ac3enc.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

            }
        }
    }
}

static int bit_alloc(AC3EncodeContext *s,
                     int16_t mask[NB_BLOCKS][AC3_MAX_CHANNELS][50],
                     int16_t psd[NB_BLOCKS][AC3_MAX_CHANNELS][N/2],
                     uint8_t bap[NB_BLOCKS][AC3_MAX_CHANNELS][N/2],
                     int frame_bits, int csnroffst, int fsnroffst)
{
    int i, ch;
    int snroffset;

    snroffset = (((csnroffst - 15) << 4) + fsnroffst) << 2;

    /* compute size */
    for(i=0;i<NB_BLOCKS;i++) {
        s->mant1_cnt = 0;
        s->mant2_cnt = 0;
        s->mant4_cnt = 0;
        for(ch=0;ch<s->nb_all_channels;ch++) {
            ff_ac3_bit_alloc_calc_bap(mask[i][ch], psd[i][ch], 0,
                                      s->nb_coefs[ch], snroffset,
                                      s->bit_alloc.floor, bap[i][ch]);
            frame_bits += compute_mantissa_size(s, bap[i][ch],
                                                 s->nb_coefs[ch]);
        }
    }
#if 0
    printf("csnr=%d fsnr=%d frame_bits=%d diff=%d\n",
           csnroffst, fsnroffst, frame_bits,
           16 * s->frame_size - ((frame_bits + 7) & ~7));
#endif
    return 16 * s->frame_size - frame_bits;
}

#define SNR_INC1 4

static int compute_bit_allocation(AC3EncodeContext *s,
                                  uint8_t bap[NB_BLOCKS][AC3_MAX_CHANNELS][N/2],
                                  uint8_t encoded_exp[NB_BLOCKS][AC3_MAX_CHANNELS][N/2],
                                  uint8_t exp_strategy[NB_BLOCKS][AC3_MAX_CHANNELS],
                                  int frame_bits)
{
    int i, ch;
    int csnroffst, fsnroffst;
    uint8_t bap1[NB_BLOCKS][AC3_MAX_CHANNELS][N/2];
    int16_t psd[NB_BLOCKS][AC3_MAX_CHANNELS][N/2];
    int16_t mask[NB_BLOCKS][AC3_MAX_CHANNELS][50];
    static int frame_bits_inc[8] = { 0, 0, 2, 2, 2, 4, 2, 4 };

    /* init default parameters */
    s->sdecaycod = 2;
    s->fdecaycod = 1;
    s->sgaincod = 1;
    s->dbkneecod = 2;
    s->floorcod = 4;
    for(ch=0;ch<s->nb_all_channels;ch++)
        s->fgaincod[ch] = 4;

    /* compute real values */
    s->bit_alloc.fscod = s->fscod;
    s->bit_alloc.halfratecod = s->halfratecod;
    s->bit_alloc.sdecay = ff_sdecaytab[s->sdecaycod] >> s->halfratecod;
    s->bit_alloc.fdecay = ff_fdecaytab[s->fdecaycod] >> s->halfratecod;
    s->bit_alloc.sgain = ff_sgaintab[s->sgaincod];
    s->bit_alloc.dbknee = ff_dbkneetab[s->dbkneecod];
    s->bit_alloc.floor = ff_floortab[s->floorcod];

    /* header size */
    frame_bits += 65;
    // if (s->acmod == 2)
    //    frame_bits += 2;
    frame_bits += frame_bits_inc[s->acmod];

    /* audio blocks */
    for(i=0;i<NB_BLOCKS;i++) {
        frame_bits += s->nb_channels * 2 + 2; /* blksw * c, dithflag * c, dynrnge, cplstre */
        if (s->acmod == AC3_ACMOD_STEREO) {
            frame_bits++; /* rematstr */
            if(i==0) frame_bits += 4;
        }
        frame_bits += 2 * s->nb_channels; /* chexpstr[2] * c */
        if (s->lfe)
            frame_bits++; /* lfeexpstr */
        for(ch=0;ch<s->nb_channels;ch++) {
            if (exp_strategy[i][ch] != EXP_REUSE)
                frame_bits += 6 + 2; /* chbwcod[6], gainrng[2] */
        }
        frame_bits++; /* baie */
        frame_bits++; /* snr */
        frame_bits += 2; /* delta / skip */
    }
    frame_bits++; /* cplinu for block 0 */
    /* bit alloc info */
    /* sdcycod[2], fdcycod[2], sgaincod[2], dbpbcod[2], floorcod[3] */
    /* csnroffset[6] */
    /* (fsnoffset[4] + fgaincod[4]) * c */
    frame_bits += 2*4 + 3 + 6 + s->nb_all_channels * (4 + 3);

