Sunday, 10 April 2016

WotIHeard

I've started a SoundCloud page where I upload recordings of radio activity, usually 145.500 at weekends and bank holidays.
These have the gaps taken out, and I try and get at least 11am - 3pm (peak time for SOTA activations etc.) when I'm not at the radio.

I'm using a Yaesu FT-7900 and Comet GP-15 triband antenna. At 145MHz the coax has about 1.5dB more loss than it really should do, because of some RG-58 extending it out of the loft. But on VHF, the receiver is limited by external noise, there is always some reading on the signal meter with an antenna plugged in (only the 7900 has a meter sensitive enough to show anything on external noise). No, even on 145MHz you can't get away from noise generated by electronic devices in a built up area.
Audio comes from the 'data' socket on the 7900, directly to the unbalanced input of a Edirol UA-25 USB audio capture. The UA-25 has a peak limiter enabled on the input, this only operates on bursts of noise or very high deviation signals.
Recording is threshold activated on the PC, at -29dB and 1 second release delay. The FT-7900's squelch only cuts the 'data' output by about 30dB when closed, so this is as low as the threshold can be without unwanted recordings.
After recording, I copy the left channel to both left and right, apply another limiter (in software) with a -6dB threshold to cut down the 'clicks' when the squelch opens/closes and level out differences between people using a mix of 2.5 and 5KHz deviation (I don't think the FT-7900 has narrow and wide filters based on the transmit deviation setting, like the more expensive Yaesu mobiles).
I should also do a low-cut filter too, it doesn't sound like the low frequencies are rolled off much in the radio, and as well as the DC clicks when the squelch opens/closes, I can hear who has their CTCSS left on when not on repeater channels.
I also use the Edirol UA-25 when I'm running the APRS gateway, with the PC's internal audio used for HF data modes. The red plug in the photo also comes from the 'data' 6-pin DIN socket, but that is from the 9600 baud packet output, giving unfiltered, unsquelched receive audio. This is good enough to use with digital voice decoding software like DSD+ to receive DMR transmissions. I don't think it works quite as well as a proper DMR radio but it's handy because if I find a DMR signal when using the FT-7900, I can quickly start up DSD+ and check it out, rather than having to change radio.

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