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

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

Audio bandwidth goes to about 3KHz so it should be possible but almost all cell phones have serial interfaces already if you know how to access them. Why not just use the built in UART directly and get the benefits of error correction and speeds of 9600 Bauds or better?

Brian.

betwixt said:

Audio bandwidth goes to about 3KHz so it should be possible but almost all cell phones have serial interfaces already if you know how to access them. Why not just use the built in UART directly and get the benefits of error correction and speeds of 9600 Bauds or better?

Brian.

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I could possibly, but "regular" users couldn't.
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.

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.

But not FSK at 110, 300 or 1200baud, they all seemed to have this problem I mentioned.

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There are several types of FSK (IIRC)- can you be somewhat more specific?

Also, try to insulate from external noises. I remember using phones fixed in rubber cups of a modem. If the fit was not tight, the connection went dead (disconnected).

My memory is rusty, but there are several dual tones that act as control signal for the modem (out of the 16 possible dual tones, only 12 are present on the keyboard).- perhaps google can help!!

What is the exact setup (some more details please!!)?

c_mitra said:

There are several types of FSK (IIRC)- can you be somewhat more specific?

Also, try to insulate from external noises. I remember using phones fixed in rubber cups of a modem. If the fit was not tight, the connection went dead (disconnected).

My memory is rusty, but there are several dual tones that act as control signal for the modem (out of the 16 possible dual tones, only 12 are present on the keyboard).- perhaps google can help!!

What is the exact setup (some more details please!!)?

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Just a simple audio cable connected to the hands free socket of the mobile phone and on the other side to the microphone and headphones jacks of the PC. There is no acoustic coupling. Bell 202 is the scheme interested the most, but I have tried other standards at lower bit rates.

I am puzzled: it should work without fuss.

Just for debugging: record a 1min fax transmission signal and play it on the smartphone. Does it still drop the connection?

Landline phone stations transferred the actual audio samples with 8 bits non-linear sampling at 8 kHz = 64 kbits/s.
Cell phones tries to get similar audio quality with a much lower bit rate. To achieve that they use a model of the human voice generation,
so it isn't audio samples that are transferred.
It is not surprising that it is difficult or impossible to use normal "analog" modems.

Cell phones tries to get similar audio quality with a much lower bit rate. To achieve that they use a model of the human voice generation,

Click to expand...

That is very interesting information: I was not aware (obviously). But how the landline to cellphone (and vice versa) calls work? And many love to listen to music on their cell phones (nothing great, but good for morning jogging or at the gym).

It will be nice to see whether it blocks DTMF frequencies used in the landline keypads.