H.120 was the first digital video compression standard. It was developed by COST 211 and published by the CCITT (now the

ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Co ...

) in 1984, with a revision in 1988 that included contributions proposed by other organizations. The video turned out not to be of adequate quality, there were few implementations, and there are no existing

codec A codec is a device or computer program that encodes or decodes a data stream or signal. ''Codec'' is a portmanteau of coder/decoder. In electronic communications, an endec is a device that acts as both an encoder and a decoder on a signal or ...

s for the format, but it provided important knowledge leading directly to its practical successors, such as H.261. The latest revision was published in March 1993.

Stream format

H.120 streams ran at 1544 kbit/s for

NTSC The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...

and 2048 kbit/s for

PAL Phase Alternating Line (PAL) is a colour encoding system for analogue television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...

. Version 1 (1984) featured conditional replenishment,

differential pulse-code modulation Differential pulse-code modulation (DPCM) is a signal encoder that uses the baseline of pulse-code modulation (PCM) but adds some functionalities based on the prediction of the samples of the signal. The input can be an analog signal or a digital ...

(DPCM),

scalar quantization Quantization, in mathematics and digital signal processing, is the process of mapping input values from a large set (often a continuous set) to output values in a (countable) smaller set, often with a finite number of elements. Rounding and t ...

, variable-length coding and a switch for sampling. Version 2 (1988) added

motion compensation Motion compensation in computing, is an algorithmic technique used to predict a frame in a video, given the previous and/or future frames by accounting for motion of the camera and/or objects in the video. It is employed in the encoding of video d ...

and background prediction.Overview of International Video Coding Standards (preceding H.264/AVC)
(

PDF Portable Document Format (PDF), standardized as ISO 32000, is a file format developed by Adobe in 1992 to present documents, including text formatting and images, in a manner independent of application software, hardware, and operating systems. ...

) ( Gary J. Sullivan,

ITU The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Union ...

, 2005) A final edition (without new technical content) was published in 1993 as a result of the creation of the

to replace the prior CCITT standardization body.

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 discrete cosine transform (DCT) block-based codecs that followed H.120, such as H.261, the first practical video encoding standard.

References

External links

H.120 : Codecs for videoconferencing using primary digital group transmission
(

) {{telecomm-stub ITU-T recommendations ITU-T H Series Recommendations H.26x Video codecs