/worksem/vica/docs/presentations/S0_P2_Sullivan.pdf Overview of International Video Coding Standards (preceding H.264/AVC)] (PDF) (Gary J. Sullivan,ITU , 2005)] A final version was published in 1993.Problems and knowledge gained
H.120 video was not of good enough quality for practical use — it had very good spatial resolution (as differential PCM works on a pixel-by-pixel basis), but very poor temporal quality. It became clear to researchers that to improve the video quality without exceeding the target bitrate for the stream, it would be necessary to encode using an average of less than one bit for each pixel. This would require groups of pixels to be coded together. This led to the block-based codecs that followed H.120, such as
H.261 , the first practical video encoding standard.References
External links
* [http://www.itu.int/rec/T-REC-H.120/en H.120 : Codecs for videoconferencing using primary digital group transmission] (
ITU )
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