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unable to generate square wave

Discussion in 'C' started by ogopa, Aug 16, 2011.

  1. ogopa

    ogopa New Member

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    Square wave = sign(sinusoidal wave)
    So then why isn't the following code creating a square wave:
    Code:
    int sgn(double d){
      if (d>=0) d=1;
       else d=-1;
      return d;
        }
    
    static void generate_square(const snd_pcm_channel_area_t *areas, 
                               snd_pcm_uframes_t offset,
                               int count, double *_phase)
    {
        static double max_phase = 2. * M_PI;
        double phase = *_phase;
        double step = max_phase*freq/(double)rate;
        unsigned char *samples[channels];
        int steps[channels];
        unsigned int chn;
        int format_bits = snd_pcm_format_width(format);
        unsigned int maxval = (1 << (format_bits - 1)) - 1;
        int bps = format_bits / 8;  // bytes per sample 
        int phys_bps = snd_pcm_format_physical_width(format) / 8;
        int big_endian = snd_pcm_format_big_endian(format) == 1;
        int to_unsigned = snd_pcm_format_unsigned(format) == 1;
        int is_float = (format == SND_PCM_FORMAT_FLOAT_LE ||
                        format == SND_PCM_FORMAT_FLOAT_BE);
        double amplitude_scale = amplitude/8.56;
    
    
        // verify and prepare the contents of areas 
        for (chn = 0; chn < channels; chn++) {
                if ((areas[chn].first % 8) != 0) {
                        printf("areas[%i].first == %i, aborting...", chn , areas[chn].first);
                        exit(EXIT_FAILURE);
                }
                samples[chn] = (((unsigned char *)areas[chn].addr) + (areas[chn].first / 8));
               if ((areas[chn].step % 16) != 0) {
                       // printf("areas[%i].step == %i, aborting...  ", chn areas[chn].step);
                       exit(EXIT_FAILURE);
                }
                steps[chn] = areas[chn].step / 8;
                samples[chn] += offset * steps[chn];
        }
        // fill the channel areas 
        while (count-- > 0) {
                union {
                        float f;
                        int i;
                } fval;
                int res, i;
                if (is_float) {
                int a = sgn(amplitude_scale * sin(phase) * maxval);
                        fval.f = (float) a;
                        res = fval.i;
                } else { 
                int b = sgn(amplitude_scale * sin(phase) * maxval);
                        res = b;
            }
                        
    
                if (to_unsigned)
                        res ^= 1U << (format_bits - 1);
                for (chn = 0; chn < channels; chn++) {
                        // Generate data in native endian format 
                        if (big_endian) {
                                for (i = 0; i < bps; i++)
                                        *(samples[chn] + phys_bps - 1 - i) = (res >> i * 8) & 0xff;
                        } else {
                                for (i = 0; i < bps; i++)
                                        *(samples[chn] + i) = (res >>  i * 8) & 0xff;
                        }
                        samples[chn] += steps[chn];
                }
                phase += step;
                if (phase >= max_phase)
                        phase -= max_phase;
        }
        *_phase = phase;
    
    }
    
    If I take out the sgn function from the code, it generates a sine wave. Any help would be greatly appreciated.
     
  2. jose_peeterson

    jose_peeterson New Member

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    hi i am studying electrical engineering. and i have not seen anything like this is in c.SORRY i cant help you but can you tell me how you did this and what you are doing.
    how do you do this in c? i might learn and try thanks.
     
  3. xpi0t0s

    xpi0t0s Mentor

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    Bit tricky as several variables that have a bearing are not defined (freq, rate, amplitude and probably others); you don't say what output you DO get or what output you get when you miss off the sgn (other than that it is a sine wave).

    At a guess you're probably running into C's fondness of converting everything to integers. Try casting maxval to a double and see what happens, i.e.:
    Code:
    int a = sgn(amplitude_scale * sin(phase) * (double)maxval);
    
    Does the sine wave that you get when you miss of sgn() vary from -amplitude_scale*maxval to +amplitude_scale*maxval or from 0 to some maximum? If so then you don't have a sine wave as such, you have sin()+offset, which would not convert to a square with a simple call to sgn(); you would have to subtract the offset, call sgn then add the offset back in.

    For more accurate analysis please upload a minimal program that is complete and that shows the output for the two test points pi/2 and 3pi/2 (i.e. the turning points, which will be +1 and -1 whether sgn is called or not).
     
  4. ogopa

    ogopa New Member

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    Hi this is actually a sample c program that I got form the ALSA website. It is called pcm.c. It is for Linux based systems.
    I can't seem to post the link. So if you google pcm.c, it is the first result.
     
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  5. ogopa

    ogopa New Member

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    Of course. How silly of me. Thanks
     
  6. ogopa

    ogopa New Member

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    Solution:
    The sgn function should only be for the sin function
    int a = amplitude_scale * sgn(sin(phase)) * maxval;
     
  7. ogopa

    ogopa New Member

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    Sorry I realised that I didn't explain fully what I was doing in my first reply to you. I am trying to build a gui that will output a sine and square wave with user inputs of frequency and amplitude. I am using the sample code form the alsa website called pcm.c which generates a sine wave. It is code for alsa on linux systems. I have built the gui and have integrated the sine wave and I am now trying to implement the square wave.
    Good luck in your learnings.
     

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