int ff_ac3_parse_header(GetBitContext *gbc, AC3HeaderInfo *hdr)
{
    int frame_size_code;

    memset(hdr, 0, sizeof(*hdr));

    hdr->sync_word = get_bits(gbc, 16);
    if(hdr->sync_word != 0x0B77)
        return AAC_AC3_PARSE_ERROR_SYNC;

    /* read ahead to bsid to distinguish between AC-3 and E-AC-3 */
    hdr->bitstream_id = show_bits_long(gbc, 29) & 0x1F;
    if(hdr->bitstream_id > 16)
        return AAC_AC3_PARSE_ERROR_BSID;

    hdr->num_blocks = 6;

    /* set default mix levels */
    hdr->center_mix_level   = 5;  // -4.5dB
    hdr->surround_mix_level = 6;  // -6.0dB

    /* set default dolby surround mode */
    hdr->dolby_surround_mode = AC3_DSURMOD_NOTINDICATED;

    if(hdr->bitstream_id <= 10) {
        /* Normal AC-3 */
        hdr->crc1 = get_bits(gbc, 16);
        hdr->sr_code = get_bits(gbc, 2);
        if(hdr->sr_code == 3)
            return AAC_AC3_PARSE_ERROR_SAMPLE_RATE;

        frame_size_code = get_bits(gbc, 6);
        if(frame_size_code > 37)
            return AAC_AC3_PARSE_ERROR_FRAME_SIZE;

        skip_bits(gbc, 5); // skip bsid, already got it

        hdr->bitstream_mode = get_bits(gbc, 3);
        hdr->channel_mode = get_bits(gbc, 3);

        if(hdr->channel_mode == AC3_CHMODE_STEREO) {
            hdr->dolby_surround_mode = get_bits(gbc, 2);
        } else {
            if((hdr->channel_mode & 1) && hdr->channel_mode != AC3_CHMODE_MONO)
                hdr->  center_mix_level =   center_levels[get_bits(gbc, 2)];
            if(hdr->channel_mode & 4)
                hdr->surround_mix_level = surround_levels[get_bits(gbc, 2)];
        }
        hdr->lfe_on = get_bits1(gbc);

        hdr->sr_shift = FFMAX(hdr->bitstream_id, 8) - 8;
        hdr->sample_rate = ff_ac3_sample_rate_tab[hdr->sr_code] >> hdr->sr_shift;
        hdr->bit_rate = (ff_ac3_bitrate_tab[frame_size_code>>1] * 1000) >> hdr->sr_shift;
        hdr->channels = ff_ac3_channels_tab[hdr->channel_mode] + hdr->lfe_on;
        hdr->frame_size = ff_ac3_frame_size_tab[frame_size_code][hdr->sr_code] * 2;
        hdr->frame_type = EAC3_FRAME_TYPE_AC3_CONVERT; //EAC3_FRAME_TYPE_INDEPENDENT;
        hdr->substreamid = 0;
    } else {
        /* Enhanced AC-3 */
        hdr->crc1 = 0;
        hdr->frame_type = get_bits(gbc, 2);
        if(hdr->frame_type == EAC3_FRAME_TYPE_RESERVED)
            return AAC_AC3_PARSE_ERROR_FRAME_TYPE;

        hdr->substreamid = get_bits(gbc, 3);

        hdr->frame_size = (get_bits(gbc, 11) + 1) << 1;
        if(hdr->frame_size < AC3_HEADER_SIZE)
            return AAC_AC3_PARSE_ERROR_FRAME_SIZE;

        hdr->sr_code = get_bits(gbc, 2);
        if (hdr->sr_code == 3) {
            int sr_code2 = get_bits(gbc, 2);
            if(sr_code2 == 3)
                return AAC_AC3_PARSE_ERROR_SAMPLE_RATE;
            hdr->sample_rate = ff_ac3_sample_rate_tab[sr_code2] / 2;
            hdr->sr_shift = 1;
        } else {
            hdr->num_blocks = eac3_blocks[get_bits(gbc, 2)];
            hdr->sample_rate = ff_ac3_sample_rate_tab[hdr->sr_code];
            hdr->sr_shift = 0;
        }

        hdr->channel_mode = get_bits(gbc, 3);
        hdr->lfe_on = get_bits1(gbc);

        hdr->bit_rate = 8LL * hdr->frame_size * hdr->sample_rate /
                        (hdr->num_blocks * 256);
        hdr->channels = ff_ac3_channels_tab[hdr->channel_mode] + hdr->lfe_on;
    }
    hdr->channel_layout = avpriv_ac3_channel_layout_tab[hdr->channel_mode];
    if (hdr->lfe_on)
        hdr->channel_layout |= AV_CH_LOW_FREQUENCY;

    return 0;
}