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

    /* skip the timecodes (or extra bitstream information for Alternate Syntax)
       TODO: read & use the xbsi1 downmix levels */
    if (get_bits1(gb))
        skip_bits(gb, 14); //skip timecode1 / xbsi1
    if (get_bits1(gb))
        skip_bits(gb, 14); //skip timecode2 / xbsi2

    /* skip additional bitstream info */
    if (get_bits1(gb)) {
        i = get_bits(gb, 6);
        do {
            skip_bits(gb, 8);
        } while(i--);
    }

    /* set stereo downmixing coefficients
       reference: Section 7.8.2 Downmixing Into Two Channels */
    for(i=0; i<ctx->nfchans; i++) {
        ctx->downmix_coeffs[i][0] = gain_levels[ac3_default_coeffs[ctx->acmod][i][0]];
        ctx->downmix_coeffs[i][1] = gain_levels[ac3_default_coeffs[ctx->acmod][i][1]];
    }
    if(ctx->acmod > 1 && ctx->acmod & 1) {
        ctx->downmix_coeffs[1][0] = ctx->downmix_coeffs[1][1] = cmixlev;
    }
    if(ctx->acmod == AC3_ACMOD_2F1R || ctx->acmod == AC3_ACMOD_3F1R) {
        int nf = ctx->acmod - 2;
        ctx->downmix_coeffs[nf][0] = ctx->downmix_coeffs[nf][1] = surmixlev * LEVEL_MINUS_3DB;
    }
    if(ctx->acmod == AC3_ACMOD_2F2R || ctx->acmod == AC3_ACMOD_3F2R) {
        int nf = ctx->acmod - 4;
        ctx->downmix_coeffs[nf][0] = ctx->downmix_coeffs[nf+1][1] = surmixlev;
    }

    return 0;
}

/**
 * Decode the grouped exponents according to exponent strategy.
 * reference: Section 7.1.3 Exponent Decoding
 */
static void decode_exponents(GetBitContext *gb, int expstr, int ngrps,
                             uint8_t absexp, int8_t *dexps)
{
    int i, j, grp, grpsize;
    int dexp[256];
    int expacc, prevexp;

    /* unpack groups */
    grpsize = expstr + (expstr == EXP_D45);
    for(grp=0,i=0; grp<ngrps; grp++) {
        expacc = get_bits(gb, 7);
        dexp[i++] = exp_ungroup_tab[expacc][0];
        dexp[i++] = exp_ungroup_tab[expacc][1];
        dexp[i++] = exp_ungroup_tab[expacc][2];
    }

    /* convert to absolute exps and expand groups */
    prevexp = absexp;
    for(i=0; i<ngrps*3; i++) {
        prevexp = av_clip(prevexp + dexp[i]-2, 0, 24);
        for(j=0; j<grpsize; j++) {
            dexps[(i*grpsize)+j] = prevexp;
        }
    }
}

/**
 * Generate transform coefficients for each coupled channel in the coupling
 * range using the coupling coefficients and coupling coordinates.
 * reference: Section 7.4.3 Coupling Coordinate Format
 */
static void uncouple_channels(AC3DecodeContext *ctx)
{
    int i, j, ch, bnd, subbnd;

    subbnd = -1;
    i = ctx->startmant[CPL_CH];
    for(bnd=0; bnd<ctx->ncplbnd; bnd++) {
        do {
            subbnd++;
            for(j=0; j<12; j++) {
                for(ch=1; ch<=ctx->nfchans; ch++) {
                    if(ctx->chincpl[ch])
                        ctx->transform_coeffs[ch][i] = ctx->transform_coeffs[CPL_CH][i] * ctx->cplco[ch][bnd] * 8.0f;
                }
                i++;
            }
        } while(ctx->cplbndstrc[subbnd]);
    }
}

/**
 * Grouped mantissas for 3-level 5-level and 11-level quantization
 */
typedef struct {
    float b1_mant[3];
    float b2_mant[3];
    float b4_mant[2];
    int b1ptr;
    int b2ptr;
    int b4ptr;
} mant_groups;

/**
 * Get the transform coefficients for a particular channel
 * reference: Section 7.3 Quantization and Decoding of Mantissas
 */
static int get_transform_coeffs_ch(AC3DecodeContext *ctx, int ch_index, mant_groups *m)
{
    GetBitContext *gb = &ctx->gb;
    int i, gcode, tbap, start, end;
    uint8_t *exps;
    uint8_t *bap;
    float *coeffs;

    exps = ctx->dexps[ch_index];
    bap = ctx->bap[ch_index];
    coeffs = ctx->transform_coeffs[ch_index];
    start = ctx->startmant[ch_index];
    end = ctx->endmant[ch_index];

