View source for G.728

{{Short description|ITU-T standard for speech coding}}
{{Infobox standard
| title           = G.728
| version         = Bit-exact fixed point (1994
}}
'''G.728''' is an [[ITU-T]] [[International standard|standard]] for [[speech coding]] operating at 16&nbsp;[[kilobit|kbit]]/s. It is officially described as ''Coding of speech at 16&nbsp;kbit/s using low-delay code excited linear prediction''.

Technology used is LD-[[CELP]], low-delay code excited linear prediction. Delay of the [[codec]] is only 5 samples (0.625&nbsp;ms). The linear prediction is calculated backwards with a 50th order [[linear predictive coding]] filter. The excitation is generated with gain scaled [[vector quantization|VQ]]. The standard was finished in 1992 in the form of algorithm exact floating point code. In 1994 a [[bit exact]] [[Fixed-point arithmetic|fixed point]] codec was released. G.728 passes low bit rate [[modem]] signals up to {{val|2400|u=bit/s}}. Also [[Computer network|network]] signaling goes through. The complexity of the codec is 30 [[Instructions per second|MIPS]]. {{val|2|ul=kilobytes}} of [[Random access memory|RAM]] is needed for codebooks. [[Mean opinion score]] for G.728 is 3.61.

The essence of CELP techniques, which is an analysis-by-synthesis approach to codebook search, is retained in LD-CELP. The LD-CELP however, uses backward adaptation of predictors and gain to achieve an algorithmic delay of 0.625&nbsp;ms.

[[RealAudio]] 28.8 is a reduced-bitrate variant of this standard, using {{val|15.2|u=kbit/s}}.<ref>{{Cite web|url=https://wiki.multimedia.cx/index.php?title=RealAudio_28.8|title=RealAudio 28.8 - MultimediaWiki|website=wiki.multimedia.cx}}</ref>

== See also ==
*[[List of codecs]]
*[[Comparison of audio coding formats]]

== References ==
<references />

== External links ==
* [http://www.itu.int/rec/recommendation.asp?type=folders&lang=e&parent=T-REC-G.728 ITU-T G.728 page]
* [ftp://svr-ftp.eng.cam.ac.uk/pub/comp.speech/coding/ldcelp-2.0.tar.gz C source code for G.728]{{dead link|date=May 2025|bot=medic}}{{cbignore|bot=medic}}

{{Compression formats}}

[[Category:Speech codecs]]
[[Category:Audio codecs]]
[[Category:ITU-T recommendations]]
[[Category:ITU-T G Series Recommendations]]