[sdiy] High pitched noise on codec problem

Eric Brombaugh ebrombaugh1 at cox.net
Thu Dec 5 19:12:27 CET 2013


Have you tried disabling the unused right channel input in the codec 
setup? I found that muting the unused channel helps sometimes.

Also - it's probably not a good idea to ground the unused input - it 
expects to be biased at ~AVDD/2, so grounding looks like a fullscale 
negative input which might do odd things to the input circuits.

FWIW, I've found that the WM873x series codecs have some unusual noise 
characteristics on the audio inputs and their performance can vary 
widely from chip to chip.

Eric

On 12/05/2013 10:55 AM, julian schmidt wrote:
> Hello list,
>
> I have a problem and need some help. ;)
>
> I've build a STM32 board with a WM8732 codec.
> at the moment I'm just using the left channel.
>
> With the gain up on my mixer I noticed a high pitched noise on the output.
> Now the strange part - as soon as I touch the unused right input
> channel, the noise slowly decays (maybe over 500ms) and is completely
> gone. The same happens if I just hold a piece of wire to the unused
> input pin.
> Wiring the input to GND does not help, neither a resistor or cap to gnd.
>
> the left input does not show this behaviour.
> I can leave the left input unconnected as well and the noise completely
> vanishes as soon as i touch the right input pin. touching the unused
> left pin has no effect.
>
> My input stage to the codec looks like this
> http://i.imgur.com/Fa9KTe0.png
> where R_IN is connected to the input pin of the codec.
>
> any hints in the right direction are welcome.
>
> cheers,
> Julian
>
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