On 16 Apr 99 at 23:21, Tony Duell wrote:
> >
> > Hi,
> > Could anyone tell me how a radio detects signals vs. static? There is a
> > little gauge on my radio that moves depending on the amount of noise vs.
>
> As you know, a radio uses some kind of freqeuncy-selective filter to
> selected the wanted station from all the others. And it then amplifies
> that signal. But since all the different signals have different strengths
> when they reach your radio (and a particular signal may even vary in
> strength), you'd have to keep on adjusting the volume control as you were
> tuning the set to ensure that (a) you didn't miss any signals and (b) you
> didn't drive everyone mad when you hit the stronger signals.
>
I've got a couple of old Eico tube amps that have both loudness and
level pots. Wasn't there also a problem related to changing amplitude.
These used pre-Crossley non-multiplexed external FM tuners so it was
related to any input not only signal detection. There's also a control for the
various phono recording types. e.g. Columbia, London, RIAA, Am78, and Eur78
> So all modern radios (the idea was first used in the 1930's, I think)
> have a circuit called an Automatic Gain Control (AGC) [Sometimes called
> Automatic Volume Control - AVC - in older books). This circuit measures
> the average level of the received signal and adjusts the gain (how much
> the set amplifies the signal) so that all signals appear to have the same
> volume. Weak signals get amplified more than strong ones.
>
> Now, firstly static is always there, even when you're listening to a
> strong station. Don't think it's stronger when there's no station - it's
> not. But if there's no station, then the AGC circuit makes the set
> amplify the (non-existent, really) signal as much as it can, so the (very
> weak) static sounds quite loud
>
> Secondly, by measuring the control signal that controls the gain you can
> get some idea of how strong the signal is. The less gain the radio is
> using, the stronger the signal. That's almost certainly what the meter on
> your radio is measuring.
>
I have some older equipment that doesn't TMK use AGC that has signal level
meters and as well there were the tube signal level indicators. Is that method
only used on the cheaper newer sets ?
> > signal. I would guess that the digitally tuned radios that skip over the
> > frequencies that are pure static work in the same way. What is this way?
>
> Yes. Unless they detect a suitably strong signal, they'll skip over it. A
> real pain if you're trying to receive a very weak signal.
>
> -tony
>
Yeah, I got rid of a mid-quality Sony receiver with a click-click and all the
conveniences but with a digital tuner, because of that. Now I have to get up
from the couch if I want to change anything. Gives me some exercise. :^))
ciao larry
lwalker_at_interlog.com
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Received on Fri Apr 16 1999 - 23:23:45 BST