Tuesday, June 15, 2010

Undesired attention

I wish Murphy, the patron saint of things that don't work, would leave me alone. It's a bit frustrating reading on other blogs that someone spent a weekend and ended up with some perfect working piece of gear while I spent several hours just trying to get a computer to talk to a radio.

Although APRSIS32 now supports transmit and receive using AGW Packet Engine which can use a soundcard as a modem I have never decoded a single packet, nor have my beacons been reported as heard by anybody. I started to wonder whether my cheap USB dongle - one of those things about the size of your thumb with a USB plug on one side and two 3.5mm stereo jacks on the other, that cost next to nothing - was up to the job. It had worked acceptably well for Echolink, but although my braaaps sounded to me like packet, they might not sound enough like packet to someone else's decoder.

My computer's on-board soundcard is in use by my K3 so there was no point in trying that. But I did have another slightly better USB audio dongle being used for PC sound, so I decided to swap them over. I removed the cheaper dongle in order to better access the audio sockets and when I replaced it I received a message from Windows that "One of the USB devices attached to the computer has malfunctioned" and it disappeared from the system.
Giving up on computer sound for the moment I proceeded to try to test APRS packet transmission using the other dongle. But although I was receiving audio OK and keying the transmitter I was not transmitting any audio!

I fished the interface out from behind the rigs - a trying task ever since I installed the new shelving - in order to test it with my netbook and FT-817. It worked perfectly with Fldigi. Perhaps a bad connection? Back in position connecting the Icom to the shack computer and still no transmit audio, even though I could plug the computer speakers into the headphone output and hear the braaps being generated by the software!

Eventually it finally dawned on me that the AGWPE soundcard software only generates audio on the left channel. My radio interface was wired to the right channel! The cheapo USB audio dongle (that was now malfunctioning) presumably had a mono output so I was getting audio on both channels. The better one was generating proper stereo so there was no output on the channel I was using. D'oh!!

Fixing this ought to have been simple. Just open up the interface and switch the wires so the one from the left channel provides the input instead of the right one. But of course, it wasn't. Even though the cable from the stereo jacks used in this interface had no less than four insulated wires within the screening, only one was connected to anything - the tip (right channel) of the plug. The ring was unconnected. So I had to take another audio patch lead, cut it in half, and use that to make new audio cables for the interface using the left hand channel.

Finally, it appears to be working, though I have still to decode an APRS packet. But now I have no computer audio because the other USB dongle is still broken and the connecting cable was cut in half to get the interface working. I came this --><-- close to saying "the hell with all this" and ordering a Signalink USB interface this morning.

1 comment:

Theodore said...

Isn't interfacing computers and radios' fun. Having spent a year and too much money on components building digital interfaces, I know how you feel.
I finally took the plunge and bought a USB signalink, and could not be happier with its performance.
It does however use a relay to key the PTT, so I am not sure how that would be with packet, never having used that mode myself.
I would expect it would be ok, but if not, you could always use a cheap 4n25 optoisolator to key an rs232 line control signal, but that is more cables and complexity.
I mainly operate HF digimodes so don't have these problems, but like watching a wrestling match, call it shadenfreude, there is a certain peverse interest in seeing you overcome adversity.
73s Julian