pcm.c

从FFMPEG转换而来的H264解码程序,VC下编译..

/*
 * PCM codecs
 * Copyright (c) 2001 Fabrice Bellard.
 *
 * This file is part of FFmpeg.
 *
 * FFmpeg is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * FFmpeg is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with FFmpeg; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

/**
 * @file pcm.c
 * PCM codecs
 */

#include "avcodec.h"

/* from g711.c by SUN microsystems (unrestricted use) */

#define         SIGN_BIT        (0x80)      /* Sign bit for a A-law byte. */
#define         QUANT_MASK      (0xf)       /* Quantization field mask. */
#define         NSEGS           (8)         /* Number of A-law segments. */
#define         SEG_SHIFT       (4)         /* Left shift for segment number. */
#define         SEG_MASK        (0x70)      /* Segment field mask. */

#define         BIAS            (0x84)      /* Bias for linear code. */

/*
 * alaw2linear() - Convert an A-law value to 16-bit linear PCM
 *
 */
static int alaw2linear(unsigned char a_val)
{
        int t;
        int seg;

        a_val ^= 0x55;

        t = a_val & QUANT_MASK;
        seg = ((unsigned)a_val & SEG_MASK) >> SEG_SHIFT;
        if(seg) t= (t + t + 1 + 32) << (seg + 2);
        else    t= (t + t + 1     ) << 3;

        return ((a_val & SIGN_BIT) ? t : -t);
}

static int ulaw2linear(unsigned char u_val)
{
        int t;

        /* Complement to obtain normal u-law value. */
        u_val = ~u_val;

        /*
         * Extract and bias the quantization bits. Then
         * shift up by the segment number and subtract out the bias.
         */
        t = ((u_val & QUANT_MASK) << 3) + BIAS;
        t <<= ((unsigned)u_val & SEG_MASK) >> SEG_SHIFT;

        return ((u_val & SIGN_BIT) ? (BIAS - t) : (t - BIAS));
}

typedef struct PCMDecode {
    short table[256];
} PCMDecode;

static int pcm_decode_init(AVCodecContext * avctx)
{
    PCMDecode *s = avctx->priv_data;
    int i;

    switch(avctx->codec->id) {
    case CODEC_ID_PCM_ALAW:
        for(i=0;i<256;i++)
            s->table[i] = alaw2linear(i);
        break;
    case CODEC_ID_PCM_MULAW:
        for(i=0;i<256;i++)
            s->table[i] = ulaw2linear(i);
        break;
    default:
        break;
    }
    return 0;
}

static int pcm_decode_frame(AVCodecContext *avctx,
                            void *data, int *data_size,
                            uint8_t *buf, int buf_size)
{
    PCMDecode *s = avctx->priv_data;
    int n;
    short *samples;
    uint8_t *src;

    samples = data;
    src = buf;

    n= av_get_bits_per_sample(avctx->codec_id)/8;
    if(n && buf_size % n){
        av_log(avctx, AV_LOG_ERROR, "invalid PCM packet\n");
        return -1;
    }

    if(buf_size > AVCODEC_MAX_AUDIO_FRAME_SIZE/2)
        buf_size = AVCODEC_MAX_AUDIO_FRAME_SIZE/2;

    switch(avctx->codec->id) {
    case CODEC_ID_PCM_ALAW:
    case CODEC_ID_PCM_MULAW:
        n = buf_size;
        for(;n>0;n--) {
            *samples++ = s->table[*src++];
        }
        break;
    default:
        return -1;
    }
    *data_size = (uint8_t *)samples - (uint8_t *)data;
    return src - buf;
}

#define PCM_CODEC(id, name)                     \
AVCodec name ## _decoder = {                    \
    #name,                                      \
    CODEC_TYPE_AUDIO,                           \
    id,                                         \
    sizeof(PCMDecode),                          \
    pcm_decode_init,                            \
    NULL,                                       \
    NULL,                                       \
    pcm_decode_frame,                           \
}

PCM_CODEC(CODEC_ID_PCM_ALAW, pcm_alaw);
PCM_CODEC(CODEC_ID_PCM_MULAW, pcm_mulaw);

#undef PCM_CODEC