How to play The RAIN Report on the Air

Written by Mark Klockson/WA9IVH.

If your local club has asked you to run the RAIN report (or other computer-based audio files) on the air using your home station, there are some issues to consider before you can successfully accomplish this task. These include heat, audio levels, hum, PTT keying, and audio source switching. Finally, you’ll need some kind of interface box to accomplish these tasks. I decided to use off-the-shelf computer A/B switches and found them easy to modify and a good and inexpensive solution.

First, the heat issue.
Our amateur gear is rated for intermittent use. So if you intend to play a lengthy recorded program, you must use one of the lower power levels on your radio. I also use a fan running directly on the radio AND on my power supply. I’ve had no issue doing this for years, but I live close enough to our repeater to put in an excellent signal at low power. If you can’t put a full-quieting signal into your repeater on low power, perhaps you could install a better antenna. Also, you should use a mobile radio, not an HT. An HT would be more likely to overheat.

Second, audio levels.
Although some radios have a line-level “TNC” input, I’ve found using the mic. input on radios works best. So this means computer audio level needs to be reduced to mic. level. Many people try to reduce the audio level coming from their computers by using the audio “slider” controls on their computer. This usually won’t go far enough, and could introduce hum as the audio level is reduced. The issue is computers typically work on “line level” audio, and the mic input is, well, at mic level. These level differences aren’t trivial. Line level is referred to as -10 dB. Mic level is typically about -30 dB. In other words, approximately 20 dB (90%) difference – a huge amount! Asking the computer “sliders” to adjust for this is not practical. You need an audio pad. I use a 20 dB “L” pad consisting of a 4.7 k series resistor and a 470 ohm resistor across the output line.

Third, hum. Most sources recommend a 1:1 audio transformer to minimize ground loops and reduce hum. Frankly, I have skipped this step most of the time, but the experts do recommend you do this. And if you do skip it and later find you have hum on your recorded audio, installing an audio transformer would be one of the first troubleshooting step to take.

Fourth, the mic switching and PTT keying. The task here is to make the switch between mic and computer audio as quick and easy as possible. There are actually 3 tasks that need to be accomplished here: Mic switching, PTT keying, and level reducing for the computer audio. As mentioned, I’ve inexpensive A/B computer switchboxes over the years. These are perfect for the task: Small, metal for good shielding, and already switch all lines. My choices have evolved from 25-pin serial switchboxes to 9-pin serial to my new favorite, RJ-45 boxes. Not only are the RJ-45 units more popular, but connectors are readily available. (I finally broke down and bought a crimp tool. Previously I just bought some cheap computer patch cords and cut them in half to avoid having to crimp the connections!). So let’s take the three tasks one at a time:
Mic switching. With modern radios and RJ-45 switch boxes, this is a no brainer! Just plug your mic into the “A” input and run a patch cable from the “common” output to your radio. Done! (Just be sure to use a “straight-through” cable, not a crossover cable!)
PTT keying: With the box in the “A” (mic) position, keying is done with your normal mic PTT button. So nothing is needed for keying on the “A” position. But when you switch to “computer,” you also want to key your radio. So you need to identify which pin is the “PTT” pin on your radio’s RJ-45 connection. Also, identify the ground pin. These need to be shorted together in the “B” position. So do some signal tracing to determine which internal wires need to be shorted. Then drill a hole in the box and run the ground wire to that hole, using a screw and nut to secure it. Then identify the PTT lead to determine which wire shorts to ground in the “B” position. (I find a VOM is very helpful with this.) Connect that wire to the RJ-45 connector so it’s grounded when the switch is in the “B” position.
Level reduction: The computer audio out needs to be connected to the “L” pad mentioned earlier (and audio transformer, if used) and the audio output line feeding the radio in the “B” switch position.

Finally, as often happens when you do something over again a few times, you see a way to improve it. Moving to the RJ-45 switchbox was one big step toward simplifying the interface. After doing a few of my interfaces this way, I finally realized I could dramatically reduce the effort needed to build each new interface. I didn’t need to drill holes and cut wires in the switch boxes at all! All I needed to do was build the level reduction and PTT keying into the cable feeding “B” input of the switchbox! So that’s what I’m doing now with new interfaces.

That’s about it, except for on-air testing to determine how well the system’s working. Also, when on the air, I listen to the repeater output on my HT to be sure it’s all working and sounding good!

73,
Mark WA9IVH
wa9ivh@arrl.net