static void audio_encode(const char *filename)
{
    AVCodec *codec;
    AVCodecContext *c = NULL;
    int frame_size, i, j, out_size, outbuf_size;
    FILE *f;
    short *samples;
    float t, tincr;
    uint8_t *outbuf;

    codec = avcodec_find_encoder(CODEC_ID_MP2);
    if(!codec)
    {
        fprintf(stderr, "codec not found\n");
        exit(1);
    }

    c = avcodec_alloc_context3(codec);

    c->bit_rate = 64000;
    c->sample_rate = 44100;
    c->channels = 1;
    c->sample_fmt = AV_SAMPLE_FMT_S16;

    if(avcodec_open(c, codec) < 0)
    {
        fprintf(stderr, "could not open codec\n");
        exit(1);
    }

    frame_size = c->frame_size;
    samples = malloc(frame_size * 2 * c->channels);
    outbuf_size = 10000;
    outbuf = malloc(outbuf_size);

    f = fopen(filename, "wb");
    if(!f)
    {
        fprintf(stderr, "could not open %s\n", filename);
        exit(1);
    }

    /* encode a single tone sound */
    t = 0;
    tincr = 2 * M_PI * 440.0 / c->sample_rate;
    for(i=0;i<200;i++) {
        for(j=0;j<frame_size;j++) {
            samples[2*j] = (int)(sin(t) * 10000);
            samples[2*j+1] = samples[2*j];
            t += tincr;
        }
        /* encode the samples */
        out_size = avcodec_encode_audio(c, outbuf, outbuf_size, samples);
        fwrite(outbuf, 1, out_size, f);
    }
    fclose(f);
    free(outbuf);
    free(samples);
    avcodec_close(c);
    av_free(c);
}