[RTTY] CocoaModem

[Bill Coleman]

On May 16, 2005, at 8:24 PM, John Merrill wrote:

> Anyone using this program for rtty? What kind of interface are using
> besides a commercial one?

Yes!

I built one. Mine is pretty simple. I have a 1k/8ohm audio  
transformer on the receive side. The 8 ohm side plugs into the  
headphone jack. The 1k side goes to the line input of my PowerBook. I  
also have a 300 ohm pot connected across the 8 ohm side to another  
jack so I can plug in a speaker or headphones. (you do want to hear  
the audio, just not very loud)

The transmit side is a little more complicated. I terminate the  
headphone output of the powerbook with two 25 ohm resistors, one for  
each channel. If you have a line output, you might not need the  
termination. I tap off the left (tip) channel and pass it through a  
500 ohm resistor and a 5k pot to a 600/600 ohm transformer. The  
output of the transformer goes directly to the rig microphone jack.

This is in a plastic box, so there's no ground loops -- the coupling  
through the transformers to the rig is all magnetic.



Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901