Did you ever wonder why in North America televisions run at a frame rate of 29.97
frames per second? I mean what a ridiculous frame rate! I came across the
inconvenience of this number recently when I was making a video, and I was
trying to manually assemble some frames back to some footage, and it got me
thinking:
where did this come from? And I couldn't find a nice coherent concise explanation
online, so I had a bit of a dig into the technical details, and I thought I'd make a
video explaining how this came to be. It comes down to how these old CRT screens
used to work. At the back is a cathode ray it sends a beam of electrons
forward. Wherever they hit the screen the screen lights up, and then electromagnets
back here can steer that dot around on the screen. To produce an image, you need to
scan it across the screen. And if that dot is small enough, and if the dot is
moving fast enough, you can vary its brightness and because of the way the
human eye works, it will perceive the brightness as an image. And so as you can
see here a rapidly scanning dot is producing a picture of me, and then
another picture of me. In fact there are infinitely many of me. That is pretty
good value! The electron beam didn't actually do the whole image in one pass;
it took two passes. The first pass it would put the top row in and then every
second row all the way down: the odd positions. It would then do a second
pass and fill in the even positions. And this is what's called interlaced video.
Because of the human persistence of vision we wouldn't see two different
passes we would just see the complete frame. And in North America TV was
broadcast with 525 horizontal rows, which you may have noticed is an odd number.
Each pass of the beam would do 260 two-and-a-half rows. The weird half thing
was because of the geometry of how the beam gets back to the top, and you want
to take the same amount of time for both of the passes, so everything stays in
alignment. But that at the basics behind interlaced video. When TVs were first
built it would have made sense to do two of those passes 24 times a second, to match
what cinema movies ran at; they were 24 frames a second.
However these home appliances were plugged into the normal household
electrical supply. And in north america that's alternating current running at 60
hertz, so to make them easier to build, and to avoid interference from other
things, they use the electricity to time the scans, and because it took two scans
of the beam for every image it meant your TV was running at 30 frames a second.
What a perfectly logical, and sensible frame rate! The problem was with the
introduction of color. The 30 frames per second system was for black and white TV,
and in 1953 color TV hit the airwaves and that ruined everything.
TV in the 1950's was sent as an analog signal over radio waves. Each
TV channel was given its own spot in the electromagnetic spectrum specifically a
six megahertz window to send all of its data. Now the first quarter of megahertz
it couldn't use, because it's kind of wasteland: a buffer between channels. It
couldn't really use the next one megahertz either because it was a build-up to the
picture signal. After that you get all the interesting data about the picture
and finally four-and-a-half megahertz later you get the audio signal and then
after that another wasted quarter of megahertz of wind down. Then above that
you would get another wasteland and the next station above it; they were packed
in fairly tight. So in reality each channel didn't get six megahertz they
just got this one four-and-a-half megahertz gap to send all of the image
and audio data when color TV came along in 1953, the color data had to be put
somewhere in that four-and-a-half megahertz window, but it needed to be
positioned carefully so it didn't disrupt the pre-existing picture and
sound information. It looked like this was going to be a major problem; the
color signal did
interfere with the picture and sound signals in a way that produced visible
artifacts. It was distorting the picture and that was not acceptable.
So the technicians had to find a way to fix that. And thankfully there's a thing
called line-by-line phase reversal. And even though I don't fully understand how
that works,
I do know what the criteria are to be able to use it. And it comes down to the two
gaps: the gap between the picture frequency and the color frequency, and
the difference between color and sound. In order for line-by-line phase reversal
to hide the artifacts both of these distances had to be an odd integer
multiple of the horizontal frequency divided by two. The horizontal frequency is the
number of horizontal lines being drawn every second. We know that if you add
these two differences together you get the complete four-and-a-half megahertz
window for the entire signal. And we can now do some simplification. Well we know
if you're adding two odd numbers together you're going to get an even
number out the other side. We can move the half over there, and if you halve any
even number you're just going to get some integer. And so the moral of the
story is that we need an integer multiple of the horizontal frequency to
equal our total interval of four-and-a-half megahertz. Which is of
course just 4,500,000. Well let's see if it works. The horizontal frequency is
equal to, well every frame is 525 horizontal rows, and we're running that
at 30 frames per second. If you multiply them together we get 15,750 out the
other side. That is our horizontal frequency. We can then try dividing both
sides up here by the horizontal frequency. And we hope to get an integer
out the other side.
