Last Modified: December 16, 2011
Contents: Basics; Dynamic Range & Things; The Microphone; Microphone Mods; Speech Compression; Speech Clippers; VOX; Food For Thought; Odds & Ends;
Readability is much more important than fidelity.
Every single type of modulation, AM, FM (both phase and true FM), SSB (Single Side-Band), etc., all have a specific set of operating parameters. Some of those parameters are set in stone, and some are dynamic. We're not going to deal with any type of modulation here, except SSB. There is a good reason why. Transceivers which transmit SSB, have microphone gain controls, and often speech compression settings as well. It is the misuse of these controls, and the misuse of measuring techniques we're going to explore.
On the other hand, FM transceivers don't have (external) microphone gains, and never have speech compression in the usual sense. They do have pre-emphasis, but that's a whole new subject, and we're not going there!
In the case of SSB, the dynamic part is specifically reliant on the individual's speech characteristics, and to a lessor degree on the microphone, microphone gain setting, compression on or off, and even the way we use the microphone.
Partially because of the dynamic nature of speech, there is a gross misunderstanding about SSB, and its inherent peak versus average power. Cutting right to the chase, the peak envelope power (PEP) is typically very close to the published specs of the transceiver in question. The PEP is not affected by nominal microphone gain settings, either. However..... (the pause is intentional), the average power depends on so many factors, you just cannot place a number, formula, or rule of thumb to it.
While reading this article, it will become very evident that correct microphone gain adjustment is the most important factor in achieving good readability on the other end of your contact. Unfortunately, most owners' manuals, and on-line articles seldom cover this important attribute, focusing instead on adjustments for the internal and/or external DSP and/or equalizers. While those adjustments are perhaps important, excessive microphone gain will effectively negate their viability.
Dynamic Range is described as the ratio of the largest to the smallest intensity of sound (speech) that can be reliably reproduced. In this case, by a human voice. The measurement method uses a frequency weighted scale, and for human voice that's dB(C). However, it isn't a finite level we need to be concerned with. After all, we do have microphone gains on our SSB radios. What we are concerned with is the ratio of the largest to the smallest intensity of speech.
To a lessor degree, we also need to be concerned with frequency dynamics. The reason is, the bandwidth in most transceivers capable of SSB transmission, are limited to about 2,400 Hz (≈300 to ≈2,700). What's more, the gain response across the range is not perfectly linear. If this wasn't enough to deal with, every individual has a different speech pattern. It is these unique patterns, the dynamic range, and the mean frequency which allows us to identify who we're talking to, even though we cannot see them.
Measuring all of the dynamics of speech is a discipline all by itself. People spend their whole working careers bathed in the study of human speech pathology. Over the years these folks have developed all manner of devices to qualify and quantify what speech is, how to measure it, and how to produce it with clarity through any given bandwidth. One rather unfortunate aspect of this is, a lot of amateurs think they know more about the subject, than the pathologists do. That fact has proliferated the wide-spread (pun intended) use of speech processors, equalizers, and other wide-banded devices, to the detriment of us all.
It should also be mentioned, that measuring the various attributes of human speech isn't easy. Nor is the effect these speech attributes have on transmitter performance. Suffice to say, very few amateurs have the requisite equipment to do either. While popular press would have you believe otherwise, you cannot use a wattmeter (damped or undamped), you cannot use a common oscilloscope (which includes a station monitor, no matter who made it), and you cannot detect inter-modulation distortion (IMD) within the bandpass aurally, until it is very crude. It is this lack of measuring capability, I believe, that there are so many lousy-sounding SSB signals on the air.
Another unfortunate fact; far too many amateurs (it really doesn't matter if you're a neophyte or not), just don't understand the relationship of PEP (peak envelope power) versus average power. Due to human voice dynamics, the measuring technique, microphone and compression gain levels, DSP settings, etc., it is not uncommon to see a peak to average ratio anywhere between 6:1, and 3:1. This fact causes too many folks to assume their transceiver isn't putting out its rated power, because their wattmeter only reads 15 to 35 watts. So, they crank up the microphone gain, kick in the compression, and end up over driving the various stages of their transceivers. The net result is distorted transmit audio due mainly to excessive IMD.
IMD is caused by non-linear behavior (internal or external) in the audio, IF, and/or RF signal path. This non-linear behavior creates spurious emissions which cause interference to nearby stations. Unless it gets really bad, in cannot be heard on-frequency, but may be clearly evident a few kHz either side. What's more, it cannot be seen on a station monitor or inexpensive oscilloscope. Equalizers, bandpass and/or lowpass filters, ALC enhancements, nor microphone mods will negate or diminish IMD, irrespective of what you read or hear to the contrary.
