[gta02-core] Audio rework

Rene Harder rehar at saweb.de
Mon Jun 22 23:27:19 CEST 2009


Right now, I'm taking care of the audio rework and there are some
questions I like to discuss.

I started already an ECN (ecn0010) with some notes and ideas what we
need to change.

The removal of the external audio amp should not be a big problem we
stay as close a possible to the reference design of wolfson.
That means earpiece connected to OUT3 and ROUT1, headphone connected to
ROUT1 and LOUT1, speaker connected to ROUT2 and LOUT2.

If we connect JACK_INSERT to the GPIO4 pin, the codec will switch
automatically between earpiece and headset. However we still can select
the output with the corresponding register settings of the codec.
What do you think, is it necessary to do that in hardware?

The current GTA02 has a 1uF cap in the left and right audio path of the
headphone, these caps act in combination with the headphone as highpass
filter, so they will decrease the lower frequencies.
If you use an headphone with an impedance of 32 OHM (+33OHM internal R)
you'll have a cutoff frequency at around 2kHz which is unacceptable for
good audio playback. It'll be better if you use an high impedance
headset but the minimal lower cutoff frequency is 160Hz without even an
headset connected.

I see 2 possible solutions:

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)

The wolfson audio codec support cap less headphones as well, here we do
not need any caps at all. However the problem is, on the audio jack we
share a common ground with the microphone and in cap less mode it is
necessary to have an offset voltage on the common pin of the headset
which will for sure interfere with our microphone. In this case we need
to change the headset and jack to get a separate ground for headphone
and microphone.

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.

I would just delete all parts U4401,R4411 R4403, H-TP4406 and H-TP4405
from the schematics to remove calypso serial interface on the headset
jack. We might need to keep the pull down resistors (R4411 R4403), I've
not checked that yet.

Did i forgot anything else, maybe there are some problems which I'm not
aware of?

So far so good,

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