Speech modulation using LPC

steveu wrote:
>>steveu wrote:
>>
>>>>steveu wrote:
>>>>
>>>>
>>>>
>>>>
>>>>>>If you need to get something quick and dirty that simply works via 
>>>>>>vocoder path, run DTMF or PSK or FSK at 300 bps.
>>>>>
>>>>>
>>>>>DTMF goes through reliably, as the cell phone systems carry that as
>>>
>>>data
>>>
>>>
>>>>DTMF can go through vocoder. It gets distorted, however it still can be
> 
> 
>>>>decoded well.
>>>
>>>
>>>Handset to network the DTMF is always turned into a digital signal,
> 
> and
> 
>>>its timing is played around with by the system. Network to handset the
> 
> DTMF
> 
>>>audio passes through the channel (at least in most cases. I don't know
> 
> if a
> 
>>>translation to a digital signal is possible in the GSM protocols).
> 
> Most
> 
>>>lowish bit rate codecs can carry DTMF pretty well, when tested in
>>>isolation. The error rates are higher than without the codec in the
> 
> way,
> 
>>>but not too bad. However, when DTMF is sent from the network to a
> 
> handset
> 
>>>the results are terrible. I've never found the time to investigate why
> 
> that
> 
>>>is.
>>
>>The tones look like bandpass noise, and there could be severe twist. 
>>Filter type DTMF decoder work, frequency counter types fail.
> 
> 
> I think you missed my point.

I guess I know what are you talking about.

> I strung together a DTMF generator, GSM FR or
> EFR codec, and a DTMF detector in a lab setup and the results were OK. I
> took the same generator and detector, and tried them across a clean GSM
> link. The results were completely unable.

I passed DTMF from one GSM mobile to the other. It was input through the 
  mic path, not by pressing keys. At the output, the signal was 
distorted pretty badly, and the 1-bit comparator decoder couldn't decode 
it. Yet the filter decoders worked reasonably well.

  I'm pretty sure the error rate on
> the link must have been low during my testing. I never had the time to go
> back and figure out the cause. It might be weird stuff happening in echo
> cancellers (some of which really screw up DTMF, for some strange and
> debilitating reason).
> 
> Steve
> 

VLV