On Tue, 11 Oct 2005 16:32:59 -0500, "Steve Nosko"
wrote:


Because a design has one or more of these, the identity of each could be
confused by combining the feedback signal and calling it only "ALC" when in
actuality, there are two or more protection schemes present along with the
(distortion limiting) ALC.

Yes Steve, once you have the gain control mechanism, it becomes the
obvious control point for various over-x closed loop control systems.
But you are right that the highest priority function is for limitation
of distortion due to exessive signal (for the current conditions), and
it is that loop that has the most onerous dynamic performance
requirements (eg fast attack) for it to perform the intended fuction
well.

Unfortunately, there is a growing common belief that unless the ALC
meter is high upscale, then the rig isn't being talked up enough (the
dumbing down of ham radio). There seems a common ignorance that most
ALC detectors have a threshold (usually the rated PEP for solid state
PAs), and that the slightest deflection of the indicator means that
some peak signal has reached that threshold, further deflection causes
greater gain reduction in the IF stages, causing the ALC to act more
and more as a compressor where overdrive of the PA is likely to occur
on transients during ALC attack time. Exceeding the red line, even for
occasional transients, is to exceed the capability of the ALC to
contain PA distortion due to overload. My thought is that if you want
audio compression, use a speech processor, not the ALC.

Mentioning speech processors. A correctly adjusted speech processor is
proably better protection against overdrive than depending on ALC
alone. The peaks are contained (clipped) and the distortion products
filtered off, before getting near the PA which does not have effective
post filtering for clipping distorion.

Owen
--