[asterisk-users] Codec Conversion

Steve Underwood steveu at coppice.org
Fri Aug 6 11:47:42 CDT 2010

  On 08/06/2010 04:43 PM, Jeff Brower wrote:
> Steve-
>>    On 08/06/2010 05:40 AM, Jeff Brower wrote:
>>> Miguel-
>>>> El 05/08/10 14:50, Tim Nelson escribió:
>>>>> ----- "michel freiha"<michofr at gmail.com>   wrote:
>>>>>> Dear Sir,
>>>>>> I tried to convert ilbc to ulaw and get the same result...Bad Voice
>>>>> Quality
>>>>>> Regards
>>>>> Again, iLBC is poor quality to begin with. You can't take a poor audio
>>>>> sample and make it better by converting it to a codec with better
>>>>> 'resolution'. An audio sample full of robot voice is going to sound
>>>>> like the same robot voice even if you transcode it to a better quality
>>>>> codec, whether that is G.729, G.711u, or the latest 'HD Voice' codecs.
>>>>> --Tim
>>>> This just made me remember some comment on the iax.conf sample file...
>>>> disallow=lpc10            ; Icky sound quality...  Mr. Roboto.
>>> LPC10 is a very old codec, from early 1980s.  LPC10 doesn't do a good job with pitch detection so it tends to have a
>>> 'robotic' sound.  With advent of MELPe, anyone needing bitrates 2400 or less should not be using LPC10.
>>> -Jeff
>> MELPe is patent encumbered,
> Not if used for govt/defense purposes.  For commercial-only purposes, TI will waive royalty fees if their chip is used
> in the product.  It would have been nice if Digium had considered the many advantages of using a DSP pioneer such as
> TI before putting a Mindspeed chip on their TC400B card.

I think all the IP for MELP is now in the hands of Compandent, and TI no 
longer has the ability to waive royalties. Either way, government use 
and use with TI silicon are two niches that might work out well, and 
everything else is a problem for several more years. If you are going to 
pay royalties for a low bit rate codec, IMBE is probably a better option.

TI is a good option, but what do you have against Mindspeed? Choosing a 
good option for this kind of card is mostly about managing the patent 
licence fees. I assume Mindspeed gave Digium the best option for doing 
that, within Digium's volume constraints.
>> so there is still a place for LPC10 [...]
> I haven't seen an LPC10 implementation with MOS higher than 2.5.  Due to its age and expiration of patents, LPC10
> might be a basis for a 2400 bps open source codec.  But enormous improvement would be needed to come close to MELPe
> performance.
MELPe is definitely a compandent thing, and TI cannot waive fees for 
that. MELP and MELPe are derived from LPC10. Any attempt to improve 
LPC10 would take you down a similar road, though you would need to skirt 
around the patents.

Do you really consider MELPe to be an enormous improvement over LPC10? 
Its still pretty lousy compared to a number of options at about 5kbps, 
and RTP overheads mean the gain from going lower than 5k isn't that big. 
The main reason LPC10 and MELPe offer a low bit rate in RTP is the 
minimum packet you can pack 22.5ms frames into sanely is a 90ms one. 
90ms RTP *really* cuts the overheads, compared to the more typical 20ms 
or 30ms packets used for G.729.

As others have mentioned, David Rowe is working on a modern 2400bps 
codec. He did a burst of work some time ago, and then put it aside while 
busy with other things. He recently told me he is restarting the work, 
and he wants to get that codec into good shape before the end of this year.


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