[asterisk-dev] Transcoder Cards - Off the wall question

Shaun Ruffell sruffell at digium.com
Thu Oct 23 12:06:40 CDT 2008

Russell Bryant wrote:
> On Oct 22, 2008, at 7:21 PM, Gregory Boehnlein wrote:
>> Can I run TCB transcoder cards in an Asterisk box along side the  
>> software
>> codec? I.E. is Asterisk smart enough to be able to realize that has  
>> run out
>> of hardware transcoding resources and switch to using software  
>> transcoding?
>> Or is it an either/or proposition?
> IIRC, you should be able to use them at the same time.  Asterisk will  
> choose the hardware transcoder until it is full, and then will fall  
> back to the software one.

Just to add my 2 cents:

Depending on the speed of your CPU, it is possible to use channels of the
software transcoder before all the hardware channels if one of the legs is
signed linear.  This would happen if conferencing several G729 calls together. 
This is due to the fact that the TC400M does not register support for SLIN, so
if converting between SLIN and g729, first codec_ulaw would convert SLIN to 
ULAW, which would then be passed to codec_dahdi.  This is calculated to take a
minimum of 2ms (1ms for each codec that converts the audio).  If codec_g729
can do the conversion in less time, which it typically can when decoding,
maybe not for encoding, Asterisk will use it instead of codec_dahdi.

In the future, Codec_dahdi will convert to and from signed linear itself even
if the underlying hardware does not support it.  This way, transcoding between
SLIN and G729/G723 would always be reported to take the minimum amount of time
(1ms in asterisk 1.4) and receive preference.  There is a branch of 1.4 with
some work on this, but it needs more testing / attention before merging.


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