libavcodec/acelp_filters.c File Reference

#include <inttypes.h>
#include "avcodec.h"
#include "acelp_filters.h"

Go to the source code of this file.

Functions
void	ff_acelp_interpolate (int16_t *out, const int16_t *in, const int16_t *filter_coeffs, int precision, int frac_pos, int filter_length, int length)
	Generic FIR interpolation routine.
void	ff_acelp_high_pass_filter (int16_t *out, int hpf_f[2], const int16_t *in, int length)
	high-pass filtering and upscaling (4.2.5 of G.729).
Variables
const int16_t	ff_acelp_interp_filter [61]
	low-pass Finite Impulse Response filter coefficients.

---

Function Documentation

void ff_acelp_high_pass_filter	(	int16_t *	out,
		int	hpf_f[2],
		const int16_t *	in,
		int	length
	)

high-pass filtering and upscaling (4.2.5 of G.729).

Parameters:

	out	[out] output buffer for filtered speech data
	hpf_f	[in/out] past filtered data from previous (2 items long) frames (-0x20000000 <= (14.13) < 0x20000000)
	in	speech data to process
	length	input data size

out[i] = 0.93980581 * in[i] - 1.8795834 * in[i-1] + 0.93980581 * in[i-2] + 1.9330735 * out[i-1] - 0.93589199 * out[i-2]

The filter has a cut-off frequency of 1/80 of the sampling freq

Note:

Two items before the top of the out buffer must contain two items from the tail of the previous subframe.

Remarks:

It is safe to pass the same array in in and out parameters.

AMR uses mostly the same filter (cut-off frequency 60Hz, same formula, but constants differs in 5th sign after comma). Fortunately in fixed-point all coefficients are the same as in G.729. Thus this routine can be used for the fixed-point AMR decoder, too.

Definition at line 85 of file acelp_filters.c.

void ff_acelp_interpolate	(	int16_t *	out,
		const int16_t *	in,
		const int16_t *	filter_coeffs,
		int	precision,
		int	frac_pos,
		int	filter_length,
		int	length
	)

Generic FIR interpolation routine.

Parameters:

	out	[out] buffer for interpolated data
	in	input data
	filter_coeffs	interpolation filter coefficients (0.15)
	precision	sub sample factor, that is the precision of the position
	frac_pos	fractional part of position [0..precision-1]
	filter_length	filter length
	length	length of output

filter_coeffs contains coefficients of the right half of the symmetric interpolation filter. filter_coeffs[0] should the central (unpaired) coefficient. See ff_acelp_interp_filter for an example.

Definition at line 43 of file acelp_filters.c.

---

Variable Documentation

const int16_t ff_acelp_interp_filter[61]

Initial value:

{ 
  29443, 28346, 25207, 20449, 14701,  8693,
   3143, -1352, -4402, -5865, -5850, -4673,
  -2783,  -672,  1211,  2536,  3130,  2991,
   2259,  1170,     0, -1001, -1652, -1868,
  -1666, -1147,  -464,   218,   756,  1060,
   1099,   904,   550,   135,  -245,  -514,
   -634,  -602,  -451,  -231,     0,   191,
    308,   340,   296,   198,    78,   -36,
   -120,  -163,  -165,  -132,   -79,   -19,
     34,    73,    91,    89,    70,    38,
      0,
}

low-pass Finite Impulse Response filter coefficients.

Hamming windowed sinc filter with cutoff freq 3/40 of the sampling freq, the coefficients are scaled by 2^15. This array only contains the right half of the filter. This filter is likely identical to the one used in G.729, though this could not be determined from the original comments with certainity.

Definition at line 28 of file acelp_filters.c.