[gta02-core] Audio ECN update

[Rene Harder]

Werner Almesberger wrote:
> Rene Harder wrote:
>   
> ECN0038: 39pF+18pF ~= 56 pF ? 
You found it! ;-)

I picked 47pF because we already have this value in our BOM, and for
this purpose i think a 47pF cap is sufficient.

> Speaking of filters, building LC with
> discrete components adds a lot of stuff. I did't realize you were
> planning to use discrete bits, sorry. How about just using an
> integrated filter ? We may need filters for SIM as well, and also
> the UART to GPS looks like a filtering candidate to me. Not sure what
> C4128 does.
>
>   

Do you mean the EMI protection of the headset jack!  Sure using an
integrated filter works with me, I just thought we do not want too many
new components.
And actually, I'm not very happy with the current bead choice (DC
resistance is quite high) so we probably would have needed a new
component anyways.

The only problem I see is finding the right one, wolfson recommends
filter with a gentle impedance profile otherwise they could introduce
ringing and those are usually unsuitable for clock or digital signal
lines (SIM/UART). So we would probably need different filters for audio
and SIM/UART.
However, I think discrete filter are more flexible regarding vendor or
parameter changes. For example, if attenuation is to low, just replace
the caps or the bead with a different one, with an integrated filter
tough you have to replace the whole filter in hope that you find one
which fits your demand  and footprint.

>> Another ECN where we haven't decided yet what we are going to do is
>> ecn0019  "Consider applying buzz fix", so comments are welcome.
>>     
>
> I asked Joerg and he thinks it would be safer this way. He also
> suggested to protect the headset mic as well. Not sure if we have
> the space, though. This is already a very busy area.
>   

Hmm, indeed that's a crowded area and I'm not complete sure if that
would be necessary.
I'll move this one to done.