[gta02-core] Audio rework

[Al Johnson]

On Monday 22 June 2009, Rene Harder wrote:
> Hi,
>
> 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)

Should be adequate.

> 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.

I've not checked the datasheet, but I thought the mic inputs were 
differential. If so we can probably connect MIC1N to the shared 'ground' at 
the jack instead of to actual ground via C3016 as it is now. I would be 
surprised if Wolfson hadn't considered this eventuality. Matching impedance 
would help the CMRR.

> 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.

Agreed.

> 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?

Making sure we don't send RF into the can via the headset mic line, or any 
other line for that matter. This may just be a layout issue, but I wouldn't 
want it to be forgotten.

Is there a reason why R4116 and R4117 are as low as 1k? Probably not an issue 
with the bigger caps in place though.