Natural Opinions

An Archive Of My Thoughts

Why does this even exist?

A fair question.  In an ideal world interlaced scanning would not exist at all.  However a problem existed in the early days of CRT TVs, a problem which interlacing solved.  Because of how CRTs display an image (by charging phosphorous compounds underneath the screen) the image on the screen quickly fades if it is not constantly and rapidly refreshed.  Therefore if the refresh rate (the number of times per second that the CRT scans across the screen) is too low it causes very noticeable and very annoying flickering.  Labratories quickly figured out that we needed a refresh rate of about 48Hz or higher to avoid the noticeable flickering.  We therefore settled on 60Hz.  Why 60Hz?  Because AC electrical outlets in north america run at 60Hz, and it made the circuits for the CRT display a lot simpler to use the same frequency.

Now here is the problem.  While we needed the displays to run at 60Hz to avoid flickering CRTs that could display 60fps were difficult to make and extremely expensive at the time.  Therefore to make CRTs cheap enough for personal use without lowering the refresh rate we created interlacing.  With interlaced scanning the display still scans over the screen 60 times per second, but it only fills in the even or odd lines at each refresh instead of an entire image, which makes the CRT design a lot simpler and cheaper.  Since these CRT TVs were designed to use a 60i signal we needed everything else to use 60i in order to be hooked up to them.  And once 60i became the standard for everything it was very hard to get rid of until new display technologies emerged which made progressive displays cheap enough for personal use.

3:2 pulldown (telecine) and inverse telecine

3:2 pulldown (also called telecine):  Even frames become 3 fields (even, odd, even), odd frames become 2 fields (odd, even).  It turns 24p into 60i.

Inverse telecine (also called pullup):  Reverses the process to turn 60i back into 24p.  Takes each group of 5 fields, combines the first 2 into frame 1, discards the 3rd field, combines fields 4 and 5 into frame 2.

Converting content

24p: 
-Output as is (24p)
-Duplicate every 4th frame to get 30p
-Use 3:2 pulldown to get 60i
-Use 3:2 duplication to get 60p

30p
-Output as is (30p)
-Use 2:2 pulldown to get 60i
-Duplicate every frame to get 60p

60i
-Output as is (60i)
-Use deinterlacing to get 30p or 60p, 30p frames will be refreshed twice by the display to get 60Hz
-Use inverse telecine (also called 3:2 pullup) to get 24p, only do this with 24p content that was converted to 60i (movies)

60p
-Output as is (60p)
-Discard odd frames to get 30p
-Discard odd lines on even frames and discard even lines on odd frames (1:1 pulldown) to get 60i

Sources of content

  1. Physical media (content has already been made and is stored on physical media)
    1. Movies (VHS, DVD, BD)
    2. Home videos (VHS, DVD, BD)
  2. Real-time source (content is being generated/rendered in real time to sent to the display)
    1. Video game consoles
    2. PC and portable computers
  3. Television broadcasting (cable, satellite, over-the-air)

All three used to be 60i only due to CRT displays being designed for a 60i signal.  Cameras for TV and home videos shot interlaced content.  VHS tapes and television broadcasts were interlaced.  Movies were shot with progressive cameras and were shown at 24p in theaters with projectors.  But they were converted to 60i using 3:2 pulldown for VHS tapes so that you could watch them on your 60i TV at home.    Video game consoles and PCs rendered frames progressively but scanning them out using an interlaced RAMDAC to produce a 60i signal.

1.  Video games are an interesting topic because video game consoles almost always render the frames in a progressive way (meaning they create progressive content) yet for a long time they used interlaced scanning to output them so that they could be used with televisions.  6th generation video game consoles (sega dreamcast, microsoft xbox, sony ps2, and nintendo gamecube) were the first video game consoles to support both interlaced and progressive scan output.  So video game consoles transitioned to progressive scan as they entered the new millennium.  Video game consoles still support both interlaced and progressive scan.

2.  PCs also render progressively.  PC monitors have almost always been progressive scan.  But when PCs first emerged in the late 70s and early 80s PC monitors were far too expensive for most users so most users used a 60i TV instead.  PC manufacturers realized this and made both interlaced and progressive output standard so that users could choose.  By the late 80s monitors were affordable and the PC began transitioning to progressive scan.  Today PCs only support progressive scan (unless you have special equipment or older hardware).

