Subject: [Megaco] OT: "equivalent" codecs

From: Troy Cauble <[email-masked]>
Date: Tue, 04 Mar 2003 10:32:29 -0500

If a gateway is asked to use G729A, which it doesn't have,
but it has a G729 implementation, should it use it?

And vice versa?

-troy

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From: "Paul Long" <[email-masked]>
Date: Tue, 04 Mar 2003 11:07:16 -0500

Troy,

G.729 and G.729a put the same bits on the wire, so the receiver cannot 
distinguish between the two; therefore, they can be used 
interchangeably. G.729a just defines a reduced-complexity 
_implementation_ of G.729. Signaling protocols should never have 
introduced Annex A as an audio codec distinct from the base standard. 
It causes needless confusion such as what you're experiencing.

Paul

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From: "Paul Long" <[email-masked]>
Date: Tue, 04 Mar 2003 12:58:44 -0500

Troy,

I have no personal experience comparing G.729 and G.729a but have 
always heard that the quality difference is neglible. This description 
in a Nortel white paper backs that up 
(http://www.nortelnetworks.com/products/01/succession/es/collateral/tb_g
729.pdf):

"Both G.729 and G.729A provide the same speech quality during normal 
operation, the data rate and frame sizes are the same, and they are 
fully interoperable. There is confusion between the two standards 
because they are identical except for complexity. The only detectable 
performance difference occurs during multiple tandem encoding (encoding 
and decoding the signal repeatedly). The speech quality of G.729A 
suffers slightly compared with G.729, however, tandem encoding causes 
speech quality degradation for all Code Excited Linear Prediction(CELP) 
codecs and is not enough of a concern to deter anyone from using 
G.729A."

Conversely, this white paper notes an MOS score difference of 3.9 and 
3.7 (http://www.adaptivedigital.com/prod/wh_pap/jan_07.htm):

"G.729 Annex A is a reduced complexity version of G.729. It requires 
about half the CPU power to operate G.729, with the drawback that it 
has slightly reduced speech quality. G.729 is rated at 3.9 on the MOS 
(Mean Opinion Score) scale, which is a subjective measure of voice 
quality. G.729 Annex A (or G.729A) achieves an MOS score of 3.7. This 
is usually considered an acceptable trade-off in order to achieve half 
the CPU requirements. The G.729 and G.729A bit streams are compatible 
and interoperable, but not identical."

Practically speaking, unless one has really extraordinary needs and 
plenty of available MIPS, one should always implement G.729a. I'm 
guessing that the MOS difference between implementations is greater 
than the MOS difference between G.729 and G.729a.

Paul

> Just playing devil's advocate...
> 
> There is a voice quality tradeoff for the complexity, yes?
> 
> What about the notion of the RGC wanting control over
> the voice quality level and the RG's resources?
> The RGC may require G729 voice quality not G729A.
> The RGC may require G729A resource consumption not G729.
> 
> -troy
> 
> Paul Long wrote:
> > Troy,
> >

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From: "Paul Long" <[email-masked]>
Date: Tue, 04 Mar 2003 12:58:44 -0500

Troy,

Paul