Monday, September 20, 2010

Tinkering about

One of the reasons I'm not endlessly filling my log with DX contacts is because I actually get more enjoyment and interest out of trying new things or just tinkering about. On Sunday I decided to try using my K2 - which has been redundant since the few mobile excursions I made earlier in the year when Olga was away - for HF 30m APRS. I connected it up to the magnetic loop. I didn't have a spare sound card I could use for the packet modem, but the microphone input of the USB dongle I use for computer audio playback was free, so I set it up for receive only.

When I previously tried to use that USB audio device to decode HF packet via the AGW Packet Engine I had no luck at all. At the time, I concluded that this was due to 300baud packet being a higher speed mode than PSK31 and the USB device was somehow losing the information necessary to decode it. However it now appears that the reason it didn't work before is because the AGW Packet Engine opens the device at a higher sample rate than most digital mode programs. For the last few weeks I have been using TrueTTY as a packet decoder and this uses a 11025Hz sample rate. I used TrueTTY to decode the audio coming from the cheap dongle and it appears to be just as sensitive as it was when the audio was coming from my best sound card.

This morning I tried listening on 14076kHz, the JT65A frequency, and decoded several stations despite the high noise level on 20m. So it seems to me that it is possible to use these cheap USB dongles for sound card digimodes as long as the software you use samples at 11025Hz. Some programs unfortunately don't let you set the sample rate and may use a rate as high as 48000Hz. This is unlikely to work if my experience is anything to go by, so I still don't recommend that you buy one of these cheap devices for digimode use unless you have a specific application in mind that you know it will work with.

I have now ordered another of these cheap USB dongles which I plan to make into a "DIY SignaLink USB" with the aid of the digital VOX circuit developed by Skip Teller, KH6TY. This will allow me to use the K2 for HF APRS, WSPR or other QRP activities using the magnetic loop, and free up the K3 for other purposes using the multiband dipole. In fact I will then be able to use four amateur bands and four different radios simultaneously if I want to!
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