Very sadly, we don't. We get 285.714 and then a bunch of other digits. And the
poor engineers must be like "oh that's close, imagine.. imagine if that was 286
that would solve all of our problems!" but it's not. For that to be 286 we would need
a different horizontal frequency, and in fact we would need a horizontal
frequency of 15,734.25. And we haven't... well we would have that if... instead of a 30
frames per second rate we had a... you got it 29.97 frames per second. And so that's
what they did they adjusted the frames per second, to make this number here an
integer and remove the interference between the new color signal and the old
picture and sound signals. So there you are, North American television has a frame
rate of 29.97 frames per second because if you multiply that by the number of
horizontal rows in each frame and then you multiply that by an integer, happens
to be 286, you get out a whole number which matches exactly the frequency
window
this data is sent over. This system of broadcast is called NTSC and it was put
in place in the 1950s by the National Television Systems Committee, and so now
you know what
NTSC stands for, it stands for Not The Smartest Choice.
surely there must be a better option than 29.97, well let's have a look what
happened in Europe. Europe has PAL television that's based on a 50 Hertz
power supply, and so at two scans a frame you get a 25 frame per second rate.
PAL has more horizontal lines, than ntsc it's got 625. Whenever you have someone
going on and on about how PAL is better quality than NTSC it's because it's
got an extra 100 horizontal lines; it has technically got better resolution. And in
Europe there's a slightly bigger window to send the data on. There's actually a
full six megahertz window, just for the data that's actually sending the TV
signals. So the PAL technicians must have been thinking oh come on how
close are we gonna be to an integer multiple? And it turns out exactly 384
precisely
And you might think wow they got lucky, but in fact this was deliberate.
PAL came into place because of color television. Europe had a look at North
America went what a mess,
let's just do a new system from the ground up and make it work. And that's
why in Europe to this day we have a nice and tidy interlace standard whereas in
North America it's this ridiculousness.
The question now is was there a better option? Instead of changing the framerate
what if instead they changed the window over which the data is sent? What if they
just moved these out slightly to make these integer multiples. Unfortunately
that wasn't possible; the standards for this were immovable they were not
allowed to go outside of that four-and-a-half megahertz range.
The only other thing they could change would be the horizontal lines, and this
in my personal opinion is what they should have done. So let's say we want to
keep the frame rate at 30 frames per second, and we're going to change the
number of horizontal lines. How many are going to need? Well assuming we only want
to increase the number of lines, we don't want to decrease them and lose quality
in the new standard, and assuming we still need an odd number, so we get the
half line geometry for the beams movement, then the next compatible number
of horizontal lines about 525 is 625, with a nice multiple of 240. Yes
the NTSC standard could have been the same number of lines as PAL, we could
have had too much more compatible standards, if they had changed the
horizontal lines instead of the frequency. But they didn't, they change
the frame rate instead and we've been stuck with this ridiculous number ever since.
Although we can't be too harsh in judgment: their motivation at the time was
to make the transition as smooth as possible, and by slightly tweaking just
the frame rate, this was very backwards compatible, almost no one would notice
this change. Their theory was if they do their job correctly
no-one would be sure they had done anything at all. The final moral of the
story is just that conventions hang around for a very long time, because of
human nature
we can't have abrupt changes in technology, people need to be
transitioned from one to the next. Standards have to be continuous for some
definition of continuous, and that makes them incredibly tenacious. Now I know a
lot of people who watch my videos work in the tech sector, and you're
responsible for coming up with standards and conventions. And a lot of young
people watch these videos. You're going to come up with the conventions and
standards of the future. So all of you, please when you're coming up with new
ideas, just spare a thought that your grandkids may one day still be locked in
to the same standard. Although that said I still got my video may just by having
to deal with 29.97 frames per second. If you're curious it was the one I did with
Henry Segerman with the spherical cameras. Because we had to export all the frames,
mess with them in Python, and then put them back together. I'll put a link to
that video in the description, so you know, people come up with conventions I
guess
actually, we don't care. A convention that exists is better than something that
doesn't, so if you can bodge it together and it works,
go for it.
I mean don't worry, the people of the future
will find out a way to deal with it.
Okay, according to my YouTube statistics at this point in a video, of all the people
who've watched it only thirty percent of them are still paying attention. Most
people watch the interesting bit, and then they don't pay attention when I'm
just rambling on at the end. And so those of you who are still paying attention,
you are my people and so I have a special announcement just for you.
You're all incredibly supportive and a lot of people ask: "when am I going to set up a
patreon page?" And I've finally done it. I have set up a patreon page and this is
kind of a soft launch. I'll do a proper launch later, and I'll follow patreon
good practice by having a video about what I'll be doing. But for now I thought
I'll just mentioned it at the end of this video. If you'd like to, please do click
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