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.