Audio Interface

        This audio interface was designed to work with my FT-1000MP radios and N1MM Logger.  Pre-recorded Audio wav files are queued up by the program and sent to the audio card in the computer.  The audio card output is connected to the audio card play terminal on the circuit below.  My station mic is plugged into the Mic In jack, and the Mic Out jack is connected to the radio mic jack.  The transceiver PTT line is connected to the Radio PTT port, and an LPT Interface PTT keyline is connected to theLPT PTT terminal.  The switch has a PTT and VOX position.  It just interupts the PTT line from the interface box so that while the computer is booting up, or you are testing messages, the transceiver does not get keyed.  The lower portion of the circuit is an option that I added to some of my units to enable recording live audio from the radio.  All of the components are generic and non critical.  One issue that I had was with hum.  I found, that for my installation, I had to isolate all of the grounds.  My computer case is grounded, and provides the return for the signal grounds and LPT interface.  The RCA style connectors on the back of the box are isolated from the box.  The interface cables, therefore only see the ground for their own piece of equipment.  I have tried to indicate this on the schematic by showing two types of ground connections, chassis ground and external equipment grounds.  The +12 volt source is from the FT-1000MP, and the audio transformers and relay are from Radio Shack.

I modified the schematic slightly to remove a potential problem with my cw interface from Project 2. The drive levels from some computers were loaded down when this unit was in line in the cw mode. This change interupts the computer LPT output in the VOX position eliminating the problem.