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Monday 24 October 2016

Interfacing radio mic to computer

Having called at Maplin Electronics today to pick up an IC and a battery for a kit my Intermediate tutor had given me to build (the kit was missing the IC which I believe to be a PLL IC), I also bought another Velleman kit, a mono pre-amplifier, and a 4-pin CB mic connector of the type found on the radio.

I built both kits this afternoon, the kit given to me by my Intermediate tutor worked first time with the replacement IC, the second kit, the pre-amp, also worked first time but required 12V DC in order to work so I connected it to my bench supply used for my radios, first was power up and connect to computer, seemed fine, next was connect a CB hand mic, for this I used the stock microphone from the TTI TCB-550, the only 4-pin CB mic I had that is wired for Uniden CBs (though TTI is not Uniden), for testing purposes all the connections were made with terminal blocks connected over the board's connector pins with the wires attached.

The test setup consisted of the TCB-550's microphone, virtually unused since I bought the radio it came with, connected to a long grey cable, though shielded I suspect was picking up noise and the wires contained inside are very thin so probably not ideal for this, a red and black wire for power, and the line-in lead salvaged from the remains of what was the PMR446 FRN gateway that I no longer operate since getting my license, connected to the mic port on the front of the computer.

Initial tests were noisy, possibly owing to how the test rig was set up, and maybe also because the tip and ring in the computer end are shorted together though I cannot be sure if this is a cause yet, especially considering the onboard sound card of the computer is Realtek HD audio, which is capable of reassigning the ports, plug something in and it lets you pick what it is.

As the machine is equipped with an older C-Media soundcard I will use this on tests tomorrow if possible and I have time.

The only outstanding matter is getting the mic to key Echolink when the PTT is pressed, I cannot do this by connecting the TX/RX from the mic up to the computer, they would need to be able to send a signal to Echolink and from what I understand the only way for a PC user to key Echolink is via the keyboard, only radios when in sysop mode can do so with other hardware.

I have seen an American operator use a Teensy 2.0 dev board to do this and the circuit looks simple enough, trouble is that dev board is hard to find in the UK as I have only found the larger version of the same board, though it can emulate a keyboard and thus code can be written for it to press the spacebar when the PTT is pressed and obviously send the keypress again when the PTT is released, this could be an option unless I find another one in the mean time, but for now I have to remember to press the spacebar.

As I had an enclosure that I was going to initially use for building the electronics for a "whistling dipole" for DF, I made the decision to use it for this, the mic connector will be fitted to one end of the box, the connection out to the computer at the other end, I also thought that I should add a switch to take the pre-amp out of circuit for microphones that don't require it, most powered desk mics for instance have their own pre-amp and should theoretically work fine.

I have, for ease of using the TTI TCB-550's mic done the wiring side on this for Uniden CB mics, so the microphone from the TCB-550 should still work as intended with the radio, I have a pre-amplified desk mic that I originally bought to use with my Moonraker FA5000 as I felt the stock hand mic supplied with it was utter rubbish, this was originally fitted with a 4-pin plug but this was then replaced with a 6-pin plug, as I originally used the TCB-550 as a CB gateway for the Free Radio Network on a part-time basis, I should have a spare 4-pin plug somewhere to allow this mic to be used with the "adaptor", of course I'll have to put a multimeter onto the wiring of the desk mic to find out which pin on it does what..

I will post more on this as it develops.

73 de M6RSQ

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