Hi!
John Hess here from FilmmakerIQ.com.
In a previous weirdly controversial video “A defense of 24 frames per second” where
I defiantly proclaimed that 24 frame rate will be around as long as there exists cinema
as a motion picture medium I made quite a few statements about technology which I thought
were rather settled and understood.
Well the comments section showed me that perhaps those concepts are not necessarily so settled
or frankly understood!
So in this and some follow up IQBiTS videos I plan to re-explore some of those topics.
Today we’re going to talk about Interlacing, the history of and I’m going to demonstrate
to you right here how 60i looks a lot like 60p.
But first a word from our sponsor corner!
Thanks John, This IQBiTs video is proudly sponsored by New Vision International Film
Festival.
Based in Amsterdam, the New Vision International Film Festival is looking for new fresh talent:
indie filmmakers, directors, writers, cinematographers, animators, producers, actors and actresses.
Submit your short films, feature lengths, webseries… they even have special categories
for animation, VR and special effects.
What makes New Vision unique is how they try to strategically set up networking opportunities
for your film and connect you with other relevant industry folks that include Emmy, Grammy,
Tony winners.
So definitely check out New Vision International Film Festival - link in the description.
Back to you John.
Thanks sponsor corner John.
No thank you.
Hey Sponsor Corner John
Yes
Do you think we could win a trophy at the New Vision International Film Festival for
this special effect we’re doing?
What Special Effect?
This one - right now - where I’m talking to you
You mean this sponsor corner gag.
Yes this very special effect right here.
Not a chance, they’re looking for talent… back to you John.
I’ll let that slide and get into today’s topic.
Let’s look at the snippet from my 24 fps video that inspired today's topic.
“That’s not the only thing that delivered 60 frames per second to the viewer
There's this old thing called Television
Yes TV since it's beginning in the Early 50s has been delivering that silky
60 frames per second right into your own living room.
But it's not 60 full frames it's 60i you say.
Well yeah... although we say it's an effective 30 full frames per second
In experience it's more like 60 half frames interlaced with one and another
So grandma watching Elvis Presley on Ed Sullivan show was watching Glorious 60hz.
And because this box delivered 60hz
That we get the Soap Opera Effect every time we talk about high frame rates.
They look like cheap television because 60 hz was the frame rate of cheap television”
Of course the internet being living breathing demonstration of the Dunning-Kruger effect
had this to say:
60i is only 60 fps technically speaking.
Never mind that technically speaking is what we’re doing here.
But no, technically speaking is fields per second but anyway...
what a lot of bolloks, go back to film school!
He says 60i are 60 frames a second when anyone that worked with such antiquated technology
knows it effectively 30.
Wait… didn’t I just say that myself?
Lol, comparing 60i to 60fps… blah blah blah - he’s not even addressing the arguments
in good faith.
And my favorite one:
If you think 60i looks anything like 60p you’re delusional
Oh why didn’t I heed the advice of therapists and loved ones who warned me about the YouTube
Comments.
I guess I’m a glutton for punishment - a proverbial Don Quixote for battling juvenile
ignorance!
It’s easy to ignore the rest as the keyboard droppings of clueless idiots but when that
last comment got 195 likes it seems clear that there’s something rotten in the state
of media literacy when comes to interlacing.
Before moving forward, a few ground rules!.
My focus today is on NTSC 60i because I live and work in North America.
Our Pals in PAL land work with 50i which works the same way but with a different number.
Next when I say 60i, know that I really mean 59.94i.
I explain the fractional oddball frequency in my course the history of frame rate but
today whenever you hear industry folks say 60i they really mean 59.94 - same with 24p
which in NTSC means 23.976.
There’s no need to correct them - It’s called shorthand and there is zero perceptual
difference with this 0.1% speed reduction.
If we mean 24 we will say REAL or TRUE 24 or 24.00 or 24HZ.
I’ll get into why this still matters in a future video.
Lastly know that “I” stands interlace and p stands for progressive and NOT pixel.
1080p is not 1080 pixels… its 1080 lines which are displayed Progressively - yes I’ve
had long heated conversations with people who insist that the p stands for pixel.
