That's not all, there are even more factors: Mic -> A/D converter -> Codec (depends on net and quality: up to 5 different codecs possible) -> Radio -> Air-Interface -> Radio ->Codec -> Voice Quality Enhancer (VQE - who knows what this will do with your data, maybe just dropping it at all) -> and the same way backwards, with different codecs. And most important: there is no TCP or similar - you just can't know what the other side will get.
On Fri, Apr 11, 2008 at 9:37 PM, Ian Stirling <[EMAIL PROTECTED]> wrote: > Diego Fdez. Durán wrote: > <snip> > > > > > Can't you initiate a voice call between to FreeRunners and then use the > > mic and mixer devs to modulate the data as sound? > > > > Broadly yes. > > The problem is: > > A human making mobile - mobile call looks something like this. > > 1 microphone > 2 analog-digital converter > 3 GSM encoder > 4 radio transmission > 5 Add errors due to sunspots > 6 radio reception > 7 GSM decoder > 8 ISDN-like 8KHz 8 bit sampling over mobile companies network > 9 Public switched telephone network > > And then back again in reverse - with all the codec senses flipped. > > Any analog modem signal has to get through the GSM codec - twice - and > survive bit errors. GSM is designed so that single bit errors make > audibly similar output - but not similar numerically. > > The GSM codec is basically designed to throw away anything that is not > voice-like. > > If you have a perfect radio channel, and can get at the digital data > that would normally go to/from the codec (step 3) it won't help much for > 'normal' modems - as you still have the GSM encode/decode cycle at the > network side. > > In reality, what you have to do to push any sort of data through this > link is to basically have a data-driven vocoder at one side, that > gabbles - but makes sounds that could in principle be made by human > throats, and a voice recogniser at the other end. This can get > 1300bits/sec. > > CSD - 'data' GSM calls are different. > Instead of using the normal GSM codec, they use a special codec that is > designed for error-free data transmission. > This can be broken out and transmitted over the normal phone network and > end up in a device like an ISDN modem. > Some ISPs 'normal' modems actually support this by default, so dialing > just works. > However, though this would be really nice to use - it's not free, and in > many countries/telcos costs significant amounts to enable. > > > > _______________________________________________ > Openmoko community mailing list > community@lists.openmoko.org > http://lists.openmoko.org/mailman/listinfo/community >
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