libavcodec/ac3.c File Reference

Common code between the AC-3 encoder and decoder. More...

#include "avcodec.h"
#include "ac3.h"
#include "bitstream.h"

Go to the source code of this file.

Functions
static int	calc_lowcomp1 (int a, int b0, int b1, int c)
static int	calc_lowcomp (int a, int b0, int b1, int bin)
void	ff_ac3_bit_alloc_calc_psd (int8_t *exp, int start, int end, int16_t *psd, int16_t *band_psd)
	Calculates the log power-spectral density of the input signal.
int	ff_ac3_bit_alloc_calc_mask (AC3BitAllocParameters *s, int16_t *band_psd, int start, int end, int fast_gain, int is_lfe, int dba_mode, int dba_nsegs, uint8_t *dba_offsets, uint8_t *dba_lengths, uint8_t *dba_values, int16_t *mask)
	Calculates the masking curve.
void	ff_ac3_bit_alloc_calc_bap (int16_t *mask, int16_t *psd, int start, int end, int snr_offset, int floor, const uint8_t *bap_tab, uint8_t *bap)
	Calculates bit allocation pointers.
void	ac3_parametric_bit_allocation (AC3BitAllocParameters *s, uint8_t *bap, int8_t *exp, int start, int end, int snr_offset, int fast_gain, int is_lfe, int dba_mode, int dba_nsegs, uint8_t *dba_offsets, uint8_t *dba_lengths, uint8_t *dba_values)
av_cold void	ac3_common_init (void)
	Initializes some tables.
Variables
static uint8_t	band_start_tab [51]
static uint8_t	bin_to_band_tab [253]

---

Detailed Description

Common code between the AC-3 encoder and decoder.

Definition in file ac3.c.

---

Function Documentation

av_cold void ac3_common_init

(

void

)

Initializes some tables.

note: This function must remain thread safe because it is called by the AVParser init code.

Definition at line 233 of file ac3.c.

Referenced by ac3_decode_init(), and AC3_encode_init().

void ac3_parametric_bit_allocation	(	AC3BitAllocParameters *	s,
		uint8_t *	bap,
		int8_t *	exp,
		int	start,
		int	end,
		int	snr_offset,
		int	fast_gain,
		int	is_lfe,
		int	dba_mode,
		int	dba_nsegs,
		uint8_t *	dba_offsets,
		uint8_t *	dba_lengths,
		uint8_t *	dba_values
	)

Definition at line 207 of file ac3.c.

static int calc_lowcomp	(	int	a,
		int	b0,
		int	b1,
		int	bin
	)			[inline, static]

Definition at line 44 of file ac3.c.

Referenced by ff_ac3_bit_alloc_calc_mask().

static int calc_lowcomp1	(	int	a,
		int	b0,
		int	b1,
		int	c
	)			[inline, static]

Definition at line 34 of file ac3.c.

Referenced by calc_lowcomp(), and ff_ac3_bit_alloc_calc_mask().

void ff_ac3_bit_alloc_calc_bap	(	int16_t *	mask,
		int16_t *	psd,
		int	start,
		int	end,
		int	snr_offset,
		int	floor,
		const uint8_t *	bap_tab,
		uint8_t *	bap
	)

Calculates bit allocation pointers.

The SNR is the difference between the masking curve and the signal. AC-3 uses this value for each frequency bin to allocate bits. The snroffset parameter is a global adjustment to the SNR for all bins.

Parameters:

[in]	mask	masking curve
[in]	psd	signal power for each frequency bin
[in]	start	starting bin location
[in]	end	ending bin location
[in]	snr_offset	SNR adjustment
[in]	floor	noise floor
[in]	bap_tab	look-up table for bit allocation pointers
[out]	bap	bit allocation pointers

Definition at line 180 of file ac3.c.

Referenced by ac3_parametric_bit_allocation(), bit_alloc(), and decode_audio_block().

int ff_ac3_bit_alloc_calc_mask	(	AC3BitAllocParameters *	s,
		int16_t *	band_psd,
		int	start,
		int	end,
		int	fast_gain,
		int	is_lfe,
		int	dba_mode,
		int	dba_nsegs,
		uint8_t *	dba_offsets,
		uint8_t *	dba_lengths,
		uint8_t *	dba_values,
		int16_t *	mask
	)

Calculates the masking curve.

First, the excitation is calculated using parameters in s and the signal power in each critical band. The excitation is compared with a predefined hearing threshold table to produce the masking curve. If delta bit allocation information is provided, it is used for adjusting the masking curve, usually to give a closer match to a better psychoacoustic model.

Parameters:

[in]	s	adjustable bit allocation parameters
[in]	band_psd	signal power for each critical band
[in]	start	starting bin location
[in]	end	ending bin location
[in]	fast_gain	fast gain (estimated signal-to-mask ratio)
[in]	is_lfe	whether or not the channel being processed is the LFE
[in]	dba_mode	delta bit allocation mode (none, reuse, or new)
[in]	dba_nsegs	number of delta segments
[in]	dba_offsets	location offsets for each segment
[in]	dba_lengths	length of each segment
[in]	dba_values	delta bit allocation for each segment
[out]	mask	calculated masking curve

Returns:

returns 0 for success, non-zero for error

Definition at line 83 of file ac3.c.

Referenced by ac3_parametric_bit_allocation(), bit_alloc_masking(), and decode_audio_block().

void ff_ac3_bit_alloc_calc_psd	(	int8_t *	exp,
		int	start,
		int	end,
		int16_t *	psd,
		int16_t *	band_psd
	)

Calculates the log power-spectral density of the input signal.

This gives a rough estimate of signal power in the frequency domain by using the spectral envelope (exponents). The psd is also separately grouped into critical bands for use in the calculating the masking curve. 128 units in psd = -6 dB. The dbknee parameter in AC3BitAllocParameters determines the reference level.

Parameters:

[in]	exp	frequency coefficient exponents
[in]	start	starting bin location
[in]	end	ending bin location
[out]	psd	signal power for each frequency bin
[out]	band_psd	signal power for each critical band

Definition at line 55 of file ac3.c.

Referenced by ac3_parametric_bit_allocation(), bit_alloc_masking(), and decode_audio_block().

---

Variable Documentation

uint8_t band_start_tab[51] [static]

Definition at line 31 of file ac3.c.

Referenced by ac3_common_init(), ff_ac3_bit_alloc_calc_bap(), and ff_ac3_bit_alloc_calc_psd().

uint8_t bin_to_band_tab[253] [static]

Definition at line 32 of file ac3.c.

Referenced by ac3_common_init(), ff_ac3_bit_alloc_calc_bap(), ff_ac3_bit_alloc_calc_mask(), and ff_ac3_bit_alloc_calc_psd().