PSK31 RTTY and more...

There's some pretty basic stuff here. If you're an old hand, you'll probably think this written for idiots by an idiot. Maybe it is, but I reckon that it sometimes makes a pleasant change not to be bogged down with formulae and heavy mathematics. Hopefully, the info below will not only be interesting to the old hands, but to newcomers. When I started with PSK31 I’d never used data modes before and knew nothing about the subject. I wrongly thought that I had to spend a fortune on a TNC and make expensive changes to my computer. As it turned out, I built the interface for nothing, didn't have to touch the computer, and am now enjoying several data modes.


Now this sounds frightening, doesn't it? There's talk of a TNC, RF feedback, earth-loops, over-driving the transmitter... It's pretty basic stuff, really. Audio from the radio's speaker socket goes into the mic socket on the computer. Audio from the computer's speaker socket goes into the mic socket on the radio. There's nothing to it. Actually, there is a little more to it than that.


I'm amazed by the number of amateurs who leave their microphones connected when they're transmitting data. The slightest sound in the shack is transmitted along with the data signal. And some of the language I've heard coming from frustrated operators when they make typing errors... Why is it the fault of the f***ing keyboard? A simple relay will switch the mic socket on the radio from the microphone to the computer audio. Problem solved.



Perhaps the heading should read: NOT OVER-DRIVING THE TRANSMITTER. There's a very simple device called a potentiometer. Stick one of these in the audio out line from the computer and use it as a volume or gain control. Turn the gain down until you're not over-driving the transmitter, and away you go. In fact, I've also stuck a pot in the audio line from the radio to the computer. OK, so we can now adjust the audio gain in and out of the radio.


With wires running from the radio to the computer, there's obviously a direct electrical connection between the two pieces of equipment. Even using screened lead, as you should for audio, there's more than a good chance of RF feedback. Here's a simple way to electrically isolate the radio and the computer. Miniature audio transformers. Cool, huh?



This mysterious device couldn't be simpler. Basically, it consists of a LED and a photocell. The small voltage from the computer's RTS line (more on that later) lights the LED which, in turn, triggers the photocell. The output from the photocell can be used to energize a relay which not only controls the radio's PTT but switches the microphone in or out and... OK, so the optoisolator isn't so scary after all.



The circuit below shows the gain controls, audio transformers, and relay contacts. You will notice that the audio lines in and out of the computer are completely isolated from the radio by the transformers. Wire the screened leads from the computer directly to the transformer connections. Do NOT earth the braids to the box or anywhere else. This, obviously, would defeat the object!

The relay contacts should be wired so that, when the relay is energized (for transmitting data) the mic socket on the radio is switched to the gain control, RV1. When de-energized, the relay should switch back to the microphone. With this configuration, on receive mode, you can grab the mic and chat to the other station. During data transmission, the microphone is switched out of circuit. Use another pair of relay contacts to switch the PTT on when the relay is energized. The optical switch will control the relay, but we'll come to that later.

The chassis connections shown relate to the radio's earth or chassis. The gain controls I used were 10k pots I found in the junk box. Anything around 5 to 10K will do. Note that the connections from the radio's loudspeaker are wired across the track of the pot, to the two outside tags. This way, when turning the gain down, the radio's loudspeaker isn't shorted out.

INTERFACEMy interface box is shown on the left. The toggle switch cuts the 12v supply to the relay so that I don't inadvertently switch the transmitter on from the computer keyboard. The LED shows that the interface is on.

Rather than cut the radio's microphone lead, I fitted a mic socket on the front of the box and took a lead from the back of the box to plug into the mic socket on the radio. I was fortunate with my Kenwood transceiver because one of the mic socket pins is audio out. I didn't have to mess about with the loudspeaker connections.

OK, now for the terrifying part - wiring up the computer's RTS line. RTS stands for Request To Send. The serial port pin-out connections are shown below.

Don't let the above diagrams frighten you. All we're going to do is use one pin and the computer chassis. Depending on your computer, you'll either have a 9 pin port or a 25 pin port. On the 9 pin, the RTS line is pin 7. If you have a 25 pin, the RTS line is pin 4. Easy, isn't it?

Rather than buy a plug, I found an old computer lead in the junk box and cut it in half. I ran a length of twin lead from the RTS wire and the outer shield of the cable to my interface box. This was a bit of a bodge as I taped up the connection from the computer lead to my twin lead. Still, it works. We'll talk about winding the wires through ferrite rings later.

You can now do one of two things. Initially, I used the circuit above showing a transistor and a relay. Not having an optoisolator, I had no choice. This worked well with my set up but do bear in mind that the computer chassis is connected to the radio chassis. This defeats the object of complete isolation.

Simply take the RTS wire from your twin lead and connect it to the resistor feeding the base of the transistor. The other lead, from the cable's shield, connects to the 12v negative ground. The voltage from the RTS line during transmit switches the transistor on which energizes the relay.

There's one very important thing I've not yet mentioned. Where to get the software... OK, no problem. I use MixW. This is a great programme covering PSK31, RTTY and loads more. You can download it by clicking HERE



If there's one thing I hate about PCs, it's the millions of cables hanging out of the back of the things. Fed up with crawling on the floor to get to the back of the old PC I use for SSTV, RTTY etc, I decided to knock up a dedicated data modes box of bits. It's nowhere near finished but, as you can see from the the photographs, it's coming on nicely - and it works very well. A major advantage is the audio in/out sockets on the front panel. With a build-in speaker and gain control on the front, monitoring my own data signals is easy. No external speakers, PSU and wires everywhere. The speaker amp, ripped out of a cheap pair of PC speakers, runs from the 5 volt rail.

I'll be adding an RTS socket on the front panel soon and there'll be an internal relay and driver transistor for this. With a relay in the unit, there'll be no need for an optoisolator. Two relay contacts will close on TX which will complete the circuit and energize the relay in my interface box. This also eliminates any RF feedback problems... I found that the transistor in my interface box was turned on by RF getting into the base and the relay wouldn't switch back to RX.



The master hard drive is a mere 2gig, but it's only used for the operating system - Windows 98. The slave drive, 10gig, is where the programs are... MMSSTV - YP LOGGING - MixW. Although the thing only has a 166 processor, with 64meg of RAM, this is more than enough for SSTV etc. It's so easy to get to the drives and eveything else and play about, if I need to. This is real PLUG AND PLAY. The hole in the front panel will house a second fan - when I get round to it. You can see the two hard rives through this hole, which will benefit from the cooling fan.


I've added a fan at the rear of the unit to assist with air flow across the mother board. Cooling is very important, even in my "box of bits" unit. I'll also be mounting the keyboard and mouse sockets on the front panel. With everything so easy to get to and play about with, this simple unit knocked up from old PCs is fun and it works well.


By the way, I've stuck a network card in the thing to allow me to go onto the internet. I wasn't going to bother with this but realized that it would be very useful to download programms from the internet rather than use my main computer and then transer the programmes via CD. I didn't bother fitting the floppy drive as I doubt that I'll ever use it. I can always take the top off the unit and plug a floppy drive in if I really need it.



This has nothing to do with SSTV RTTY etc. I just thought I'd show off my 24 port 100mHz switch.