Friday, 10 November 2017

Isolated sound card interface

I've been working towards getting my girlfriends HF rig remotely controlled. We have it installed in the sun room at the back of the house but her office is up in the attic on the 3rd floor.  This presents a problem as not only do we need to control the radio but we also need to hear what it receives and also send it some audio to transmit.

The radio control is easily achieved by using Ham Radio Deluxe and is very well documented elsewhere. For the audio portion I am using RemAud by DF3CB which is also very well documented on his website. What's not so well documented is the hardware that one might need be able to both receive and transmit audio from another location. It should be noted that whilst I am doing this from another room 3 floors away it could equally be done from 3000 miles away over the Internet.

If you are familiar with Skype then you'll know that it uses the sound system installed into your PC. The microphone is usually sourced from your webcam and the speakers let you hear the other person. RemAud is a similar application. It uses the same audio sources. And that's where the difficulty begins. Your HF rig cannot use the mic on your webcam.

Luckily this problem has been solved many times before. A simple radio-computer sound interface is all that's needed on the radio end. It needs to move audio from the PC to the radio and back. It also needs to key the PTT to enable the radio's transmitter. This is what I came up with.

It's an isolated audio interface with USB based PTT keying. I made it from the junk box. The Audio part is a copy of the many transformer based sound card interfaces one can find on the 'net. The transformers isolate the electrical path between the radio and the computer in an effort to reduce computer noise getting into the radio.

The USB part is a USB to serial interface that I had meant to use in an Arduino project but never got around to it. I added a small bit of circuitry to invert the signal from controller chip and also isolate the PTT from the computer.

I also added a USB sound card adapter to the PC to manage the sound instead of using its built in sound facilities. I'll be moving it all to the rear sockets of the PC and there are no sound outputs there.

It works very well. However, I am not happy with it. The PTT should really be isolated via a opto-isolator and it's larger than I'd like. So I set about designing another interface. This time it's more compact and has "proper" sockets etc. It's also fully isolated and can also key some CW too.

I designed a board for it too. You can get it at Oshpark

And here's the finished result. It works perfectly. It's also much smaller and now fully isolated. I used a PS/2 male-male extension cable that I got from Microcenter to connect the data port on the rig to the interface. At $8 it was not worth my time or effort to try soldering a cable myself.

There was a caveat that took me a few days to work out. Direct from the factory the FT232 USB-serial chip has its signals inverted. This is correct as it actually converts from TTL to USB, expecting another chip to do level shifting up to the +/-12v that is RS232. But the chip makers have thought of that. They have a tool on their drivers website that allows you to change the signalling on any of the pins. I inverted DTR and RTS so that they would go high when applied thus powering the LED in the opto-isolator. 

The Bill Of Materials is quite trivial;

  • FTDI FT232RL USB to Serial chip x1
  • 10K x1
  • 1K x3
  • 3MM LED x1
  • 0.1uf x2
  • 10uF x1
  • Mini-USB 'B' x1
  • 4N25 x2
  • 3.5mm audio socket x3
  • 42TM016 600:600 audio transformer x2
  • PS/2 6 pin mini DIN socket x1

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