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.
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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.
Thanks again to our sponsor New Visions International
Film Festival - check out their site and submit
your film.
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That’s all for today friends - I’m John
Hess and I’ll see you at Filmmaker IQ.com
