Audio path gain, was: Re: Bluetooth module

Sun Jan 23 19:57:53 CET 2011

Al Johnson píše v Pá 21. 01. 2011 v 10:34 +0000:
> On Friday 21 January 2011, Hrabosh wrote:
> > Hi guys,
> > 
> > yesterday I opened my freerunner to check whether mine FR is affected by
> > R3004 issue [1].
> > 
> > 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:
http://hnet.endofinternet.net/elektro/Freerunner/ALSA/sound_setting.html
I was very surprised how LOW I had to set the audio controls to get rid
of distortion:
Mic2: 0
Sidetone: 1
Strange ...
>> > But ... when I was dismounting the phone, I dammaged the bluetooth
> > module. The module itself may be OK, but the flex cable is torn off.
> > 
> > Don't you have any idea where can I buy some spare?
>> I've not seen anyone list them as a spare for sale, but any of the 
> distributors might have them available.

Nicholas from Golden Delicious answered my desperate E-mail :-)
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