Further, most (affordable) wattmeters sold to amateurs have an accuracy around ±10% of their full scale reading. Peak reading wattmeters aren't any better. Whatever yours reads while transmitting a dead carrier (CW) into a 50 ohm dummy load, will be very close to the peak power in SSB mode. For a 100 watt radio, and nominal wattmeter accuracy, the reading may be anywhere between 90 to 110 watts! If that same meter reads from 15 to 35 watts while transmitting SSB, you're probably very close to where you should be. Much more than this, and you'll most likely have distorted transmit audio.
Since we're talking about mobile operation, here's an important point. Designing an amateur transceiver to operate on a nominal 13.8 volts DC, and maintain a respectable (and legal) level of IMD, is a difficult task. Even harder to design in this respect, are mobile amplifiers. Overdriving any stage between your lips and the antenna, will most assuredly cause excessive IMD products. The real truth is, when the transceiver in question is properly adjusted, a 100 watt signal will be easier to copy, than a 500 watt one full of distortion.
While slightly off the subject, there is one modification which should never be done; boosting the power output. The finals of modern transceivers are selected to provide a given power level, yet stay within their linear response region. When the drive and bias levels are adjusted to increase output power, they force the finals to operate outside their linear response region. The result is highly increased levels of IMD, all at the expense of ≤1 dB of power increase. Most mobile operators would do better by properly mounting their antennas!
If you'd like a technical explanation of what IMD is, how it's measured, and its effects, this article by Tom Rauch, W8JI, is as good as it gets! When reading the article, pay attention to the necessary hardware needed to accurately measure IMD.
Modern solid state transceivers (almost universally) use a low impedance (nominally 500 ohm) microphone. The elements are usually electret condenser types, but may be dynamic. Some do have preamps built in, but unlike a power microphone, their gain and impedance matching is fixed. Gain and DSP adjustments aside, the way you use your microphone can have a major affect on your audio quality. One of the best articles on this subject of microphone use, was written by Steve Katz, WB2WIK/6. It points out several common user faults.
For example, the output level of electret and/or noise canceling microphones drops off rather quickly as the distance between your lips and the microphone increases. Therefore, you should speak directly into the microphone, not across it as is often suggested. This is especially important when using (background) noise canceling microphones, which the majority of mobile transceivers come equipped with. Your lips and the microphone should almost be touching. In some cases, two inches is too far! Some folks don't like to eat their microphone, so they turn up the gain to compensate. About all this does is increase the background noise. If you get excited, and talk closer up, your audio becomes distorted.
One very good way to assure the correct speaking distance is to use a headset with a built in microphone, such as the Heil Traveler® series. This headset can be ordered to match almost any transceiver, and comes in both single and dual sided. It should be noted, that some jurisdictions do not allow headsets (especially dual sided ones) to be used by the driver. Make sure you know what your local law allows if you take this route.
The real truth is, in a mobile setting, the stock microphone which came with your radio is as good a choice as you can make. However, there are the select few who seemingly cannot overcome their gadget obsession, so they install power microphones with Roger Beeps and Echo Effects. Using one on any amateur radio frequency will net you a lot of ill will, and label you as a LID (poor operator).
Most late model transceivers incorporate some form of microphone DSP (digital signal processing). This allows tailoring the audio response to suit your specific speech dynamics. However, until you read Steve's article, you're probably better off leaving the adjustments in their default setting.
As mentioned above, most modern transceivers, SSB and FM alike, are supplied with electret condenser microphones. There are several reasons why this is so, not the least of which is their diminutive size, and power requirements. You almost cannot over drive one either, unless you're in full-shout mode, which for some of us is a ubiquitous attribute!
Probably the least known reason, however, is their almost flat frequency response (as shown in the chart at right). No other commonly used microphone type can even come close. This fact makes designing the requisite audio stages easier.
If they have a drawback, it is improper use. As noted above, you should speak directly into the microphone, not across it. Follow the rules, and you won't need to replace the microphone, or perform any mods to get good audio reports. All you have to do is use a moderate amount of microphone gain. The key here is to watch the ALC indication. While there is a lot of variation between transceivers (make and models), the fact remains, any ALC indication means the internal electronics are reducing the drive level to the finals to keep them within their linear response curve. If yours is constantly showing, it is an indication the microphone gain is too high. If the truth be known, having to set the microphone gain higher than about 10% to 15% is an indication you're not using your microphone correctly. Again, read Steve's article.
