Avionics System Design: Huh? What Did He Say?

By Walter Shawlee 2 | October 1, 2001
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On a recent flight to the bustling metropolis of Nelson, British Columbia (population 8,120), I witnessed airborne communications in the general aviation world. Too bad I didn’t pack my secret decoder ring because, generally, I missed about 40 to 50 percent of the radio traffic.

Despite the advances in noise suppression, I simply could not understand a word said by the pilots in many other aircraft. My hat’s off to the local controllers who seemed to decipher at least some of the messages. Of course, they could refer to the transponder tags on their radar displays and at least knew most of the pilots in the area. During the 90-minute flight, the tower sounded fine. Too bad we couldn’t listen to just the controllers.

Even the air transport aircraft had problems, mainly inverter hum overlaid on the audio and a distinct echo, probably from lightweight acoustically coupled boom microphones in cockpits. Communications were appalling. Noise, distortion, over- and undermodulation, generator whine, clipping and all kinds of signal breakup were the norm, regardless of aircraft type or cost.

Voice communications from a helicopter close to our departing airport suffered from such severe "rotor modulation" that every word the rotorcraft pilot spoke was unintelligible. This caused a real concern, as he was in our immediate airspace on takeoff and we could have flown right into him. The helicopter pilot sounded as if he was gargling his messages in Urdu during a monsoon.

While people point to various issues they feel are important to flight safety, audio quality clearly has lacked attention. Part of the problem is the way pilots monitor their radio operations. Often they monitor only the audio signal (sidetone) and do not actually hear a demodulated signal, so their efforts are of minor use as a gauge of transceiver health.

Many pilots incorrectly set the sidetone and microphone levels for individual taste, without realizing it may affect their transmissions. Some pilots casually change headsets and microphones. In both cases, the result is often poor radio modulation. Pilots may try to correct the modulation by adjusting the sidetone. But the outgoing transmission remains over- or undermodulated, and since the pilot doesn’t know how he sounds to the rest of the world, a genuine problem ensues.

There are some little-known facts about sidetone level. The ideal level for the right psychoacoustic behavior is about 7dB below the received audio, according to RTCA specification DO-214. If the headset signal level is set too high, the pilot will inevitably speak more softly, resulting in even less modulation to the transmitter. And he will do so even though the headset level has almost nothing to do with the modulation level. If the headset sidetone level is too low, the pilot will speak louder, often overmodulating and clipping the signal. This drastically reduces intelligibility. Setting the sidetone level is just one adjustment, but it’s an important one.

A radio’s test bench performance differs greatly from its airborne performance. During all my radio installations, I’ve examined both the radio’s static performance and its performance in flight surrounded by mechanical and electrical noise. I monitor the radios in flight long enough to correct system levels, as well as the intelligibility of audio on the ground.

From my experience on flights like the one to Nelson, I often find that pilots do not understand correct mic operation. The microphone has to be close enough to touch your lips. Thanks to TV programs like "Magnum PI," many people believe the mic should be far from the mouth, say five inches (12.7 cm), no doubt to allow a clean camera shot. Sad to say, the differential noise cancellation of most mics has a limited operational range.

While DO-214 describes a general specification range for microphones and headsets in the aircraft and defines common system levels, it is more honored in the breach than the observance. It certainly is not true that you can switch mics and headsets at random.

Also, if an antenna inspection has not been performed within several years, a serious radio-frequency (RF) mismatch and resulting distortion on transmit can occur. In helicopters, rotating blade interference with a closely spaced, top-mounted antenna can be detrimental. This rotor modulation will shut off the transmitter although the effect will not show up in a static ground test.

Currently, the only aircraft receiving a thorough audio flight check seem to be those contracted by the U.S. Forest Service or provincial forestry services in Canada. Radios on those aircraft get a detailed check of every headset position and thorough modulation tests with the headsets. Everyone else appears to be winging it.

Radio adjustments should be a system exercise. It could be an enlightening one, too, because most people are astounded to hear their own voices. They often say, "I don’t sound like that." Well, yes, they do. What you hear when you speak is largely colored by bone conduction and resonance inside your head. What everyone else hears is quite different.

Now imagine your voice competing with blaring cockpit noise and broadcast through a poorly adjusted radio suite, via a bad mic. You may not understand yourself, let alone recognize that the voice is yours.

What is needed is a simple service at a fixed-base operator, avionics shop or airport, where pilots can talk into a radio for a few seconds and then listen to their rebroadcast voice on the same radio to sample the "real" signal. The shock from what the pilots hear may make them park their planes. It certainly would cut down on a lot of problems in the air.

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