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| 1200 baud data transfer over audio passband of cellphone. Is that possible? |
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| coppice:
--- Quote from: ogden on October 29, 2019, 04:25:55 pm --- --- Quote from: coppice on October 29, 2019, 01:34:31 pm --- --- Quote from: ogden on October 29, 2019, 09:44:21 am --- --- Quote from: coppice on October 29, 2019, 02:00:01 am ---Most of the speech codecs used for cellular assign a single pitch for each 5ms --- End quote --- Then use OFDM modulation with symbol length > 5ms. --- End quote --- What does the modulation have to so with the codec being used? They are completely unrelated issues. --- End quote --- You talked about standard FSK and PSK modulations, their short symbol length (update every 1.7ms or 0.83ms). I offer OFDM modulation because it has slong(er) symbols that can be stretched depending on FFT order used --- End quote --- Oh, sorry, I was looking at what you said in the wrong way. OFDM won't really help, though. It gets high throughput through parallelism. Good luck getting any parallel signals through a speech codec. |
| ogden:
--- Quote from: coppice on October 29, 2019, 06:26:38 pm ---OFDM won't really help, though. It gets high throughput through parallelism. Good luck getting any parallel signals through a speech codec. --- End quote --- Agreed. I mentioned OFDM just because it kinda solves "too short symbol problem". Modulation shall be tuned for LPC-based speech codec to pass through. Anyway paper I mentioned and many references in it, not mentioning commercial "secure phone" products, proves that it is possible indeed. |
| sv3ora:
Anyway I have just tested it. I connected a smart phone from it's hands free port to the PC mic and phones. then I called the smart phone from a land line at home. It could not even send 110 baud. It started sending them ok but after 1-3 seconds they were suppressed. Continuously changing the AF volume to 0% and then back to a percentage tricked the codec somehow. Sending pulses of a single tone passed ok. Also, sending a dual tone passed ok. But not FSK at 110, 300 or 1200baud, they all seemed to have this problem I mentioned. Also when sending the above and tried to also send FSK at the same time, again everything was ceased at the channel and nothing heard at the other end, even my sound card was output tones. I have also tried SSTV and the result was that some tines refused to be transmitted. Now, I do not know if this is due to the mobile phone codecs or the landline phone codecs, as I have not tried it with two mobile phones yet. I may have a more detailed analysis on it, maybe note which tones pass ok and which not, or if it is not a tone problem, identify why the codecs cause silence on the channel. |
| borjam:
--- Quote from: ogden on October 29, 2019, 06:01:55 pm --- --- Quote from: borjam on October 29, 2019, 05:03:01 pm ---The problem with voice compression algorithms is that they degrade the signal in a way that is mostly harmless for speech intelligibility but they can make it impossible for a data modem to work. Especially if you use more complex modulation systems. --- End quote --- BS. Many research papers prove you wrong, achieves >1200 bps data transmission rates over GSM-FR (worst case codec, rarely used in modern networks). There are some "secure phone" products that uses not only encrypted VoIP but audio channels as well. You may start here: https://www.researchgate.net/publication/304296823_Data_transmission_via_GSM_voice_channel_for_end_to_end_security --- End quote --- Amazing work, thanks :) I agree that I shouldn't say "impossible" but "very hard". |
| ogden:
--- Quote from: sv3ora on October 29, 2019, 09:48:55 pm ---Now, I do not know if this is due to the mobile phone codecs or the landline phone codecs --- End quote --- Landline usually have 8ksps @8bit PCM, no codecs. Who uses landline today anyway :-// |
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