[Ham-Mac] Digital Interface for PSK with PTT

[John Townend]

I have been using cocoaModem for PSK with good results but found RX - 
TX switching a slight problem.        Unfortunately running OS10.2.8 
W7AY's cocoaPTT software is not supported and I have found USB to 
serial convertors somewhat hit and miss on my G4 iMac.      It seemed 
an audio triggered switch was the answer and browsing through copies of 
QST came across a solution in the April 2005 issue.

Larry Coyle, K1QW, has an excellent article entitled "An Improved 
Digital Communication Interface" in this issue.  With two 600 ohm 1:1 
line transformers, a quad op-amp, a meter and a handful of components a 
reliable and flexible interface can be made.  Like Larry I used perf 
board and - surprise - it worked first time.  Coupled from the iMic it 
provides reliable switching of the PTT line, gives DC isolation of the 
rig from the computer and the preset potentiometers allow the input and 
output levels to be set.  The audio level indicator section is a handy 
add-on and could be constructed on its own with a single op-amp and 
used as an audio level probe.

All in all it's a useful and versatile piece of gear to have around 
and, if you're handy with the soldering iron, an alternative to 
commercial units such as  SignaLink.    CU on PSK..................73 
John G3BBD / N3YZR