[alsa-devel] [PATCH BAT V1 4/7] BAT: Add signal generator

[Liam Girdwood]

On Fri, 2015-09-18 at 09:50 -0700, Caleb Crome wrote:
> Hi Han, Liam, all,
> 
> It looks like you have run into the old problem that the math library
> actually stinks at generating sine waves right :-)
> 
> > +       for (k = 0; k < length; k++) {
> > +               for (c = 0; c < bat->channels; c++) {
> > +                       idx = k * bat->channels + c;
> > +                       out[idx] = sinf(val[c] * i) * factor;
> > +               }
> > +               i++;
> > +               if (i == bat->rate)
> > +                       i = 0; /* Restart from 0 after one sine wave period */
> > +       }
> 
> If I'm reading this right, you're resetting the argument to the sinf
> function.  This can generate unwanted distortion in the sine wave if
> the frequency isn't an exact fraction of the sample rate.
> 
> Here's a little library (along with a test program) I just whipped up
> using a phasor as the sine wave generator.
> The test program runs it for a simulated 3 days, then prints a python
> script to stdout that you can then run and plot to see the generated
> wave.
> 
> This will work for any frequency sine wave, and as far as I know, it's
> the best way to generate sine waves. (Not for calculating the sin of a
> value though!)
> 
> Here is the generator and test program.  Sorry If this isn't the right
> way to submit a file/patch, but I have no idea how to do it properly
> :-/
> 

No worries, I dont think Han has put a link to his public git repo so it
will be a bit more difficult to create proper patches. 

> BTW, this isn't actually integrated into the bat program, it's just
> the little library for generating the sine waves.
> 

Thanks, we can integrate and add to the codebase for resending V2
patches. :)

Liam

> Author: Caleb Crome <caleb at signalessence.com>
> Date:   Fri Sep 18 09:45:43 2015 -0700
> 
>     added sin_generator
> 
> diff --git a/bat/sin_generator.c b/bat/sin_generator.c
> new file mode 100644
> index 0000000..e67f376
> --- /dev/null
> +++ b/bat/sin_generator.c
> @@ -0,0 +1,120 @@
> +#include "math.h"
> +#include "sin_generator.h"
> +/*
> + * Copyright (C) 2015 Caleb Crome
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation; either version 2 of the License, or
> + * (at your option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + *
> + */
> +
> +/*
> + * This is a general purpose sine wave generator that will stay stable
> + * for a long time, and with a little renormalization, could stay stay
> + * stable indefinitely
> + */
> +
> +
> +/* Initialize the sine wave generator.
> + * sin_generator:  gets initialized by this call.
> + * frequency:      the frequency for the sine wave.  must be < 0.5*sample_rate
> + * sample_rate:    the the sample rate...
> + * returns 0 on success, -1 on error.
> + */
> +int sin_generator_init(struct sin_generator *sg, float magnitude,
> float frequency, float sample_rate)
> +{
> +    // angular frequency:  cycles/sec / (samp/sec) * rad/cycle = rad/samp
> +    float w = frequency/sample_rate*2*M_PI;
> +    if (frequency >= sample_rate/2)
> +        return -1;
> +    sg->phasor_real = cos(w);
> +    sg->phasor_imag = sin(w);
> +    sg->magnitude   = magnitude;
> +    sg->state_real  = 0.0;
> +    sg->state_imag  = magnitude;
> +    sg->frequency = frequency;
> +    sg->sample_rate = sample_rate;
> +    return 0;
> +}
> +
> +/*
> + * Generates the next sample in the sine wave.
> + * should be much faster than calling a sin function
> + * if it's inlined and optimized.
> + *
> + * returns the next value.  no possibility of error.
> +*/
> +float sin_generator_next_sample(struct sin_generator *sg)
> +{
> +    // get shorthand to pointers
> +    const double pr = sg->phasor_real;
> +    const double pi = sg->phasor_imag;
> +    const double sr = sg->state_real;
> +    const double si = sg->state_imag;
> +    // step the phasor -- complex multiply
> +    sg->state_real = sr * pr - si * pi;
> +    sg->state_imag = sr * pi + pr * si;
> +    return sr;  // return the input value so sine wave starts at
> +            // exactly 0.0
> +}
> +/* fills a vector with a sine wave */
> +void sin_generator_vfill(struct sin_generator *sg, float *buf, int n)
> +{
> +    int i;
> +    for (i = 0; i < n; i++) {
> +        *buf++ = sin_generator_next_sample(sg);
> +    }
> +}
> +
> +#ifdef TEST_PHASOR
> +#define TESTSIZE (48000/1000)
> +// compile with this command to create executable that will
> +// auto-generate a self plotting plot :-)
> +
> +// gcc -g sin_generator.c -o sin_generator -lm -DTEST_PHASOR
> +
> +#include <stdio.h>
> +
> +void dumpbuffer(const char *name, float *buffer, int n)
> +{
> +    int i;
> +    printf("%s = [\n", name);
> +    for (i = 0; i < n; i++) {
> +        printf("   %f,\n", buffer[i]);
> +    }
> +    printf("]\n");
> +    printf("plot(%s)\n", name);
> +}
> +
> +int main(int argc, char *argv[])
> +{
> +    float buffer[TESTSIZE];
> +    long int  i;
> +    struct sin_generator sg;
> +
> +    // test the first few.
> +    sin_generator_init (&sg, 3.0, 1000, 48000);
> +    sin_generator_vfill(&sg, buffer, TESTSIZE);
> +    printf("from pylab import *\n");
> +    dumpbuffer("s", buffer, TESTSIZE);
> +
> +    // now wind the phasor for 3 days worth of samples to see
> +    // if magnitude drifts.
> +    for (i = 0; i < (3L*24*60*60*48000); i++)
> +        sin_generator_next_sample(&sg);
> +
> +    sin_generator_vfill(&sg, buffer, TESTSIZE);
> +    // and let's see what that one looks like
> +    dumpbuffer("o", buffer, TESTSIZE);
> +    printf("show()\n");
> +
> +    return 0;
> +}
> +#endif
> diff --git a/bat/sin_generator.h b/bat/sin_generator.h
> new file mode 100644
> index 0000000..2ab7561
> --- /dev/null
> +++ b/bat/sin_generator.h
> @@ -0,0 +1,23 @@
> +/*
> + * Here's a generic sine wave generator that will work indefinitely
> for any frequency.
> + *
> + * Note:  the state & phasor are stored as doubles (and updated as
> + * doubles) because after a million samples the magnitude drifts a
> + * bit.  If we really need floats, it can be done with periodic
> + * renormalization of the state_real+state_imag magnitudes.  I was
> + */
> +
> +struct sin_generator;
> +struct sin_generator {
> +    double state_real;
> +    double state_imag;
> +    double phasor_real;
> +    double phasor_imag;
> +    float frequency;
> +    float sample_rate;
> +    float magnitude;
> +};
> +
> +int   sin_generator_init(struct sin_generator *sg, float magnitude,
> float frequency, float sample_rate);
> +float sin_generator_next_sample(struct sin_generator *sg);
> +void  sin_generator_vfill(struct sin_generator *sg, float *buf, int n);