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1200 baud data transfer over audio passband of cellphone. Is that possible?

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tooki:

--- Quote from: ollopa on November 05, 2019, 12:36:55 am ---It's not rubbish.  One of the early phases of dial-up negotiation is signaling to the channel (AKA phone company) to disable echo cancellation for full-duplex operation.  That's completely in-line with "the landline network has the ability to detect modems and change something in the codecs to make them work."

--- End quote ---
Thank you! That's what it was.

Knowing the specifics, I was able to find more, e.g. ITU-T Recommendation G.16: Interaction aspects of signal processing network equipment, sections 5.2.1–5.2.2, pp 5–9.

coppice:

--- Quote from: tooki on November 04, 2019, 10:16:56 pm ---
--- Quote from: coppice on November 04, 2019, 06:20:24 pm ---
--- Quote from: tooki on November 04, 2019, 09:45:26 am ---
--- Quote from: sv3ora on October 30, 2019, 08:42:55 am ---Do you think that I could pass the 1200 baud 1200Hz/2200Hz through the wired telephone instead, or will I have the same problems as the GSM?

--- End quote ---
Should work fine. AFAIK, the landline network has the ability to detect modems and change something in the codecs to make them work. Fax machines have relied on this for decades.

--- End quote ---
Why do people make up rubbish like this?

--- End quote ---
The possibility of inaccurate recollection (as explained above) is not the same as “making up rubbish”. (And absent documentation either way, I’m not certain that my memory is actually wrong.)

Edit: documentation found, thanks to ollopa remembering the specifics. So yeah, I was right. Definitely not "rubbish", and definitely not "made up". (Also, how much more could I have couched it in uncertainty? I made it clear that I wasn't 100% sure, prefacing it with "AFAIK" and only saying that "something" got changed.)

--- End quote ---
What ollopa noted is that modems generally signal to disable echo suppressors and echo cancellers. This has nothing to so with the codecs that are used.

tooki:
OK, I see what you’re saying. Sorry I chose the word “codec” and didn’t just leave it at “...changes something in how the PSTN handles calls”. I defer to your infinite knowledge and boundless grace and generosity of spirit in this matter...  ::)

sv3ora:
Well despite said it can't be done, I have managed to pass 1200 baud AFSK (Bell202) through GSM and also between GSM and landline.
I have made a small page for it with the experiments I did
http://qrp.gr/gsmhack

I want to investigate how valid is this through out the countries.
Would anyone be kind enough to download the file on the page and test it for me in your country's network?

To test this yourself, I have created a zip file in the page, which contains two audio wav recordings. One of them is random FSK data at 1200 baud following the Bell202 tones protocol. The other is a single continuous DTMF tone, the number 2. Set up your audio player so it can open multiple instances and play these files at the same time, at about the same audio volume (enable repeat track so that they play continuously). If you do not have an audio jack connection from your phone (hands free plug) to the PC, you could try acoustically coupling the phone to the PC, although I have not tested this. If you can hear the FSK data uninterrupted on the other phone, then it works for you. Note that I have deliberately introduced a few short pauses in the original FSK audio data, as an indication of which part is played, without looking at the screen.

jhpadjustable:
Holding the channel open with DTMF! Fine business! :clap:

Have you verified with a proper demodulator yet? What sort of bit error rates are you getting?

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