External microphone design

Hi,

I’ve spotted a few posts/questions over time about using external microphones, so thought I’d share my experiences.

About a year ago I built a small module to add an external microphone which works well for me.

Some of the following is what I’ve worked out from trial and error/investigation and may well not be completely correct - I accept no responsibility or liability for the consequences of the use of this design or information. Use at your own risk :slight_smile:

MOVI integral mic circuitry seems to be based around a MAX9814 ic, which is an electret mic amplifier and auto gain control/compressor chip. I suspect that some effort has gone into tweaking this setup to work as well as it does. The issue with the MOVI ext_mic input is that it bypasses all that clever on-board conditioning circuitry and feeds almost straight into the mic_in pin on the A13 - so just plugging in an external mic, whether it’s a raw electret, moving-coil, or pre-amplified type, either simply won’t work or, at best, will likely give poor results compared with the on-board electret.

About a year ago I had a project that needed an external mic about a metre away from the main enclosure containing the MOVI board, so I came up with solution described in the attached jpeg (ext_mic_scheme.jpg).

The design is based around an Adafruit 1713 module with on board electret mic, and which uses the same MAX9814 compressor ic as MOVI. I added a remote voltage regulator, and some output conditioning to make it compatible with the movi ext_mic input. I recall it took a lot of tweaking, experimenting, fine-tuning and probing around on the movi board to get it to work well, but it gives me virtually identical results to the on-board mic. At the time I did not know about the ‘LowLevelInterface’ code with it’s MIC DEBUG feature, but I tried it a few weeks ago and, to me, the results sound indistinguishable from the on-board mic.

The design sets the gain of the Adafruit 1713 module to 50db by connecting its gain set pin to 0v. The audio output from the module is fed back to the MOVI board enclosure via mono screened cable, and is then passed to a voltage divider and some capacitive filtering and then fed into the ext_mic input on MOVI. The 2 x 5k1 and 470R resistor values gave the best results after a lot of experimentation. C1 provides a degree of RF pick-up immunity, C2 rolls off the response above the audio band, and C3 decouples the dc component. I suggest using polypropylene capacitors all round, especially as C3 is biased at about 2.5v on both sides so using an electrolytic here as normal might have issues.

You could use the 5v supply from MOVI/Arduino rather than the 12v supply plus 5v regulator, but I was uncomfortable running the internal 5v logic supply outside of the main enclosure. See comment about battery supply below.

Finally, one thing I remember is that this was all quite sensitive to earth loops and hum pick-up, especially if you are using a switched-mode power supply for the 12v. I strongly recommend using the LowLevelInterface code with its MIC_DEBUG feature (which loops the mic input back around to the headphone output) and listen to what MOVI is actually hearing from the mic, then play around with 0v connections, power supplies, cable screening etc to minimise hum/pickup. I recall that taking the 0v and 12v from the edge connector on the MOVI board rather than directly from the power supply gave noticeably better results.
If you can get away with using a 9v battery (and 5v regulator) at the microphone end this is perfect - the current consumption is about 7mA at 9v so should give a reasonable life if switched off when not in use.
Also, I didn’t try it, but I suspect that a long cable run will potentially exceed the maximum capacitive loading of the Adafruit module output, cause excessive hum pick-up, and possibly cause instability. It’s a guess, but I wouldn’t expect this to work well with more than about 3m of cable.

I’ve attached a picture of the final hardware (ext_mic_hw.jpg)

. In the end I only needed to move the mic to the side wall of the main enclosure, so ended up building everything on one board. Ignore the LED and piece of wood!

Mark.