[aprssig] squelched audio

Steve Noskowicz noskosteve at yahoo.com
Fri Sep 21 19:57:51 CDT 2007

  I expand a little on Ray's below.  Take what you
want.   Blow off what you want.

73, Steve, K9DCI

--- Ray Wells <vk2tv at exemail.com.au> wrote:

> Ben Lindner wrote:
> > Can some kind person explain what squelched audio
> > Ben Lindner
> > VK5JFK
> >

> 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. 

    Squelch = to silence.  This circuit type
frequently, but not always used, is called a "noise
squelch".  Another common, but not as effective for FM
opens simply on signal strength or is a "S-Meter"

>  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.

    When an FM receiver receives a signal, the noise
(that is present with no signal) is reduced.  This is
called "quieting".  As we increase the signal level
from a very low level, this higher frequency noise
quiets faster (reduces more) than the noise in the
lower audio frequencies.   This makes the squelch very
sensitive to weak signals and opens the audio path for
rather weak signals ... that is, it does this when we
want this to happen, by adjusting the squelch control
close to the "threshold" point where the no-signal
noise just goes off.  This is called "threshold
   When we turn the squelch control (usually
clockwise) so that it takes a very strong signal,
producing a lot of quieting (noise reduction),  to
finally unsquelch (open the audio path), this is
called "Tight Aquelch".
  The squelch control is actually a gain control and
in the tight squelsh setting is putting the most noise
into the squelch circuit, so it takes more of quieting
to open the squelch.

> 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.

While it takes a little time to unsquelch (un mute or
turo on) the audio, any delay here is kept to a
minimum in desiging the circuitry.  Otherwise we are
likely to miss the beginning of a transmission.  
  On ther other hand, it is desirable to leave the
audio on for a little time should the signal drop. 
This helps during varrying signal conditions.  When
the signal drops very close to the noise and may drop
below the noise, the squelch stays open and we hear
this noisy signal.  It is easier for our ear to pick
out the most intelligence this way.
    Because the squelch stays open when the signal
drops, we hear the classical noise burst at the end of
an FM transmission.  This noise burst is called the
"squelsh Tail".    Some receivers eliminate this
squelch tail on strong signals with a slightly more
complex circuit.

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

     This disconnects the *internal* speaker and the
audio output from the radio should be on the external
speaker just plugged in.

> Ray vk2tv

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