[LAU] status usb2 for sound

Martin Homuth-Rosemann linuxaudio at cryptomys.de
Sun Nov 8 10:53:51 EST 2009

Am Samstag, 7. November 2009 schrieb Dominic Sacré:
> On Friday 06 of November 2009 19:55:25 Martin Homuth-Rosemann wrote:
> > you're right - the pcm2902 codec is broken by design. The internal
> > voltage reference shows a terrible distortion because the 1 kHz usb
> > clock isn't decoupled properly. The data sheet
> > (http://focus.ti.com/lit/ds/symlink/pcm2902.pdf) recommends the use
> > of an external voltage reference for "a high-performance application"
> > (figure 37) - unfortunately Behringer uses the circuit for "a simple
> > application" (like figure 39).
> > I tested my UCA202 with Fons' jaaa and got a "nice" peak at 1 kHz -
> > it went away after adding a REG103-A to my UCA (you can get free
> > samples from TI).
> Hi Martin,
> How do you "add" a REG103-A? Do I need a deep knowledge of electronics,
> or would a soldering iron be enough? ;)
> I've got a UCA202 which causes a very audible, pulsating noise at
> multiples of 1 kHz while running Jack. If there's a way to fix it, I'd
> like to try. Of course the device was cheap, and as it is it's pretty
> much useless to me, so I'm not all too afraid of breaking it...
> Dominic
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Hi Dominic,

if you compare figure 37 and 39 you'll see the difference. I've soldered the two 
resistors 27k and 13k and the diode D1 directly onto the IC1 (these 4 devices 
are SMD parts - I had to use a magnifying glass an a very fine pitch soldering 
iron) and made connections to pin 3, 10, 11 and 28 of pcm2902. This "piece of 
crap" is fixed with hot glue  and I'm done. 
You can substitute the voltage regulator by any other low drop type - the 
enable input isn't imperative. The pcm2902 codec needs about 3.6..4.0 volts at 
pin 10 to override it's internal voltage source. 
Good luck.

Ciao Martin

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