    /* auxdatae, crcrsv */
    frame_bits += 2;

    /* CRC */
    frame_bits += 16;

    /* calculate psd and masking curve before doing bit allocation */
    bit_alloc_masking(s, encoded_exp, exp_strategy, psd, mask);

    /* now the big work begins : do the bit allocation. Modify the snr
       offset until we can pack everything in the requested frame size */

    csnroffst = s->csnroffst;
    while (csnroffst >= 0 &&
           bit_alloc(s, mask, psd, bap, frame_bits, csnroffst, 0) < 0)
        csnroffst -= SNR_INC1;
    if (csnroffst < 0) {
        av_log(NULL, AV_LOG_ERROR, "Bit allocation failed, try increasing the bitrate, -ab 384 for example!\n");
        return -1;
    }
    while ((csnroffst + SNR_INC1) <= 63 &&
           bit_alloc(s, mask, psd, bap1, frame_bits,
                     csnroffst + SNR_INC1, 0) >= 0) {
        csnroffst += SNR_INC1;
        memcpy(bap, bap1, sizeof(bap1));
    }
    while ((csnroffst + 1) <= 63 &&
           bit_alloc(s, mask, psd, bap1, frame_bits, csnroffst + 1, 0) >= 0) {
        csnroffst++;
        memcpy(bap, bap1, sizeof(bap1));
    }

    fsnroffst = 0;
    while ((fsnroffst + SNR_INC1) <= 15 &&
           bit_alloc(s, mask, psd, bap1, frame_bits,
                     csnroffst, fsnroffst + SNR_INC1) >= 0) {
        fsnroffst += SNR_INC1;
        memcpy(bap, bap1, sizeof(bap1));
    }
    while ((fsnroffst + 1) <= 15 &&
           bit_alloc(s, mask, psd, bap1, frame_bits,
                     csnroffst, fsnroffst + 1) >= 0) {
        fsnroffst++;
        memcpy(bap, bap1, sizeof(bap1));
    }

    s->csnroffst = csnroffst;
    for(ch=0;ch<s->nb_all_channels;ch++)
        s->fsnroffst[ch] = fsnroffst;
#if defined(DEBUG_BITALLOC)
    {
        int j;

        for(i=0;i<6;i++) {
            for(ch=0;ch<s->nb_all_channels;ch++) {
                printf("Block #%d Ch%d:\n", i, ch);
                printf("bap=");
                for(j=0;j<s->nb_coefs[ch];j++) {
                    printf("%d ",bap[i][ch][j]);
                }
                printf("\n");
            }
        }
    }
#endif
    return 0;
}

static int AC3_encode_init(AVCodecContext *avctx)
{
    int freq = avctx->sample_rate;
    int bitrate = avctx->bit_rate;
    int channels = avctx->channels;
    AC3EncodeContext *s = avctx->priv_data;
    int i, j, ch;
    float alpha;
    static const uint8_t acmod_defs[6] = {
        0x01, /* C */
        0x02, /* L R */
        0x03, /* L C R */
        0x06, /* L R SL SR */
        0x07, /* L C R SL SR */
        0x07, /* L C R SL SR (+LFE) */
    };

    avctx->frame_size = AC3_FRAME_SIZE;

    ac3_common_init();

    /* number of channels */
    if (channels < 1 || channels > 6)
        return -1;
    s->acmod = acmod_defs[channels - 1];
    s->lfe = (channels == 6) ? 1 : 0;
    s->nb_all_channels = channels;
    s->nb_channels = channels > 5 ? 5 : channels;
    s->lfe_channel = s->lfe ? 5 : -1;

    /* frequency */
    for(i=0;i<3;i++) {
        for(j=0;j<3;j++)
            if ((ff_ac3_freqs[j] >> i) == freq)
                goto found;
    }
    return -1;
 found:
    s->sample_rate = freq;
    s->halfratecod = i;
    s->fscod = j;
    s->bsid = 8 + s->halfratecod;
    s->bsmod = 0; /* complete main audio service */

