Thanks Eleon, that makes sense. The application is industrial and runs on 24V DC so no power issues. I am liking the digital microphones for their ease of use and dynamic range. Would be nice to have them packaged in a conventional 1/2" cartridge* so a 94dB acoustic calibrator could be supplied to verify the measurement chain back to the DCS (will be adding a 4-20mA output). I may have to get an engineered housing to accommodate the MEMS microphone. Only need acoustic frequencies so 20Hz to 10kHz is perfectly adequate and the ~30dB Noise Floor is not a problem as the equipment being monitored operates around 95dB(A)
On the topic of what protocol to use, I am edging towards I2S for no other reason than it seems quite well documented. Now I just need to find some good working examples of the following quasi-code:
T=125 or 1000 Milliseconds
Power-Up > Initialise >
Read Loop 1: Start reading WaveForm into DMA Buffer 'A'> Has T milliseconds expired ? > Yes > Raise Interrupt Event > Start Loop 2
Read Loop 2: Start reading WaveForm into DMA Buffer 'B'> Has T milliseconds expired ? > Yes > Raise Interrupt Event > Start Loop 1
Interrupt Event ? > Yes > Perform analysis of sound bite from Buffer A or B:
- FFT calculations t
- Band Pass Filtering
- A-Weighting
- RMS and Peak calculations on both Band Passed, A Weighted and Unfiltered Values
As you can see I need the signal to be continuous (no nasty phase mismatch on signal hence constant write to one of two buffers) and I need the analysis to be complete within the 125 millisecond minimum. Hence my choice of a high speed core. This is due to me wanting to demonstrate compliance to Sound Level Meter international standards and the testing involves injecting various crest factor signals and monitoring the accuracy of the peak detector.
The code in bold is my first problem and one I am now actively investigating (or will once my Digilent Analog 2 Logic Analyser arrives !) If you know of any good demo projects that capture this concept I'd be most grateful.
