Speech Processing & VOX

The use of VOX and speech processing (compression) are not prudent operating practices while mobile-in-motion. The main reason is, the average passenger vehicle traveling at 60 mph has a noise level ranging from 45 dBa to well over 65 dBa. Add a little wind noise from a couple of antennas, and perhaps an open window, and the background level will be enough to trip any VOX regardless of its settings. Besides, everyone listening will know exactly what you think of the guy who just pulled out in front of you.

There is another factor to consider. When you over drive the various stages of a transceiver, the level of IMD (inter-modulation distortion) increases. Unless the IMD gets really bad, you don't hear the distortion on-frequency (you can't hear it using a built-in monitor function either). However, if you tune off to one side, it can be clearly heard. IMD can be especially troublesome in a mobile installation.

Most vehicle electrical systems as fairly stable voltage wise, even under fairly heavy loads. However, as the voltage at the transceiver's input drops, the IMD increases. This fact is of extra importance if you run high power. It also points out the need for proper wiring practices, but that's not all!

Speech Compression

The use of speech processing has become the major bane of amateur radio operation, mobile or otherwise. Most amateurs who use it do not know how to properly set the microphone gain and compression controls, or use it when it is not necessary (i.e.: 24/7)! They add insult by not using (or improperly adjusting) ALC, and have a propensity to tell you what you can do about your splatter complaint.

It's far too common to hear amplifier fan noise, the wife vacuuming, or the fact that the operator in question has a very bad cold, or at least sounds like it. Using speech processing while mobile is even worse! Every little nuance of engine noise, the AC fan, the kids in the back seat, and that squeak in the left quarter panel will be plainly heard. It's important to remember, the average vehicle traveling at 60 mph, is at least 25 dB louder than the average living room, and most are over 40 dB louder.

If you run an mobile amplifier, it increases the current load enough, that in most cases the average alternator can't keep up with it. My advice is obvious; don't use speech compression while operating mobile-in-motion. Alas, for some people, this admonition just isn't enough.

The term VOX stands for Voice Operated Transmit. It came into vogue with the advent of SSB (Single Sideband). A well executed VOX system will have VOX gain, VOX delay, and anti-VOX.

The VOX gain is separate from the mic gain, and is commonly set just a little higher. The VOX delay is the length of time the transmitter stays keyed after the input level drops below the VOX gain setting. The anti-VOX is set just above the level where the incoming audio trips the VOX. Obviously, there is some interaction, and successful adjustment isn't always easy.

If you set the VOX gain too high, every little nuance of background noise will trip it, even if you use a noise canceling mic. Set the VOX delay too long, and you might miss the first word or two from the other station. If the anti-VOX is set too low, the incoming audio might trip the VOX on peaks. Set too high, and you won't be able to trip the VOX with your voice. And, if you set the gain higher to compensate you end up with a bunch of false trips.

Since mobile mics aren't always noise canceling, this adds a level of complication to VOX (and to speech compression) operation. If you just have to use VOX, at least use a headset with a noise canceling microphone. However, be aware as explained below.

Noise canceling mics come in a variety of configurations. How they work varies with the make, but the short answer is this; there are two ports for the mic element. One is short and direct, and other long. A wave front closer to the main port will arrive sooner than the same wave at the second port, so the waves do not cancel each other. A distant wave (several inches to several feet away) arrives at both ports about the same time, thus canceling each other out.

There is one very important point to make about noise canceling mics; you have to close-talk the mic (less than an inch away from your lips), or you won't be heard. Not understanding this important point, far too many amateurs just turn up the mic gain distorting what audio does come through.

Microphone Mods

There are at least three enterprising amateurs modifying stock microphones (primarily the Icom HM-151) with the supposition of improved audio quality, and output level. The truth is, if you learn use your microphone correctly as outlined above, you do not need any mods. For example, I personally use an IC-7000, and its companion HM-151 hand microphone. The microphone gain is set at 7% (you read that right!), not 60% as one modification expert suggests. Yet, my output is a full 100 watts PEP on SSB, with the average hovering around 35%, exactly where is should be (Read the Odds & Ends below).

Making matters worse, most of these mods remove the noise canceling capabilities of the microphone; a required attribute when operating from a vehicle. After 37 plus years of operating mobile, my sage advice is, don't!

The fact that SSB doesn't have an inserted carrier like AM (it's actually removed on purpose), a lot of neophyte amateurs don't understand about power output meter readings. They think (incorrectly) that their wattmeter should read the full output power their transceiver is rated for. When they don't see it, they turn up the mic gain, sometimes, full-tilt! The truth is, depending on the dynamics of the wattmeter, and a few dozen other variables, the actual power reading maybe as low as 20 watts. Yet, the peak is a full 100 watts.

Even if you have a peak-reading wattmeter, the actual displayed value may be plus or minus 20%. The only real way to tell is look at the transmitted envelope on an oscilloscope. Even that might not tell you the story if it isn't calibrated correctly. So, what ever you do, don't use the mic gain, and/or compression gain, to make up for what you don't see!

Speaking of oscilloscopes (and station monitors). Regardless of what you hear to the contrary, it takes a very good quality storage scope to see the IMD in a transmitted signal. In fact, most of the time, it will be heard long before you can "see" it. And it's just as inane to rely on the ALC indication. Again, moderation is the key to good Transmit Audio.