Why is TV 29.97 frames per second?

Why is TV 29.97 frames per second?


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
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Author:

100 thoughts on “Why is TV 29.97 frames per second?”

  • Johnny Quest says:

    That 1 MHz wasted spectrum at the low end of the channel is not wasted. It's necessary. If it weren't there you'd have SSB … and you'd have to have an accurate carrier injection at the receiver to recover the video signal. That's expensive. And pointless.

  • Johnny Quest says:

    You're talking about PAL. And you're talking about American TV. Those two things don't go together. America never used PAL.

  • Couldn't choose a sub so this'll work
    https://chrome.google.com/webstore/detail/threelly-ai-for-youtube/dfohlnjmjiipcppekkbhbabjbnikkibo

  • My video tech career goes back to the mid 1970's. As a teacher and mentor I've often explained the 29.97 fps mystery to students of digital media,, but always from the aspect of backwards compatibility with 30fps televisions (-; so consumers could continue using their existing hardware, back when tech corp. profiteering through planned obsolescence, walled gardens and forced migration was limited by proper regulation and standards enforcement ;-). This explanation from the parallel concern of broadcast bandwidth allocations was enlightening and appreciated (as with most all your YouTube offerings)!. Still, I find the engineering from the hardware signal and CRT compatibility side even more amazing, The final compromises, however, meant that accurate color reproduction in NTSC was very problematic compared to the European standards, thus, for many in the U.S. television industry, the NTSC acronym stood for "Never Twice the Same Color", while PAL was considered "Paradise At Last". The engineering of producing "moving" images from a single amplitude modulated signal will remain genius to me in any case.

  • Anugerah Rompis says:

    But if 30 fps was initially chosen to match the AC frequency to make engineering TV sets easy, wouldn't reducing it to 29.97 fps cause problems in earlier TVs?

  • HappyBeezerStudios - by Lord_Mogul says:

    Couldn't they just reduce the used frequency slightly? As in, leaving a bit of the 4.5 MHz to do nothing.
    Like how 527 lines with an iteger of 248 would result in a 4,490,040 Hz signal, leaving 0.00996 MHz or less than 0.3% difference empty.
    The additional lines would dissapear into overscan anyway and the miniscule amount of not used frequency could easily fall into manufacturing accuracy on the older B&W sets.

  • I am gonna forward people to this video next time I get asked what's the difference between 50/60Hz on Console games 🙂 Thanks

  • Do these standards still apply to HDTV? I know there is NTSC and PAL TVs but now they have way more resolution and I would’ve though more then 29.97 frames per second. Our digital TV broadcast standards here now either come in 720p(maybe I) and 1080p (or i?). Would the old SD tv NTSC standard still be effecting the newer HDTV broadcasts till today?

  • NTSC in the 50s: tweak this framerate down a notch for interlanced backwards compat

    Tech corps in the 21st century: yeah so we're gonna progressively scan everything at multiples of 60Hz; suck it

    144Hz monitor manufacturers: hold my beer

    165Hz monitor manufacturers: no, hold my beer

  • Patrick Rannou says:

    Very interesting video.

    One thing which weighted on the reason they did not want to change the number of lines:

    Back then. things were built to last (at least, to last much more than now) and the rate of change/progress was much lower.

    They had to respect all the people that were already owning a B&W TV, and that would not buy a color set before many years later.

    This meant the new signal had to be compatible. The 525 lines had to be respected. They could not simply say:

    "Hey, all of you with B&W TV owners, from now on, our TV signals will have too many lines for your TV sets, so throw them in the garbage and buy new ones!"

    Heck, they couldn't even change the number of lines lines to 524 or 526, either. That would mean that the slightly-too-small image (524 lines) would not reach physical line 525, the "half a line of zapping back to to of screen" signal would be confused with the the data for line 525 (making that line look like ugly junk), and the line 1 data of the next image would be interpreted as "jump back signal) (and skipped), and the all with a half-line delay desynch. Ouch. So you'd get 2 overlapped images offset by half the width of the screen, and that also slowly scroll up the screen every 17.5 seconds. Wow. Talk about looking at pure garbage! Same for 526 lines, but scrolling down instead. Yeepee.

    Basically, all the stores TV inventories would be ruined, or they warn well in advance and B&W TV sales would drop drastically in the couple years before the swap. You can easily see the huge uproar that that would have caused.

