The original concept of ALC was devised by Collins engineers as part of
their development program for speech processing. Speech clipping
flattens the peak amplitude, but also introduces some splatter which
then has to be filtered out - but the filtering re-introduces a small
amount of level variation (called 're-peaking'). Collins then added a
very small amount of ALC, only 2-3dB, to keep the peak envelope power
more constant. This mild ALC introduced very little additional splatter
of its own, so the result was a large increase in intelligibility with
very little increase in bandwidth.
Where ALC got a bad name was when other manufacturers started mis-using
it for other purposes - in particular for manual power control from 100W
down to 1W or less. ALC was also being used to compensate for variations
in TX gain between different bands. All of this required much more gain
in the control loop (20-30dB) which made teh ALC much more aggressive
and created major problems with transient behavior - in other words,
quite serious splatter due to the ALC system itself.
Then someone decided it would be a good idea to apply ALC from the power
amplifier, thus wrapping an external control loop around the existing
ALC loop within the transceiver, so the transient behavior became
totally unpredictable. Add to this the tendency of many hams to
perpetually overdrive their transmitters while *also* applying speech
processing, and the result was that both ALC and speech processing got a
very bad name.
Enter the K3. With a little persuasion from early adopters, Elecraft
returned to the original Collins concept of applying only a small amount
of ALC to minimise re-peaking *without* significantly re-introducing
splatter. The SDR concept allowed other means to be used to implement
the non-dynamic power control functions such as manual power setting and
TX gain compensation. When correctly configured according to the User
Manual, the K3 family of transceivers are capable of excellent speech
processing without worrying about also creating splatter. (Also note
that speech processing is automatically disabled for data modes.)
As far as I know, there is still no other manufacturer who is doing this.
References:
[1] W E Sabin and E O Schoenike (Collins), 'Single-Sideband Systems and
Circuits'. McGraw-Hill Book Company 1987. ISBN 0-07-054407-7
[2] Leif Åsbrink, SM5BSZ (http://sm5bsz.com), ‘Real Life Dynamic Range
of Modern Amateur Transceivers’ and ‘Speech Processing for SSB
Transmitters’.
73 from Ian GM3SEK
On 13/07/2019 08:35, Richard Corfield wrote:
The compression modulates the signal. That will have a fourier transform of
its own. If we think of it as a simple amplitude modulation then, as for AM
signals, the spectrum of the original signal will be combined (convolved)
with upper and lower sidebands representing the spectrum of the compressor.
Our ears don't notice (unless we're sound guys listening to a pop song
thinking "They've squashed those dynamics to death" ;-) ) but maybe a
digital mode will.
The transmitted signal is constrained to the IF passband so you stay within
band. The compression frequencies and hopefully amplitude are both small so
the artefacts it introduces into the spectrum should also be both narrow
and small.
- Richard
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