[Asterisk-Dev] channel driver sample - Coding/Transcoding/Conferences

Steve Underwood steveu at coppice.org
Sat Jul 3 10:42:28 MST 2004

Jeremy McNamara wrote:

> Miroslav Nachev wrote:
>> Dear Jeremy,
>> JM> Why?  You are going to have two PCI bus transits adding totally
>> JM> unecessary latency.
>>    Also the latency from Memory Switching combining with DSP coding
>> Speed is smaller than Host CPU Coding where you need of very power
>> Host CPU which is expensive variant.
> Say what?
> With todays CPU power there is absolutely no need for expensive DSPs. 
> This is why Digium didn't design hardware with DSPs on board.

I'm all in favour of host DSP code, and put much of the DSP into *.

Say what?

A high end DSP is *far* cheaper than a Pentium, and consumes a tiny 
fraction of the power. Also, a Pentium doesn't really have the 
processing grunt needed for serious DSP - say, several E1s worth of echo 
cancellation + G.729 coding. You need clusters of DSP chips for that. 
However, most people only use a few channels, or do lightweight 
processing, and for them host CPU DSP is the effective choice.

The *appearance* of DSPs being expensive is an artifact of the way the 
industry runs. DSP cards are priced at an extreme multiple of their 
actual cost, leading to a vicious circle - DSP cards are expensive so 
they have a small market. Why are they expensive? because they have a 
small market. E1 cards were like that too, until recently. Apply the 
Digium pricing strategy to a DSP card and it would not look too bad. Add 
an H.100 port to Digium's E1/T1 cards and the DSP card and maybe you are 
starting to build something nice. Sure, it starts to look a little like 
a Dialogic card. That isn't all bad, though, if its a reasonably priced 
open platform, instead of a Dialogic high priced straight-jacket.


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