[prev in list] [next in list] [prev in thread] [next in thread] 

List:       elecraft
Subject:    Re: [Elecraft] What happened to ALC?
From:       Fred Jensen <k6dgwnv () gmail ! com>
Date:       2022-01-31 21:40:50
Message-ID: d7310ce2-4096-7346-3715-f0f3ad77f3a8 () gmail ! com
[Download RAW message or body]

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)
> 



-- 
This email has been checked for viruses by AVG.
https://www.avg.com
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to elecraft@progressive-comp.com 


[prev in list] [next in list] [prev in thread] [next in thread]