    for (i = start; i < end; i++) {
        tbap = bap[i];
        switch (tbap) {
            case 0:
                coeffs[i] = ((av_random(&ctx->dith_state) & 0xFFFF) / 65535.0f) - 0.5f;
                break;

            case 1:
                if(m->b1ptr > 2) {
                    gcode = get_bits(gb, 5);
                    m->b1_mant[0] = b1_mantissas[gcode][0];
                    m->b1_mant[1] = b1_mantissas[gcode][1];
                    m->b1_mant[2] = b1_mantissas[gcode][2];
                    m->b1ptr = 0;
                }
                coeffs[i] = m->b1_mant[m->b1ptr++];
                break;

            case 2:
                if(m->b2ptr > 2) {
                    gcode = get_bits(gb, 7);
                    m->b2_mant[0] = b2_mantissas[gcode][0];
                    m->b2_mant[1] = b2_mantissas[gcode][1];
                    m->b2_mant[2] = b2_mantissas[gcode][2];
                    m->b2ptr = 0;
                }
                coeffs[i] = m->b2_mant[m->b2ptr++];
                break;

            case 3:
                coeffs[i] = b3_mantissas[get_bits(gb, 3)];
                break;

            case 4:
                if(m->b4ptr > 1) {
                    gcode = get_bits(gb, 7);
                    m->b4_mant[0] = b4_mantissas[gcode][0];
                    m->b4_mant[1] = b4_mantissas[gcode][1];
                    m->b4ptr = 0;
                }
                coeffs[i] = m->b4_mant[m->b4ptr++];
                break;

            case 5:
                coeffs[i] = b5_mantissas[get_bits(gb, 4)];
                break;

            default:
                /* asymmetric dequantization */
                coeffs[i] = get_sbits(gb, qntztab[tbap]) * scale_factors[qntztab[tbap]-1];
                break;
        }
        coeffs[i] *= scale_factors[exps[i]];
    }

    return 0;
}

/**
 * Remove random dithering from coefficients with zero-bit mantissas
 * reference: Section 7.3.4 Dither for Zero Bit Mantissas (bap=0)
 */
static void remove_dithering(AC3DecodeContext *ctx) {
    int ch, i;
    int end=0;
    float *coeffs;
    uint8_t *bap;

    for(ch=1; ch<=ctx->nfchans; ch++) {
        if(!ctx->dithflag[ch]) {
            coeffs = ctx->transform_coeffs[ch];
            bap = ctx->bap[ch];
            if(ctx->chincpl[ch])
                end = ctx->startmant[CPL_CH];
            else
                end = ctx->endmant[ch];
            for(i=0; i<end; i++) {
                if(bap[i] == 0)
                    coeffs[i] = 0.0f;
            }
            if(ctx->chincpl[ch]) {
                bap = ctx->bap[CPL_CH];
                for(; i<ctx->endmant[CPL_CH]; i++) {
                    if(bap[i] == 0)
                        coeffs[i] = 0.0f;
                }
            }
        }
    }
}

/**
 * Get the transform coefficients.
 */
static int get_transform_coeffs(AC3DecodeContext * ctx)
{
    int ch, end;
    int got_cplchan = 0;
    mant_groups m;

    m.b1ptr = m.b2ptr = m.b4ptr = 3;

    for (ch = 1; ch <= ctx->nchans; ch++) {
        /* transform coefficients for full-bandwidth channel */
        if (get_transform_coeffs_ch(ctx, ch, &m))
            return -1;
        /* tranform coefficients for coupling channel come right after the
           coefficients for the first coupled channel*/
        if (ctx->chincpl[ch])  {
            if (!got_cplchan) {
                if (get_transform_coeffs_ch(ctx, CPL_CH, &m)) {
                    av_log(ctx->avctx, AV_LOG_ERROR, "error in decoupling channels\n");
                    return -1;
                }
                uncouple_channels(ctx);
                got_cplchan = 1;
            }
            end = ctx->endmant[CPL_CH];
        } else {
            end = ctx->endmant[ch];
        }
        do
            ctx->transform_coeffs[ch][end] = 0;
        while(++end < 256);
    }

    /* if any channel doesn't use dithering, zero appropriate coefficients */
    if(!ctx->dither_all)
        remove_dithering(ctx);

    return 0;
}

/**
 * Stereo rematrixing.
 * reference: Section 7.5.4 Rematrixing : Decoding Technique
 */
static void do_rematrixing(AC3DecodeContext *ctx)
{
    int bnd, i;
    int end, bndend;
    float tmp0, tmp1;

    end = FFMIN(ctx->endmant[1], ctx->endmant[2]);

    for(bnd=0; bnd<ctx->nrematbnd; bnd++) {
        if(ctx->rematflg[bnd]) {
            bndend = FFMIN(end, rematrix_band_tab[bnd+1]);
            for(i=rematrix_band_tab[bnd]; i<bndend; i++) {
                tmp0 = ctx->transform_coeffs[1][i];
                tmp1 = ctx->transform_coeffs[2][i];
                ctx->transform_coeffs[1][i] = tmp0 + tmp1;
                ctx->transform_coeffs[2][i] = tmp0 - tmp1;
            }
        }
    }
}

