Alternative Architecture mode (ALT_ARCH)

The Alternative Architecture mode, when enabled, improves Echo Cancellation performance in reverberate environments. It operates by re-configuring the audio pipeline by switching out either the AEC or the IC, depending on the energy in the AEC reference signal, to recover resources to be used to increase the specification of the remaining pipeline.

The two audio pipeline configurations are summarised below:

• ALT_ARCH disabled ALWAYS apply echo-cancelling AND interference cancelling; or

• ALT_ARCH enabled apply ONLY echo-cancelling when a reference signal is available, otherwise ONLY apply interference cancelling

The figure below expands the implementation details of the alternative mode switching. Multiplexers permit the AEC and/or the IC to be bypassed. When the IC is bypassed, only a single channel from the AEC is used, allowing it to be reconfigured, extending the filters to support a longer tail length. An internal module which collects statistics about the reference is used to dynamically control these multiplexers and memory allocation during runtime.

NOTE: Manually bypassing the IC using the Control Interface does not apply the memory reallocation.

The figure below highlights the audio signal path when the Alternative Architecture is disabled (ie. standard operation).

Fig. 15 Audio pipeline configuration, [ALT_ARCH=0] mode

Whenever ALT_ARCH=1, then the pipeline dynamically switches between AEC alone, or IC alone. In this condition the AEC is able to make use of additional memory increasing the echo cancelling period, and making it more resilient to echo in highly reverberant conditions.

Fig. 16 Audio pipeline configuration, [ALT_ARCH=1] when reference signal is present

The dynamic switching uses statistics collected from the reference signal to establish if echo cancelling is required.

Fig. 17 Audio pipeline configuration, [ALT_ARCH=1] when reference signal is absent

The following table summarises the audio characteristics for standard and alternative architectures.

Table 20 Alternative pipeline mode characteristics

Pipeline configuration	Far-end audio (AEC Ref) status	Pipeline functionality	AEC characteristics
ALT_ARCH = 0	With and without Far-end audio present	IC enabled AEC enabled	Max echo delay = 150ms
ALT_ARCH = 1	No far-end audio	IC enabled AEC disabled	No cancellation
ALT_ARCH = 1	Far-end audio present	IC disabled AEC enabled	Max echo delay = 225ms

The following table describes the configuration parameters for the Alternative Architecture.

Table 21 Alternative pipeline mode characteristics

Command	type	Value	Description
SET_ALT_ARCH_ENABLED	uint32	[0,1]	Enable or disable alternate architecture (alt arch)
		0 - Alt arch is disabled
		1 - Alt arch is enabled	When alt arch is enabled, the system works in either AEC mode (when far end signal is detected) or IC mode (when far end signal is not detected). When in AEC mode in Alt arch, AEC processing happens on only one Mic channel with 15 phases per mic-ref AEC filter.