sending data over gsm voice like bell 202 modem

Do you have any idea how I could test this using my 2 GSM phones?
I'd like to have an answer directly from hardware tests. I think about connecting the output of sound card to the jack of my android phone and then the output of my sony ericson to the input of sound card to see if the signal has been deteriored or not.

Do you know how work hands free kits of phone? what are the cables for the headphones and the one of the incorporated microphone. I think I'm gonna have to dissect the cable of my SE..:/

EDIT: nevermind, SE hands free are well made, just have to plug a jack to jack connector.
hopefully I'll give it a try tonight with square signals from reason

but DTMF can be achived from GSM network this is a plus point rather than using 56k modem.

56k modem can transfer data at up to 56,000 bit/s (7 kB/s) over the phone line.
here is a limitation of using POTS

yes data rate for DTMF using PSTN is 8 Kbyte
http://www.cs.cf.ac.uk/Dave/Multimedia/node150.html

coolmirza143

I can't see any reference to DTMF tones here, just telephones in general.

I was a bit suspicious about the rate we could reach with DTMF but from your link:

DTMF tones 8 kHz sample rate at 8 bit resolution

Does it mean we can send one character (8 bits), 8 000 times per seconds?

mutonic

In http://forums.techarena.in/off-topic-chat/1234038.htm
"DTMF tones 8 kHz sample rate at 8 bit resolution",
is talking about recording DTMF tones to playback later.
This is not the data rate you could get over a phone line.

The author could have suggested sampling at 44100Hz to get CD quality sound,
but it would not have improved the data rate over the phone.

Mobile phones digitise and compress the audio before
sending it, so you should look for figures relating to GSM phones.

So it is better to use only 2 frequencies or the DTMF method described by coolmirza.

mutonic

I don't know
There are arguments on both sides.
I think I would go for the normal two tone "Bell" system.
Start with the lowest speed , 300 baud.. and work up.

I won't be offended if you go for the DTMF system though

What data rate do you need ?

Allright, I just played a music over my phone and listended to it on the other phone. And... quality is just awfull :/

In http://forums.techarena.in/off-topic-chat/1234038.htm
"DTMF tones 8 kHz sample rate at 8 bit resolution",
is talking about recording DTMF tones to playback later.
This is not the data rate you could get over a phone line.

ExperimenterUK

That's right,the given values are theoretical.

Mobile phones digitise and compress the audio before
sending it, so you should look for figures relating to GSM phones.

ExperimenterUK

Indeed they are doing a lot of crap there so the sound takes the less place possible.

What data rate do you need ?

ExperimenterUK

300 bauds would be perfect given the poor quality of the sound transmitted. The purpose is just to receive the output of a shell terminal in Linux. If it takes 2 or 3 s to display, it's not a problem


I'm gonna try to send simple waveforms first to see how they look after transmission.
I think with some processing we can get rid of the noise but even with this, it doesn't sounds good, sound is very bad..

i didn't have any idea about Android
does it support DTMF?
http://droid.corky.net/dtmfdial/

ya u can try DTMF using two GSM fones and a DTMF decoder, 1st gsm phone will generate DTMF code and 2nd one will receive it and provide this DTMF tone to decoder this decoder can be made by using "MT8870" chip.

http://www.siongboon.com/projects/2005-07-18_dtmf_circuits/

http://vrhome.net/vassilis/category/afsk-modem/

On this link a guy realized it for his FPV Flight system. It uses the audio stream of his video camera to send GPS signals encoded in AFSK. It works, but he is not using GSM.That way, the sound is probably much better but it gives an idea..

here is an adress for a morse encoder/decoder which supports AFSK. I'm gonna try to make it working, I don't know why under 7 it didn't recognise any sound drivers.

http://www.polar-electric.com/Morse/MRP40-EN/

Coolmirza, I'm gonna check what the android apps is doing exactly but I'm not sure if it is actually sending data through dtmf. I may also develop my own android apps to support sending/decoding of commands through DTMF/AFSK.

I think I wont bother using hardware to decode the signal, I'll use input of sound card. On the device side, the 'Beagle Board' I'm planning to use has a sound input/output so I'm not gonna miss this opportunity

and the DTMF encoder/decoder:
http://www.polar-electric.com/DTMF/Index.html

See dis Application of DTMF[yt]video code##SANITISED## width="425" height="344">##SANITISED## src="http://www.youtube.com/v/4QkbXOUkNKA&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344">##SANITISED##>##SANITISED##>[/yt]

which os u r using on ur Beagle Board?
if u r using Windows OS then try this code to decode DTMF data
http://www.phrack.com/issues.html?issue=50&id=13&mode=txt

DTMF Decoder

http://www.twopaths.com/Software/index.htm