It’s quite apparent to me that there are a lot of folks using digital modes
that do not understand that louder is not always better!
Anything that modifies the TX (or even the RX) signal in a non-linear way
(equalization, compression or distortion) will impair the quality of your
signal, and if you are transmitting this over the air, you may also be in
violation of several sections of 47CFR97!
Mike’s paper is a very good explanation of why ALC is BAD, but each of us is
licensed to know the technical specifications of our equipment and how to
operate our station and be in compliance with applicable FCC regulations.
The simple explanation is that with proper adjustment in the audio chain to
your rig, you should never experience the ALC overshoot issues. If you are
seeing this issue, you have some turned up way too far! If your TX level out of
WSJT-X needs to be above “0 dB” there are other issues! The best starting point
is to “zero” all of the gains to unity gain or no gain at all!
Secondly, you must understand the difference between microphone levels and line
levels. There’s about a 40 dB difference here!
If you are feeding line level (0 dB) into a microphone level input, you are
about 40 dB too hot! This will result in an inability to transmit a decodable
signal!
For digital modes using older rigs that have no USB interfaces, you must use an
audio interface that:
1) Has positive PTT control (this is not possible with the Signalink USB!)
The RigBlaster Advantage does have this feature!
2) You must provide the proper input level to the interface!
If you are using a microphone level input, you must attenuate the line
level output from your PC 30-40 dB in order to
not overload the input to the transmitter.
You should be able to meter the ALC or compression levels on your rig to ensure
that you do not reduce the high level input
by “compressing” the peaks (this is a non-linear action) with the ALC circuits
serving to protect the output levels by reducing the input levels. The desired
operating point is that the full dynamic range of the input signal is presented
to the modulation circuits
without any distortion. This means that you will control your RF output with
the rigs “drive” control not the input level (audio drive). If you try to
reduce the output level by adjusting the volume controls, you are not
accomplishing what you think you want!
The desired effect is to be able to replace any digital mode software and not
have to readjust any level controls.
You must disable all equalization of the input audio and output as well. The
equalization circuits change the amplitude and phase
relationships in unpredictable ways that the decoders cannot adjust for.
The “tune” button in WSJT-X can be used to set the audio levels. If you engage
the Tune button and you see any variation in your
RF output level, something is reducing the level and this is something that you
don’t want. With “Tune engaged and a properly
adjusted TX path the level should be constant. If the level starts out high and
then reduces, there is some overdrive happening
and your levels are too high.
If you cannot reach an RF output level of about 75% or your peak capability,
then you are not providing enough modulation
to the transmitter (a level mismatch). As Mike describes, some radios have AGC
circuits always engaged to protect the radio,
some have none. You must know how your radio works to be able to operate it
successfully.
Oh, here’s a nugget of knowledge:
If you think that you are going to be heard at a greater distance by using a
linear amplifier with an already distorted or
improperly adjusted TX Path, you are only fooling yourself! Amplifying a bad
signal only makes it worse! And you could
find yourself in violation of part 97 rules! Know that your station is “clean”
and you will be respected on the bands. Operate
with a “sloppy” signal and you will not be welcomed.
Senator Jack S. Phogbound (U)
Dogpatch, Kentucky USA
Thanks you for your time and efforts!
Mitch – AE2A
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