How to remove noise from microphone input?

Question

The Issue

I've made a project centred around the XS3868 module which is based on OVC3860. The circuit works fine and I was able to get the microphone to work except for just one thing, the mic input is noisy as hell. The audio input is clear enough when I'm speaking something to the mic, but it gets noisy in an echoing manner whenever there is silence.
I want to get rid of this distracting noise. Is there any way to do so? If so, then please help me in making my project reach closer to perfection.

My Setup

Here is the schematic which I followed:

The schematic is simply based on the manufacturer's application schematic. I'm a beginner with digital signals and hence I simply combined multiple schematics into one. However, I'm ready to learn and improve this schematic.

The details about my microphone connections are as follows:

• My circuit uses the microphone of TRRS earphones.
• The mic input is fetched through the MIC and GND ring and sleeve.
• Since XS3868 has different GND pads for audio and power supply, therefore, instead of grounding one of the mic terminals according to the schematic, I connected it to AGND, or the Audio-Ground pad, of XS3868. AGND is at around 0.6V voltage level.

  This connection to AGND is indirectly made because I also use the earphones inserted to the TRRS jack for audio output and earphones intrinsically connect MIC- to GND. This has been done to prevent shorting AGND with GND.

What I've Tried

I tinkered around with the circuit and here's what I've observed:

• Connecting an external condenser microphone according to the schematic(which is based on the manufacturer's schematic), i.e. with MIC- grounded properly, I still heard the noise, which subjectively felt slightly lesser in magnitude.

Recording of the noise: https://clyp.it/gap2kc0z

Answer 1

Electret mics often have the rear open with an open weave cloth to keep out dust. It varies a lot between mics that look the same as balancing the front and rear air pressure to cancel is hard to match perfectly.

This helps to cancel far-field sounds like echoes off walls and distant noise that degrades the near-field voice signal to noise ratio.

Check your mic quality for this feature and test for far-field cancellation. Attenuating the input with the exact amount of cloth might help but is hard to balance. So often, mic selection is trial and error, unless they say this has excellent distant noise cancellation.

Also if you are using a speaker instead of a headset, positive feedback with full duplex audio will sound like echoes unless it has echo cancellation technology. ( side tone cancellation)

Answer 2

The electret microphone should be powered. Please connect a 4.7k resistor between the microphone # 1 terminal and the positive capacitor terminal (1.8V). It works for me.

Answer 3

Documentation is sparse for the interface chip, but as there is no gain control for the mic, most likely there is AGC. So when you have no signal, gain goes to max and the self noise of the preamp becomes audible. If you clearly hear the noise "pump" as signal is applied and removed, this is what is going on. Not much you can do, unless there are some commands you can find to suppress the AGC. But this will bring other issues, it's there for quite a good reason.

EDIT : of course there is something you might try, which is to have your own (variable gain) mic amp, and mix it with the line level input to the chip. Extra work and circuitry but it might be that AGC is only applied to the mic input - this would make sense.

Answer 4

A guess:

Add to the mic signal some continuous, but unaudible signal. Test if it fools the AGC to keep the gain low enough. A pure sine subbass, even few millivolt DC through a resistor to the right end of C4 can do the job. OVC3860 datasheet gives no hints does it work or not.