decode.cpp

mpeg2编码解码源程序代码

/* 
 *	Copyright (C) Aaron Holtzman - May 1999
 *
 *  This file is part of ac3dec, a free Dolby AC-3 stream decoder.
 *	
 *  ac3dec is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2, or (at your option)
 *  any later version.
 *   
 *  ac3dec 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 General Public License for more details.
 *   
 *  You should have received a copy of the GNU General Public License
 *  along with GNU Make; see the file COPYING.  If not, write to
 *  the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA. 
 *
 */
#define AC3
#define BIT
#include "../global.h"
#include "ac3.h"
#include "bitstream.h"

void bit_allocate(uint_16 fscod, bsi_t *bsi, audblk_t *audblk);

static audblk_t audblk;
static bsi_t bsi;
static syncinfo_t syncinfo;

// the floating point samples for one audblk
static stream_samples_t samples;

// the integer samples for the entire frame (with enough space for 2 ch out)
// if this size change, be sure to change the size when muting
static sint_16 s16_samples[2 * 6 * 256];

// storage for the syncframe
#define BUFFER_MAX_SIZE 4096
static uint_8 buffer[BUFFER_MAX_SIZE];
static sint_32 buffer_size;

uint_32 decode_buffer_syncframe(syncinfo_t *syncinfo, uint_8 **start, uint_8 *end)
{
	uint_8 *cur = *start;
	uint_16 syncword = syncinfo->syncword;
	uint_32 ret = 0;

	// find an ac3 sync frame
resync:
	while(syncword != 0x0b77)
	{
		if(cur >= end)
			goto done;
		syncword = (syncword << 8) + *cur++;
	}

	// need the next 3 bytes to decide how big the frame is
	while(buffer_size < 3)
	{
		if(cur >= end)
			goto done;

		buffer[buffer_size++] = *cur++;
	}
	
	parse_syncinfo(syncinfo, buffer);

	if (syncinfo->frame_size==0)		// CRITICAL CONDITION
		goto done;

	while(buffer_size < syncinfo->frame_size * 2 - 2)
	{
		if(cur >= end)
			goto done;

		buffer[buffer_size++] = *cur++;
	}

	// check the crc over the entire frame
	crc_init();
	crc_process_frame(buffer, syncinfo->frame_size * 2 - 2);

	if(!crc_validate())
	{
		syncword = 0xffff;
		buffer_size = 0;
		goto resync;
	}

	// if we got to this point, we found a valid ac3 frame to decode
	bitstream_init(buffer);
	// get rid of the syncinfo struct as we already parsed it
	bitstream_get(24);

	// reset the syncword for next time
	syncword = 0xffff;
	buffer_size = 0;
	ret = 1;

done:
	syncinfo->syncword = syncword;
	*start = cur;
	return ret;
}

void decode_mute(void)
{
	// mute the frame
	memset(s16_samples, 0, sizeof(sint_16) * 256 * 2 * 6);
	error_flag = 0;
}

void ac3_init(ac3_config_t *config)
{
	memcpy(&ac3_config, config, sizeof(ac3_config_t));
	imdct_init();

	sanity_check_init(&syncinfo, &bsi, &audblk);
}

sint_32 ac3_decode_data(uint_8 *data_start, int size)
{
	uint_8 *data_end = data_start + size;
	uint_32 i, j=0;

	while (decode_buffer_syncframe(&syncinfo, &data_start, data_end))
	{
		if (error_flag)
		{
			decode_mute();
			continue;
		}

		parse_bsi(&bsi);

		for (i=0; i<6; i++)
		{
			// Initialize freq/time sample storage
			memset(samples, 0, sizeof(float) * 256 * (bsi.nfchans + bsi.lfeon));

			// Extract most of the audblk info from the bitstream
			// minus the mantissas
			parse_audblk(&bsi, &audblk);

			// Take the differential exponent data and turn it into
			// absolute exponents 
			exponent_unpack(&bsi, &audblk); 
			if(error_flag)
				goto error;

			// Figure out how many bits per mantissa 
			bit_allocate(syncinfo.fscod, &bsi, &audblk);

			// Extract the mantissas from the stream and
			// generate floating point frequency coefficients
			coeff_unpack(&bsi, &audblk, samples);
			if(error_flag)
				goto error;

			if(bsi.acmod == 0x2)
				rematrix(&audblk, samples);

			// Convert the frequency samples into time samples 
			imdct(&bsi, &audblk, samples);

			// Downmix into the requested number of channels
			// and convert floating point to sint_16
			downmix(&bsi, samples, &s16_samples[i * 512]);

			sanity_check(&syncinfo, &bsi, &audblk);
			if(error_flag)
				goto error;
		}

		memcpy(&AC3Dec_Buffer[j*6144], s16_samples, 6144);
		j++;

error:
		;
	}

	return j*6144;
}

void InitialAC3()
{
	error_flag = 0;
	lfsr_state = 1;
	buffer_size = 0;

	ac3_config.num_output_ch = 2;
	ac3_config.flags = 0;
	ac3_init(&ac3_config);
}