[Freeswitch-users] echo cancellation on PRI cards

[Steve Underwood]

David Knell wrote:
> Steve Underwood wrote:
>> David Knell wrote:
>>   
>>> Steve Underwood wrote:
>>>     
>>>>> When there is Echo being generated from the far end, usually in a 
>>>>> bridged call. If you application is just an IVR, with no far end 
>>>>> connectivity, then you shouldn't need an echo can. If you are bridging 
>>>>> calls, then at some point you may need it, depending on what else is 
>>>>> in the loop.
>>>>>     
>>>>>         
>>>> This is VERY VERY WRONG. IVRs badly need echo cancellation. Without it 
>>>> they give very poor reliability detecting DTMF while the prompts are 
>>>> playing. If the system uses voice recognition, its reliability will be 
>>>> even worse.
>>>>   
>>>>       
>>> With respect, this is at best half true.  DTMF detection has always 
>>> worked just fine
>>> without echo cancellation - the Dialogic, Aculab and Rhetorex cards 
>>> which I used
>>> in the late 1990s managed it perfectly well; if the DTMF detection 
>>> code in * and FS
>>> can't, then maybe that's something for its author to look at ;-)
>>>     
>> Try reading the Dialogic and Aculab documentation. Those cards used 
>> quite a bit of their DSP capability to remove the spillback of outgoing 
>> voice into their DTMF receivers. You'll find the DTMF detector in 
>> spandsp (not necessarily the ones in * or FS, which have been altered a 
>> bit) is superior to either Dialogic or Aculab's.
>>   
> The first bit of that's a tad patronising, isn't it,
You are the one who started out being offensive.
> and, in the case of the decade-old Aculab
> cards which which I'm most familiar, is also untrue.
I can't find too much about the old cards on the web now, but I found 
http://www.amdevcomm.com/voice-mail-products/voice-mail-components/dialogic/dti_sc.html 
which is pretty much a copy and paste from the old Dialogic web pages, 
and you'll see it says "Cut through : Local echo cancellation permits 
100% detection with a >4.5 dB return loss line". The Aculabs did the 
same thing for sure. They just couldn't work without cancellation. There 
were some very early Dialogic cards, using DTMF receiver chips and OKI 
ADPCM chips, and had no general purpose DSPs. They performed really 
badly because of the lack of cancellation, and were quickly replaced 
with cards that put the OKI ADPCM, DTMF anf echo cancellation algorithms 
into a Motorola 56k DSP chips.
>
> As for the second, do you have any test results to back that up?  I'm 
> more curious than
> setting out for an argument..
>>> ASR - yes, maybe, but L&H's ASR1500 used to work perfectly well on the 
>>> same
>>> hardware above back in the day.  I'd be interested to see results of 
>>> testing an ASR
>>> engine in with echo; unfortunately, most vendors appear to prohibit 
>>> the publication
>>> of test results in their licensing.
>>>     
>> L&H used to work fine with the J series Dialogic cards. The Dialogic 
>> documents go into considerable details about the echo cancellation 
>> arrangements to make that happen.
>>
>>   
> You've missed the point I was trying to make.  It used to work fine 
> with no echo cancellation
> at all.
I think you've missed the point. These things don't work by pixey dust. 
They work by engineering. If you have any old J or JCT cards around 
record the signal from the far end. You'll find only the tiniest trace 
of the outgoing signal mixed in with it. How do you think that happens?

Steve