Offer an optional longer filter with better performance.

[astewart-xmos]

@Allan-xmos has identified that we can further improve our audio performance by using longer or more complicated filters for decimation. For example, using a 125-tap second stage filter instead of a 65, or using a 3 stage filter design.

The risk in making such a filter the default filter is that it may substantially increase MIPS or memory usage, breaking existing applications that have been tuned to make full use of MIPS and memory. Making the longer filter opt-in eliminates that risk.

Because there are several possible filter structures we could use for this, some exploratory work is also necessary to determine the trade-offs in terms of audio performance parameters, MIPS and memory.

NOTE: Nothing currently prevents lib_mic_array users from using their own longer filter within the current 2-stage decimator structure -- it just requires a bit more work than using the default coefficients. Other custom filter structures can also be implemented by users without any need to change the mic array library but require writing a bit of C++.

[Allan-xmos]

A comparison of different filter structures:

	MIPS	Memory (bytes)
Current	25.6	1280
Large 2S	28.672	2048
Large 3S	27.024	1484
Small 3S	25.808	1212
Tiny 2S	12.8	1152

Estimation of unoptimised MIPS = (rate*taps/decimation) for different filter topologies
Memory estimated as 4 bytes per tap

More details for each structure below:

Current

	Rate	Taps	Decimation	MIPS	Memory per mic
Stage 1	3072000	256	32	24.576	1024
Stage 2	96000	64	6	1.024	256
Stage 3	-	-	-	-	0
			TOTAL	25.6	1280

Notes: OK performance, early rolloff and aliases

Large 2 stage

	Rate	Taps	Decimation	MIPS	Memory per mic
Stage 1	3072000	256	32	24.576	1024
Stage 2	96000	256	6	4.096	1024
Stage 3	-	-	-	0	0
			TOTAL	28.672	2048

Notes: Good performance

Large 3 stage

	Rate	Taps	Decimation	MIPS	Memory per mic
Stage 1	3072000	256	32	24.576	1024
Stage 2	96000	19	2	0.912	76
Stage 3	48000	96	3	1.536	384
			TOTAL	27.024	1484

Notes: Good performance, 3rd stage reduces MIPS and memory

Small 3 Stage

	Rate	Taps	Decimation	MIPS	Memory per mic
Stage 1	3072000	256	32	24.576	1024
Stage 2	96000	15	2	0.72	60
Stage 3	48000	32	3	0.512	128
			TOTAL	25.808	1212

Notes: OK performance, early rolloff and aliases

Tiny 2 Stage

	Rate	Taps	Decimation	MIPS	Memory per mic
Stage 1	1536000	256	32	12.288	1024
Stage 2	48000	32	3	0.512	128
Stage 3	-	-	-	0	0
			TOTAL	12.8	1152

Notes: OK performance, early rolloff and aliases, 3dB higher noise floor due to lower PDM rate