    /* bitrate & frame size */
    bitrate /= 1000;
    for(i=0;i<19;i++) {
        if ((ff_ac3_bitratetab[i] >> s->halfratecod) == bitrate)
            break;
    }
    if (i == 19)
        return -1;
    s->bit_rate = bitrate;
    s->frmsizecod = i << 1;
    s->frame_size_min = ff_ac3_frame_sizes[s->frmsizecod][s->fscod];
    s->bits_written = 0;
    s->samples_written = 0;
    s->frame_size = s->frame_size_min;

    /* bit allocation init */
    for(ch=0;ch<s->nb_channels;ch++) {
        /* bandwidth for each channel */
        /* XXX: should compute the bandwidth according to the frame
           size, so that we avoid anoying high freq artefacts */
        s->chbwcod[ch] = 50; /* sample bandwidth as mpeg audio layer 2 table 0 */
        s->nb_coefs[ch] = ((s->chbwcod[ch] + 12) * 3) + 37;
    }
    if (s->lfe) {
        s->nb_coefs[s->lfe_channel] = 7; /* fixed */
    }
    /* initial snr offset */
    s->csnroffst = 40;

    /* mdct init */
    fft_init(MDCT_NBITS - 2);
    for(i=0;i<N/4;i++) {
        alpha = 2 * M_PI * (i + 1.0 / 8.0) / (float)N;
        xcos1[i] = fix15(-cos(alpha));
        xsin1[i] = fix15(-sin(alpha));
    }

    avctx->coded_frame= avcodec_alloc_frame();
    avctx->coded_frame->key_frame= 1;

    return 0;
}

/* output the AC3 frame header */
static void output_frame_header(AC3EncodeContext *s, unsigned char *frame)
{
    init_put_bits(&s->pb, frame, AC3_MAX_CODED_FRAME_SIZE);

    put_bits(&s->pb, 16, 0x0b77); /* frame header */
    put_bits(&s->pb, 16, 0); /* crc1: will be filled later */
    put_bits(&s->pb, 2, s->fscod);
    put_bits(&s->pb, 6, s->frmsizecod + (s->frame_size - s->frame_size_min));
    put_bits(&s->pb, 5, s->bsid);
    put_bits(&s->pb, 3, s->bsmod);
    put_bits(&s->pb, 3, s->acmod);
    if ((s->acmod & 0x01) && s->acmod != AC3_ACMOD_MONO)
        put_bits(&s->pb, 2, 1); /* XXX -4.5 dB */
    if (s->acmod & 0x04)
        put_bits(&s->pb, 2, 1); /* XXX -6 dB */
    if (s->acmod == AC3_ACMOD_STEREO)
        put_bits(&s->pb, 2, 0); /* surround not indicated */
    put_bits(&s->pb, 1, s->lfe); /* LFE */
    put_bits(&s->pb, 5, 31); /* dialog norm: -31 db */
    put_bits(&s->pb, 1, 0); /* no compression control word */
    put_bits(&s->pb, 1, 0); /* no lang code */
    put_bits(&s->pb, 1, 0); /* no audio production info */
    put_bits(&s->pb, 1, 0); /* no copyright */
    put_bits(&s->pb, 1, 1); /* original bitstream */
    put_bits(&s->pb, 1, 0); /* no time code 1 */
    put_bits(&s->pb, 1, 0); /* no time code 2 */
    put_bits(&s->pb, 1, 0); /* no addtional bit stream info */
}

/* symetric quantization on 'levels' levels */
static inline int sym_quant(int c, int e, int levels)
{
    int v;

    if (c >= 0) {
        v = (levels * (c << e)) >> 24;
        v = (v + 1) >> 1;
        v = (levels >> 1) + v;
    } else {
        v = (levels * ((-c) << e)) >> 24;
        v = (v + 1) >> 1;
        v = (levels >> 1) - v;
    }
    assert (v >= 0 && v < levels);
    return v;
}

/* asymetric quantization on 2^qbits levels */
static inline int asym_quant(int c, int e, int qbits)
{
    int lshift, m, v;

    lshift = e + qbits - 24;
    if (lshift >= 0)
        v = c << lshift;
    else
        v = c >> (-lshift);
    /* rounding */
    v = (v + 1) >> 1;
    m = (1 << (qbits-1));
    if (v >= m)
        v = m - 1;
    assert(v >= -m);
    return v & ((1 << qbits)-1);
}

