Message-ID: <57FD3F9F.80409@lineone.net>
Date: Tue, 11 Oct 2016 20:38:07 +0100
From: LineOne <dhchurch@lineone.net>
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To: rsgb_lf_group@blacksheep.org
Subject: LF: Audio link
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Status: O

To avoid having  laptop running in the shack, which occasionally 
produces some QRM,  I can send received audio from the shack to a 
desktop computer in the house on a twisted telephone pair. This works 
but any wire connection between shack and this desktop introduces some 
spurious signals on 136khz even if I use an audio isolating transformer.

I bought a cheap digital baby alarm, disconnected the microphone and 
padded down the input to the sender's microphone pads including a 
potentiometer to adjust the input. Audio seems to reach the house OK but 
I think the automatic gain control in the sender has a strange effect 
with some sort of attack time allowing the start of, say, an Opera 
signal to be seen but not the whole of a pulse. The background noise 
does come up slightly between signals also. Nevertheless I can hear the 
audio normally and did send a decent Opera report for G3XDV this 
evening. WSPR doesn't seem to decode at all but I will persevere tonight 
before abandoning the project. Has anyone else tried these baby alarms 
nad managed to overcome the agc?

My next attempt is likely to be a 36kHz PWM laser link which will take 
some time to build.

73, Hugh, M0DSZ