ac3enc.c

来自「杜比AC-3编码解码器（参考程序）」· C语言 代码 · 共 1,461 行 · 第 1/3 页

        }
        exp1[i] = exp_min;
        k += group_size;
    }

    /* constraint for DC exponent */
    if (exp1[0] > 15)
        exp1[0] = 15;

    /* Iterate until the delta constraints between each groups are
       satisfyed. I'm sure it is possible to find a better algorithm,
       but I am lazy */
    do {
        recurse = 0;
        for(i=1;i<=nb_groups;i++) {
            delta = exp1[i] - exp1[i-1];
            if (delta > 2) {
                /* if delta too big, we encode a smaller exponent */
                exp1[i] = exp1[i-1] + 2;
            } else if (delta < -2) {
                /* if delta is too small, we must decrease the previous
               exponent, which means we must recurse */
                recurse = 1;
                exp1[i-1] = exp1[i] + 2;
            }
        }
    } while (recurse);
    
    /* now we have the exponent values the decoder will see */
    encoded_exp[0] = exp1[0];
    k = 1;
    for(i=1;i<=nb_groups;i++) {
        for(j=0;j<group_size;j++) {
            encoded_exp[k+j] = exp1[i];
        }
        k += group_size;
    }
    
#if defined(DEBUG)
    printf("exponents: strategy=%d\n", exp_strategy);
    for(i=0;i<=nb_groups * group_size;i++) {
        printf("%d ", encoded_exp[i]);
    }
    printf("\n");
#endif

    return 4 + (nb_groups / 3) * 7;
}

/* return the size in bits taken by the mantissa */
int compute_mantissa_size(AC3EncodeContext *s, UINT8 *m, int nb_coefs)
{
    int bits, mant, i;

    bits = 0;
    for(i=0;i<nb_coefs;i++) {
        mant = m[i];
        switch(mant) {
        case 0:
            /* nothing */
            break;
        case 1:
            /* 3 mantissa in 5 bits */
            if (s->mant1_cnt == 0) 
                bits += 5;
            if (++s->mant1_cnt == 3)
                s->mant1_cnt = 0;
            break;
        case 2:
            /* 3 mantissa in 7 bits */
            if (s->mant2_cnt == 0) 
                bits += 7;
            if (++s->mant2_cnt == 3)
                s->mant2_cnt = 0;
            break;
        case 3:
            bits += 3;
            break;
        case 4:
            /* 2 mantissa in 7 bits */
            if (s->mant4_cnt == 0)
                bits += 7;
            if (++s->mant4_cnt == 2) 
                s->mant4_cnt = 0;
            break;
        case 14:
            bits += 14;
            break;
        case 15:
            bits += 16;
            break;
        default:
            bits += mant - 1;
            break;
        }
    }
    return bits;
}


static int bit_alloc(AC3EncodeContext *s,
                     UINT8 bap[NB_BLOCKS][AC3_MAX_CHANNELS][N/2],
                     UINT8 encoded_exp[NB_BLOCKS][AC3_MAX_CHANNELS][N/2],
                     UINT8 exp_strategy[NB_BLOCKS][AC3_MAX_CHANNELS],
                     int frame_bits, int csnroffst, int fsnroffst)
{
    int i, ch;

    /* 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_channels;ch++) {
            parametric_bit_allocation(s, bap[i][ch], encoded_exp[i][ch], 
                                      0, s->nb_coefs[ch], 
                                      (((csnroffst-15) << 4) + 
                                       fsnroffst) << 2, 
                                      fgaintab[s->fgaincod[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 bap[NB_BLOCKS][AC3_MAX_CHANNELS][N/2],
                                  UINT8 encoded_exp[NB_BLOCKS][AC3_MAX_CHANNELS][N/2],
                                  UINT8 exp_strategy[NB_BLOCKS][AC3_MAX_CHANNELS],
                                  int frame_bits)
{
    int i, ch;
    int csnroffst, fsnroffst;
    UINT8 bap1[NB_BLOCKS][AC3_MAX_CHANNELS][N/2];

    /* init default parameters */
    s->sdecaycod = 2;
    s->fdecaycod = 1;
    s->sgaincod = 1;
    s->dbkneecod = 2;
    s->floorcod = 4;
    for(ch=0;ch<s->nb_channels;ch++) 
        s->fgaincod[ch] = 4;
    
    /* compute real values */
    s->sdecay = sdecaytab[s->sdecaycod] >> s->halfratecod;
    s->fdecay = fdecaytab[s->fdecaycod] >> s->halfratecod;
    s->sgain = sgaintab[s->sgaincod];
    s->dbknee = dbkneetab[s->dbkneecod];
    s->floor = floortab[s->floorcod];

