ac3dec.c

ffmpeg的完整源代码和作者自己写的文档。不但有在Linux的工程哦

    /* block switch flags */
    for (ch = 1; ch <= nfchans; ch++)
        ctx->blksw[ch] = get_bits1(gb);

    /* dithering flags */
    ctx->dither_all = 1;
    for (ch = 1; ch <= nfchans; ch++) {
        ctx->dithflag[ch] = get_bits1(gb);
        if(!ctx->dithflag[ch])
            ctx->dither_all = 0;
    }

    /* dynamic range */
    i = !(ctx->acmod);
    do {
        if(get_bits1(gb)) {
            ctx->dynrng[i] = dynrng_tab[get_bits(gb, 8)];
        } else if(blk == 0) {
            ctx->dynrng[i] = 1.0f;
        }
    } while(i--);

    /* coupling strategy */
    if (get_bits1(gb)) {
        memset(bit_alloc_stages, 3, AC3_MAX_CHANNELS);
        ctx->cplinu = get_bits1(gb);
        if (ctx->cplinu) {
            /* coupling in use */
            int cplbegf, cplendf;

            /* determine which channels are coupled */
            for (ch = 1; ch <= nfchans; ch++)
                ctx->chincpl[ch] = get_bits1(gb);

            /* phase flags in use */
            if (acmod == AC3_ACMOD_STEREO)
                ctx->phsflginu = get_bits1(gb);

            /* coupling frequency range and band structure */
            cplbegf = get_bits(gb, 4);
            cplendf = get_bits(gb, 4);
            if (3 + cplendf - cplbegf < 0) {
                av_log(ctx->avctx, AV_LOG_ERROR, "cplendf = %d < cplbegf = %d\n", cplendf, cplbegf);
                return -1;
            }
            ctx->ncplbnd = ctx->ncplsubnd = 3 + cplendf - cplbegf;
            ctx->startmant[CPL_CH] = cplbegf * 12 + 37;
            ctx->endmant[CPL_CH] = cplendf * 12 + 73;
            for (bnd = 0; bnd < ctx->ncplsubnd - 1; bnd++) {
                if (get_bits1(gb)) {
                    ctx->cplbndstrc[bnd] = 1;
                    ctx->ncplbnd--;
                }
            }
        } else {
            /* coupling not in use */
            for (ch = 1; ch <= nfchans; ch++)
                ctx->chincpl[ch] = 0;
        }
    }

    /* coupling coordinates */
    if (ctx->cplinu) {
        int cplcoe = 0;

        for (ch = 1; ch <= nfchans; ch++) {
            if (ctx->chincpl[ch]) {
                if (get_bits1(gb)) {
                    int mstrcplco, cplcoexp, cplcomant;
                    cplcoe = 1;
                    mstrcplco = 3 * get_bits(gb, 2);
                    for (bnd = 0; bnd < ctx->ncplbnd; bnd++) {
                        cplcoexp = get_bits(gb, 4);
                        cplcomant = get_bits(gb, 4);
                        if (cplcoexp == 15)
                            ctx->cplco[ch][bnd] = cplcomant / 16.0f;
                        else
                            ctx->cplco[ch][bnd] = (cplcomant + 16.0f) / 32.0f;
                        ctx->cplco[ch][bnd] *= scale_factors[cplcoexp + mstrcplco];
                    }
                }
            }
        }
        /* phase flags */
        if (acmod == AC3_ACMOD_STEREO && ctx->phsflginu && cplcoe) {
            for (bnd = 0; bnd < ctx->ncplbnd; bnd++) {
                if (get_bits1(gb))
                    ctx->cplco[2][bnd] = -ctx->cplco[2][bnd];
            }
        }
    }

    /* stereo rematrixing strategy and band structure */
    if (acmod == AC3_ACMOD_STEREO) {
        ctx->rematstr = get_bits1(gb);
        if (ctx->rematstr) {
            ctx->nrematbnd = 4;
            if(ctx->cplinu && ctx->startmant[CPL_CH] <= 61)
                ctx->nrematbnd -= 1 + (ctx->startmant[CPL_CH] == 37);
            for(bnd=0; bnd<ctx->nrematbnd; bnd++)
                ctx->rematflg[bnd] = get_bits1(gb);
        }
    }

    /* exponent strategies for each channel */
    ctx->expstr[CPL_CH] = EXP_REUSE;
    ctx->expstr[ctx->lfe_ch] = EXP_REUSE;
    for (ch = !ctx->cplinu; ch <= ctx->nchans; ch++) {
        if(ch == ctx->lfe_ch)
            ctx->expstr[ch] = get_bits(gb, 1);
        else
            ctx->expstr[ch] = get_bits(gb, 2);
        if(ctx->expstr[ch] != EXP_REUSE)
            bit_alloc_stages[ch] = 3;
    }

