Speech Processing & VOX
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Contents: Basics; VOX; Speech Compression; Noise Cancelling Mics; Other No-Nos; Odd & Ends;
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 m.p.h. 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.
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 cancelling 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 cancelling, this fact 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 cancelling microphone. However, be aware as explained below.
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 (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. 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.
Noise cancelling 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. Properly designed (that's a poor assumption at best), 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 (maybe).
A distant wave (several inches to several feet away) arrives at both ports about the same time, thus cancelling each other out. Good in theory, but poor in practice in most cases. But, let's be honest. Some noise cancelling mics are so good, you have to close-talk the mic (less than an inch away from your lips), or you won't be heard. Believe it or not, this is the best scenario, especially if you just-have-to use VOX. To be honest, anything less can't really be considered a noise cancelling device.
Alas, there is a drawback. Noise cancelling mics wreak havoc with speech compression. This is yet another reason to avoid using compression.
There are several devices commonly used on the CB frequencies which have no place in amateur radio, unless you want to be know as a LID (very poor operator).
One of these is the Roger Beep. I suppose there are instances when it's important to know when the other operator is finished speaking. However, using a Roger Beep mic is not the way to do it. As trite as it may sound, using the words over, or back to you, should be used as a curtesy if nothing else.
Another is the echo chamber. If you use one of these, don't expect me to answer your CQ!
Yet another is the power mic. Contrary to popular belief, 99% of the amateur transceivers have enough mic gain to make up for even the weakest and/or timidest of voices. About all a power mic will do, is create excessive IMD (Inter-Modulation Distortion). Read that as splatter and/or wide signals.
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!