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Voice Compression for CISCO VoIP

By basit_jafri ·
Does anybody know about what algorithm does CISCO use for the voice compression for their VoIP thing? In the H.323 suit what is G.711 and G.729 algo???? I'm stuck with my project, pls help

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Voice Compression for CISCO VoIP

by christine In reply to Voice Compression for CIS ...

There are two approaches you can take to encoding. First is to sample the signal strength at a rate higher than the frequency of the signal.
To reproduce the original signal from a digital sample we must sample at a rate at least 2.2 times the maximum frequency represented in the underlying signal.This is called Pulse Code Modulation (PCM). This sampling approach can be refined by doing some additional processing. Adaptive Differential Pulse Code Modulation (ADPCM). PCM and ADPCM are used inITU standards G.711 and G.726, respectively.
The second approach to encoding is to split the voice signal up into larger chunks, which represent whole, recognizable sounds used in human speech. This approach is used for Codebook Excited Linear Prediction & its variants; prevalent examples include MPE/ACELP (ITU Standard G.723.1) and CS-ACELP (G.729).The standard by which telephone voice quality is measured is so-called "toll quality," which in effect means the quality delivered by PCM (G.711) With PCM the algorithm itself is pretty straightforward, so not too much processing power is required. The downside of PCM is that it uses up a whole 64kbps for each voice circuit ? not so bad if you?re a carrier with bandwidth to burn, but less optimal if you?re a corporate customer getting heavily charged for that bandwidth. CELP makes a big difference in bandwidth requirements because the sampling frequency can be much lower. In order to do that CELP does a lot more processing than PCM, whichoriginally meant substantially higher costs and lower throughput. All that said, 8kbps is simply four times better use of bandwidth than PCM and twice as good as ADPCM (which realistically needs 32kbps for near toll quality). For that reason, you?llgenerally want to look for G.723.1 or G.729 encoding ? the two standard CELP implementations.
For more info?.. check here http://www.nwc.com/netdesign/1109voipfull.html

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Voice Compression for CISCO VoIP

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