[aprssig] Mic-E and HTs

scott at opentrac.org scott at opentrac.org
Mon Dec 12 09:08:59 CST 2005

> Ahhh.... but perhaps Scott is talking bout the tiny track 
> _sensing_ the
> mic line.  First it has to sense what the status of PTT is so that it
> can know when you release PTT so it can inject the packet.
> Wouldn't this be an integrating op amp?  Just bypass any 
> frequency above
>  100hz, right?

Right, it's the sensing I'm worried about.  I did it with a transistor to
detect the mic PTT (basically duplicating the radio's PTT detect circuit),
and then AC coupling the audio to the radio side through a capacitor.  The
tracker's PTT out functions as usual.  It was enough to prove that it
worked, but a 1 uF capacitor lowered the audio level a bit too much.  An op
amp or maybe just a common emitter amplifier should fix that.

I was just curious if there was already an accepted circuit out there for
doing this.  No one ever talks much about mic encoding, and it's usually in
the context of mobiles.


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