[gta02-core] Audio rework

[Rene Harder]

Rask Ingemann Lambertsen wrote:
> On Mon, Jun 22, 2009 at 04:27:19PM -0500, Rene Harder wrote:
>
>   
>> We increase the cap size to at least 100uF (better more). This results
>> in a dramatically increased physical size of the capacitor. I think the
>> maximum capacity for a common 1206 ceramic cap is 100uF at 6.3V.
>> (f_cutoff=50Hz @Z_headphone=32OHM and C=100uF)
>>     
>
>    200 uF if possible, perhaps as 2x100 uF to keep the height down. See
> https://lists.openmoko.org/pipermail/openmoko-kernel/2008-March/001996.html
> https://lists.openmoko.org/pipermail/community/2009-March/043203.html
> The WM8753L datasheet uses 220 uF, see page 47.
>
>   

Well, I partially agree with increasing capacity we will reduce the
cutoff even more 22Hz @32OHM and 45Hz @16OHM but we will increase the
size of the caps dramatically and they are already quite big.
Furthermore , what's the benefit of an even lower cutoff, most
headphones (e.g earbuds) will not have an outstanding sound for
frequencies <50Hz anyways.
If you want higher quality connect the output to an high impedance input
(e.g high impedance headphone, active speaker, amplifier, etc.) then you
do not have this problem at all. The high impedance will shift the
cutoff to lower frequencies (<2Hz @1kOhm input impedance).

I think with 100uF we have the right balance between audio quality and
cap size.

>> Also in the GTA02 schematics there are 33 OHM resistors in series of the
>> headphone stereo path, they will increase the lower cutoff frequency but
>> will limit the maximum volume of the headphone.
>> Although it will increase our lower cut off frequency, I think we should
>> remove those completely. Also wolfson doesn't recommend any resistor in
>> the headphone paths.
>>     
>
>    Removing the 33 Ohm series resistors would make it possible to attach
> unpowered external speakers, wouldn't it? So IMHO go for it.
>   

If your external speaker will be happy with 40mW you can do so.
But I think most of them need at least 200-300mW, so unfortunately this
will not work.