MFJ-1205 DigiLINKTM USB Radio Interface Instruction & Technical Manual
2.2 DIGITAL-MODE AUDIO ADJUSTMENT
Operating digital modes is very different than operating traditional phone or CW modes. For
starters, not every mode works well at all power levels. PSK31, for example, is an excellent
low-power mode (<20 W); however, driving PSK31 at 100 W not only can damage most radios
(they typically are not built for 100% duty cycle), but it can distort your signal into something
undecipherable, create spurious harmonics, and increase your bandwidth to the point where you
are wiping out everyone else around you. This is the epitome of bad operating, but fortunately it is
fairly easy to avoid with a basic understanding of how your radio is generating its signal.
Most analogue HF radios simply inject audio from a digital interface somewhere along the audio
input circuit. Radios built before the rise of digital modes often require the audio to be injected at
the microphone port, while newer radios typically have an auxiliary input that bypasses some of
the initial audio conditioning at the mic for a more stable signal. Regardless of the exact method,
the audio is injected onto the same line, and therefore must pass through the ALC circuit on the
way to the finals.
Most of us are used to simply adjusting the power control to where we want to be, speaking into
the mic, and then adjusting the mic gain to minimize distortion. For digital modes, you want to do
something a little different. For less-complex analogue radios (i.e. the ones for which you need an
external digital interface) you actually want to open up your power control all of the way and use
the audio controls on your interface to adjust transmit power.∗The reason for this is to keep the
ALC from turning on and distorting your signal. The drawback is that it is easy to undermodulate
your signal, increasing your signal-to-noise ratio. Another common method is to set the power to
the desired level, then slowly increase the audio volume until the ALC “just barely flickers.” This
method makes full use of your dynamic range and will produce the best signal-to-noise ratio;
however, it does involve using the ALC as the upper limit, runs the risk of overmodulating your
signal, and requires constant fine tuning as you move around the band. Both of these techniques
are valid, and the best one to use will depend on your radio and your operating style.
In order to make the audio adjustments easier, the MFJ-1205 has two sets of audio controls. The
first is the direct control of the audio chip’s output level through the computer. the second is the
pair of potentiometers on the front of the unit. As discussed earlier, we do not want to overdrive
the digital audio; however, interfaces such as the MFJ-1205 require a certain audio threshold to
engage the PTT circuit. This threshold is met by increasing the volume using the Pi until the
PTT LED is solidly on. It is quite likely that this will be too much audio drive for the desired
power output; therefore, a 10 kΩ potentiometer has been placed in the output audio circuit.
Adjusting this pot will lower the audio drive while still maintainin the level required to activate
the PTT circuit.
∗Remember that the power of an AM or sideband signal is directly proportional to the volume of the input. This
is why shouting at the mic on AM or sideband will get you more power.
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