Overview

DSP Concepts proprietary Single-Channel Noise Reduction Gain Calculator. For version 4

Discussion

This module calculates gain coefficients for single-channel noise reduction. It is part of the SCNRv4 design.

The module works in conjunction with the WOLA filterbank modules.

The Yk input pin expects the squared-magnitude of the WOLA-domain noisy signal.

The Pn input pin expects a signal with noise levels per subband. This signal should be in dBs, and typically comes directly from the output of the NoiseTracking module.

The output pin provides the gain coefficients per subband, which can then be applied to the WOLA-domain samples of the input signal. These are real numbers and represent magnitude only.

The module is single channel, expects real data, and operates on any block size and any sampling rate.

The minGaindB parameter is the lower clipping threshold of gain for each frequency bin. The value for minGaindB is a negative number in dB, and it corresponds to the maximum attenuation allowed for each subband.

Before being output, the gain signal is smoothed out using cross-bin averaging in logarithmic groupings. The smoothOctave parameter defines how many octaves the averaging should span. Smoothing can be disabled by setting smoothOctave to 0.

The timeAlphaPrioriMs parameter controls the recursive averaging parameter in the SNR calculation. The default value is 250ms. Higher values mean more smoothing, resulting in a lower risk of musical noise, but a higher risk of distorting onset transients.

Type Definition

typedef struct _ModuleSCNRv4GainCalculator
{
    ModuleInstanceDescriptor instance;            // Common Audio Weaver module instance structure
    FLOAT32 prioriSnrScaler1Gain;                 // Scaler gain.
    FLOAT32 prioriSnrScaler2Gain;                 // Scaler gain.
    FLOAT32 timeAlphaPrioriMs;                    // Averaging time for a-priori SNR computation, in ms.
    FLOAT32 minGaindB;                            // Minimum gain (maximum reduction) per subband, in dB.
    FLOAT32 smoothOctave;                         // Number of octaves for cross-bin logarithmic smoothing.
    FLOAT32 postSnrClipAsymUpper;                 // Asymmetric-clip upper limit.
    FLOAT32 prioriSnrClipAsymUpper;               // Asymmetric-clip upper limit.
    FLOAT32 gainCalcLogScaleGain;                 // Scale Offset gain.
    FLOAT32 smoothStartFreq;                      // Start frequency for cross-bin smoothing.
    FLOAT32* aAPrioriFb;                          // A-priori SNR estimation feedback line.
    FLOAT32* aPrioriSnrDelay;                     // Block Delay single-block delay buffer.
    INT32* aSmoothStartIdx;                       // Starting index for the averaging process.
    INT32* aSmoothEndIdx;                         // Ending index for the averaging process.
    FLOAT32* aSmoothOneOverN;                     // Precomputed 1/N for averaging.
} ModuleSCNRv4GainCalculatorClass;

Variables

Properties

Name	Type	Usage	isHidden	Default value	Range	Units
prioriSnrScaler1Gain	float	derived	0	1	Unrestricted
prioriSnrScaler2Gain	float	derived	0	1	Unrestricted
timeAlphaPrioriMs	float	parameter	0	250	0:1000000	ms
minGaindB	float	parameter	0	-12	-30:0	dB
smoothOctave	float	parameter	0	0.1	0:1
postSnrClipAsymUpper	float	const	1	1000000	Unrestricted
prioriSnrClipAsymUpper	float	const	1	10000	Unrestricted
gainCalcLogScaleGain	float	const	1	0.033333	Unrestricted
smoothStartFreq	float	const	1	0	Unrestricted
aAPrioriFb	float*	state	1	[1 x 32]	Unrestricted
aPrioriSnrDelay	float*	state	1	[1 x 32]	Unrestricted
aSmoothStartIdx	int*	derived	1	[32 x 1]	Unrestricted
aSmoothEndIdx	int*	derived	1	[32 x 1]	Unrestricted
aSmoothOneOverN	float*	derived	1	[32 x 1]	Unrestricted

Pins

Input Pins

Name: Yk

Description: Input signal power

Data type: float

Channel range: 1

Block size range: Unrestricted

Sample rate range: Unrestricted

Complex support: Real

Name: Pn

Description: Input noise power

Data type: float

Channel range: 1

Block size range: Unrestricted

Sample rate range: Unrestricted

Complex support: Real

Output Pins

Name: out

Description: Output signal

Data type: float

Scratch Pins

Channel count: 1

Block size: 33

Sample rate: 48000

MATLAB Usage

File Name: scnrv4_gain_calculator_module.m

 M = scnrv4_gain_calculator_module(NAME)
 Single-channel noise-reduction gain calculator. This module
 takes a noise-level estimate and calculates gain coefficients to be used
 in noise reduction.
 Arguments:
 NAME - name of the module