It does not.
Okay - now I've preemptively addressed a good chunk of the comments this video will get
and will still get regardless… let’s talk about the original way of creating an image
on a screen electronically… the Cathode Ray Tube.
It started with this guy, Karl Ferdinand Braun who invented the Cathode Ray Tube Oscilloscope
as a scientific curiosity in 1897.
A Cathode Ray is just a stream of electrons shot from a cathode ray gun.
This stream falls on a screen coated with phosphors.
Where the electrons hit, they cause the phosphors to glow - sort of like what happens when I
fire this laser this piece of diffusion gel.
But unlike a laser, cathode rays can be bent with a magnetic field- they are after all
negatively charged electrons.
By controlling the magnetic field we can direct the beam to create shapes.
You’ve seen Oscilloscopes before - old fashioned heart rate monitors..
Now if instead of a shape, what if we draw a zig-zag pattern on the screen really fast.
I mean really really fast, so it looks like the entire screen is lit up.
If we do this then precisely control the brightness of the glow of the cathode ray as it zig zags
across the screen say by using a radio wave signal, we could conceivably create an image!
And that right there is the basis of television!
If you want to learn more, there is a channel called Technology Connections which I highly
recommend…
He goes into much more detail on the technology and history that I can here.
But back to this interlacing..
Here's the problem: early television sets in the 1930s could only produce about 200
zig zaggy lines in a 60th of a second given the limitations of radio technology.
But showing only 200 or so lines was pretty potato resolution even for those days.
You really need about 400 lines to get a decent picture.
We could take a 30th of a second and scan 400 lines but that would flicker like crazy
- we really need the refresh rate up to 50 or 60 hz to get past the critical flicker
fusion rate for our television to appear as a solid image.
Fritz Schröter in 1930 working with Telefunken in Germany and Randal C Ballard working for
RCA in the US 1932 both devised a system to solve this dilemma - interlacing.
We’ll stick to our 200 lines per 60th of a second but on each consecutive zig zag we
offset the pattern down slightly.
What does this give us?
Well first we get to keep our high refresh rate of 60 hertz but we also get more spatial
resolution - more lines in our zigzag.
The United States eventually settled on 525 interlaced lines - that’s 262.5 lines per
1/60th of a second scan whereas PAL coming a decade later has 625, 312.5 lines which
are possible using the slower clock of 1/50th a second cycle.
So to fill the entire screen we need 2 scans - that is why we say 60i is effectively 30
frames a second…
When you’re editing 60i you can’t pull apart the fields so most editing software
will call it 29.97fps.
BUT the effect really doesn’t look like 30 fps has all - What we’re really doing
is drawing in half of the screen every 1/60th of a second.
These half screens - called fields (lower-then-upper in the standard definition days) are captured
and displayed sequentially.
So what you’re seeing when you’re watching real 60i footage is one field followed by
the next field captured and displayed in the next 60th of a second.
Ignoring the obvious resolution difference, 60i is indeed 60fps
Still think I’m delusional?
Well it's not like I spent my entire childhood watching 60i and then five years of my professional
career working with 60i footage… before doing a stint with a non-profit that produced
60p content.
No, what do I know…
I’m just an old delusional fogey...
You’ve probably seen images like this on the web demonstrate an artifact of interlacing
called combing.
How can someone look at that and think 60i and 60p look alike?
I must be delusional…
Here’s the bombshell - combing doesn’t exist on CRT in normal viewing environment.
Two things: Television in the interlace days weren’t as big as they are today.
480 or so lines viewed on a 25 inch screen from 10 feet away makes it hard to differentiate
the actual interlace lines.
Those diagrams you see online are designed to show you the lines but in reality you don’t
see separate lines.
Secondly you never ever saw both fields on a CRT screen at the same time.
Enough talk!
Let's do a demonstration.
And with a snap of my finger we are now recording and playing back 60 frames per second.
Eww, I looks so smooth and soap opera-y
This Sony television set on my desk was the same one I use as my confidence monitor when
I was working with 60i.
On screen you will see a video I shot with my Sony PD-150 at 60i.
