iMM6 low frequency roll off with Pixel 3A

[Slartibartfast]

After getting a UMIK-1 it became obvious that the iMM6 I was using was rolling off seriously when plugged into the headphone socket of my Pixel 3A. You can see a comparison of the iMM6 to the UMIK-1 in the attached image. Green is UMIK-1 and Red is iMM6. Any thoughts on the reason? Is it something to do with using the VOICE_RECOGNITION mic stream instead of the UNPROCESSED stream?

[Julian Bunn]

That's a big difference! I suppose it's possible that the Pixel3 firmware is treating the mic samples from the headset jack differently to those from the UMIK USB microphone, and passing them through an HP filter, but as far as AudioTool is concerned, it always uses VOICE_RECOGNITION which is defined to be an unprocessed stream of non-filtered, non-AGC data.

Another possibility is a difference in setup between how these two measurements were made.

Maybe others have better theories?

Julian

[Niall Gray]

I tried my old Samsung S5 with the iMM6 and it was better but still rolled off sharply at 60Hz. I wonder if the iMM6 works correctly with any phones.

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[pa sk]

as I am aware, in the domain we are interested in, Android audio specs are pretty well defined for audio input in the frequency range from 100 Hz to 4 kHz, and this is only for VOICE RECOGNITION channel;

IMO, we have to reconcile to what we get with a particular phone and calibrate (= measure) the device separately with each mike we use

if you want to have some understanfing of what is going on in your case you may try:

 - get some other "jacked" mike, e.g. from a headset, and look how it behaves (mike from headset i have has exactly the same characteristic as iMM6, not counting 3 dB offset)

 - check how the raw data from the VOICE RECOGNITION channel look like and adjust the calibration coefficients accordingly

 - get the USB-jack adapter, connect iMM6 and check what you have

(btw, requirements for UNPROCESSED are much more loose and although they strictly require turning off any DSP stuff, they resemble MOPS for the input circuitry)

[pa sk]

last but not least - iMM6 does work properly (this is just a piece of thin alu-foil with as much as one transistor, you know :D)

unless it is damaged

two examples of silence:

POCO F1 (AT v. 8.6.1, original cal file):

Honor 7S (AT v. 8.6.2, hugely modified cal file):