Saturday, June 19, 2010

Unsquelched

I did a quick lash-up of the UHF FM transmitter module I received a few days ago which I was planning to use in a low-power Echolink node, mainly just to see if it worked. It did, but I discovered a small snag. The audio output isn't squelched.

The Echolink software requires either normal squelched FM audio or a carrier operated squelch signal on one of the serial port control lines. As neither of those is provided I guess I am going to need to generate a squelch signal derived from noise. But as I am not knowledgeable enough about electronics to design my own circuit I am going to have to find one first. This appears easier said than done, though there seem to be a surprising number of patents on the subject.

It isn't an urgent problem as I'm not in any hurry to build this project. I'll probably have more enthusiasm for it once the gloomy autumn weather starts.

3 comments:

Juha said...

Try software squelch. IMHO Echolink
should provide that, but if/as it doesn't,
jumper one output pin back into input
for Echolink.
A sum of abs difference between
a sample and the average of it's neighbours works surprisingly well
as an approximation of FM noise level.
A bandpass for half the samplerate, of
sorts. Decay the sum, threshold and delay
open/close.

James said...

You wouldn't happen to have a source on any other types of modules like these would you? I'm interested and I've been googling around but most of my results have turned up nothing but regular radios. Mobile, base or ht.

Julian said...

Sorry, no. There are lots of low power data modules available on eBay, but this is the only one I found capable of voice audio.