Oh yeah... some may need to smoke a fatty to digest the following information. It can get deep.
In it's basic form, audio processing performs one task... to keep your audio constant and clean. That doesn't sound too bad, does it? Well, it really isn't, but it surely can get that way!
The basic audio processing you need it a PEAK LIMITER to keep those audio peaks out of the mixer that exceed 100% from going over into the transmitter. For AM, you can use something called "asymmetrical processing" to let your positive go over 100% while keeping NEGATIVE peaks from exceeding 99%. A little crazy indeed. However, not all AM transmitters can even do these peaks past +100%, so we will address that entire headache in a future post. SSB, FM, TV, digital modes and basic AM transmitters WILL NOT and SHOULD NOT do asymmetrical modulation. It's mainly for commercial AM broadcasters, but some hams embrace it as well (Hi TimTron!!).
What the peak limiter does is "ATTACK" these peaks quickly and RELEASE them just as quickly when they're over. The limiter attacks when a peak is above a certain THRESHOLD, Usually set to 0dB/100%. Anything above that is limited, what's below is not. How drastically peaks attacked is what is called the COMPRESSION RATIO. Typically, not always though, a 4dB rise above THRESHOLD point results in ONE dB rise in the output, and this is known as a *4 to 1* (4:1) COMPRESSION RATIO. When this limiting is occuring, how much is called GAIN REDUCTION.
This, in it's basic form, keeps you from overmodulating your transmitter on loud peaks. That is ALL it's meant to do. Because of it's really fast ATTACK and RELEASE times, pushing more into the peak limiter will literally mash the life out of your audio, making it sound super squashed in the receiver.
We've determined that you need to protect your transmitter from peaks in excess of +100%. The peak limiter does that. It should never be used to push up your quieter passages because it's basically a hand grinder for audio signals. So then, how do we keep the levels into the limiter from dropping below threshold or really squashing the life out of your audio? Simple...
AUTOMATIC GAIN CONTROLLER. Aka, AGC, AUTOMATIC LEVEL CONTROL, LEVELER. All the same thing.
Yes, works just like AGC in shortwave radios, just on your audio chain. An AGC is basically the same as a limiter circuit EXCEPT that it attacks and releases the peaks MUCH SLOWER... so it more or less pulls loud audio down and pushes soft audio up... just like a radio's AGC does to the signal.
"Well, why can't I just rely on the radio's AGC to level my signal??" Good question, and I have the answer! The AGC in the radio is also got all the noise between your antenna and the radio's antenna to deal with. If your signal is weak and the AGC in the radio is pulling up hard, it pulls up all the static and noise floor into your audio. That simple.
And in the same vein, you do not want your AGC in your audio chain to pull all kinds of hiss, buzz from RF, ground loop hum, feedback from the mic, etc, when the level drops past a certain point. When your faders are all set all the way down, you do not want any noise pulling up. To do so just plain sounds bad. All you're accomplishing by pinning your agc is pushing more noise to the listener's ears than they already have to deal with. JUST DON'T DO IT!!!
Now then... if you have an AGC and a limiter after your mixing console, you want your hottest signal to push it down just enough to not push the peak limiter too hard. I never recommend over 8dB of gain reduction on limiting, and the same on AGC.
So, we're at the AGC stage again... it can be just a simple unit like an old CBS Audimax, which compresses everything alike, or it can have multiple bands for bass, midrange and presence/highs. The advantage to these multiband units is, they do not cause PUMPING or HOLE-PUNCHING in your audio. What is that??? Well, imagine a moog organ note that's -10dB on your mixer... then, add a drum track at +2 dB over that. What is going to happen if ran really hard, is the bass drum will kick in, punch into gain reduction, but release so slow that the moog note is knocked practically to nothing and then slowly pulls up just in time to get mauled by the next bass beat. It gives a really choppy, "up-and-down in the wrong places" sound output. Think early 1970's AM radio. A song is ending, a loud jingle hits, song fading just disappears into the jingle, then the next song's notes slowly pull up from the big hole they were kicked into.
This doesn't happen (theoretically) when using a multiband compressor/AGC, because each band is narrow and is controlled without affecting the gain reduction on bands to either side of it. Unless, your AGC unit is running too hard, then all bets on sound quality are off.
So, the AGC and Peak Limiter now provide your transmitter a controlled, safe level. However, that does not mean all is hunky-dorey yet.
Your audio may sound too bassy, muffled, "stale" or even lifeless on the receiving end. Your midrange may be too hot, giving you a "tin can" response, your highs like saying "S" or cymbal crashes (called SIBILANCE, FWIW) may be turning into static of their own. While the flat processed audio may sound splendid on your studio speakers, all hell can break loose when it is fed to a transmitter input. And usually does!
How you approach this part of the quagmire will be covered in the next part, "Adjusting Your Station's Airsound. But I have to eat before I faceplant!!!
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