modulated data over GSM voice (was Re: Data over normal GSM call)
OpenMoko at mauve.plus.com
Mon Apr 14 22:16:31 CEST 2008
Harald Welte wrote:
> Just to give you a "summary judgement":
> Running any kind of voice-encoded data over a regular voice channel of a
> GSM phone is _extremely_ unlikely to work.
> There are a number of different codecs in use. Which codec is
> determined by the network. There is echo cancellation at potentially
> multiple locations during the call. There might be one or multiple
> transcoders of the voice codec along the road.
> If you can manage to design a modulation and coding scheme that survives
> all (or even most) of the stages above, I think you have achieved
> something great. I doubt you will get more than 300bps though :)
It has been done - for the minimal case of going through two actual real
Basically a voice synthesizer driven by data, and a voice recognition
system on the other end.
It uses the very properties that GSM/... codecs for speech are intended
to preserve in the face of small amounts of bit errors - synthesizing an
artificial throat, and exciting this model with data, so you don't get
into parts of the codec that are chaotic or horribly lossy.
Unfortunately, I can't find the original paper I found.
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