ac3enc.c

杜比AC-3编码解码器（参考程序）

        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_channels;ch++) {
            put_bits(&s->pb, 4, s->fsnroffst[ch]);
            put_bits(&s->pb, 3, s->fgaincod[ch]);
        }
    }
    
    put_bits(&s->pb, 1, 0); /* no delta bit allocation */
    put_bits(&s->pb, 1, 0); /* no data to skip */

    /* mantissa encoding : we use two passes to handle the grouping. A
       one pass method may be faster, but it would necessitate to
       modify the output stream. */

    /* first pass: quantize */
    mant1_cnt = mant2_cnt = mant4_cnt = 0;
    qmant1_ptr = qmant2_ptr = qmant4_ptr = NULL;

    for (ch = 0; ch < s->nb_channels; ch++) {
        int b, c, e, v;

        for(i=0;i<s->nb_coefs[ch];i++) {
            c = mdct_coefs[ch][i];
            e = encoded_exp[ch][i] - global_exp[ch];
            b = bap[ch][i];
            switch(b) {
            case 0:
                v = 0;
                break;
            case 1:
                v = sym_quant(c, e, 3);
                switch(mant1_cnt) {
                case 0:
                    qmant1_ptr = &qmant[ch][i];
                    v = 9 * v;
                    mant1_cnt = 1;
                    break;
                case 1:
                    *qmant1_ptr += 3 * v;
                    mant1_cnt = 2;
                    v = 128;
                    break;
                default:
                    *qmant1_ptr += v;
                    mant1_cnt = 0;
                    v = 128;
                    break;
                }
                break;
            case 2:
                v = sym_quant(c, e, 5);
                switch(mant2_cnt) {
                case 0:
                    qmant2_ptr = &qmant[ch][i];
                    v = 25 * v;
                    mant2_cnt = 1;
                    break;
                case 1:
                    *qmant2_ptr += 5 * v;
                    mant2_cnt = 2;
                    v = 128;
                    break;
                default:
                    *qmant2_ptr += v;
                    mant2_cnt = 0;
                    v = 128;
                    break;
                }
                break;
            case 3:
                v = sym_quant(c, e, 7);
                break;
            case 4:
                v = sym_quant(c, e, 11);
                switch(mant4_cnt) {
                case 0:
                    qmant4_ptr = &qmant[ch][i];
                    v = 11 * v;
                    mant4_cnt = 1;
                    break;
                default:
                    *qmant4_ptr += v;
                    mant4_cnt = 0;
                    v = 128;
                    break;
                }
                break;
            case 5:
                v = sym_quant(c, e, 15);
                break;
            case 14:
                v = asym_quant(c, e, 14);
                break;
            case 15:
                v = asym_quant(c, e, 16);
                break;
            default:
                v = asym_quant(c, e, b - 1);
                break;
            }
            qmant[ch][i] = v;
        }
    }

    /* second pass : output the values */
    for (ch = 0; ch < s->nb_channels; ch++) {
        int b, q;
        
        for(i=0;i<s->nb_coefs[ch];i++) {
            q = qmant[ch][i];
            b = bap[ch][i];
            switch(b) {
            case 0:
                break;
            case 1:
                if (q != 128) 
                    put_bits(&s->pb, 5, q);
                break;
            case 2:
                if (q != 128) 
                    put_bits(&s->pb, 7, q);
                break;
            case 3:
                put_bits(&s->pb, 3, q);
                break;
            case 4:
                if (q != 128)
                    put_bits(&s->pb, 7, q);
                break;
            case 14:
                put_bits(&s->pb, 14, q);
                break;
            case 15:
                put_bits(&s->pb, 16, q);
                break;
            default:
                put_bits(&s->pb, b - 1, q);
                break;
            }
        }
    }
}

/* compute the ac3 crc */

#define CRC16_POLY ((1 << 0) | (1 << 2) | (1 << 15) | (1 << 16))

static void ac3_crc_init(void)
{
    unsigned int c, n, k;

    for(n=0;n<256;n++) {
        c = n << 8;
        for (k = 0; k < 8; k++) {
            if (c & (1 << 15)) 
                c = ((c << 1) & 0xffff) ^ (CRC16_POLY & 0xffff);
            else
                c = c << 1;
        }
        crc_table[n] = c;
    }
}

static unsigned int ac3_crc(UINT8 *data, int n, unsigned int crc)
{
    int i;
    for(i=0;i<n;i++) {
        crc = (crc_table[data[i] ^ (crc >> 8)] ^ (crc << 8)) & 0xffff;
    }
    return crc;
}

static unsigned int mul_poly(unsigned int a, unsigned int b, unsigned int poly)
{
    unsigned int c;

    c = 0;
    while (a) {
        if (a & 1)
            c ^= b;
        a = a >> 1;
        b = b << 1;
        if (b & (1 << 16))
            b ^= poly;
    }
    return c;
}

static unsigned int pow_poly(unsigned int a, unsigned int n, unsigned int poly)
{
    unsigned int r;
    r = 1;
    while (n) {
        if (n & 1)
            r = mul_poly(r, a, poly);
        a = mul_poly(a, a, poly);
        n >>= 1;
    }
    return r;
}


