[AMRadio] re: Apache audio not good

[Bill Smith]

Yes, that is how I understand it.  Understanding the difference between a
"compressor" and "limiter" is difficult, because they do almost the same
thing.  I think the real distinction is that a limiter is a device used as
the last processor before a transmitter's modulation stage.  The limiter is
in position to make sure the transmitter is never overmodulated.   This is
particularly important in FM transmitters because they can splatter, or
excessive modulation can exceed the bandpass of the receiver and activate
the receiver's squelch circuitry.

A compressor adjusts the audio path to provide the largest average signal,
and is in position to provide maximum "punch".  Usually the difference
between a limiter and a compressor is a matter of a longer time-constant
used in the voltage-controlled amplifier stage that is automatically
adjusted to control the volume.  A compressor will usually not fully respond
to a strong, very loud pulse, or a "rogue" pulse (similar to a "rogue wave"
at an ocean beach).

A simple "brick wall" limiter can be made from a couple of LED diodes, wired
back to back.  Positive audio peaks will light one LED, and negative audio
peaks will light the other LED.  One can soften the limiting action somewhat
by placing resistors (or a pot) in series with each LED diode, with a diode
wired across each pot so that the resistance is effective for only one
polarity.

The advantage of LED's is they will light when they are limiting (the
circuit is low-impedance).  The problem, of course is that severe limiting
occurs when the diodes conduct.  But the circuit provides a concept of how a
limiter operates, and the pots can be adjusted to provide asymmetrical
(positive peak) limiting.

73 de Bill, AB6MT
billsmith at ispwest.com




----- Original Message -----
From: <SBJohnston at aol.com>
To: <amradio at mailman.qth.net>
Sent: Saturday, April 13, 2002 4:40 PM
Subject: Re: [AMRadio] re: Apache audio not good


>
>  billsmith at ispwest.com writes:
>
> >Not necessarily.  A clipper is a limiter.  A limiter need not introduce
> >the severe distortion a "brick wall" diode clipper will contribute.   :-)
>
> That's true and you say it well:  "A clipper is a limiter."  If the stage
in
> the Apache is a clipper, then the fellow who called it a limiter isn't
wrong.
>  The control behind the CW key jack sets the drive to the stage where the
> peak-limiting is taking place.  Set the drive toward the low end and
you're
> just clipping off the peaks that you don't readily hear.  Drive it harder
and
> now you're tearing off nasty chunks of the waveform and you can really
hear
> it.
>
> I've always thought of a limiter as a device that reduces the peak values
of
> a signal down toward the average level.  It can rip and tear those peaks
off
> (simple back-to-back diodes) or it can gently smooth them off (as in
> sophisticated audio processing).
>
> Some confusion can come in as some folks use the term limiter to cover a
> device that I know as a "compressor" which adjusts its gain to maintain a
set
> average output level.  It has the effect of reducing the dynamic range of
the
> signals applied.  But it doesn't peak-limit unless its action is set fast
> enough top respond to the transients - but then it is a limiter.
>
> Words, words, words... sigh.   -grin-
>
> 73  Steve
>
> sbjohnston at aol.com
>
> _______________________________________________
> AMRadio mailing list
> AMRadio at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/amradio