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

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sv3ora:
1200 baud data transfer over audio passband of cellphone. Is that possible through the use of their codecs? Has anyone tried it? There are videos about comms on 300 baud but how about 1200 baud, has anyone tried it?

amyk:
Tried, yes. Successfully, probably not. GSM audio codecs compress the signal a lot.

NiHaoMike:
The modern "HD Voice" codecs should give a better chance of working. But if you have that, why not just write an app to interpret the tones on the smartphone itself?

coppice:

--- Quote from: sv3ora on October 28, 2019, 08:57:34 pm ---1200 baud data transfer over audio passband of cellphone. Is that possible through the use of their codecs? Has anyone tried it? There are videos about comms on 300 baud but how about 1200 baud, has anyone tried it?

--- End quote ---
Most of the speech codecs used for cellular assign a single pitch for each 5ms of speech, which is a fast enough update rate for smooth speech. This is true whether you use the narrow band or the wide band HD voice codecs. However, even the Bell 103 or V.21 modems at 300bps need a pitch update every 3.3ms, so the cellular codecs can't cope. The standard FSK and PSK modems for 1200bps (V.23, Bell 212, V.22) need a pitch update every 1.7ms or 0.83ms, so they are even more heavily corrupted by the speech codecs. You could design a custom modem that will get some data through fairly reliably, by being adapted to the qualities of low bit rate speech codecs. I don't know if anyone has tried to do that.

BrianHG:

--- Quote from: coppice on October 29, 2019, 02:00:01 am ---
--- Quote from: sv3ora on October 28, 2019, 08:57:34 pm ---1200 baud data transfer over audio passband of cellphone. Is that possible through the use of their codecs? Has anyone tried it? There are videos about comms on 300 baud but how about 1200 baud, has anyone tried it?

--- End quote ---
Most of the speech codecs used for cellular assign a single pitch for each 5ms of speech, which is a fast enough update rate for smooth speech. This is true whether you use the narrow band or the wide band HD voice codecs. However, even the Bell 103 or V.21 modems at 300bps need a pitch update every 3.3ms, so the cellular codecs can't cope. The standard FSK and PSK modems for 1200bps (V.23, Bell 212, V.22) need a pitch update every 1.7ms or 0.83ms, so they are even more heavily corrupted by the speech codecs. You could design a custom modem that will get some data through fairly reliably, by being adapted to the qualities of low bit rate speech codecs. I don't know if anyone has tried to do that.

--- End quote ---
Being a digital system, there is a lack of analog noise, so, you might be able to take advantage of that.  For example, try a 100hz sine wave or square wave tone with 64QAM levels.  At this low frequency, you are relying on the cell networks speech codec just to cope with 100hz, but, modulate the amplitude to 16 levels delivering 4 bits.  Scope the in and out, if it works, you have 600 baud.  If you get lucky enough, operating on the peaks and troffs of the sine wave with 64QAM, just maybe you can squeeze out 1200 baud.  Such a dumb codec shouldn't be too difficult just to try and play with.  It should also be easy enough at this point to try 150Hz, 16QAM to achieve the same 600/1200 baud.

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