/* compute log2(max(abs(tab[]))) */
static int log2_tab(INT16 *tab, int n)
{
    int i, v;

    v = 0;
    for(i=0;i<n;i++) {
        v |= abs(tab[i]);
    }
    return log2(v);
}

static void lshift_tab(INT16 *tab, int n, int lshift)
{
    int i;

    if (lshift > 0) {
        for(i=0;i<n;i++) {
            tab[i] <<= lshift;
        }
    } else if (lshift < 0) {
        lshift = -lshift;
        for(i=0;i<n;i++) {
            tab[i] >>= lshift;
        }
    }
}

/* fill the end of the frame and compute the two crcs */
static int output_frame_end(AC3EncodeContext *s)
{
    int frame_size, frame_size_58, n, crc1, crc2, crc_inv;
    UINT8 *frame;

    frame_size = s->frame_size; /* frame size in words */
    /* align to 8 bits */
    flush_put_bits(&s->pb);
    /* add zero bytes to reach the frame size */
    frame = s->pb.buf;
    n = 2 * s->frame_size - (s->pb.buf_ptr - frame) - 2;
    assert(n >= 0);
    memset(s->pb.buf_ptr, 0, n);
    
    /* Now we must compute both crcs : this is not so easy for crc1
       because it is at the beginning of the data... */
    frame_size_58 = (frame_size >> 1) + (frame_size >> 3);
    crc1 = ac3_crc(frame + 4, (2 * frame_size_58) - 4, 0);
    /* XXX: could precompute crc_inv */
    crc_inv = pow_poly((CRC16_POLY >> 1), (16 * frame_size_58) - 16, CRC16_POLY);
    crc1 = mul_poly(crc_inv, crc1, CRC16_POLY);
    frame[2] = crc1 >> 8;
    frame[3] = crc1;
    
    crc2 = ac3_crc(frame + 2 * frame_size_58, (frame_size - frame_size_58) * 2 - 2, 0);
    frame[2*frame_size - 2] = crc2 >> 8;
    frame[2*frame_size - 1] = crc2;

    //    printf("n=%d frame_size=%d\n", n, frame_size);
    return frame_size * 2;
}

int AC3_encode_frame(AVEncodeContext *avctx,
                     unsigned char *frame, int buf_size, void *data)
{
    AC3EncodeContext *s = avctx->priv_data;
    short *samples = data;
    int i, j, k, v, ch;
    INT16 input_samples[N];
    INT32 mdct_coef[NB_BLOCKS][AC3_MAX_CHANNELS][N/2];
    UINT8 exp[NB_BLOCKS][AC3_MAX_CHANNELS][N/2];
    UINT8 exp_strategy[NB_BLOCKS][AC3_MAX_CHANNELS];
    UINT8 encoded_exp[NB_BLOCKS][AC3_MAX_CHANNELS][N/2];
    UINT8 bap[NB_BLOCKS][AC3_MAX_CHANNELS][N/2];
    INT8 exp_samples[NB_BLOCKS][AC3_MAX_CHANNELS];
    int frame_bits;

    frame_bits = 0;
    for(ch=0;ch<s->nb_channels;ch++) {
        /* fixed mdct to the six sub blocks & exponent computation */
        for(i=0;i<NB_BLOCKS;i++) {
            INT16 *sptr;
            int sinc;

            /* compute input samples */
            memcpy(input_samples, s->last_samples[ch], N/2 * sizeof(INT16));
            sinc = s->nb_channels;
            sptr = samples + (sinc * (N/2) * i) + ch;
            for(j=0;j<N/2;j++) {
                v = *sptr;
                input_samples[j + N/2] = v;
                s->last_samples[ch][j] = v; 
                sptr += sinc;
            }

            /* apply the MDCT window */
            for(j=0;j<N/2;j++) {
                input_samples[j] = MUL16(input_samples[j], 
                                         ac3_window[j]) >> 15;
                input_samples[N-j-1] = MUL16(input_samples[N-j-1], 
                                             ac3_window[j]) >> 15;
            }
        
            /* Normalize the samples to use the maximum available
               precision */
            v = 14 - log2_tab(input_samples, N);
            if (v < 0)
                v = 0;
            exp_samples[i][ch] = v - 8;
            lshift_tab(input_samples, N, v);

            /* do the MDCT */
            mdct512(mdct_coef[i][ch], input_samples);
            
