Comm Radio Distortion and Clipping

[gryph0n]

Hello Friends,

I wanted to recount a technical issue I had with my communications radio recently- when depressing the yoke-mounted push to talk switch, I would hear a lot of engine noise in the headphone sideband. This would happen especially on my radio call at crosswind, or during climb out. Sometimes, my voice got very distorted and would cut in and out intermittently, so other pilots couldn't hear me.

Originally, I assumed this was ignition noise coming from an unshielded p lead or from the mag harness, however reception on the radio was loud and clear. I thought it could be a faulty connection on the ptt, however the problem went away when I was on final or while taxiing.

I took the aircraft to Carl Ahlquist at Three Crowns Avionics in Sussex, NJ and he was able to determine the cause: the microphone gain on the comm radio had been set way too high. Microphone gain (or mic gain as its usually referred to) is the volume level setting for sound going in to the microphone, as opposed to volume, which is the level for sound coming out of the headphones. If mic gain is set too high, regardless of headphone volume, the microphone pulls in a very high amount of sound, so high that the amplitude of the sound waves exceeds the range that the comm radio can process, and the peaks of the waves are "clipped" off. This was resulting in the transmission cutting in and out. The high mic gain setting was pulling in a lot of engine noise from the cockpit, and when I added my radio calls to the racket, the radio couldn't handle it.

Reducing the mic gain to almost zero solved the problem. Carl was amazed at how noisy the cockpits of our little airplanes are.

Steve S

[6352]

I had a very similar issue with my com (a Garmin SL40). Mic gain is user adjustable and was set too high. It took me quite a bit of fiddling with settings to get it to work well in the C140 cockpit.