Noise canceling microphones 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 microphone element. One is short and direct, and the other long and indirect. 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 microphones; you have to close-talk the microphone (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 microphone gain distorting what audio does come through.
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. They do this by defeating the noise canceling feature. If you've been paying attention, you know what the ramifications are.
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 it should be. How did I measure this? By using a 100 MHz storage scope with an RF detector module, which is about the only way to get an accurate measurement.
As alluded to above, there is no cheap-and-dirty way to measure peak to average speech levels. It takes a decent storage scope, astute knowledge of how to interpret the resulting readout, and a handful of other laboratory-grade hardware. Nor is there a cheap-and-dirty way to measure the frequency response of any given microphone. You certainly can't do it with your ear alone, as some mod-experts would have you believe. Caveat Emptor!
The use of speech compression (aka speech processing) has become the major bane of amateur radio operation, mobile or otherwise. The major reason is, most amateurs who use it do not know how to properly set the microphone gain and/or compression controls, or use it when it is not necessary (i.e.: 24/7)! Using speech compression while mobile, allows every little nuance of engine noise, the AC fan, the kids in the back seat, and that squeak in the left quarter panel to be plainly heard. It is important to remember, the average vehicle traveling at 60 mph, is at least 25 dB louder than the average living room, and some are over 40 dB louder.
My advice is, don't use speech compression while operating mobile-in-motion. This is especially true if your microphone has one of the aforementioned mods, as they eliminate the noise canceling feature.
Assuming that the gain controls are set correctly (and prudently!), speech clipping is a more effective way to increase one's talk power, without undue distortion or excessive background noise levels typically associated with speech compression. Unlike speech compression, the audio gain remains the same regardless of one's speech pattern. What goes on inside the clipper, is rather different as well. Basically, the base band audio is mixed to a higher frequency (≈100 kHz), clipped, filtered, and then converted back to base band audio. Properly done, distortion products are eliminated. Unfortunately, no one makes a speech clipper specifically for mobile use, although both Idiom Press®, and TenTec® make base station models.
From the Idiom Press web site: RF clipping is simply the most effective way to increase the average power of your SSB signal. LogiKlipper® works by converting your audio to SSB at an IF of 125 kHz, amplifying the signal, and then clipping it. This results in a signal with higher average power, but also with harmonics. Fortunately, the harmonics don't fall within the passband, and are easily filtered out. The signal is then downconverted back to audio to be sent to your rig. The result is clean, splatter-free audio that sounds louder on the other end.
The term—splatter-free audio—is only true if the microphone gain settings are properly adjusted, and the mobile installation in question can handle the increased average input level. This includes the transceiver, amplifier if any, the wiring, and most importantly, the power source (the alternator in most cases). This is one aspect of mobile operation that cannot be overlooked!
Remember, clipping increases the average SSB power by as much as 6 dB or more, which can result in over-taxing a vehicle's electrical system, especially if high power is being used. Alternators are designed to provide a specific level of peak power (typically their published ratings). Their continuous power rating may be as low as 50% of the peak power rating! This raises the question about the specific ratings of any-given alternator? The best place to obtain this information, is from service manuals. Besides this, they (service manuals) are a great source for anyone installing amateur radio gear in any vehicle!
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 microphone 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 microphone. 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 microphones aren't always noise canceling, this adds a level of complexity 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.
There are several common mistakes to avoid if your goal is to be heard at the far end. Avoiding the use of power microphones, and speech compression are but two of them. Another important one is much more difficult to over come, and that is shouting. It is human nature to increase one's oral volume level when excited, or when the background level increases. In the closed cabin of a vehicle, your brain interprets the reflected sound from your own voice as an increase in background level. Add in a little traffic noise, and by the end of your transmission, you're in full shout mode! One way to avoid this is to use a headset and the built in monitor function. Doing so gives you direct feedback (not a time-delayed echo), and your brain won't get confused. If you doubt this premise, do a little listening the next time you hear a mobile station.
Another important aspect of being heard clearly, is the way you give your call phonetically. Skip all the funny acronyms, and use the standard ICAO (International Civil Aeronautical Organization) alphabet.
Unless you want to be know as a LID (poor operator), forget the 10-4-good-buddy, QSL-QSL-QSL, and roger-that jargon as well.
The old saw about an ounce of prevention being worth a pound of cure is perhaps true. But two ounces of prevention can be worse than the cure. As stated above, the key to far end readability is moderation. It would behoove all of us to practice it.