Noise from DMICs on Wolfson Sound Card

The only problem I have had is the DC component that gets recorded. That can be stripped out in Audacity (Normalise) or by introducing an High-Pass-Filter in the mixer.

The problem may be that there are several volume controls involved. I am on my Pi2 ATM and am going by memory (not a good thing, in my case!). There is a 'Recording' AIF1TX Input Volume of 0-48 (set it to 32, which is 0dB), but there are two DMIC volume Controls, as well. There is an IN2 Digital Volume control of 0-191 (try 116) and another which I think is in the analog domain of 0-48 (try 32).

If the two IN2 levels are set too low and the deficit made up on AIF1TX Digital, then noise will be increased.

I find that not using an HPF (patching in an LHPF and setting it to high pass) means that the level measured in Audacity includes a substantial DC component. This conceals the true record level.

There are two different versions of the amixer driver (Wolfson and Cirrus) and the commands for each are different. I have written a mixer for both, but I can't think of a way of sharing them. However, typing alsamixer into terminal will provide the appropriate  character based interface for volume controls, patches and switches. They can be changed using the keyboard. All the volume controls are given the range 0-100 so you will have to experiment.

Hi Roy. Thanks for your reply!

So I followed your suggestion and set the volumes to the recommended values. But still, I got high white noise such that rec with sox cannot stop by detecting silence at the end. I tried to configure the parameters with alsamixer. But none of them seem to work, including noise gate threshold, noise generator, noise in.

If you set noise volume ('Noise Generator Volume') to zero it should cut it out completely. However you might also want to set 'Noise Input 1 [-4] Volume' to zero, as well. I think these inputs inject a comfort noise signal when the microphone is muted. In addition, check all the inputs to all the channels and filters you are using and make sure all inputs you are not using are set to 'None'. Steer clear of the dynamic range control (DRC) as it may boost the noise during quiet periods.

I have got good results with the Noise Gate set to the following defaults;

'Noise Gate Switch' : on

'Noise Gate Threshold Volume' : 7

'Noise Gate Hold' : 120ms

Try recording with the HPF in circuit using Audacity. If you get low noise results, it points to a configuration or a levels problem with sox. With Audacity in 'Record' and 'Pause' you can monitor the input level to ensure that you are using reasonable record levels.

If you set noise volume ('Noise Generator Volume') to zero it should cut it out completely. However you might also want to set 'Noise Input 1 [-4] Volume' to zero, as well. I think these inputs inject a comfort noise signal when the microphone is muted. In addition, check all the inputs to all the channels and filters you are using and make sure all inputs you are not using are set to 'None'. Steer clear of the dynamic range control (DRC) as it may boost the noise during quiet periods.

I have got good results with the Noise Gate set to the following defaults;

'Noise Gate Switch' : on

'Noise Gate Threshold Volume' : 7

'Noise Gate Hold' : 120ms

Try recording with the HPF in circuit using Audacity. If you get low noise results, it points to a configuration or a levels problem with sox. With Audacity in 'Record' and 'Pause' you can monitor the input level to ensure that you are using reasonable record levels.