Audio path gain
giacomo 'giotti' mariani
giacomomariani at yahoo.it
Fri Feb 4 12:12:47 CET 2011
>> On Friday 21 January 2011, Hrabosh wrote:
>>> Hi guys,
>>> yesterday I opened my freerunner to check whether mine FR is affected by
>>> R3004 issue .
>>> It was, I put a piece of wire there instead of those two caps and volume
>>> got better!
>>> I have also connected sine wave from generator to the WD8753 mic input
>>> and cheched it on the WD8753's output with the oscilloscope. It seemes
>>> that the audio codec chip gets into trouble (distortion, oscillations)
>>> if the overall audio path has higher gain than 1. I'm going to write
>>> more about this soon, with the scope pictures included (taken by my
>>> camera, I don't have DSO at home :-/ )
>> That would be interesting to see, but your summary seems wrong or perhaps
>> oversimplified. Distortion is inevitable if you overload an amp, and
>> oscillation isn't unusual immediately after overload. It isn't a function of
>> the overall path gain though. The wolfson has a chain of amps and volume
>> controls, and it only needs overload at one of them to introduce distortion.
>> Stick the gain too high on the first stage (mic input) and the distortion
>> won't go away no matter how far you turn down the gain on later stages. This
>> has been a long standing problem with the default volume settings.
> Yes, it is oversimplified. It is obvious that you can't fix distortion
> by lowering the gain of next-in-chain amplifier. You are just
> controlling the volume of already distorted signal.
> I put that picture from my scope with a short description to my web
> page, here:
> I was very surprised how LOW I had to set the audio controls to get rid
> of distortion:
> Mic2: 0
> Sidetone: 1
> Strange ...
Great work, thank you very much!
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