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Avionics System Design: Voodoo Audio: Part II

By Walter Shawlee 2 | December 1, 2001
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In last month’s issue, I began telling the history of the poorly understood subject of cockpit audio. I discussed the introduction of noise-canceling microphones and the accompanying low-impedance headsets. And I mentioned that fully TSO-C50/DO-214-compliant, 600-ohm output in an audio system must be able to drive a 150-ohm load and thus must have a source impedance of 150 ohms or less. This technique usually provided an adequate audio signal for pilots but was quite unattractive from a design standpoint.

However, a chance design decision made in the 1970s by David Clark Co. Inc. changed the evolution of audio systems dramatically. The company had used a 300-ohm ear element in each headpiece and connected them in series for 600 ohms. There were a few inevitable field failures from broken wires and open ear elements, and in response, David Clark engineers solved the problem by hooking the two 300-ohm earpieces in parallel, so that a failure in one ear couldn’t affect the other ear. This straightforward solution dramatically improved headset reliability and, incidentally, also significantly increased volume.

David Clark’s technique worked perfectly, but it also changed the headset from a 600-ohm to a 150-ohm load. This was perfectly acceptable under DO-214 (which defines the 4:1 ratio of operational loading) but had repercussions in the cockpit that were not immediately obvious to pilots or designers.

In essence, it was as if four 600-ohm headsets were now hooked up in parallel, in terms of loading on the audio bus. (Four 600-ohm headsets in parallel equals a 150 ohm load, as a quick ohms law calculation will show.) Many radio output stages simply could not drive this load well, especially when other radios and more headsets were connected in parallel in a typical cockpit situation, driving the total load impedance far below 150 ohms, often to 50 ohms or even lower. This can be a load mismatch of 10:1 in many situations.

The result of driving this lower impedance is generally twofold: lower audio level (especially when fed through series resistors) and increased distortion (usually in the form of early clipping). This is especially aggravating when crews use headsets of different impedances from a common audio panel: the headsets will often operate at dramatically different volume levels.

Because the David Clark headset was so well made and works well in high ambient noise environments, thanks to its excellent ear cup and cushion design, it soon generated many imitators. The imitators copied every trim feature, nut and bolt of headsets like the H10-30 series; they also copied the parallel element wiring. Thus the de facto standard of 150 ohms for headsets was established.

Meanwhile, other cockpit electronics continued to be made to the 600-ohm design criteria, so attaching even one headset already represented the maximum design load. Designers moderated this effect to a degree by adding volume controls to some later-model headsets, so that additional resistance usually appears in series with the earpieces.

Unfortunately, most general aviation aircraft now have two or more headsets on board, adding to the design load on the headset audio bus. Typically, at least one headset is significantly different from and of lower quality than the other(s). When everything is shorted together in many audio interconnects, the results are quite unpredictable.

Better civil headsets soon brought better microphones, usually in the form of amplified dynamic microphones. Early versions were handheld mics like the famous Shure 488T (one of my all-time favorites) and Telex 100TRA for most Cessna cockpits. David Clark’s M1/DC soon appeared in the H10-30 headset, significantly elevating the quality of communication for the cockpit. Later electret microphones (also borrowed from professional recording) would push the quality much higher and further increase signal levels.

The microphone "design standard" remained modeled after a carbon mic element, but amplified dynamic microphones differed from each other in significant ways, especially in characteristic impedance, required excitation current and voltage, and output level. Pilots often would discover eerie incompatibilities between their radios, intercoms and audio panels, and their headsets and microphones–all approved items by themselves. Just as headset impedance was now typically much lower than the design standard, amplified dynamic mics had their own strange quirks and sometimes needed more excitation voltage or current than the radio or intercom supplied.

They also had a brand-new issue: adjustable output level. This would introduce many operational problems to the cockpit, as the various microphone levels were adjusted without regard for the downstream radio implications.

Unfortunately, at this point, the supposedly better "peripherals" ran into the now firmly entrenched poor practices of the civil world. Bad audio interconnect, poor system grounding and poor bus practices all worked in tandem to deliver poor over-all audio quality. Flight crews became annoyed because the much better fidelity of the headsets and mics made every unwanted noise fully and exquisitely audible.

Noise remained an issue for pilots. This led audio system manufacturers to rethink the system interconnects and create better grounding techniques, especially for the mic jacks. Intercoms not holding a Technical Standard Order (TSO)–and some early TSO’d intercoms–attempted to improve noise performance by simply bypassing the mic audio with fairly large capacitors to control cabin noise pickup. This tended to create a muffled audio, rich in low frequencies but lacking the consonants’ high-frequency components. This was not ideal and it often reduced the intelligibility of outgoing transmissions. But it sounded quieter in the cockpit, so pilots were happier.

Headset makers attempted to fix the cabin noise pickup problem by incorporating active noise rejection into their headsets, as well as better acoustic shielding. Bose, the first real player in this field, was soon followed by Telex and many others. This added another layer of electronics between the person and the radio and not always with ideal results. Keep in mind, this technique still had a grounded microphone and depended on a grounded headset for power return to drive the electronics, so escape from ground loop interference was not possible on the way to the radio. Pilots heard less noise, however, reducing flight fatigue.

All of these cockpit alterations affected the pilot’s communication link with the outside world. Minor changes in AM radio modulation can seriously alter the perceived transmission intelligibility, especially as the radio nears 80 to 100 percent modulation, and distortion rises dramatically. Ground loop interference between the radio and the headset may introduce serious noise, whine or other interference that degrades the ability to understand what the pilot said.

Additional audio processing, filtering and bypassing on the microphone signal may combine badly with existing response curves in the radio to create oddly colored audio, hard to understand and indistinct. Loss of consonant edges in particular (high frequency data) makes the human voice difficult to understand.

Design or operational decisions seem good individually, but in an integrated system they may combine in the worst way for communication with the outside world. These problems are invisible when the radio is bench-tested; only live testing in the aircraft, under power, reveals the real situation. Unfortunately, such tests are rarely carried out.

Today’s cockpit combines past decisions and new technology but reflects many years of grounding and system mistakes. As a result, there is no longer any real practical standard for audio levels. A microphone may have virtually any level from 100 to 500 millivolts rms (root mean square), different acoustic efficiencies and wildly varying excitation requirements. Headset loads are significantly lower in impedance, so amplifiers have to deliver much higher currents than before, and may distort significantly. The problem is that all this eventually connects to radios that have standardized (sort of) inputs. Thus the resulting transmissions can vary dramatically, just by changing the headset. Pilots really don’t know how they sound to others or how to test the results of adjustments they make to the mic.

But it is not the pilots’ fault. It’s just the accumulated weight of history.

Walter Shawlee 2 may be reached by e-mail at walter2@sphere.bc.ca.

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