    Nowadays however, things are different, We're used to "use and throw away" and a much faster pace of change. When TV went digital, all analog TV owner got told (YEARS in advance): either buy a new TV with digital capability, or buy a 100$ separate special digital-to-analog TV Tuner & converter, that will convert the digital signals back into analog for your old TV. For years people bought TVs that had both digital and analog inside, until the actual signal swap which forced only those relatively few with really old TVs to either buy a new one or buy the special tuner. all in all a relatively painless migration. (well, that was when I stopped using my 10 years old TV lol to watch TV, only for my playstation and video cassettes, until the internet got me too).

    Some grumblings, but the transition went without too much of a itch. Even then, it took years and was planned for a very long time.
    Still, a 100$ tuner is still costly, and such a tuner/converter is a relatively simple piece of electronics. But much less than a brand new TV.

    But that kind of thing would have been impossible to think of back then. Can you imagine taking a 625 lines image, and having a "tuner re-parser" that rebuilds a 525 frame image "just for your old B&W TV". Skipping some lines? That thing would have need of a special "buffer" and been quite complex and would have overall cost much more proportionately to buying a new TV anyway. Many old TVs had straight out antennas jutting directly out of them, without "cable input". Because that was entirely before Cable TV! In fact, a TV back then looked more like a radio that also has a screen!

    So, there was no capability to "add a tuner". People also had the tendency to use their TV set not for a mere 5 to 10 years, but fully expected it to last at least 20 or more. Same as refrigerators, truly: refrigerators of that era didn't have all the bells and whistles, but those things are seemingly invincible and eternal, while today's refrigerators will last you about 8 years. If you're lucky. so they couldn't just

    Also, If a device able to grab in radio waves and convert them back to the old format for your old TV existed, that would flow the airwaves with way too much interference, too. And keeping separate frequency spans for B&W and for color would also have been wasteful. There was just no nice little cable input for a converter to be designed to attach to the TV.

    So it was the signal itself that just had to be fully compatible.

    Changing the number of lines was completely impossible for economical reasons, while very slightly changing the frequency was in fact possible, and the old tvs would still work ok with the new airwave signal.

    It's the initial 525 lines which was a bad choice. Or the assigning of a 4.5 Mhz wide airwave band. But to their defense it is extremely hard to predict in advance how tech will need to change. And I'm quite sure there was a strong reason why "525" was the number of lines initially chosen.

  • In your solution to the NSTC problem you simply picked a lower integer factor of 240, with 625Hlines and 30 FPS. However, if you can just arbirarity modify that integer, why not go for a smaller factor such as 120 and get 1250 Hlines? I'm sure there are some consequences to altering that factor, which I'm sure you know too, so please explain why you can't simply pick 120 as the integer factor?

  • I love how you used an altered CRT to present your points visually as opposed to mainly graphics in post-production. You really don't see this amount of creativity on a regular basis.

  • Increasing the number of horizontal lines wouldn't have increased the bandwith needed for the transmission?
    Just wondering

  • I assume they could've reduced the 4.5MHz, and just not used their entire allotment. Slightly less bandwidth, but cleaner. Would that adjustment have killed backwards compatibility for existing black and white devices?

  • Michael John Little says:

    NTSC: Never The Same Colour
    SECAM: System Entirely Contrary to the American Method
    PAL: Peace At Last.

    What about the French SECAM system?

  • Michael John Little says:

    07:16… would you get a beat frequency between the line frequency of 60 Hz and the 29.97 frame rate frequency?

  • 4:36 He doesn't fully understound how line by line reversal works:

    https://www.youtube.com/watch?v=3GJUM6pCpew&feature=youtu.be&t=281

  • I have a 29.97fps movie. When I burn it to dvd, which automatically converts it to 25fps, (I can't alter that), my movie comes out slightly jerky! What do I need to do to the movie to make the motion come out onto dvd, looking smooth, please?

  • The Cando Railfan says:

    I wish they just did progressive colour from the beginning, instead of starting with interlaced black and white. Or flat out ignored compatibility.

  • Nikolay Rybkin says:

    Please don't tell that framerate was determined by AC frequency, it's ridiculous. VSync triggers next frame rendering.

  • Why is not possible to change with our digital smart TV's today, Why cannot the US change now change that most of our systems are streamed and non streamed because of fully digital media

  • Why would someone make a video about TV frame rates and post it on Youtube?

    Why would I then watch it to the end?

    Who is the bigger nerd?

  • Next, can you do a video on why hotdog packages don’t match the number of buns in bun packages? I imagine it has something to do with the different resonate frequencies of the two foods.

  • тупые американцы до сих пор меряют в инчах, наверное удобно складывать 2/3 и 5/8. идиоты

  • How would it be if companies changed standards at will? They would be able to obsolete your equipment so you have to buy new. Very convenient when a company wants to exploit its users for mo' money, mo' money, mo' money. Like when a website keeps "suggesting" you update your browser to avoid interruptions with a version that won't work with your machine. You have to buy a new one. Particularly suspicious is the case when the site doesn't serve anything different from what other sites serve with no interruptions, and no suggestions you upgrade.

