Audio Jack Sampling Rate?

Al Johnson openmoko at
Mon Sep 22 01:46:13 CEST 2008

On Sunday 21 September 2008, Charles Pax wrote:
> On Sat, Sep 20, 2008 at 8:42 PM, Al Johnson
> <openmoko at>wrote:
> > You probably want to look at these two. The main limitation is that
> > you've only got one input channel via the jack, but you might be able to
> > get round that using the mic switch detection.
> I incorrectly assumed there is stereo input on the jack. I'll have to go
> back and rethink this whole thing. What is the mic detection switch?

There's a switch on the headset that shorts the mic when pressed. I've not 
looked into how this appears on the freerunner.

> Can I 
> record audio from the mic input and the built-in microphone at the same
> time?

Yes. Route headset (mic1) to the left ADC and handset (mic2) to the right ADC, 
then set the record mode to stereo.

> > Since you need external bits for the IR anyway you may be better off
> > making a
> > USB device instead. The arduino and other similar devices make the USB
> > part relatively easy. Put a narrow bandpass filter on your audio
> > transducer and you make the audio signal a simple on/off - use multiple
> > frequencies if you need to track different markers or indicate different
> > states.
> I'm hoping to find some novel way of doing this with the most simple
> hardware setup possible. Maybe we can have each transducer connected to a
> bandpass filter that activates an oscillator. If the two oscillators
> operate at different frequencies on the same mic-in channel, we should be
> able to process this in software on the Freerunner to calculate when each
> oscillator was triggered. This would allow us to use only dumb and cheap
> hardware that should work on any computer with a mic-in. The IR detector(s)
> can also be connected to a oscillator operating at a third frequency.

Sounds more complicated than making a usb device to me :-)

> What do you mean by "external bits for the IR"? Data bits or just little
> pieces of hardware? I figure a photodiode can be connected between signal
> and ground. When suficient IR light hit it the diode should make a spike on
> the mic-in channel.

Bits as in components. I've not tried a photodiode on a mic input, but it 
sounds like the sort of thing someone might have done for lirc hardware.

> -Charles Pax

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