A couple of years back, I learned that I have a superpower. Turns out this little oddity is common among people with ADD, though as far as I know I do not have that disorder. What was actually surprising to me was not that I had this power, but that most people did not.
I can hear (or perhaps "hear") a cathode ray tube in operation. There is a very quiet high pitched whine made by monitors and televisions in operation. Most of the time I am unaware of it. But whenever a computer is turned off I can hear the monitor until it is turned off, whenever a monitor is turned off it makes quite a bit of noise, and if I wake up in the middle of the night and a television is going in another room I am almost always well aware of it even if it is completely silent (playing a DVD player main screen, for example) and before I am in a positio to see light from it.
I've known this for a while. Somewhere I have some links to a slashdot discussion on the phenomenon.
But just now I made quite a discovery. My previous monitor at work responded strangely when the general color composition of the screen changed: switching from a screen that was mostly white (text document on a black background) to a screen that was mostly dark (selecting all the text on that page) would actually resize the visible image. Really weird.
That monitor went nuts a couple of months ago and I appropriated the monitor of a departing colleague. So today I'm working late in a very quiet office and as I was selecting and deselecting a couple of paragraphs, I noted a cricket chirping. Then it fell strangely silent. A minute later I performed a selection, and the cricket chirped again. I alternated between ctrl-A and clicking my mouse to select and deselect, and realized that the "cricket chirp" is my monitor. This one doesn't resize anything when the color composition changes, but apparently the monitor sound gets louder and/or changes pitch.
Yep, known phenomenon. Humans have varying upper acustic frequency range limits, with some people’s ears going up as far as 20.5KHz, whereas on average people top out around 16.5KHz.
Now, the way CRTs work is that there’s a gun in the back end of the tube which produces a ray of electrons, and there’s an electromagnet which deflects the beam to controls where it hits the phosphor coating of the screen. The electromagnet’s field is controled by a sawtooth generator, so that the electron beam constantly scans horizontally across the screen.
The horizontal scan frequency is generally on the order of tens of KHz; nowadays, generally around 80. That’s way above human-audible frequencies, but due to aliasing, it can still be audible to a human as a lower frequency with lower intensity.
I could hear my old 15” monitor, and my even older 14” was clearly perceptible. My current 17” with its high refresh rate is inaudible to me. Likewise I used to be able to tell whether someone nearby was watching TV, sometimes through a wall, before 100Hz screens became common. I remember the TV my parents had when I was little – back when TV screens generally ran at nominal signal frequency, instead of some multiple as they do today, so the horizontal scan frequency was around 16.7KHz, and almost everyone could hear them. That thing would emit a constant high-pitched whistle and would audibly fwinch and crackle when the picture cut from one scene to another and the brightness changed abruptly. (Well, the crackle was partly due to static, of which that thing built up a lot – you could collect enough charge from the screen to zap someone painfully.)
The principle behind CRTs also explains the pumping image that you saw happen between bright vs dark images. Old CRTs used to be really bad about this. It is because the electron beam needs to be more intense to project brighter images, which affects the effect of the deflector field. Modern monitors are much better about regulating the field’s strength – and I suppose that’s the chirping that you heard: the sound of a monitor cranking the deflector field up and down to react to sudden brightness changes.
Re:
jdavidb on 2005-10-10T04:44:32
Cool. I appreciate all the additional information. Sounds like it may not be related to ADD at all.
I remember being able to shock someone after touching our old televisions! I had forgotten about that.
