[gta02-core] Audio rework

Rene Harder rehar at saweb.de
Tue Jun 23 04:28:20 CEST 2009

Al Johnson wrote:
> 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.

The inputs are differential but I'm more worried about the biasing of
the 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.
> 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.
I'll try my best to prevent any RF in the audio subsystem but you never

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

I think R4116 is the pull down resistors for JACK_INSERT if we increase
this resistor we'll increase the logic low Level (e.g. with R4116=10k
LOW=0.3V) I guess that might cause already confusion on the input
detection of the cpu (I need to check the data sheet)

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