    /* channel bandwidth */
    for (ch = 1; ch <= nfchans; ch++) {
        ctx->startmant[ch] = 0;
        if (ctx->expstr[ch] != EXP_REUSE) {
            int prev = ctx->endmant[ch];
            if (ctx->chincpl[ch])
                ctx->endmant[ch] = ctx->startmant[CPL_CH];
            else {
                int chbwcod = get_bits(gb, 6);
                if (chbwcod > 60) {
                    av_log(ctx->avctx, AV_LOG_ERROR, "chbwcod = %d > 60", chbwcod);
                    return -1;
                }
                ctx->endmant[ch] = chbwcod * 3 + 73;
            }
            if(blk > 0 && ctx->endmant[ch] != prev)
                memset(bit_alloc_stages, 3, AC3_MAX_CHANNELS);
        }
    }
    ctx->startmant[ctx->lfe_ch] = 0;
    ctx->endmant[ctx->lfe_ch] = 7;

    /* decode exponents for each channel */
    for (ch = !ctx->cplinu; ch <= ctx->nchans; ch++) {
        if (ctx->expstr[ch] != EXP_REUSE) {
            int grpsize, ngrps;
            grpsize = 3 << (ctx->expstr[ch] - 1);
            if(ch == CPL_CH)
                ngrps = (ctx->endmant[ch] - ctx->startmant[ch]) / grpsize;
            else if(ch == ctx->lfe_ch)
                ngrps = 2;
            else
                ngrps = (ctx->endmant[ch] + grpsize - 4) / grpsize;
            ctx->dexps[ch][0] = get_bits(gb, 4) << !ch;
            decode_exponents(gb, ctx->expstr[ch], ngrps, ctx->dexps[ch][0],
                             &ctx->dexps[ch][ctx->startmant[ch]+!!ch]);
            if(ch != CPL_CH && ch != ctx->lfe_ch)
                skip_bits(gb, 2); /* skip gainrng */
        }
    }

    /* bit allocation information */
    if (get_bits1(gb)) {
        ctx->bit_alloc_params.sdecay = ff_sdecaytab[get_bits(gb, 2)] >> ctx->bit_alloc_params.halfratecod;
        ctx->bit_alloc_params.fdecay = ff_fdecaytab[get_bits(gb, 2)] >> ctx->bit_alloc_params.halfratecod;
        ctx->bit_alloc_params.sgain  = ff_sgaintab[get_bits(gb, 2)];
        ctx->bit_alloc_params.dbknee = ff_dbkneetab[get_bits(gb, 2)];
        ctx->bit_alloc_params.floor  = ff_floortab[get_bits(gb, 3)];
        for(ch=!ctx->cplinu; ch<=ctx->nchans; ch++) {
            bit_alloc_stages[ch] = FFMAX(bit_alloc_stages[ch], 2);
        }
    }

    /* signal-to-noise ratio offsets and fast gains (signal-to-mask ratios) */
    if (get_bits1(gb)) {
        int csnr;
        csnr = (get_bits(gb, 6) - 15) << 4;
        for (ch = !ctx->cplinu; ch <= ctx->nchans; ch++) { /* snr offset and fast gain */
            ctx->snroffst[ch] = (csnr + get_bits(gb, 4)) << 2;
            ctx->fgain[ch] = ff_fgaintab[get_bits(gb, 3)];
        }
        memset(bit_alloc_stages, 3, AC3_MAX_CHANNELS);
    }

    /* coupling leak information */
    if (ctx->cplinu && get_bits1(gb)) {
        ctx->bit_alloc_params.cplfleak = get_bits(gb, 3);
        ctx->bit_alloc_params.cplsleak = get_bits(gb, 3);
        bit_alloc_stages[CPL_CH] = FFMAX(bit_alloc_stages[CPL_CH], 2);
    }

    /* delta bit allocation information */
    if (get_bits1(gb)) {
        /* delta bit allocation exists (strategy) */
        for (ch = !ctx->cplinu; ch <= nfchans; ch++) {
            ctx->deltbae[ch] = get_bits(gb, 2);
            if (ctx->deltbae[ch] == DBA_RESERVED) {
                av_log(ctx->avctx, AV_LOG_ERROR, "delta bit allocation strategy reserved\n");
                return -1;
            }
            bit_alloc_stages[ch] = FFMAX(bit_alloc_stages[ch], 2);
        }
        /* channel delta offset, len and bit allocation */
        for (ch = !ctx->cplinu; ch <= nfchans; ch++) {
            if (ctx->deltbae[ch] == DBA_NEW) {
                ctx->deltnseg[ch] = get_bits(gb, 3);
                for (seg = 0; seg <= ctx->deltnseg[ch]; seg++) {
                    ctx->deltoffst[ch][seg] = get_bits(gb, 5);
                    ctx->deltlen[ch][seg] = get_bits(gb, 4);
                    ctx->deltba[ch][seg] = get_bits(gb, 3);
                }
            }
        }
    } else if(blk == 0) {
        for(ch=0; ch<=ctx->nchans; ch++) {
            ctx->deltbae[ch] = DBA_NONE;
        }
    }

