Re: FDM-DUO ALC

G8DQX list
 

Tony,

it may help to think of it this way. A Duo does not have ALC, nor does it have a slow-start heater supply, nor a thermionic valve output stage. The traditional reasons for needing ALC in a transceiver no longer apply. Nor do the reasons which required back-coupling of a so-called ALC voltage from an external amplifier to a transceiver.

A digital transceiver, such as the Duo, which generates RF directly from a D/A converter at the output frequency can, in essence, run open-loop, because the gain of the PA stage(s) post D/A is relatively small, in dB terms, and relatively unvarying. Thus any needed control may be applied in the DSP, rather than the analogue, domain. A similar argument applies if the transceiver drives an external amplifier of modern, solid-state, design.

So why did we ever use ALC? It was a bodge which worked using '60's technology. It allowed for both a degree of speech compression and avoidance of overdriving PAs (or external amplifiers). But note that speech compression and avoidance of overdriving are incompatible given a common mechanism. How do we know that it was a bodge? Just try finding a specification for what a given ALC voltage means, in terms of gain reduction or power output or even IMD. It's not there, merely a phono socket labelled ALC, but more accurately identified by Neil earlier!

The 2018 ARRL handbook puts it more prosaically in section 14.1.2:

"…Since we have absolute control over the sig-
nal level by altering samples in the radio, it
is generally better to set a maximum power
out in the radio and allow the signal chain to
make the appropriate adjustments than to rely
on ALC to adjust the levels when they are too
high for an amplifier.
"An ALC subsystem that is too active can
modulate the transmit signal. This results in
distortion that could be prevented by adjusting
the proper levels in the signal chain to start
with. This is why many digital mode guide-
lines specify that the ALC system be switched
off — to avoid unintentional distortion of the
signal by power control systems."
73,

Robin, G8DQX


On 04/11/2019 12:30, Tony EI8JK wrote:
Am I right in thinking that it's not possible to access the Duo's ALC?
In fact I can't find any reference to ALC in any Elad documentation.
Or is it a case of carefully setting the Windows audio drive levels, which is fraught with problems as every Windows update seems to reset the audio out to whatever MS thinks fit.
Thanks
Tony, EI8JK

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