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All the Perl that's Practical to Extract and Report

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  • 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.

    • 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.

      --
      J. David works really hard, has a passion for writing good software, and knows many of the world's best Perl programmers