Sound

This is a discussion on Sound within the C Programming forums, part of the General Programming Boards category; I'd like to create a .wav file containing a sound wave that starts at a certain frequency and fades to ...

  1. #1
    int x = *((int *) NULL); Cactus_Hugger's Avatar
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    Sound

    I'd like to create a .wav file containing a sound wave that starts at a certain frequency and fades to another frequency.

    I believe that y = sin(2 * pi * x * f) will graph a sine wave of frequency f where x is time in seconds. So, I planned to just vary f.
    Given freq_start & freq_end, then the change in frequency over time wave_length is:
    y = (freq_end - freq_start) / wave_length * x + freq_start
    Right? (Basic y = mx+b line)
    So, I replaced f in the first equation with that. The resulting wave faded down in freqency, hit bottom, and when back up. I just want it to fade downwards. The code that does all this stuff is:
    Code:
    void write_short(FILE *fp, unsigned short i)
    {
    	int x;
    	for(x = 0; x < 2; x++) fputc((i >> (x * 8)) & 0xFF, fp);
    }
    
    ...
    
    int main()
    {
    	...
    	// Begin writing data:
    	freq_diff = freq_end - freq_start;
    	printf("Frequency difference is %d Hz.\n", freq_diff);
    	for(x = 0; x < SAMPLE_RATE * samp_time; x++)
    	{
    		ctime = (double) x / (double) SAMPLE_RATE;
    		hz = (double) freq_diff / (double) samp_time * ctime + (double) freq_start;
    		amp = sin(2.0 * M_PI * ctime * hz);
    		//printf("ctime = %g, hz = %g, amp = %g\n", ctime, hz, amp);
    		//scanf("%d", &y);
    		iamp = amp * 32767.0;
    		write_short(fp, (unsigned short) iamp);
    	}
    
    	fclose(fp);
    
    	return 0;
    }
    Compiles with no errors (Using gcc -Wall -o wavegen.exe wavegen.c) but gives a bad wave, as described above. ctime is gradually going up to the length, in seconds, of the wave file, as it should. Hz is gradually going down from freq_start to freq_end, again, as it should. SAMPLE_RATE is 44100. Yet the wave goes down in pitch, then back up. It should only go down. Full source is attached.
    Attached Files Attached Files
    long time; /* know C? */
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  2. #2
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    Cleaned up a bit: http://uranther.pastebin.com/432741
    Works fine for me Nice program btw :O

  3. #3
    Just Lurking Dave_Sinkula's Avatar
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    I don't know what the actual frequencies produced are, nor have I fully investigated this "fix", but this seems to produce steadily increasing or decreasing tones.
    Code:
          if ( freq_diff < 0 )
          {
             amp = sin(2.0 * M_PI * (samp_time - ctime) * hz);
          }
          else
          {
             amp = sin(2.0 * M_PI * ctime * hz);
          }
    What started me on this (bad habit of throwing code at an issue without understanding what I am doing) was this:
    http://en.wikipedia.org/wiki/Sound

    Perhaps you can do better investigation than my cursory attempt.
    Last edited by Dave_Sinkula; 11-16-2005 at 10:21 PM.
    7. It is easier to write an incorrect program than understand a correct one.
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