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Channel coupling is an optional AC-3 feature that increases quality by combining high frequency information from multiple channels into a single channel. The per-channel high frequency information is sent with less accuracy in both the frequency and time domains. This allows more bits to be used for lower frequencies while preserving enough information to reconstruct the high frequencies.
180 lines
6.1 KiB
C
180 lines
6.1 KiB
C
/*
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* Common code between the AC-3 encoder and decoder
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* Copyright (c) 2000, 2001, 2002 Fabrice Bellard
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*
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* This file is part of Libav.
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*
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* Libav is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; either
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* version 2.1 of the License, or (at your option) any later version.
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*
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* Libav is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with Libav; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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*/
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/**
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* @file
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* Common code between the AC-3 encoder and decoder.
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*/
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#ifndef AVCODEC_AC3_H
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#define AVCODEC_AC3_H
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#define AC3_MAX_CODED_FRAME_SIZE 3840 /* in bytes */
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#define AC3_MAX_CHANNELS 7 /**< maximum number of channels, including coupling channel */
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#define CPL_CH 0 /**< coupling channel index */
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#define AC3_MAX_COEFS 256
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#define AC3_BLOCK_SIZE 256
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#define AC3_MAX_BLOCKS 6
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#define AC3_FRAME_SIZE (AC3_MAX_BLOCKS * 256)
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#define AC3_WINDOW_SIZE (AC3_BLOCK_SIZE * 2)
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#define AC3_CRITICAL_BANDS 50
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#define AC3_MAX_CPL_BANDS 18
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#include "ac3tab.h"
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/* exponent encoding strategy */
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#define EXP_REUSE 0
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#define EXP_NEW 1
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#define EXP_D15 1
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#define EXP_D25 2
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#define EXP_D45 3
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/* pre-defined gain values */
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#define LEVEL_PLUS_3DB 1.4142135623730950
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#define LEVEL_PLUS_1POINT5DB 1.1892071150027209
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#define LEVEL_MINUS_1POINT5DB 0.8408964152537145
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#define LEVEL_MINUS_3DB 0.7071067811865476
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#define LEVEL_MINUS_4POINT5DB 0.5946035575013605
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#define LEVEL_MINUS_6DB 0.5000000000000000
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#define LEVEL_MINUS_9DB 0.3535533905932738
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#define LEVEL_ZERO 0.0000000000000000
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#define LEVEL_ONE 1.0000000000000000
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/** Delta bit allocation strategy */
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typedef enum {
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DBA_REUSE = 0,
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DBA_NEW,
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DBA_NONE,
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DBA_RESERVED
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} AC3DeltaStrategy;
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/** Channel mode (audio coding mode) */
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typedef enum {
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AC3_CHMODE_DUALMONO = 0,
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AC3_CHMODE_MONO,
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AC3_CHMODE_STEREO,
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AC3_CHMODE_3F,
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AC3_CHMODE_2F1R,
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AC3_CHMODE_3F1R,
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AC3_CHMODE_2F2R,
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AC3_CHMODE_3F2R
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} AC3ChannelMode;
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typedef struct AC3BitAllocParameters {
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int sr_code;
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int sr_shift;
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int slow_gain, slow_decay, fast_decay, db_per_bit, floor;
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int cpl_fast_leak, cpl_slow_leak;
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} AC3BitAllocParameters;
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/**
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* @struct AC3HeaderInfo
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* Coded AC-3 header values up to the lfeon element, plus derived values.
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*/
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typedef struct {
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/** @defgroup coded Coded elements
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* @{
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*/
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uint16_t sync_word;
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uint16_t crc1;
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uint8_t sr_code;
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uint8_t bitstream_id;
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uint8_t bitstream_mode;
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uint8_t channel_mode;
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uint8_t lfe_on;
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uint8_t frame_type;
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int substreamid; ///< substream identification
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int center_mix_level; ///< Center mix level index
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int surround_mix_level; ///< Surround mix level index
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uint16_t channel_map;
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int num_blocks; ///< number of audio blocks
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/** @} */
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/** @defgroup derived Derived values
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* @{
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*/
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uint8_t sr_shift;
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uint16_t sample_rate;
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uint32_t bit_rate;
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uint8_t channels;
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uint16_t frame_size;
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int64_t channel_layout;
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/** @} */
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} AC3HeaderInfo;
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typedef enum {
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EAC3_FRAME_TYPE_INDEPENDENT = 0,
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EAC3_FRAME_TYPE_DEPENDENT,
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EAC3_FRAME_TYPE_AC3_CONVERT,
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EAC3_FRAME_TYPE_RESERVED
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} EAC3FrameType;
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void ff_ac3_common_init(void);
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/**
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* Calculate the log power-spectral density of the input signal.
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* This gives a rough estimate of signal power in the frequency domain by using
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* the spectral envelope (exponents). The psd is also separately grouped
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* into critical bands for use in the calculating the masking curve.
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* 128 units in psd = -6 dB. The dbknee parameter in AC3BitAllocParameters
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* determines the reference level.
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*
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* @param[in] exp frequency coefficient exponents
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* @param[in] start starting bin location
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* @param[in] end ending bin location
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* @param[out] psd signal power for each frequency bin
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* @param[out] band_psd signal power for each critical band
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*/
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void ff_ac3_bit_alloc_calc_psd(int8_t *exp, int start, int end, int16_t *psd,
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int16_t *band_psd);
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/**
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* Calculate the masking curve.
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* First, the excitation is calculated using parameters in s and the signal
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* power in each critical band. The excitation is compared with a predefined
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* hearing threshold table to produce the masking curve. If delta bit
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* allocation information is provided, it is used for adjusting the masking
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* curve, usually to give a closer match to a better psychoacoustic model.
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*
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* @param[in] s adjustable bit allocation parameters
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* @param[in] band_psd signal power for each critical band
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* @param[in] start starting bin location
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* @param[in] end ending bin location
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* @param[in] fast_gain fast gain (estimated signal-to-mask ratio)
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* @param[in] is_lfe whether or not the channel being processed is the LFE
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* @param[in] dba_mode delta bit allocation mode (none, reuse, or new)
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* @param[in] dba_nsegs number of delta segments
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* @param[in] dba_offsets location offsets for each segment
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* @param[in] dba_lengths length of each segment
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* @param[in] dba_values delta bit allocation for each segment
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* @param[out] mask calculated masking curve
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* @return returns 0 for success, non-zero for error
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*/
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int ff_ac3_bit_alloc_calc_mask(AC3BitAllocParameters *s, int16_t *band_psd,
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int start, int end, int fast_gain, int is_lfe,
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int dba_mode, int dba_nsegs, uint8_t *dba_offsets,
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uint8_t *dba_lengths, uint8_t *dba_values,
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int16_t *mask);
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#endif /* AVCODEC_AC3_H */
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