Modulate digital data into audio using AFSK

[tharaka.nw]

Hi,
I want to modulate digital data into audio. Then communicate it through any audio channel and demodulate at the destination from audio to data again. To do this I hope to use computer sound card and software modem without using any hardware implementation. In the internet, I found that this can be through the technique called Audio Frequency-Shift Keying(AFSK). I want to know that can I obtain bit rate more than 1200bps from AFSK and if it is no what the reason behind that this limitation.

Is there any technique efficient than AFSK for this purpose ?

 

[zorro]

Hi,

what you are looking for is a voice band modem (or voiceband modem).
There are many modulation systems and techniques for this purpose. Standards used universally over telephone lines are ITU-T reccomendations (V.21, V.22, V.26, etc.). Using sophisticated techniques, modems acording to V.34 standard allow 33600 bps over analog (less than 4 kHz bandwidth) channels.
Binary FSK is the simplest modulation/demodulation system for this purpose. Speed limitation is bandwidth. Standard modems for nominal telephone lines bandwidth give 1200 bps with FSK. For more speed, other modulation methods are used.
If you have an audio channel of enough bandwidth you can increase speed over the 1200 bps using Audio FSK. For example, if you want 4800 bps you would need on the order of 10 kHz of bandwidth.
Regards

Z

 

[tharaka.nw]

Hi,
First thanx for your reply zorro.
If I can increase speed over the 1200 bps using Audio FSK how I implement it. Will I want to use more than two tones for that. Can u explain it little bit more or is there any references?
(I believe that these techniques which u said can be implemented using software without any additional hardware other than computer with sound card; even without modem)

regards,
tharaka

 

[zorro]

Hi Tharaka,

It is not needed to use more than two tones, it would not help to increase speed for a given channel bandwidth.
For binary FSK you have to choose:
a) the duration of a symbol: Tbit = 1/BitRate
b) the two frequencies. They must have a separation according to BitRate and they must fit well into the channel bandwidth, so the spectrum (that spans beyond the two frequencies) is contained into the bandwidth.
Please look for FSK in books on Digtal Communicactions.
Care that the generation must be continuous-phase.
As you said, you don't meed additional hardware for generation and detection (eventually, only for the interfaces).
Regards

Z