Re: [Freetel-codec2] cascaded ulaw, Alaw and,AMBE etc
On 17/09/16 18:28, glen english wrote: > Yeah. > > 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) . Transcoding is going from one format to another, and when the destination is a lossy format, that will mean a reduction in quality. Decoding to uncompressed shouldn't incur loss compared with a hypothetical conversion from AMBE2 direct to µ-law, more likely the assumptions made by the µ-law CODEC don't hold true for the synthesized voice from the AMBE2 CODEC, and that would be why it sounds so terrible. Regards, -- Stuart Longland (aka Redhatter, VK4MSL) I haven't lost my mind... ...it's backed up on a tape somewhere. -- ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
Re: [Freetel-codec2] cascaded ulaw, Alaw and,AMBE etc
Yeah. 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) . regards 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 -- ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
Re: [Freetel-codec2] cascaded ulaw, Alaw and,AMBE etc
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 > > > > > -- > ___ > Freetel-codec2 mailing list > Freetel-codec2@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > -- ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
[Freetel-codec2] cascaded ulaw, Alaw and,AMBE etc
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 -- ___ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
