H.120 was the first digital video encoding standard. It was developed by COST 211 and published by the CCITT 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 existing codecs for the format, but it provided important knowledge leading directly to its practical successors, such as H.261.
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.[2]
