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[aprssig] squelched audio

Ray Wells vk2tv at exemail.com.au
Fri Sep 21 23:57:21 UTC 2007

Ben Lindner wrote:

> Can some kind person explain what squelched audio is.
> I have a Ic281H and when the data jack and the speaker jack are in use 
> for APRS no noise can be heard, this is standard I beleive.
> Is this what is meant by squelched audio or not
> Ben Lindner
> _______________________________________________
> aprssig mailing list
> aprssig at lists.tapr.org
> https://lists.tapr.org/cgi-bin/mailman/listinfo/aprssig
Squelched or muted (either terminology can be used) audio is that audio 
that is held or gated off in the absence of an incoming signal. High 
frequency noise (typically around 12khz) that appears at the 
discriminator is passed through a high pass filter, amplified and 
rectified. The resulting DC is used to turn an audio switch off. When a 
signal appears the noise goes away and so does the DC, so the audio 
switch is turned on.

For applications such as packet (including APRS) unmuted audio is 
desireable to decrease receiver response time (it takes time for a 
receiver to unmute). For selective calling, and CTCSS applications, 
pre-muted audio is essential so the tone decoder has something to listen to.

The speaker jack on most radios disconnects the speaker when a plug is 
inserted. Maybe this is why you hear nothing from the speaker.

Ray vk2tv

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