Radio Broadcast Audio Processing... Part Three - Tweaking your Airsound

Okay... thus far we have our mixer set up so that we have reasonable control over all of our sources, and our processing set up so the levels are controlled and stable no matter what the mixer is doing. However, once on the air the sound may be too bassy, too shrill, distorted highs or other issues. This step is probably the most subjective of them all, because what sounds great to you locally may not in fact sound so good "in the wild'. Or, you may absolutely HATE how it sounds locally, but it sounds awesome far away. But, we shall continue nevertheless.

Once the audio reaches the transmitter input, there are many things to consider. Matching your audio output to your transmitter's input may happen in several different ways, from microphones taped to headphones to a full-out balanced line input. Or maybe you've bypassed your transmitter's microphone input altogether. It isn't that important but you should be aware that sometimes there are better ways to get your audio transmitted and you should always investigate that further with regards to your rig. You should also learn and know your transmitter's input stage as some have virtually no processing while others have built-in peak limiters, EQ and filters, which will all matter a LOT.

One thing that *really* makes a big difference at this stage is EQUALIZATION. The old tone control on steroids. This beast is the golden tweak for any transmitter. With equalization, you can cure a range of airsound issues from popping bass, screechy voice and scratchy highs.

This is a Behringer Equalizer. A *decent* piece of gear, not total crap and not "high-end" either, but it will do the job. A lot of people, especially in broadcasting, *hate* Behringer gear, mainly because they do have kinda weak power supplies. I will not go into that here...

This unit has two channels, for stereo use. Unless you are transmitting a stereo signal, all you would need is one channel. The large section on the unit are your tone controls. To the left is the gain structure settings, input/output metering and a peak limiter.  The thing I do like about this particular EQ is that it includes a PEAK LIMITER after the equalizer stage. Unless you are only boosting a little bit (1 - 2 dB), you should ALWAYS have a peak limiter after an EQ.

Starting out, you want to adjust the OUTPUT control almost all the way down. Adjust the INPUT control until your processed audio reads around 0dB. Adjust the LIMITER THRESHOLD until the gain reduction lights one or two LEDs. You don't want excessive limiting at this point, just enough to keep peaks controlled. Any more will lead to squashing or pumping.

Let's look at that left section a second... There are LOW and HIGH CUT filters. You should set these to roll off any frequencies your transmitter will not pass. So, if your transmitter has a filter which cuts everything over 5kHz, there's no real reason to push that in there as all it will do is cause problems, like distortion. Same with the low end... if your TX doesn't pass anything below 150Hz, cut it. The lucky operator has a TX with no filtering and passes audio as high as you can put into it. Most Ham transmitters in the last 30 years or so probably have some kind of filters. Some also have limiters or clippers. If so, reduce your peak limiter after the EQ so it barely flickers, or disable it altogether. Also, do not run your input too hard or the built-in limiter will grind your nice audio to a pulp.

Set your processed audio level to the transmitter so you are not exceeding 100% modulation or running your TX internal processing too hard.