/**
 * Perform the 256-point IMDCT
 */
static void do_imdct_256(AC3DecodeContext *ctx, int chindex)
{
    int i, k;
    DECLARE_ALIGNED_16(float, x[128]);
    FFTComplex z[2][64];
    float *o_ptr = ctx->tmp_output;

    for(i=0; i<2; i++) {
        /* de-interleave coefficients */
        for(k=0; k<128; k++) {
            x[k] = ctx->transform_coeffs[chindex][2*k+i];
        }

        /* run standard IMDCT */
        ctx->imdct_256.fft.imdct_calc(&ctx->imdct_256, o_ptr, x, ctx->tmp_imdct);

        /* reverse the post-rotation & reordering from standard IMDCT */
        for(k=0; k<32; k++) {
            z[i][32+k].re = -o_ptr[128+2*k];
            z[i][32+k].im = -o_ptr[2*k];
            z[i][31-k].re =  o_ptr[2*k+1];
            z[i][31-k].im =  o_ptr[128+2*k+1];
        }
    }

    /* apply AC-3 post-rotation & reordering */
    for(k=0; k<64; k++) {
        o_ptr[    2*k  ] = -z[0][   k].im;
        o_ptr[    2*k+1] =  z[0][63-k].re;
        o_ptr[128+2*k  ] = -z[0][   k].re;
        o_ptr[128+2*k+1] =  z[0][63-k].im;
        o_ptr[256+2*k  ] = -z[1][   k].re;
        o_ptr[256+2*k+1] =  z[1][63-k].im;
        o_ptr[384+2*k  ] =  z[1][   k].im;
        o_ptr[384+2*k+1] = -z[1][63-k].re;
    }
}

/**
 * Inverse MDCT Transform.
 * Convert frequency domain coefficients to time-domain audio samples.
 * reference: Section 7.9.4 Transformation Equations
 */
static inline void do_imdct(AC3DecodeContext *ctx)
{
    int ch;
    int nchans;

    /* Don't perform the IMDCT on the LFE channel unless it's used in the output */
    nchans = ctx->nfchans;
    if(ctx->output_mode & AC3_OUTPUT_LFEON)
        nchans++;

    for (ch=1; ch<=nchans; ch++) {
        if (ctx->blksw[ch]) {
            do_imdct_256(ctx, ch);
        } else {
            ctx->imdct_512.fft.imdct_calc(&ctx->imdct_512, ctx->tmp_output,
                                          ctx->transform_coeffs[ch],
                                          ctx->tmp_imdct);
        }
        /* For the first half of the block, apply the window, add the delay
           from the previous block, and send to output */
        ctx->dsp.vector_fmul_add_add(ctx->output[ch-1], ctx->tmp_output,
                                     ctx->window, ctx->delay[ch-1], 0, 256, 1);
        /* For the second half of the block, apply the window and store the
           samples to delay, to be combined with the next block */
        ctx->dsp.vector_fmul_reverse(ctx->delay[ch-1], ctx->tmp_output+256,
                                     ctx->window, 256);
    }
}

/**
 * Downmix the output to mono or stereo.
 */
static void ac3_downmix(float samples[AC3_MAX_CHANNELS][256], int nfchans,
                        int output_mode, float coef[AC3_MAX_CHANNELS][2])
{
    int i, j;
    float v0, v1, s0, s1;

    for(i=0; i<256; i++) {
        v0 = v1 = s0 = s1 = 0.0f;
        for(j=0; j<nfchans; j++) {
            v0 += samples[j][i] * coef[j][0];
            v1 += samples[j][i] * coef[j][1];
            s0 += coef[j][0];
            s1 += coef[j][1];
        }
        v0 /= s0;
        v1 /= s1;
        if(output_mode == AC3_ACMOD_MONO) {
            samples[0][i] = (v0 + v1) * LEVEL_MINUS_3DB;
        } else if(output_mode == AC3_ACMOD_STEREO) {
            samples[0][i] = v0;
            samples[1][i] = v1;
        }
    }
}

/**
 * Parse an audio block from AC-3 bitstream.
 */
static int ac3_parse_audio_block(AC3DecodeContext *ctx, int blk)
{
    int nfchans = ctx->nfchans;
    int acmod = ctx->acmod;
    int i, bnd, seg, ch;
    GetBitContext *gb = &ctx->gb;
    uint8_t bit_alloc_stages[AC3_MAX_CHANNELS];

    memset(bit_alloc_stages, 0, AC3_MAX_CHANNELS);