ac3dec.c

            /* Compute excitation function, Compute masking curve, and
               Apply delta bit allocation */
            ff_ac3_bit_alloc_calc_mask(&s->bit_alloc_params, s->band_psd[ch],
                                       s->start_freq[ch], s->end_freq[ch],
                                       s->fast_gain[ch], (ch == s->lfe_ch),
                                       s->dba_mode[ch], s->dba_nsegs[ch],
                                       s->dba_offsets[ch], s->dba_lengths[ch],
                                       s->dba_values[ch], s->mask[ch]);
        }
        if(bit_alloc_stages[ch] > 0) {
            /* Compute bit allocation */
            ff_ac3_bit_alloc_calc_bap(s->mask[ch], s->psd[ch],
                                      s->start_freq[ch], s->end_freq[ch],
                                      s->snr_offset[ch],
                                      s->bit_alloc_params.floor,
                                      s->bap[ch]);
        }
    }

    /* unused dummy data */
    if (get_bits1(gbc)) {
        int skipl = get_bits(gbc, 9);
        while(skipl--)
            skip_bits(gbc, 8);
    }

    /* unpack the transform coefficients
       this also uncouples channels if coupling is in use. */
    if (get_transform_coeffs(s)) {
        av_log(s->avctx, AV_LOG_ERROR, "Error in routine get_transform_coeffs\n");
        return -1;
    }

    /* recover coefficients if rematrixing is in use */
    if(s->channel_mode == AC3_CHMODE_STEREO)
        do_rematrixing(s);

    /* apply scaling to coefficients (headroom, dynrng) */
    for(ch=1; ch<=s->channels; ch++) {
        float gain = 2.0f * s->mul_bias;
        if(s->channel_mode == AC3_CHMODE_DUALMONO) {
            gain *= s->dynamic_range[ch-1];
        } else {
            gain *= s->dynamic_range[0];
        }
        for(i=0; i<s->end_freq[ch]; i++) {
            s->transform_coeffs[ch][i] *= gain;
        }
    }

    do_imdct(s);

    /* downmix output if needed */
    if(s->channels != s->out_channels && !((s->output_mode & AC3_OUTPUT_LFEON) &&
            s->fbw_channels == s->out_channels)) {
        ac3_downmix(s);
    }

    /* convert float to 16-bit integer */
    for(ch=0; ch<s->out_channels; ch++) {
        for(i=0; i<256; i++) {
            s->output[ch][i] += s->add_bias;
        }
        s->dsp.float_to_int16(s->int_output[ch], s->output[ch], 256);
    }

    return 0;
}

/**
 * Decode a single AC-3 frame.
 */
static int ac3_decode_frame(AVCodecContext * avctx, void *data, int *data_size, uint8_t *buf, int buf_size)
{
    AC3DecodeContext *s = avctx->priv_data;
    int16_t *out_samples = (int16_t *)data;
    int i, blk, ch, err;

    /* initialize the GetBitContext with the start of valid AC-3 Frame */
    init_get_bits(&s->gbc, buf, buf_size * 8);

    /* parse the syncinfo */
    err = ac3_parse_header(s);
    if(err) {
        switch(err) {
            case AC3_PARSE_ERROR_SYNC:
                av_log(avctx, AV_LOG_ERROR, "frame sync error\n");
                break;
            case AC3_PARSE_ERROR_BSID:
                av_log(avctx, AV_LOG_ERROR, "invalid bitstream id\n");
                break;
            case AC3_PARSE_ERROR_SAMPLE_RATE:
                av_log(avctx, AV_LOG_ERROR, "invalid sample rate\n");
                break;
            case AC3_PARSE_ERROR_FRAME_SIZE:
                av_log(avctx, AV_LOG_ERROR, "invalid frame size\n");
                break;
            default:
                av_log(avctx, AV_LOG_ERROR, "invalid header\n");
                break;
        }
        return -1;
    }

    /* check that reported frame size fits in input buffer */
    if(s->frame_size > buf_size) {
        av_log(avctx, AV_LOG_ERROR, "incomplete frame\n");
        return -1;
    }

    /* check for crc mismatch */
    if(avctx->error_resilience > 0) {
        if(av_crc(av_crc_get_table(AV_CRC_16_ANSI), 0, &buf[2], s->frame_size-2)) {
            av_log(avctx, AV_LOG_ERROR, "frame CRC mismatch\n");
            return -1;
        }
        /* TODO: error concealment */
    }

    avctx->sample_rate = s->sample_rate;
    avctx->bit_rate = s->bit_rate;

    /* channel config */
    s->out_channels = s->channels;
    if (avctx->request_channels > 0 && avctx->request_channels <= 2 &&
        avctx->request_channels < s->channels) {
        s->out_channels = avctx->request_channels;
        s->output_mode  = avctx->request_channels == 1 ? AC3_CHMODE_MONO : AC3_CHMODE_STEREO;
    }
    avctx->channels = s->out_channels;

    /* parse the audio blocks */
    for (blk = 0; blk < NB_BLOCKS; blk++) {
        if (ac3_parse_audio_block(s, blk)) {
            av_log(avctx, AV_LOG_ERROR, "error parsing the audio block\n");
            *data_size = 0;
            return s->frame_size;
        }
        for (i = 0; i < 256; i++)
            for (ch = 0; ch < s->out_channels; ch++)
                *(out_samples++) = s->int_output[ch][i];
    }
    *data_size = NB_BLOCKS * 256 * avctx->channels * sizeof (int16_t);
    return s->frame_size;
}

/**
 * Uninitialize the AC-3 decoder.
 */
static int ac3_decode_end(AVCodecContext *avctx)
{
    AC3DecodeContext *s = avctx->priv_data;
    ff_mdct_end(&s->imdct_512);
    ff_mdct_end(&s->imdct_256);

    return 0;
}

AVCodec ac3_decoder = {
    .name = "ac3",
    .type = CODEC_TYPE_AUDIO,
    .id = CODEC_ID_AC3,
    .priv_data_size = sizeof (AC3DecodeContext),
    .init = ac3_decode_init,
    .close = ac3_decode_end,
    .decode = ac3_decode_frame,
};