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PODTMF Goertzel Algorithm Not Working
 

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    <p>So I am opening a .raw file of a DTMF tone I generated in audacity. I grabbed a canned goertzel algorithm similar to the one on the wikipedia article. It doesn't seem to decode the correct numbers though.</p> <p>The decoded number also changes depending on what value of N I pass to the algorithm. As far as I understood a higher value of N gives it better accuracy but should not change what number would get decoded correct?</p> <p>Here is the code,</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;math.h&gt; double goertzel(short samples[], double freq, int N) { double s_prev = 0.0; double s_prev2 = 0.0; double coeff, normalizedfreq, power, s; int i; normalizedfreq = freq / 8000; coeff = 2*cos(2*M_PI*normalizedfreq); for (i=0; i&lt;N; i++) { s = samples[i] + coeff * s_prev - s_prev2; s_prev2 = s_prev; s_prev = s; } power = s_prev2*s_prev2+s_prev*s_prev-coeff*s_prev*s_prev2; return power; } int main() { FILE *fp = fopen("9.raw", "rb"); short *buffer; float *sample; int sample_size; int file_size; int i=0, x=0; float frequency_row[] = {697, 770, 852, 941}; float frequency_col[] = {1209, 1336, 1477}; float magnitude_row[4]; float magnitude_col[4]; double result; fseek(fp, 0, SEEK_END); file_size = ftell(fp); fseek(fp, 0, SEEK_SET); buffer = malloc(file_size); buffer[x] = getc(fp); buffer[x] = buffer[x]&lt;&lt;8; buffer[x] = buffer[x] | getc(fp); while(!feof(fp)) { x++; buffer[x] = getc(fp); buffer[x] = buffer[x]&lt;&lt;8; buffer[x] = buffer[x] | getc(fp); } for(i=0; i&lt;x; i++) { //printf("%#x\n", (unsigned short)buffer[i]); } for(i=0; i&lt;4; i++) { magnitude_row[i] = goertzel(buffer, frequency_row[i], 8000); } for(i=0; i&lt;3; i++) { magnitude_col[i] = goertzel(buffer, frequency_col[i], 8000); } x=0; for(i=0; i&lt;4; i++) { if(magnitude_row[i] &gt; magnitude_row[x]) x = i; } printf("Freq: %f\t Mag: %f\n", frequency_row[x], magnitude_row[x]); x=0; for(i=0; i&lt;3; i++) { if(magnitude_col[i] &gt; magnitude_col[x]) x = i; } printf("Freq: %f\t Mag: %f\n", frequency_col[x], magnitude_col[x]); return 0; } </code></pre>
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    1. COTry to verify the length of each tone. N cannot be larger than the length of the tone you are measuring.
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    2. COI'm not sure how to determine this...
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    3. COThe decoded number varies widely depending on the value of N, but any value I try I can't get it to decode the actual number I want. Is it possibly the algorithm is just plain wrong? If we could figure out what needs to be done to make it work it would be a nice to update wikipedia with something that works!
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