    /* Bit allocation */
    for(ch=!ctx->cplinu; ch<=ctx->nchans; ch++) {
        if(bit_alloc_stages[ch] > 2) {
            /* Exponent mapping into PSD and PSD integration */
            ff_ac3_bit_alloc_calc_psd(ctx->dexps[ch],
                                      ctx->startmant[ch], ctx->endmant[ch],
                                      ctx->psd[ch], ctx->bndpsd[ch]);
        }
        if(bit_alloc_stages[ch] > 1) {
            /* Compute excitation function, Compute masking curve, and
               Apply delta bit allocation */
            ff_ac3_bit_alloc_calc_mask(&ctx->bit_alloc_params, ctx->bndpsd[ch],
                                       ctx->startmant[ch], ctx->endmant[ch],
                                       ctx->fgain[ch], (ch == ctx->lfe_ch),
                                       ctx->deltbae[ch], ctx->deltnseg[ch],
                                       ctx->deltoffst[ch], ctx->deltlen[ch],
                                       ctx->deltba[ch], ctx->mask[ch]);
        }
        if(bit_alloc_stages[ch] > 0) {
            /* Compute bit allocation */
            ff_ac3_bit_alloc_calc_bap(ctx->mask[ch], ctx->psd[ch],
                                      ctx->startmant[ch], ctx->endmant[ch],
                                      ctx->snroffst[ch],
                                      ctx->bit_alloc_params.floor,
                                      ctx->bap[ch]);
        }
    }

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

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

    /* recover coefficients if rematrixing is in use */
    if(ctx->acmod == AC3_ACMOD_STEREO)
        do_rematrixing(ctx);

    /* apply scaling to coefficients (headroom, dialnorm, dynrng) */
    for(ch=1; ch<=ctx->nchans; ch++) {
        float gain = 2.0f * ctx->mul_bias;
        if(ctx->acmod == AC3_ACMOD_DUALMONO) {
            gain *= ctx->dialnorm[ch-1] * ctx->dynrng[ch-1];
        } else {
            gain *= ctx->dialnorm[0] * ctx->dynrng[0];
        }
        for(i=0; i<ctx->endmant[ch]; i++) {
            ctx->transform_coeffs[ch][i] *= gain;
        }
    }

    do_imdct(ctx);

    /* downmix output if needed */
    if(ctx->nchans != ctx->out_channels && !((ctx->output_mode & AC3_OUTPUT_LFEON) &&
            ctx->nfchans == ctx->out_channels)) {
        ac3_downmix(ctx->output, ctx->nfchans, ctx->output_mode,
                    ctx->downmix_coeffs);
    }

    /* convert float to 16-bit integer */
    for(ch=0; ch<ctx->out_channels; ch++) {
        for(i=0; i<256; i++) {
            ctx->output[ch][i] += ctx->add_bias;
        }
        ctx->dsp.float_to_int16(ctx->int_output[ch], ctx->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 *ctx = (AC3DecodeContext *)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(&ctx->gb, buf, buf_size * 8);

    /* parse the syncinfo */
    err = ac3_parse_header(ctx);
    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;
    }

    avctx->sample_rate = ctx->sampling_rate;
    avctx->bit_rate = ctx->bit_rate;

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

    /* channel config */
    ctx->out_channels = ctx->nchans;
    if (avctx->channels == 0) {
        avctx->channels = ctx->out_channels;
    } else if(ctx->out_channels < avctx->channels) {
        av_log(avctx, AV_LOG_ERROR, "Cannot upmix AC3 from %d to %d channels.\n",
               ctx->out_channels, avctx->channels);
        return -1;
    }
    if(avctx->channels == 2) {
        ctx->output_mode = AC3_ACMOD_STEREO;
    } else if(avctx->channels == 1) {
        ctx->output_mode = AC3_ACMOD_MONO;
    } else if(avctx->channels != ctx->out_channels) {
        av_log(avctx, AV_LOG_ERROR, "Cannot downmix AC3 from %d to %d channels.\n",
               ctx->out_channels, avctx->channels);
        return -1;
    }
    ctx->out_channels = avctx->channels;

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

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