AMR narrowband decoder. More...

#include <string.h>
#include <math.h>
#include "libavutil/channel_layout.h"
#include "libavutil/float_dsp.h"
#include "avcodec.h"
#include "libavutil/common.h"
#include "libavutil/avassert.h"
#include "celp_math.h"
#include "celp_filters.h"
#include "acelp_filters.h"
#include "acelp_vectors.h"
#include "acelp_pitch_delay.h"
#include "lsp.h"
#include "amr.h"
#include "internal.h"
#include "amrnbdata.h"

Classes
struct	AMRContext

Macros
#define	AMR_BLOCK_SIZE 160
	samples per frame

#define	AMR_SAMPLE_BOUND 32768.0
	threshold for synthesis overflow

#define	AMR_SAMPLE_SCALE (2.0 / 32768.0)
	Scale from constructed speech to [-1,1]. More...

#define	PRED_FAC_MODE_12k2 0.65
	Prediction factor for 12.2kbit/s mode.

#define	LSF_R_FAC (8000.0 / 32768.0)
	LSF residual tables to Hertz.

#define	MIN_LSF_SPACING (50.0488 / 8000.0)
	Ensures stability of LPC filter.

#define	PITCH_LAG_MIN_MODE_12k2 18
	Lower bound on decoded lag search in 12.2kbit/s mode.

#define	MIN_ENERGY -14.0
	Initial energy in dB. More...

#define	SHARP_MAX 0.79449462890625
	Maximum sharpening factor. More...

#define	AMR_TILT_RESPONSE 22
	Number of impulse response coefficients used for tilt factor.

#define	AMR_TILT_GAMMA_T 0.8
	Tilt factor = 1st reflection coefficient * gamma_t.

#define	AMR_AGC_ALPHA 0.9
	Adaptive gain control factor used in post-filter.

Typedefs
typedef struct AMRContext	AMRContext

Variables
AVCodec	ff_amrnb_decoder

Detailed Description

AMR narrowband decoder.

This decoder uses floats for simplicity and so is not bit-exact. One difference is that differences in phase can accumulate. The test sequences in 3GPP TS 26.074 can still be useful.

Comparing this file's output to the output of the ref decoder gives a PSNR of 30 to 80. Plotting the output samples shows a difference in phase in some areas.
Comparing both decoders against their input, this decoder gives a similar PSNR. If the test sequence homing frames are removed (this decoder does not detect them), the PSNR is at least as good as the reference on 140 out of 169 tests.

Macro Definition Documentation

§ AMR_SAMPLE_SCALE

#define AMR_SAMPLE_SCALE (2.0 / 32768.0)

Scale from constructed speech to [-1,1].

AMR is designed to produce 16-bit PCM samples (3GPP TS 26.090 4.2) but upscales by two (section 6.2.2).

Fundamentally, this scale is determined by energy_mean through the fixed vector contribution to the excitation vector.

§ MIN_ENERGY

#define MIN_ENERGY -14.0

Initial energy in dB.

Also used for bad frames (unimplemented).

§ SHARP_MAX

#define SHARP_MAX 0.79449462890625

Maximum sharpening factor.

The specification says 0.8, which should be 13107, but the reference C code uses 13017 instead. (Amusingly the same applies to SHARP_MAX in g729dec.c.)

Variable Documentation

§ ff_amrnb_decoder

AVCodec ff_amrnb_decoder

Initial value:

= {
    .name           = "amrnb",
    .long_name      = NULL_IF_CONFIG_SMALL("AMR-NB (Adaptive Multi-Rate NarrowBand)"),
    .type           = AVMEDIA_TYPE_AUDIO,
    .id             = AV_CODEC_ID_AMR_NB,
    .priv_data_size = sizeof(AMRContext),
    .init           = amrnb_decode_init,
    .decode         = amrnb_decode_frame,
    .capabilities   = AV_CODEC_CAP_DR1,
    .sample_fmts    = (const enum AVSampleFormat[]){ AV_SAMPLE_FMT_FLT,
                                                     AV_SAMPLE_FMT_NONE },
}

Classes

Macros

Typedefs