Transmit Audio
Last Modified: September 1, 2010
Contents: Basics; Important Points; The Microphone; Stock Microphones; Microphone Mods; Food For Thought; Odds & Ends;
Intelligibility is much more important than fidelity.
These are a bold statements, but all too true: The vast majority of amateur operators use too much microphone gain and/or compression level; Far to many use speech processing when they shouldn't; And too many don't know how to properly use a microphone. All of these contribute to poor sounding, transmit audio. When coupled with the noisy environment of a vehicle in motion, the resulting distortion is all to evident. It pays to remember that readability is much more important than bandwidth and fidelity.
What follows are a few suggestions on how to maintain good transmit audio. If there is just one point to be made here, it's moderation. You might want to read this article on speech processing as well.
Most modern, SSB, solid state transceivers are designed to output 100 watts PEP. Some like the Icom IC-703 are only 10 watts PEP, and some like the Kenwood TS480Hx are 200 watts PEP. The key term here is, PEP (Peak Envelope Power). Unlike FM and AM, a SSB signal has no continuous carrier. In other words, the RF output power follows the audio input power.
Depending on how we measure the power out, it is not uncommon to see a peak to average ratio of 4:1. Due to variations in the human voice, the ratio might be 3:1, particularly if you have a deep set voice. A women's voice might have a ratio of 5:1. These apparently unknown facts cause too many folks to assume their transceiver isn't putting out its rated power, because their wattmeter only reads 20 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 (inter modulation distortion).
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. If that same meter reads from 20 to 35 watts while transmitting SSB, you're probably very close to where you should be. Any more than this, and you'll most likely have distorted transmit audio.
Digressing for a moment. IMD (Inter-Modulation Distortion) is caused by non-linear behavior 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 can't be heard on-frequency, but may be clearly evident a few kHz either side. What's more, contrary to popular belief, it can't 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.
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.
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 indeed have preamps built in, but unlike a power mic, their gain and impedance matching is fixed.
Gain and DSP adjustments aside, the way you use your microphone can have a major effect on your audio quality. 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. So here are a few tips.
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 most cases, two inches is too far! Some folks don't like to eat their mic, so they turn up the gain to compensate. About all this does is increase the background noise, and 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 can't 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). The Icom IC-706MkIIg, adds an adjustable BFO (beat frequency oscillator). The offset can be from -200 to +200 Hz. I've always found the standard, medium bandwidth, settings to be just fine most of the time, but in the case of the 706, -200 seems to work better for the majority of users. However you set yours, don't set the high end cutoff too high as this increases background hiss and noise.
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 can't over drive one either, unless you're in full-shout mode, which for some is a required attribute!
Probably the least known reason, however, is their almost flat frequency response (as shown in the chart at left). 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's 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.
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 use your microphone as outlined above, you do not need any mods. I personally use an IC-7000, and the microphone gain is set at 7%, not 60% as one modification expert suggests. My output is a full 100 watts PEP on SSB, and the FM deviation is 4.85 kHz, as factory settings dictates. If I hold the microphone 6 inches away, the PEP drops to 80 watts, and the deviation to 4 kHz. Do you really think you need that mod?
Incidentally, if you're using an Icom IC706MkIIg, and you want the best transmit audio, set the BFO offset to -200 (Q6 Carrier menu item). Then close talk the microphone (one inch away), and set the microphone gain no higher than 10%. And, don't use compression!
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 processing are but two of them. Another important one is much more difficult to over come, and that is shouting. It's 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 intelligibility is moderation. It would behoove all of us to practice it.