Bluetooth headsets to the rescue

[Paul Fertser]

"arne anka" <openmoko at ginguppin.de> writes:
>> And don't forget that the proposed rework doesn't fix wired headset
>> mic, it will still produce a lot of buzz.
>
> how's that?
> i lived under the impression that the buzz is basically one buzz and the  
> hw fix is intended to end all buzz.
>
> any sources proving your statement available?

Schematics. Or ask Joerg.

Basically the idea is that the buzz gets to the MICBIAS line via the
4th pin of the headphone receptable (that is mic input). The longer
antenna you attach to it, the more RF you get.

Then it's detected somewhere inside the Wolfson and comes back via the
same MICBIAS line this time as a very strong audio frequency signal.

With internal mic you can use a large cap to filter it on one side of
the mic and get the signal from the other side because it's connected
in a differential way. With the headset mic it's not possible because
it has only one line.

The rework for that would take lifting the can, desoldering one small
R, soldering two of them in parallel to the same place (already tricky
enough!) and adding a big cap to gnd (and free space inside the can is
very limited). So, not practically possible.

Read the schematics.

OTOH you can use headphones and _internal_ mic for GSM after
buzz-fixing it.

NB: Factory-produced A8 will have ferrite beads in place (so RF will
not get anywhere it can be detected in the first place), and that
should probably solve all known buzz problems.

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