/* Output one audio block. There are NB_BLOCKS audio blocks in one AC3
   frame */
static void output_audio_block(AC3EncodeContext *s,
                               uint8_t exp_strategy[AC3_MAX_CHANNELS],
                               uint8_t encoded_exp[AC3_MAX_CHANNELS][N/2],
                               uint8_t bap[AC3_MAX_CHANNELS][N/2],
                               int32_t mdct_coefs[AC3_MAX_CHANNELS][N/2],
                               int8_t global_exp[AC3_MAX_CHANNELS],
                               int block_num)
{
    int ch, nb_groups, group_size, i, baie, rbnd;
    uint8_t *p;
    uint16_t qmant[AC3_MAX_CHANNELS][N/2];
    int exp0, exp1;
    int mant1_cnt, mant2_cnt, mant4_cnt;
    uint16_t *qmant1_ptr, *qmant2_ptr, *qmant4_ptr;
    int delta0, delta1, delta2;

    for(ch=0;ch<s->nb_channels;ch++)
        put_bits(&s->pb, 1, 0); /* 512 point MDCT */
    for(ch=0;ch<s->nb_channels;ch++)
        put_bits(&s->pb, 1, 1); /* no dither */
    put_bits(&s->pb, 1, 0); /* no dynamic range */
    if (block_num == 0) {
        /* for block 0, even if no coupling, we must say it. This is a
           waste of bit :-) */
        put_bits(&s->pb, 1, 1); /* coupling strategy present */
        put_bits(&s->pb, 1, 0); /* no coupling strategy */
    } else {
        put_bits(&s->pb, 1, 0); /* no new coupling strategy */
    }

    if (s->acmod == AC3_ACMOD_STEREO)
      {
        if(block_num==0)
          {
            /* first block must define rematrixing (rematstr)  */
            put_bits(&s->pb, 1, 1);

            /* dummy rematrixing rematflg(1:4)=0 */
            for (rbnd=0;rbnd<4;rbnd++)
              put_bits(&s->pb, 1, 0);
          }
        else
          {
            /* no matrixing (but should be used in the future) */
            put_bits(&s->pb, 1, 0);
          }
      }

#if defined(DEBUG)
    {
      static int count = 0;
      av_log(NULL, AV_LOG_DEBUG, "Block #%d (%d)\n", block_num, count++);
    }
#endif
    /* exponent strategy */
    for(ch=0;ch<s->nb_channels;ch++) {
        put_bits(&s->pb, 2, exp_strategy[ch]);
    }

    if (s->lfe) {
        put_bits(&s->pb, 1, exp_strategy[s->lfe_channel]);
    }

    for(ch=0;ch<s->nb_channels;ch++) {
        if (exp_strategy[ch] != EXP_REUSE)
            put_bits(&s->pb, 6, s->chbwcod[ch]);
    }

    /* exponents */
    for (ch = 0; ch < s->nb_all_channels; ch++) {
        switch(exp_strategy[ch]) {
        case EXP_REUSE:
            continue;
        case EXP_D15:
            group_size = 1;
            break;
        case EXP_D25:
            group_size = 2;
            break;
        default:
        case EXP_D45:
            group_size = 4;
            break;
        }
        nb_groups = (s->nb_coefs[ch] + (group_size * 3) - 4) / (3 * group_size);
        p = encoded_exp[ch];

        /* first exponent */
        exp1 = *p++;
        put_bits(&s->pb, 4, exp1);

        /* next ones are delta encoded */
        for(i=0;i<nb_groups;i++) {
            /* merge three delta in one code */
            exp0 = exp1;
            exp1 = p[0];
            p += group_size;
            delta0 = exp1 - exp0 + 2;

            exp0 = exp1;
            exp1 = p[0];
            p += group_size;
            delta1 = exp1 - exp0 + 2;

            exp0 = exp1;
            exp1 = p[0];
            p += group_size;
            delta2 = exp1 - exp0 + 2;

            put_bits(&s->pb, 7, ((delta0 * 5 + delta1) * 5) + delta2);
        }

        if (ch != s->lfe_channel)
            put_bits(&s->pb, 2, 0); /* no gain range info */
    }

    /* bit allocation info */
    baie = (block_num == 0);
    put_bits(&s->pb, 1, baie);
    if (baie) {
        put_bits(&s->pb, 2, s->sdecaycod);
        put_bits(&s->pb, 2, s->fdecaycod);
        put_bits(&s->pb, 2, s->sgaincod);
        put_bits(&s->pb, 2, s->dbkneecod);
        put_bits(&s->pb, 3, s->floorcod);
    }

    /* snr offset */
    put_bits(&s->pb, 1, baie); /* always present with bai */
    if (baie) {
        put_bits(&s->pb, 6, s->csnroffst);
        for(ch=0;ch<s->nb_all_channels;ch++) {