[aprssig] Fill-in Digis

Stephen H. Smith wa8lmf2 at aol.com
Sat Mar 25 11:09:46 CST 2006

tim_cunningham at mindspring.com wrote:
> When selecting a packet controller to work as a packet decoder on our 
> local voice repeater, I found the add-on TRUE-DCD board worked rather 
> poorly with voice audio and it could not be utilized in muting the 
> audio on the output of the repeater. This add-on board falsely 
> triggered the mute control line to the point that it was very annoying 
> to listen to a conversation on the output of the voice repeater. We 
> then went to a Kantronics KPC-9612+ and it worked acceptably well. The 
> add-on board for the TNC 2 clones may be adequate to work with open 
> squelch radio outputs, but they are far inferior when using the DCD 
> signal to mute audio on a voice repeater to prevent the packet data 
> from being heard on the output of the voice repeater.
> The point is that the DCD decoding works much differently on the TNC2 
> add-on (hardware decoding) versus the KPC (firmware decoding) units.
> 73's,
> Tim - N8DEU
> Huntsville, Alabama

I've had the same experience.  The TNC2, no matter what kind of enhanced 
DCD is added,  is useless for Mic-E-on-voice-repeater  applications.  It 
simply false-detects on voice peaks and DTMF signaling constantly.  

Oddly one brute-force approach to DCD, borrowed from the past works 
fairly well.   I built a high-Q passive single tone filter using  a 
tuned circuit made of a classic 88mH toroid and styrene capacitors, in 
the style of early RTTY terminal units.  I resonated the toroid at 1200 
Hz, yielding a bandwidth narrow enough that only the low tone of a 
packet burst had significant energy within the passband. Voice energy 
and DTMF were ignored completely.    Total parts:   1 88mH toroid, two 
styrene or mylar caps, an LM324 op-amp, three resistors and a diode.   

Stephen H. Smith    wa8lmf (at) aol.com
EchoLink Node:      14400    [Think bottom of the 2M band]
Home Page:          http://wa8lmf.com

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