IC Overview

The Interference Canceller (IC) suppresses static noise from point sources such as cooker hoods, washing machines, or radios for which there is no reference audio signal available. When the Voice to Noise Ratio estimator (VNR) input indicates the absence of voice, the IC adapts to remove noise from point sources in the environment. When the VNR signal indicates the presence of voice, the IC suspends adaptation which allows the voice source to be passed but maintains suppression of the interfering noise sources which have been previously adapted to.

It can offer much greater, and automatic, cancellation of broad-band noise sources when compared to beam forming techniques.

It is designed to work at a sample rate of 16kHz and has a fixed configuration of two input microphones and a single output channel.

The interference canceller is based on an AEC architecture and attempts to cancel one microphone signal from the other in the absence of voice. In this way, it builds an estimate of the difference in transfer functions between the two microphones for any present noise sources. Since the transfer function includes spatial information about the noise sources, applying this filter to the mic input allows any signals originating from the noise source to be cancelled.

The IC uses an adaptive filter which continually adapts to the acoustic environment to accommodate changes in the room created by events such as doors opening or closing and people moving about. However, it will hold the current transfer function in the presence of voice meaning it does not adapt to desired audio sources, which can be a person speaking.

The cancellation is performed on a frame by frame basis. Each frame is made of 15msec chunks of data, which is 240 new samples at 16kHz input sampling frequency, per input channel. This is combined with previous audio data to form a 512 sample frame which allows for sufficient overlap for effective operation of the filter.

The first channel of input microphone data is referred to as y when in time domain and Y when in frequency domain. The second channel of input microphone data is referred to as x when in time domain and X when in frequency domain. The y signal is effectively used as the signal containing noise that needs to be cancelled and the x signal is the reference from which the transfer function is estimated and consequently the noise signal estimated before it is subtracted from y.

In general throughout the code, names starting with lower case represent time domain and those beginning with upper case represent frequency domain. For example error is the filter error and Error is the spectrum of the filter error. The filter coefficient array referred to as h_hat in time domain and H_hat in frequency domain.

The filter has multiple phases each of 15ms. The term phases refers to the tail length of the filter. A filter with more phases or a longer tail length will be able to model a more reverberant room response leading to better interference cancellation but, as with all normalised LMS based architectures, will be slower to converge in the case of a transfer function change.

Before starting the IC processing the user must call ic_init() to initialise the IC. If the configuration parameters are to be set to non-defaults please modify these after ic_init() or in the lib_ic API Definitions file. Once the IC is initialised, the library functions can be called in a order to perform interference cancellation on a frame by frame basis.