Not sure I see the difference. AVC [now called AGC aka audio
compression], at it's simplest rectifies the audio from the detector and
uses it as variable bias on one or more of the IF stages such that the
IF gain is inversely proportional to the audio level. The original goal
was to provide a constant volume in the presence of a fading RF signal.
The K3 takes that concept to exceptional heights of course with multiple
attack times, thresholds, slopes, and release times, maybe a tad too
many for folks like me. ALC, at its simplest rectifies the RF at the
input to the PA and feeds the variable DC back into the transmit chain
to control the power applied to the PA with the goal of preventing PA
saturation.
Both introduce distortion by the very fact that they modify the waveform
of the original signal. Both reduce the dynamic range of the
transmitted signal, that is, both increase the average energy in the RF
envelope. The original question however was simply, "If ALC was so great
at reducing the effects of PA saturation in the early days of SSB, why
isn't it that great now?" Granted, one manufacturer's amplifier may not
feed back the correct varying DC levels to another manufacturer's
transceiver, but that's just adjustment.
Around the time SSB was being born, most [or maybe nearly all] AM
broadcast stations ran at least two audio limiters in the chain to the
transmitter. The first had a fairly short time constant and sort of
shaved off the peaks. The second ["Sta-Level"] had a much longer time
constant and responded to the average audio level. The result was
modulation that hung around 100% all the time. AM broadcast has changed
a lot since then and I don't know for sure if they are as heavily
limited, but listening to several local ones, they sure sound like they
have almost zero dynamic range.
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
Walter Underwood wrote on 1/30/2022 5:07 PM:
I expect that ALC is being used for two different parts of the transmission
chain.
Audio ALC would reduce the dynamic range of the audio. That automatically
reduces the microphone gain to maintain a consistent level of modulation. That
would be inside the audio part of transmitter, before the modulator. No
external connector involved.
ALC between a transmitter and amplifier does use an external connector. That
kind of ALC reduces the transmitter power to try and avoid overdriving the
amplifier. It can cause overdriving and splatter.
Unlike the audio version, in amp ALC the two parts of the control loop (sensor
and gain control) are in different boxes, maybe made by different
manufacturers. Because the response time and sensitivity of the transmitter
gain control are unknown, the system relies on luck to have a stable control
loop that doesn’t oscillate. The way to avoid oscillation is to have a slow
time constant on the sensor. That means that it just cannot respond quickly, so
overshoot will happen and will cause splatter. With audio ALC, the control loop
is an integrated whole and can be designed to be quick and stable.
wunder
K6WRU
Walter Underwood
CM87wj
http://observer.wunderwood.org/ (my blog)
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