Noise Suppressor

The Noise Suppressor (NS) reduces stationary noise in an audio signal so that speech is clearer and easier to process. It estimates the noise present in the signal and attenuates it in the frequency domain, producing a cleaner output.

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Fig. 19 The Noise Suppressor topology.

Overview

The NS operates in the frequency domain, continuously estimating noise and adapting to changing conditions. It continuously estimates the noise spectrum by tracking the smoothed input spectrum and its local minima, adapting the smoothing rate based on the probability of speech presence. The estimated noise is subtracted from the input spectrum (spectral subtraction), and the resulting signal is transformed back to the time domain for output.

The NS component in lib_voice works on a single input channel. If multiple channels need to be processed by the application, or multiple outputs are required, an independent instance of the NS must be run for each channel.

Signal Representation

Noise suppression is performed on a frame-by-frame basis. Each frame consists of 15 ms of audio (240 samples at 16 kHz input sampling rate), with input data expected in fixed-point 32-bit 1.31 format. The output is the noise-suppressed signal in the same format.

Processing Flow

For each frame, the NS performs the following steps:

  1. Form the input frame from new samples and previous samples.

  2. Apply an analysis window and transform the signal into the frequency domain using a 512-point FFT.

  3. Compute the magnitude spectrum.

  4. Estimate the noise spectrum and apply spectral subtraction to attenuate noise.

  5. Rescale the frequency-domain signal and transform back to the time domain using an inverse FFT.

  6. Apply a synthesis window and form the output using overlap-add.

Usage

Before processing any frames, the application must configure and initialise the NS instance by calling ns_init(). Then for each frame, ns_process_frame() will update the NS instance’s internal state and produce the output frame by applying the NS algorithm to the input frame. Refer to the Pipeline example to see how the APIs above are used.

Parameters

The Noise Suppressor has no user configurable parameters.