            /* compute "exponents". We take into account the
               normalization there */
            for(j=0;j<N/2;j++) {
                int e;
                v = abs(mdct_coef[i][ch][j]);
                if (v == 0)
                    e = 24;
                else {
                    e = 23 - log2(v) + exp_samples[i][ch];
                    if (e >= 24) {
                        e = 24;
                        mdct_coef[i][ch][j] = 0;
                    }
                }
                exp[i][ch][j] = e;
            }
        }
        
        compute_exp_strategy(exp_strategy, exp, ch);

        /* compute the exponents as the decoder will see them. The
           EXP_REUSE case must be handled carefully : we select the
           min of the exponents */
        i = 0;
        while (i < NB_BLOCKS) {
            j = i + 1;
            while (j < NB_BLOCKS && exp_strategy[j][ch] == EXP_REUSE) {
                exponent_min(exp[i][ch], exp[j][ch], s->nb_coefs[ch]);
                j++;
            }
            frame_bits += encode_exp(encoded_exp[i][ch],
                                     exp[i][ch], s->nb_coefs[ch], 
                                     exp_strategy[i][ch]);
            /* copy encoded exponents for reuse case */
            for(k=i+1;k<j;k++) {
                memcpy(encoded_exp[k][ch], encoded_exp[i][ch], 
                       s->nb_coefs[ch] * sizeof(UINT8));
            }
            i = j;
        }
    }

    compute_bit_allocation(s, bap, encoded_exp, exp_strategy, frame_bits);
    /* everything is known... let's output the frame */
    output_frame_header(s, frame);
        
    for(i=0;i<NB_BLOCKS;i++) {
        output_audio_block(s, exp_strategy[i], encoded_exp[i], 
                           bap[i], mdct_coef[i], exp_samples[i], i);
    }
    return output_frame_end(s);
}

#if 0
/*************************************************************************/
/* TEST */

#define FN (N/4)

void fft_test(void)
{
    IComplex in[FN], in1[FN];
    int k, n, i;
    float sum_re, sum_im, a;

    /* FFT test */

    for(i=0;i<FN;i++) {
        in[i].re = random() % 65535 - 32767;
        in[i].im = random() % 65535 - 32767;
        in1[i] = in[i];
    }
    fft(in, 7);

    /* do it by hand */
    for(k=0;k<FN;k++) {
        sum_re = 0;
        sum_im = 0;
        for(n=0;n<FN;n++) {
            a = -2 * M_PI * (n * k) / FN;
            sum_re += in1[n].re * cos(a) - in1[n].im * sin(a);
            sum_im += in1[n].re * sin(a) + in1[n].im * cos(a);
        }
        printf("%3d: %6d,%6d %6.0f,%6.0f\n", 
               k, in[k].re, in[k].im, sum_re / FN, sum_im / FN); 
    }
}

void mdct_test(void)
{
    INT16 input[N];
    INT32 output[N/2];
    float input1[N];
    float output1[N/2];
    float s, a, err, e, emax;
    int i, k, n;

    for(i=0;i<N;i++) {
        input[i] = (random() % 65535 - 32767) * 9 / 10;
        input1[i] = input[i];
    }

    mdct512(output, input);
    
    /* do it by hand */
    for(k=0;k<N/2;k++) {
        s = 0;
        for(n=0;n<N;n++) {
            a = (2*M_PI*(2*n+1+N/2)*(2*k+1) / (4 * N));
            s += input1[n] * cos(a);
        }
        output1[k] = -2 * s / N;
    }
    
    err = 0;
    emax = 0;
    for(i=0;i<N/2;i++) {
        printf("%3d: %7d %7.0f\n", i, output[i], output1[i]);
        e = output[i] - output1[i];
        if (e > emax)
            emax = e;
        err += e * e;
    }
    printf("err2=%f emax=%f\n", err / (N/2), emax);
}

void test_ac3(void)
{
    AC3EncodeContext ctx;
    unsigned char frame[AC3_MAX_CODED_FRAME_SIZE];
    short samples[AC3_FRAME_SIZE];
    int ret, i;
    
    AC3_encode_init(&ctx, 44100, 64000, 1);

    fft_test();
    mdct_test();

    for(i=0;i<AC3_FRAME_SIZE;i++)
        samples[i] = (int)(sin(2*M_PI*i*1000.0/44100) * 10000);
    ret = AC3_encode_frame(&ctx, frame, samples);
    printf("ret=%d\n", ret);
}
#endif

AVEncoder ac3_encoder = {
    "ac3",
    CODEC_TYPE_AUDIO,
    CODEC_ID_AC3,
    sizeof(AC3EncodeContext),
    AC3_encode_init,
    AC3_encode_frame,
    NULL,
};