I'm trying to implement a software audio modem with FSK to transmit and receive short digital data between two devices (e.g. a PC and an iPhone) so I'm simulating simple cases study using Scilab/MATLAB.

While I can reconstruct the original signal when no noise nor background sound is present, problems arise when playing the afsk in presence of normal ambient background sounds: trying to reconstruct the digital data reading from the PC microphone is extremely difficult.

To generate the AFSK signal I use this simple script below, then I save the wav file to disk for the next steps.

function s = fsk (bit, f1, f2, fs, T)
% bit : Array of bit
% f1: Frequency 1
% f2: Frequency 2
% fs: Sampling frequency
% T : Tramsision period for 1 bit
    for i=1:length(x)
        s=[s (bit(i)==0)*sin(2*pi*t*f1)+(bit(i)==1)*sin(2*pi*t*f2)];
  • The frequency f1, f2 I choose can vary: $3000\textrm{ Hz}$ and $4000\textrm{ Hz}$ or $3000\textrm{ Hz}$ and $5000\textrm{ Hz}$.
  • The frequency sampling is $44100\textrm{ Hz}$ and the period of transmission is $0.01\textrm{ sec}$ (for the moment I don't bother with higher trasmission rate).
  • The bit array is a very simple $[\textrm{0 1 0 1 0...}]$ sequence.

When I read-back the wav file I decode it using a simply coherent demodulation in these 3 step scheme:

  1. Pass-band the signal with a FIR filter around $5000\textrm{ Hz}$ created using the command fir1(91,[2*4900/fs 2*5100/fs])

  2. Multiply the filtered signal by the f2 (mark) carrier sin(2*pi*f2*t) (or multiply the signal with a 1 sample-delayed version of itself)

  3. Low-pass the multiplied signal obtained in step 2 to isolate the low frequency component of the original digital signal using LP FIR filter generated with fir1(61, 2*500/fs)

The original sequence of bit is now almost perfectly reconstructed. I've played a bit around with the filters parameters to obtain a good recontruction.

Reconstructed bit stream

Then I decided to play the wav file on my stereo with normal background noise present (actually very low), I recorded it with the pc microphone and saved with Audacity, read it back with MATLAB/scilab and I tried to reconstruct the original bit sequence with the 3 steps above but the signal is a bit different from what I expected and I can't see the original pattern and I don't know I to recontruct the bit sequence using any treshold value.

Noisy bit-stream

I suppose the cause is the SNR is awesome but I don't know where I'm wrong and I don't understand why the results are so different than the previous case.

How could I reconstruct the correct bit stream sequence in presence of background noise ?

UPDATE: Do I have to low-pass filter the modulated signal before send it to the receiver ?

Any suggestion will be very appreciated.

Well...it seems that my recording system was buggy ,as Jim pointed out ! I played and recorded the sound using the speakers and mic on the same PC so this introduced some indesiderable feedback or filtering or something else I suppose. Now I'm using an external device to play the sound (a smartphone) and the recorded signal is much better. This is the signal transmitted

Hello world !

and this is the decoded signal

fsk decoded

I'm using a bandpass filter centered around the mark and space frequency before transmit it. The decoding steps are the same as before but in step 2 I'm using the slicing method correlating the signal with the delayed version of itself.

The reconstructed FSK signal has now a much better SNR than before. Now should be easy to recover the exact bit sequence....

  • 1
    $\begingroup$ Your pass-band filter destroys the f1 portions of the signal. How do you recover f1? $\endgroup$
    – Jim Clay
    Aug 28 '12 at 16:37
  • 2
    $\begingroup$ Your synthesis script makes a signal with phase jumps between symbols (you stitch together 2 incoherent sine waves). Normally AFSK signals have continuous phase. Your case will generate a wider spectrum and will be probably more difficult to decode. $\endgroup$
    – Juancho
    Aug 28 '12 at 17:50
  • $\begingroup$ I've also made a bandpass FIR filter centered on the mark and space frequencies (about 100/200 Hz width) to let them pass both while stopping out-of-band noise but with the same discouraging results. How could I build a continuous phase AFSK signal in practice ? Please keep in mind that these scheme works perfectly in absence of noise while it's seriously degraded with some low-level background noise. $\endgroup$
    – Yozek
    Aug 31 '12 at 8:51
  • $\begingroup$ Actually for restoring the bit stream I have to transmit a bit for a period of 0.1/0.05 seconds (symbol rate). If I try to transmit with a faster rate (<0.01 sec) I recover just a lot of garbage. $\endgroup$
    – Yozek
    Aug 31 '12 at 14:11
  • $\begingroup$ @Randagio How do you know that your recording setup works? That looks like a garbage recording. $\endgroup$
    – Jim Clay
    Sep 3 '12 at 15:14

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