3. Algorithm

The recipe for a basic crossfade loop is pretty simple: Apply a linear fade in N seconds long at the beginning of the sample. Fade out the last N seconds. Take those last N seconds and mix it into the beginning of the file. Boom. Crossfade loop.

In my version of this crossfade algorith, the loop time is relative to the sample. More often than not, it makes more musical sense to think it relative terms than in precise seconds. When the crossfade loop time is 100% of the file, it means that the fade in/out times add up to the total duration of the file, making the fade time half the file. A 50% loop time would therefore be quarter the file duration for the fade times.

#define BUFSIZE 1024

int do_cfloop(const char *in, const char *out, float cf)
{
    float buf[BUFSIZE];
    float endbuf[BUFSIZE];
    drwav infile;
    drwav *outfile;
    drwav_data_format format;
    int rc;
    int count;
    unsigned long dur;
    unsigned long fade;
    unsigned long pos;
    unsigned long n;
    unsigned long endpos;

    rc = drwav_init_file(&infile, in);

    if (!rc) {
        fprintf(stderr, "Error opening %s\n", in);
        return 1;
    }

    format.container = drwav_container_riff;
    format.format = DR_WAVE_FORMAT_IEEE_FLOAT;
    /* format.format = DR_WAVE_FORMAT_PCM; */
    format.channels = 1;
    format.sampleRate = infile.sampleRate;
    format.bitsPerSample = 32;
    outfile = drwav_open_file_write(out, &format);

    dur = infile.totalSampleCount;
    fade = floor(dur * cf * 0.5);
    dur -= fade;
    endpos = dur;

    pos = 0;

    while (1) {
        count = drwav_read_f32(&infile, BUFSIZE, buf);

        if (pos < fade) {
<<read_end>>
        }

        for (n = 0; n < count; n++) {
            if (pos < fade) {
<<scale_and_sum>>
            }
            pos++;
        }

        if (pos >= dur) {
            count -= (pos - dur);
            drwav_write(outfile, count, buf);
            break;
        } else {
            drwav_write(outfile, count, buf);
        }
    }

    drwav_uninit(&infile);
    drwav_close(outfile);
    return 0;
}

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