[aprssig] Mic-E and HTs

Robert Bruninga bruninga at usna.edu
Tue Dec 13 08:37:37 CST 2005

>>> Ian ZL1VFO <zl1vfo at yahoo.co.uk> 12/13/05 12:48 AM >>>
<http://www.byonics.com/tinytrak/tinytrak3.zip> --> TinyTrak3_Schematic.png
>The Tinytrak3 senses the PTT in line from the microphone.
> When the line goes from low to high,[operator has
> released the PTT], so it ... turns on Q1, and pulls down 
>the PTT line again, so it can add it's data burst on the end 

As you described it,  this is scarry.
THe original Mic-E design has the output PTT
go low the instant the input goes low, so that
there is no transient induced on the PTT line
when the user releases his PTT.  Some radios
will react to that transient and the extremely 
short Mic-E burst might get munged in the
transceiver's turn-around.   Or you have to
set the TXD delay so great to avoid this tht
the Mic-E burst now is tripled in size and more
obnoxious to the other listeners on the channel.

But this is a firmware issue and maybe we are
both missinterpreting what the code really does.

Also, I notice in the TT3 schematic that the 2.2k
resistor that mixes the audio and PTT for
combined handhelds seems to be in the wrong
place.  It should be in series with the Q1
output so that the Q1 does not load down
the audio.  As shown, the audio is essentially
shorted by the PTT transistor when it keys.

Or at least that is how it looks here...
Bob, Wb4APR

of the over. (Mic-E PTT action, as
opposed to a Mic-E compressed packet.)
D6 is there to stop it retriggering itself.

> From: Wes Johnston <aprs at kd4rdb.com>
> Subject: Re: [aprssig] Mic-E and HTs
> To: TAPR APRS Mailing List <aprssig at lists.tapr.org>,    Scott Miller

> 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?

> Wes

> Stephen H. Smith wrote:
>> scott at opentrac.org wrote:

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