Cascading codecs is always trouble.

BTW My understand of the word TRANSCODING is going between one encoded 
method and another without going back all the way to uncompressed. Going 
between different video encoding methods usually done by transcoding. 
Most video encoding is all DCT / macroblock based so is probably more 
relationship between codecs than speech codec variations....

In this problematic voip case:  uncoded PCM (microphone)  >> AMBE2 >> 
over air >> decoded >> PCM >> uLaw encode>> ulaw decode >> headset. (and 
reverse) .


On 17/09/2016 3:46 PM, David Rowe wrote:
> It's a good question Glen, off the top of my head I'm not sure.  When
> one or more codecs are combined it's called transcoding and IIRC often
> causes problems.
> Alaw/mulaw are rather non-linear operations.  That could upset the
> parameter estimation algorithms.
> - David
> On 17/09/16 11:38, glen english wrote:
>> David
>> you are the man to ask this one
>> Why do (or why do you think) relatively benign companding algorithms
>> like Alaw, uLaw,  that are commonly used for encoding for VOIP links for
>> radio systems (and other) , sounds so awful when they pass AMBE/ AMBE2
>> etc processed speech ?
>> Must be something to do we the re distribution of the quantizating noise
>> characteristics?
>> Not sure what the effect is yet with codec2, but that would be a cinch
>> to test up.
>> g

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