[asterisk-users] Codec Conversion

Jeff Brower jbrower at signalogic.com
Sun Aug 8 11:18:35 CDT 2010


>   On 08/07/2010 03:15 AM, Jeff Brower wrote:
>> Steve-
>>>>>>> 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.
>> That is not correct.  Compandent has filed copyrights on certain files associated with a C549 chip assembly language
>> implementation they did under contract to NSA around 2001.  TI has patent rights on 2400 bps, TI + Microsoft on 1200
>> bps, and TI + Microsoft + Thales Group on 600 bps.  Microsoft's IP came about as a result of acquiring a company
>> called SignalCom around 2001.  If the noise pre-processor is used, then there is some AT&T IP.  To verify this, you
>> can search dsprelated.com (specifically, look for posts discussing this issue on comp.dsp), and you can also read
>> the
>> "Compandent IPR" section of the MELPe Wikipedia page
>> (http://en.wikipedia.org/wiki/Mixed_Excitation_Linear_Prediction).  That section was authored by the Compandent's
>> founder, Oded Gottesman.  Oded is a super sharp, very hard working guy.
>> Compandent also claims a copyright on some C code in the file melp_syn.c (synthesis filter).  I have read
>> discussions
>> by DSP experts indicating the copyrighted section of code can be implemented in alternative ways, but Oded may say
>> that's not accurate.
> That guy is PITA. He must have driven a lot of people away from MELP by
> the way he acts. He really annoys the regulars in the comp.dsp group by
> posting astroturf questions about MELP, and giving astroturf replies
> about how fantastic it is. That probably shapes a lot of my attitude to
> MELP. :-)
>>> 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.
>> I would disagree because IMBE source is not available.  MELPe source is available and can be downloaded online.
> Depends what you mean by available. IMBE is patented, just like MELP is
> patented. Licence either, and implementations are available.

I meant that MELPe C source code is available for non-commercial purposes (academic, R&D, bug fixes and other source
level improvements) without payment and without signing a license agreement with a corporation (such as Digital Voice
with IMBE).

> IMBE has
> the great benefit of being widely used for commercial and amateur low
> bit rate channels. For example, amateur radio uses IMBE - an anomaly
> which is one of the drivers for David Rowe's work on an open low bit
> rate codec. Transcoding at low bit rates is a disaster, so using a codec
> you won't need to transcode is a big plus.

Yes all good points.  IMBE and AMBE have surely been successful, testaments to the Digital Voice guys and their
pioneering work in the LBR codec area.

>>> 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.
>> My understanding in talking to Digium engineers at Globalcom and other trade shows back in 2006 is they were worried
>> about interfacing the TI TNET series devices over the PCI bus.  They would have needed an FPGA with some non-trivial
>> logic programming, so I understand their decision.  But if they had got past their FPGA "writer's block", they could
>> have put one TNETV3010 chip on there, even smaller than the Mindspeed and without the heat sink, and had twice the
>> channel capacity as they do now.
> TI have had DSP chips with a PCI interface for years, so that
> explanation doesn't make a lot of sense. Of course, these days you need
> a PCI-E interface. I'm not so sure about the status of those in DSP chips.

Good insight, but there's another layer of subtlety.  The TNETV3010 was the most "channel dense" VoIP chip that TI had
during the 2002-2008 time frame (192 chan, 16x16 mm package).  Underneath the hood, the chip is a 6-core C55x device,
but doesn't have a PCI bus interface like so many of their other DSPs.


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