Mic Input Gain

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omnyacoustics
Posts: 7
Joined: Wed Dec 13, 2017 3:45 am

Mic Input Gain

Post by omnyacoustics » Fri Jan 19, 2018 11:41 am

(Main Question) Is there any way to adjust the mic input gain other than the "Mic Boost" in alsamixer? You mentioned in another post that the "Capture" control should also adjust this, but it only seems to work on the line level inputs.

(Q2) As an add on question, do you know if any adjustments to the Mic Boost or Capture levels affects the dynamic range limitations/capabilities of the AISC? (I'm hoping to have the highest dynamic range as possible, and thus I want to avoid adjusting anything that would limit that. I'm happy to add an external preamp/attenuator if necessary to avoid this.)

(Q3) Is there any way for you to enable searching in this forum? Regardless of what I search, it always returns "The following words in your search query were ignored because they are too common words". This makes the usage of this forum as a reference very difficult to use.

To recreate the Main Question issue:

Mic Input Test
  • Attached my own electret mic to mic input
  • Turned on 94 dB Sound Level Calibrator over the mic
  • Start arecord via

    Code: Select all

    arecord -vv -f cd test1.wav
  • In a new shell window I open alsamixer and adjust Mic Boost. I notice a significant change in the arecord VU meter as expected
  • Adjusting "Capture" levels does nothing to the VU meter
Line Input Test
  • Attach my laptop soundcard output to the line level input of the AISC
  • Generate 1kHz tone in Audacity and play
  • Start arecord via

    Code: Select all

    arecord -vv -f cd test1.wav
  • In a new shell window I open alsamixer and adjust Mic Boost. No change in the arecord VU meter as expected
  • Adjusting "Capture" levels does increase/decrease levels on the arecord VU meter
Edit1:

I tested Q2 a bit and answered this part of my own question, using a few sound level calibrators (94dB@1kHz, 114dB@1kHz, and 124dB@250Hz). I thought the results might be helpful to others, so here they are. Levels are rounded to nearest dBV:
Test 1
  • Mic Boost ON, Mic Input attached to MicW i436 (-62 dBV)
  • Mic Boost OFF, Mic Input attached to MicW i436 (-75 dBV)
  • Mic Boost ON, Mic Input detached (open circuit) (-67 dBV)
  • Mic Boost OFF, Mic Input detached (open circuit) (-75 dBV)
  • Mic Boost ON, Mic Input attached to equivalent resistance as MicW i436 (1.4 kOhms in my case). No microphone attached (closed circuit) (-67 dBV)
  • Mic Boost OFF, Mic Input attached to equivalent resistance as MicW i436 (1.4 kOhms in my case). No microphone attached (closed circuit) (-75 dBV)
Conclusions:
  • Mic Boost ON, Max SPL = 135 dB, Min SPL = 60 dB, DynRng = 75 dB
  • Mic Boost OFF, Max SPL = 115 dB, Min SPL = 48 dB, DynRng = 67 dB
  • These Min and Max SPL levels could be adjusted one way or the other via gains, but the only gain for the mic I can find is that 20 dB Mic Boost. As measuring above 135 dB seems unlikely in most scenarios, all I would likely need for adjustment is a POT, but flatmax expressed that this likely will further reduce the already fairly tight dynamic range.
Test 2
To get a better idea of the true noise floor, I also tested the following with Mic Input and Line In circuits Detached (Open Circuit). Tested each scenario below for 10 seconds. Resulting noise floor levels in parenthesis to the right of each.
  • All inputs off (no capture), all capture and mic boosts at lowest (-93 dBV)
  • Maximize Capture levels (-93 dBV)
  • Maximize Mic Boost (-93 dBV)
  • Turned on Line capture (-77 dBV)
  • Minimized Mic Boost (-77 dBV)
  • Minimized Capture levels (-76 dBV)
  • Turned off Line capture and turned on Mic capture and switch Input Mux to Mic(-75 dBV)
  • Maximize Capture levels (-75 dBV)
  • Maximize Mic Boost (-67 dBV)
Conclusions:
  • I guess the noise floor of -93 dBV comes from the Raspberry Pi itself
  • Minimum noise floor (max SNR) for the AISC appears to be around the 75-77 dBV range. Not great, and certainly makes recording in 32-bit rather meaningless since 16-bit already provides 96 dB of SNR possibility.
Test 3
I then looked at the spectrum of the noise floor and there was a peak at 69 Hz. I suspected my power source, so I started another test starting from the 2nd to last step from the previous test list (Mic Input Open Circuit, Alsamixer Settings: Mic Capture ON, Mic Boost OFF, Capture levels Max, Input Mux = Mic.). The only external connections to the Pi 3 B a this time were the USB power connected to my laptop, and ethernet to my router. I attempted to isolate both via the following steps. I followed the below steps and noted levels again.
  • Initial Levels (-75 dBV)
  • Laptop power supply unplugged (running on battery) (-75 dBV)
I'll try to figure out the source of the 69 Hz tonal noise later if I have time. Ran out of time on this today. Hopefully there is a way to increase the dynamic range.

omnyacoustics
Posts: 7
Joined: Wed Dec 13, 2017 3:45 am

Re: Mic Input Gain

Post by omnyacoustics » Wed May 30, 2018 6:42 pm

I still have not found a solution for this. Is there no way to adjust the mic input gain other than the ON/OFF Mic Boost?

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