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

    /* audio blocks */
    for(i=0;i<NB_BLOCKS;i++) {
        frame_bits += s->nb_channels * 2 + 2;
        if (s->acmod == 2)
            frame_bits++;
        frame_bits += 2 * s->nb_channels;
        for(ch=0;ch<s->nb_channels;ch++) {
            if (exp_strategy[i][ch] != EXP_REUSE)
                frame_bits += 6 + 2;
        }
        frame_bits++; /* baie */
        frame_bits++; /* snr */
        frame_bits += 2; /* delta / skip */
    }
    frame_bits++; /* cplinu for block 0 */
    /* bit alloc info */
    frame_bits += 2*4 + 3 + 6 + s->nb_channels * (4 + 3);

    /* CRC */
    frame_bits += 16;

    /* 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, bap, encoded_exp, exp_strategy, frame_bits, csnroffst, 0) < 0)
        csnroffst -= SNR_INC1;
    if (csnroffst < 0) {
        fprintf(stderr, "Error !!!\n");
        return -1;
    }
    while ((csnroffst + SNR_INC1) <= 63 && 
           bit_alloc(s, bap1, encoded_exp, exp_strategy, frame_bits, 
                     csnroffst + SNR_INC1, 0) >= 0) {
        csnroffst += SNR_INC1;
        memcpy(bap, bap1, sizeof(bap1));
    }
    while ((csnroffst + 1) <= 63 && 
           bit_alloc(s, bap1, encoded_exp, exp_strategy, frame_bits, csnroffst + 1, 0) >= 0) {
        csnroffst++;
        memcpy(bap, bap1, sizeof(bap1));
    }

    fsnroffst = 0;
    while ((fsnroffst + SNR_INC1) <= 15 && 
           bit_alloc(s, bap1, encoded_exp, exp_strategy, frame_bits, 
                     csnroffst, fsnroffst + SNR_INC1) >= 0) {
        fsnroffst += SNR_INC1;
        memcpy(bap, bap1, sizeof(bap1));
    }
    while ((fsnroffst + 1) <= 15 && 
           bit_alloc(s, bap1, encoded_exp, exp_strategy, frame_bits, 
                     csnroffst, fsnroffst + 1) >= 0) {
        fsnroffst++;
        memcpy(bap, bap1, sizeof(bap1));
    }
    
    s->csnroffst = csnroffst;
    for(ch=0;ch<s->nb_channels;ch++)
        s->fsnroffst[ch] = fsnroffst;
#if defined(DEBUG_BITALLOC)
    {
        int j;

        for(i=0;i<6;i++) {
            for(ch=0;ch<s->nb_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(AVEncodeContext *avctx)
{
    int freq = avctx->rate;
    int bitrate = avctx->bit_rate;
    int channels = avctx->channels;
    AC3EncodeContext *s = avctx->priv_data;
    int i, j, k, l, ch, v;
    float alpha;
    static unsigned short freqs[3] = { 48000, 44100, 32000 };

    avctx->frame_size = AC3_FRAME_SIZE;
    avctx->key_frame = 1; /* always key frame */
    
    /* number of channels */
    if (channels == 1)
        s->acmod = 1;
    else if (channels == 2)
        s->acmod = 2;
    else
        return -1;
    s->nb_channels = channels;

    /* frequency */
    for(i=0;i<3;i++) {
        for(j=0;j<3;j++) 
            if ((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 ((bitratetab[i] >> s->halfratecod) == bitrate)
            break;
    }
    if (i == 19)
        return -1;
    s->bit_rate = bitrate;
    s->frmsizecod = i << 1;
    s->frame_size_min = (bitrate * 1000 * AC3_FRAME_SIZE) / (freq * 16);
    /* for now we do not handle fractional sizes */
    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;
    }
    /* initial snr offset */
    s->csnroffst = 40;

    /* compute bndtab and masktab from bandsz */
    k = 0;
    l = 0;
    for(i=0;i<50;i++) {
        bndtab[i] = l;
        v = bndsz[i];
        for(j=0;j<v;j++) masktab[k++]=i;
        l += v;
    }
    bndtab[50] = 0;

    /* 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));
    }

    ac3_crc_init();

    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, NULL, NULL);

    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 == 2) {
        put_bits(&s->pb, 2, 0); /* surround not indicated */
    }
    put_bits(&s->pb, 1, 0); /* no 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)) >> 25;
        v = (levels >> 1) + v;
    } else {
        v = (levels * ((-c) << e)) >> 25;
        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 exp_strategy[AC3_MAX_CHANNELS],
                               UINT8 encoded_exp[AC3_MAX_CHANNELS][N/2],
                               UINT8 bap[AC3_MAX_CHANNELS][N/2],
                               INT32 mdct_coefs[AC3_MAX_CHANNELS][N/2],
                               INT8 global_exp[AC3_MAX_CHANNELS],
                               int block_num)
{
    int ch, nb_groups, group_size, i, baie;
    UINT8 *p;
    UINT16 qmant[AC3_MAX_CHANNELS][N/2];
    int exp0, exp1;
    int mant1_cnt, mant2_cnt, mant4_cnt;
    UINT16 *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 == 2) {
        put_bits(&s->pb, 1, 0); /* no matrixing (but should be used in the future) */
    }

#if defined(DEBUG) 
    {
        static int count = 0;
        printf("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]);
    }
    
    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_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);
        }