Audio I/O questions

Robert Michel openmoko at
Wed Mar 28 00:52:20 CEST 2007


I'm very happy that the Neo1973 has 
- stero headphone out
- mono mic in (for headset)
this give us much power for having
much fun with the neo. :))))
 Analog wired Headset
 There's a four-ring 2.5mm stereo jack which provides connectivity to
 old-fashioned wired headsets.
 The headsets used by Motorola smartphones (A780,A1200, ...) and the
 V-360 have a compatible configuration. 

Nahhhh this audio I/O is not only good for a headset,
playing the music from a Neo1973 on the next party
and more would be fun - so an adapter would be usefull
- but how?

I went to the electronic component shop just
around my corner and bought a 2,5 mm 4 pin jack
(Klinkenstecker) and 3 chinch jacks.
testing the headset with a (very cheap) mulitmeter:
1-2 and 1-3 has 33-35 Ohm
1-4 1170 Ohm
shows that I guessed right that the jack is used with:

-+1 2 3 4

2 l audio out (tested this with alsamixer and my adapter)
3 r audio out
4 mono audio in/ mic with bias

(is this correct?)

So 33-35 Ohm is quite low - my headphone has 600 Ohm
and the audio in of my hifi amp 60kOhm. 

  Sorry, I'm no electrical expert but I'm very interested in how 
  we can use the power of Neo1973's audio I/O - so excuse me when this
  mail raise more questions then answers...

AFIK does the resistant number of the connected device matter and
so the Neo1973 output should have different profiles to switch from
- neo1973 headset to
- headphone and line out.
Would does need the support of the hardware or could we change the
driver settings to get a good line output?

I thought my adaper would be fine, *but*...

1 and 2  gaves a very low output of the right channel- even when headphone 
and pcm are on 100% 
1 and 3 gaves a pulsating noise (about every 0,5s) but with the neo1973
headset it sounds fine.
   Hmm, does somebody have an idea? 
Ahh.. I can hear it with the Neo1973 headset as well, wenn I pull out
the headset for 0,7mm - mybe my 2,5 mm 4 pin jack is not accurat enough.

Does anybody else start to play to use the neo1973 audio for more
then the official equipement?

My plan was to have a good adapter to linie in of my usb audio device
egosys U2A and do some recorings of the neo1973's audio I/O (and to 
share this recordings) and to solder an adaper to normal headphones.


PS: My V3 has also some noice on the left channel in depandances
    of the screen - alsamixer on the screen with much black gives
    other noise than the shell..
    ogg123 playback is bucking (ruckelt)

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