Why Old Screens Make A ᴴᶦᵍʰ ᴾᶦᵗᶜʰᵉᵈ Noise
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Why Old Screens Make A ᴴᶦᵍʰ ᴾᶦᵗᶜʰᵉᵈ Noise


Last week, I put up a video that was filmed
here at the Cambridge Centre for Computing History. And folks seemed to like the video, but a
few comments made it clear that there was a slight problem. And it was a problem that made me
really angry with myself. There was a very subtle high-pitched noise
in the background of the video, and I hadn’t noticed it before uploading. So I thought I should explain why: first, why the noise was there at all, and second, why I didn’t notice it. This museum is filled with old computers with
cathode ray tube screens: CRTs. Some of these screens are
specialised computer monitors, but others, like these over here, are basically
just old-school televisions. At the back of each monitor
is an electron gun, and it fires out a beam of electrons. Wherever they hit,
the phosphors on the screen light up. But the electron gun doesn’t move at all. It couldn’t possibly move fast enough. Instead, it just fires in one direction, and electromagnets bend the beam of electrons. The more power that’s sent to the magnets,
the more the beam bends. So for every frame of video,
that beam scans the whole screen, left to right, top to bottom. I’m going to ignore the vertical movement, what we’re interested in is the horizontal. Inside there is a clever bit of kit called
a flyback transformer. It outputs a sawtooth signal to the electromagnet, which means that it ramps up, and then drops
very quickly back down. And as it ramps up, the electromagnet bends
the beam across the screen before it quickly snaps straight back ready
for the next line. That electron beam scans across 625 lines
of the TV signal, 25 times a second, so that flyback transformer needs to
ramp up and drop down 15,625 times a second. And because of something called magnetostriction, it changes shape a tiny, tiny, tiny amount
every time it does that, which generates a noise
at that high-pitched frequency. It’s the same frequency that
every old-school TV produces. At least in Europe. The numbers in America are a little different, but they still end up at about the same frequency. Now, with modern audio editing tools it is really easy to isolate that frequency
and remove it. Until now, I’ve been removing it
from this video, but let me tell you: it’s here, and in this room,
with all these monitors on, it is loud. Now I knew the sound was recorded originally, because I could hear it in this room, I knew
my microphone was picking it up. I just thought the automatic noise reduction
stuff I use would take care of it. It didn’t: but it also turns out these headphones
that I use to check audio aren’t great at high frequencies, and so with my voice drowning it out I just
didn’t notice the sound. There aren’t many quiet bits in that video,
I don’t shut up. And for a while, I was angry with myself,
because it was an unforced error. I wasn’t under time pressure,
I checked through half a dozen times, I just didn’t think to check the sound
with my eyes. That mistake might have stopped the message
getting as far as it could. But I think there was another reason
that I was angry. Spider Robinson once wrote that
“anger is fear in disguise”, and I’m not sure that’s always true, but… it wasn’t just my headphones. As you get older, your range of hearing
starts to narrow. It starts to get more difficult
to hear high frequencies. And I used to pride myself
on how good my hearing was, I could hear higher frequencies than I was
supposed to for my age. Those 17,000Hz mosquito alarms designed to
make teenagers go away, they irritated me through my twenties. And, sure, I can hear the loud, high-pitched
noise here, ‘cos it’s really obvious, but in that video? Nope. I have to strain to even notice it, even as
some folks in the comments said it was loud enough to stop them watching. I did a hearing test. I can’t hear mosquito alarms any more. I can’t hear anything higher than these monitors. That video, that mistake I made, was a sudden and sharp reminder
that I’m getting older. That some of the things I used to do,
I can’t any more. So protect your hearing, and next time someone forgets to check for a high pitched noise in the background and
it annoys you… well, at least you can still hear it.

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