Transmit Audio

The following paragraph appears in my Audio Filtering article: I first wrote (parts of) this article for publication on eham.net. It received the usual number of rebuffs particularly from the HiFi SSB, wideband AM, and other enhanced mode aficionados. Apparently, they've never operated HF mobile where readability is much more important than bandwidth and fidelity. However you limit, adjust, or modify the receiving audio bandpass, the desired result should be increased readability. After all, if you can't make out what they're saying, signal strength doesn't mean much.

Other factors effecting audio readability include band conditions, the level of RFI egress from the various on-board electronic devices, and even how good your antenna installation is. As I point out in my Efficiency article, the actual received Signal plus Noise to Noise ratio (S+N/N) is the only measure which counts.

There is another aspect to readability, and that is how good is the other station's transmit audio. If it is distorted from too much microphone gain, too much audio compression, or a poor quality microphone, no amount of receive audio processing will increase readability.

My personal motto is; Set the example, don't be one. In other words, you can't complain too much about the other guy's audio quality if yours is just as bad!

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 too.

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 TS480H 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 output power follows the input power.

Depending on how we measure the power out, it is not uncommon to see a ratio of 4:1. That is to say, the average power will be about one fourth the peak power. Due to variations in the human voice, the ratio might be 3:1, particularly if you have a deep set voice.

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 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.

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.

There are a few things you just don't do, and one of the worst is installing an amplified Power Mic. 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.

Then there are the select few who seemingly can't overcome their gadget obsession, so they install 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).

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. That is, if you adjust and use it correctly. The chart at left (click for a full-sized view) is from Heil Sound, and is used here with permission. These settings are a good place to start, but some variation might be necessary depending on your specific voice characteristics.

It should be noted that the settings shown are for Heil microphones, and not necessarily an OEM one. Nonetheless, they're fairly close with one exception, and that is the compression level settings.

Mobiles are much noisier than your average radio room, and for this reason speech compression isn't recommended while mobile-in-motion. Even sitting still, over use of compression will allow every nuance of wind and interior noise to be transmitted. Again, we're trying to enhance the far end readability, not distract from it.

If you're really sharp eyed, you'll notice that the Icom IC-706MkIIg, is the only radio with a true BFO (beat frequency oscillator) adjustment. No other, currently available, transceiver has this feature including Icom's top-of-the-line IC-7800. Unlike DSP bandpass tuning, adjusting the BFO frequency shifts the entire audio bandpass. This is the main reason the IC-706 has such good transmit audio quality when the BFO is customized for a specific users voice characteristics.

Gain and DSP adjustments aside, the way you use your microphone can have a major effect on your audio quality. You should speak into the microphone, not across it as is often suggested. This is especially important when using (background) noise cancelling microphones, which the majority of mobile transceivers come equipped with.

There are a couple of other aspects which needs to be mentioned. The output level of electret and/or noise cancelling microphones drops off rather quickly as the distance between your lips and the microphone increases. In some cases, six 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. If you get excited and talk closer up, your audio could become distorted. Either way, you're reducing readability on the other end.

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. Incidentally, I use a Traveler when my wife travels with me for obvious reasons.

If you want to know what's coming soon from Heil Sound, look here.

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 over use of compression 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.