AEC Overview

The lib_aec library provides functions that can be put together to perform Automatic Echo Cancellation on input microphone data by using input reference data to model the echo characteristics of the room.

The echo canceller takes in one or more channels of microphone (mic) input and one or more channels of reference input data. The mic input is the input captured by the device microphones. Reference input is the audio that is played out of the device speakers. The echo canceller uses the reference input to model the room echo characteristics for each mic-loudspeaker pair and outputs an echo cancelled version of the mic input. AEC uses adaptive filters, one per mic-speaker pair to constantly remove echo from the the mic input. The filters continually adapt to the acoustic environment to accommodate changes in the room created by events such as doors opening or closing and people moving about.

Echo cancellation is performed on a frame by frame basis. Each frame is made of 15msec chunks of data, which is 240 samples at 16kHz input sampling frequency, per input channel. For example, for a 2 mic channel and 2 reference channel input configuration, an input frame is made of 2x240 samples of mic data and 2x240 samples of reference data. Input data is expected to be in fixed point 32bit 1.31 format. Further, in this example, there will be a total of 4 adaptive filters; \(\hat{H}_{y0x0}\), \(\hat{H}_{y0x1}\), \(\hat{H}_{y1x0}\) and \(\hat{H}_{y1x1}\), monitoring the echo seen in mic channel 0 from reference channel 0 and 1 and echo seen in mic channel 1 from reference channel 0 and 1.

Microphone data is referred to as \(y\) when in time domain and \(Y\) when in frequency domain. 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. Reference input is referred to as \(x\) in time domain and \(X\) when in frequency domain. Filter is referred to as \(\hat{h}\) in time domain and \(\hat{H}\) in frequency domain.

A filter has multiple phases. 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 echo cancellation.

There are 2 types of adaptive filters used in the AEC. These are referred to as main filter and shadow filter. The main filter as the name suggests is the main filter that is used to generate the echo cancelled output of the AEC. Shadow filter is a filter that used to quickly detect and respond to changes in the room transfer function. There is one main filter and one shadow filter per \(x\)-\(y\) pair. Typically the main filter has more phases than the shadow filter. Fewer phases in the shadow filter enable it to rapidly detect and respond to changes while more phases in main filter lead to deeper convergence and hence better echo cancellation at the AEC output.

Before starting AEC processing or every time there’s a configuration change, the user needs to call aec_init() to initialise the echo canceller for a desired configuration. Once the AEC is initialised, the library functions can be called in a logical order to perform echo cancellation on a frame by frame basis. Refer to the aec_1_thread and aec_2_threads examples to see how the functions are called to perform echo cancellation using one thread or 2 threads.