Tuesday, February 10, 2009

Squelch

This is a picture after I took my sister's family for a ride in a Cessna 172. From left is Emmett, Eli and then me (and it's a few years old). I learned to fly helicopters in the military and transitioned to multiengine fixed wing after getting only 30 flight hours in a 182. I did most of my flying in Germany, where all airspace is controlled, or in the US, always under an IFR flight plan. So, for me, flying a single engine airplane under VFR, with my family on board no less, was a little, shall we say, stressful?


Anyway, I include the picture because I wanted to talk about "Squelch." Say it enough times and it just sounds weird. Emmett, when he was young, was obsessed about my blush. So every time I'd visit, he'd watch me putting my makeup on and just keep saying, "Bluuuush." A tenuous link, but there it is.


Squelch has always been to me just a "SQ" button on airplane communications radios. If I tried to listen on a frequency of a transmitter that was some distance away, I could press this button and possibly hear earlier (assuming I'm traveling to it) but the frequency would have static as well. This comes in handy, btw, if you switch to a different radio and you want to make sure it's working and the volume is set adequately - press the squelch switch, hear a burst of static and then press switch again.


So, what am I doing, really? Here is the definition of carrier squelch from Wikipedia.

A simple carrier squelch or noise squelch operates strictly on the signal strength, such as when a television mutes the audio or blanks the video on "empty" channels, or when a walkie talkie mutes the audio when no signal is present. In some designs, the squelch threshold is preset. For example, television squelch settings are usually preset.

In devices such as radiotelephones (also known as two-way radios), the squelch can be adjusted with a knob, others have push buttons. This setting adjusts the threshold at which signals will open the audio channel. Backing off the control will turn on the audio, and the operator will hear white noise if there is no signal present. The usual operation is to adjust the control until the channel just shuts off - then only a small threshold signal is needed to turn on the speaker. However, if a weak signal is annoying, the operator can adjust the squelch to open only when stronger signals are received. If you hold the squelch open you will also get a lot of noise.



I am not kidding when it took me three times to understand all that. So, let me sum up: when I tune in a random frequency where nothing is being transmitted, I would just hear static - same as you would if you selected channel 3 on your TV and there's no Channel 3 TV station in your area (there may be one in the next town over - but not close enough for you to receive it). The TV blocks that static so you don't have to listen to it if you so chose to keep the tv on that channel.

The comm (short for communication) radios in my airplane do me a favor by shutting this static noise out as well when it "knows" there's nothing to hear. If I know that I just might be close enough to hear the station (because, let's face it, who's smarter? Me or the radio?). I can disable this feature so the radio will let me hear whatever's there. If I'm lucky, I'll hear the weather at my destination farther out than I would if I waited until I could hear it without adjusting the preset threshold.

3 comments:

  1. Greetings,

    Nice bonding with your sister. I guess it was stressful as long as you've enjoyed. Keep bonding to your sister as it will create a healthy relationship...

    ReplyDelete
  2. Anonymous1:44 PM

    nice answer

    ReplyDelete
  3. Anonymous11:58 AM

    Thanks for helping me understand this. I have been flying for two years without ever fully knowing what the squelch knob did.

    ReplyDelete