  • millimetreperfect says:

    PAL has better quality no just because of the 625 lines but because of the ‘Phase alternate Line’ to lock the colour signal to the monochrome signal where the colour signal flipped every line and the receiver anticipated this therefore it knew exactly where the beginning of each new line was, NTSC did not do this so the colour drifted. ‘Steam’ tv was brilliantly engineered as the b&w reverse compatibility was designed in from the beginning.

  • changing horizontal lines would not have been backward compatible and you know it
    only a complete idiot would do that..
    Americans invented the t.v. and the transistor and the electrical transmission system you snippy little schoolgirl.
    bodge that together..

  • PAL was released in 1963,a full 10 years later, after NTSC, so of cource they can make improvements. The Key to NTSC was it needed to work with 100 million B&W televisions in existence and with the new color tv standards, it worked and it was BRILLIANT! NTSC was also racing against a mechanical color wheel system being pushed by Columbia (CBS Labs). so they had to make a working prototype very quickly before the industry was saddled with a mechanical system.

  • I think a lot of design decisions you criticise were made to keep the signal compatible with existing black and white tvs

  • I know I'm a bit late to the party, but I just wanna say that I love what you did with the visuals on the CRT throughout the video. It was all lined up perfectly and helped explain your point very well!

  • Why give a hoot? A machine does the working out, so 29.97 is as good as 28.87, 30.00, or 30.26. It makes no difference, nobody needs to care … colour TV was a superb technical achievement.

  • Why are there so many 9's in life?
    29.97 adds up to 9
    60hz is a 9 upside down.
    360p adds up to nine.
    720p same.
    1080p same
    1440p same
    Now 144hz is popular… You guessed it… Adds to 9.

    We are up to the i9 CPU
    Windows never made windows 9
    iPhone skipped over the 9

    9 lives in a cat
    Cloud 9
    The whole 9 yards
    9 months in pregnancy.
    We start work at 9. With an hour lunch break… We get out at 6 (upside down 9) which is also 1800 hours military time (adds to 9)
    A yard is 3 feet (36 inches) adds to 9.
    6 feet tall is 72 inches. Adds to 9.
    72 is the typical temperature people put on their ac's.
    360 degrees in a circle.

    The number 9 itself is like a swirl. That swirl is the shape of the milky way galaxy.
    Also the shape of tornados from an above view.
    Also of a toilette flushing. Also of your ear. Also of the Fibonacci spiral.

    Your Mead workbook had a times table. Why did they always end it at 12×12? Answer is 144… That equals 9

    It's also the only number besides 0 that you can add to a group of numbers and reduce by adding those numbers down and still get the same answer…

    1+1=2.
    1+1+9= 11… Which still equals 2 when you add the 1's together in your answer of 11.

    Wtf is going on here man?
    Is there some shit being hidden from us?

    And how come binary computer (doubling circuit) code never shows any 3's 6's or 9's (3+6=9)

    Example…
    1 doubled… 2
    2 doubled… 4
    4 doubled… 8
    8 doubled.. 16 (equals 7)
    16… 32 (5)
    32… 64 (10… 1)
    64… 128 (11… 2)
    So basically you get a repeat of the following sequence infinitely:
    124875
    No 3's. No 6's. No 9's. No 0's.

    W.T.F. !

  • I’d also like to point out that NTSC was backward compatible w black and white TV whereas when you switched to PAL the black and white TVs were effectively bricked.

  • I love that at the end of the video the image on the screen has a visibly moving scan line because you recorded a 29.97FPS TV at 30FPS (used by Youtube and most modern video recording equipment but they also have a 25FPS option in these cameras as they are now often sold internationally, on a side note it's also why some lights flicker when filmed because of the camera being set to the wrong frame rate OR the lights are running through a cheap transformer that alters the Hz of the light to the standard in the country where the unit was manufactured).

  • 24 fps is quite literally a terrible framerate for youtube. it does not scale up to 60 hz monitors well at all.
    interlaced video is the DEVIL

  • Did they also change the power grid frequency to 59.94Hz? Or did TV sets that used 60Hz go out of sync with the new signal?

  • 01:40 that is a lot of tv’s

    If you dont get it,
    in the 1st tv, there is a tv
    in the 2nd tv, there is another tv
    in the 3rd tv, there is a 3rd tv
    in the 1562nd tv, there is a 1563rd tv

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