Notice how the motion looks pretty much identical to the motion in the 60 progressive stream
you are watching.
Let’s slow this down and go frame by frame in this 60 frames per second video.
Notice how in each frame the television shows a different image - just like each frame of
the 60 fps video.
There’s no combing whatsoever because each field is shown independently.
Besides the phosphors actually stop glowing after a few scan lines are complete..
Check out this video from the Slow Mo Guys when they turned their high speed cameras
on a CRT. 6 or 7 lines later, the phosphor is almost completely died out.
So when people say that young kids today are getting used to watching high-frame-rate 60fps
video and gaming… remember the fact is young kids have been watching that kind of motion
since the beginning of television some 70 years ago.
I even brought out my old NES, whacked it a couple of times to get it to start and yes…
Even gaming on this Nintendo original entertainment center from 1982 was done at 60 frames per
second - notice how each frame in this 60 fps stream contains a new image.
And the NES this by tricking the old CRT television into drawing the next field over the same
field creating a 240 line image - every 1/60th of a second.
Switching gears for a moment - Even 24 frames per second is handled quite well over a 60i
stream..
Remember you can’t actually see both fields at the same time.
So a 24 frame source would show one frame for three Fields the next frame for two fields
and alternates back and forth.
This famous chart showing the 3:2 pulldown with the two jumbled frames - it doesn’t
happen when you actually watch it on a CRT.
Like I said, you never see two fields at the same time.
I even found out that the 3:2 pulldown was done extensively throughout television history.
In fact almost every show that originated on film shot at 24 frames per second and uses
3:2 pulldown broadcast.
One of the most notable was Twilight Zone.
In the middle of the second season the show was going over budget so CBS decided to try
to cut costs by film 6 episodes on Video Tape instead of film.
Well It looks totally different and frankly wrong.
Video tape didn’t save them much money and the whole thing was deemed a failure and never
tried again.
Even condensed to 30p which are available on Netlfix - where you can see the combing
artifacts left over from the 60i master, these following episodes have a very distinctly
different look than classic Twilight Zone shot on film at 24.
Speaking of which…
>snap< ah that’s better!
Now interlacing was a wonderful idea for cathode ray tube televisions.
It gave us a faster refresh rate, better resolution and could handle 24 frames a second pretty
well.
But when the television world started moving toward Progressive displays like LCD and plasma
and OLED…
Now that we’re not using a cathode ray gun to sweep across a phosphor screen… now interlace
starts to rear its ugly ugly head.
Because of progressive technology, now you could start seeing the combing artifact!
Furthermore MPEG compression is really stifled when it comes to interlace images.
MPEG compression looks at pictures and determines what moves between frames.
Well when you have to interlace a frame in fields that can get kind of tricky and the
compressor can make all kinds of mistakes.
In the late 90s a new technique of sending Progressive Scan images over interlaced streams
was developed called Partial segmented Frames or PsF.
This was originally developed for the movie industry to get progressively scanned 24 frames
per second onto legacy interlaced equipment.
The first digital movie cameras utilized it including the Sony CineAlta that George Lucas
used for Star Wars: Episode 2.
Despite some technical objections, PsF is still a very common thing especially when
sending progressive streams over HDMI and SDI cables.
Where does 60i stand in today's ecosystem?
Well 60i is a legacy format and a lot of Television studios are still equipped with 60i Networks.
The latest over the air broadcast system ATSC 3.0 does include support for legacy 60i for
standard def and HD broadcasts but there is no interlace functions for the new 4K UHD
formats.
Chances are if you end up delivering a show to a TV station, you most likely will have
to deliver 60i even if you shot it at 30p or 24p.
So what was once a brilliant workaround that led to be a better television experience turned
into a terrible artifact on today’s progressive screens I hope this gives you a little more
insight about what interlaced is.
Today, you should simply avoid shooting interlace at all - shoot 60p or 30p if you want that
live television/video look, shoot 24p (23.976 for us NTSCers) if you want that cinematic
look.
It’s really quite that simple.
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That’s all for today friends - I’m John Hess and I’ll see you at Filmmaker IQ.com