Audio Jack Sampling Rate?

Charles Pax charles.pax at
Mon Sep 22 02:36:12 CEST 2008

On Sun, Sep 21, 2008 at 7:46 PM, Al Johnson
<openmoko at>wrote:

> > 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.

This is good news.

> > > 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 :-)

It probably is, but would make for an inexpensive piece of hardware.
However, now that you've told me we can record from both the handset and
headset the only necessary external hardware should be some wire with a 2.4
mm plug, a transducer, and a photodiode. Now we're pretty much back to my
original idea, but with less necessary hardware. Thanks.

> > 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.

One time I decoded the signal from an IR personal response system [1] by
pointing the transmitter at a photodiode connected to the mic-in port of my
laptop, so I know that would work.

-Charles Pax

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