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 the link in the description; go and check
it out if you’re not familiar with Patreon. You can support me a bit like you would
a kickstarter but it’s ongoing. The idea is people who can afford it donate money so I can do these videos
better, and in return I have all sorts of rewards. So have a look at it, give me
some feedback, let me know if there are thing’s there you do want things there
you don’t want. I’m going to have fun making a bit of extra content for
Patreon. In fact I’m going to do a behind-the-scenes video of this video
I’ve just made, because you wouldn’t believe the ridiculous tech around me it
took to make this all work, and because I haven’t got any Patreon supporters yet
I’m just going to put it on my Patreon channel and anyone can see it. So do go
check out my patreon, the link is at the very top of the description
unsurprisingly. I would love to make more videos and do them better and so your
support is hugely appreciated, so go check it out and I’d love some feedback on
it before I do the proper launch, let me know if the rewards are what you want, or
if there’s anything else I can add in there for you. I really appreciate you all
supporting these videos.