3.  Movies have always been shot using progressive cameras.  Yet just like PCs and video game consoles they had to be converted to interlaced in order to be used with 60i TVs.  In the late 1990s DVDs began to replace VHS.  VHS only supported 60i content and therefore all movies had to be converted to 60i using 3:2 pulldown before they could be put on a VHS tape.  But DVDs supported 24p, 30p, and 60i content.  Movies were therefore stored at 24p on DVDs and DVD players supported both interlaced and progressive output.  The same is true of BD and BD players.

4.  Progressive camcorders for home video also began to take off in the late 1990s.  This made it possible to record progressive home videos and store them on DVDs.  Progressive camcorders could often record content in either progressive (30p) or interlaced (60i) mode depending on which you preferred.  Interlaced would produce smoother video since the shutter fires 60 times per second instead of 30 times per second but it would have combing.  Todays video cameras can do 24p, 30p, 60i, and 60p but keep in mind that the DVD standard does not support 60p.

5.  TV broadcasts took the longest to make the transition.  The transition of TV broadcasts to progressive scan, digital broadcasting, and HD broadcasting all happened at around the same time in 2002-2007.  This is because progressive scan and HD broadcasting were practically impossible until the switch to digital broadcasting was made.  Today interlaced and progressive TV broadcasts are both common depending on the channel.

6.  Of course progressive content is useless without a progressive display system to show it on.  The various sources of content listed above did not begin transitioning to support progressive scan until progressive displays were starting to become common.  PC monitors almost always supported progressive scan even in the 80s but progressive TVs didn't start to become common until the turn of the millennium, which is why it took so long for everything else to make the transition.  This is because even though PC monitors and TVs were both based off of CRT technology PC monitors were much smaller in screen size, which made it a lot easier and cheaper to implement support for progressive scanning.  Building a large CRT that can do 60p was extremely difficult. 


Today most PC monitors and TVs are LCDs, which are a type of progressive display.  Because LCDs can only display progressive content they need to have a deinterlacer built in in order to accept an interlaced signal.  The deinterlacer will take the interlaced signal and change it into a progressive signal that the display can display.  All HDTVs have a built in deinterlacer.  Modern PC monitors do not have a built in deinterlacer, they can only accept progressive signals as input.

How do I know?

Simple.  Look at the cable being used.  Here in north america we have:
Composite:  interlaced only
S-Video:  interlaced only
Component:  interlaced or progressive
HDMI:  interlace or progressive
VGA (PC):  progressive only
DVI (PC):  progressive only
Displayport (PC):  interlaced or progressive
RF Coaxial:  interlaced or progressive

What if I'm using a cable that support both interlaced and progressive signals? 
Check the source.  A cable carries a signal/information from one device to another.  The source device whether it be a DVD player, video game console, or whatever will have a setting somewhere that determines whether it operates in interlaced or progressive output mode. 

If it's a TV channel you should be able to hit the info. or guide button on your remote to see if the channel is broadcasting an interlaced or progressive signal. 

If the device does not have a progressive option anywhere then it is safe to assume that it uses interlaced scanning.

Questions to ask

1.  How was the content captured?  Progressive or interlaced (cameras can record progressive or interlaced content, real time sources are usually progressive)

2.  How was the content stored?  Progressive or interlaced (for example DVDs support storing progressive content, but VHS does not)

3.  Does the content player support progressive scan? (DVD player, BD player, video game console, cablebox, etc.)

4.  Does the cable you are using support progressive scan?

5.  Does the display you are using support progressive scan? (some CRT TVs support progressive scan but most don't, CRT PC monitors do, LCD/plasma/DLP/projector all do)

6.  Does the display you are using support interlaced scan? (PC monitors don't, but everything else does)

7.  If all of the above checks out is the content player set to progressive or interlaced output? (most content players will be set to interlaced output by default, sometimes you need to change the settings)

Common deinterlacing filters


Description
Output
Fluid motion
Interlacing artifacts
Still sharpness
Motion Sharpness
Complexity
None
Each frame consists of the odd and even field
30p
Moderate
Yes
Sharp
Sharp
Simple
Blend
Blend each pair of odd/even fields together
30p
Moderate
No
Slightly blurry
Slightly blurry
Simple
Area
Blend only the mice teeth (only during fast motion), otherwise do nothing
30p
Moderate
No
Sharp
Slightly blurry
Complex
Discard
Discard every odd field, stretch height of even fields to 2x
30p
No
No
Very blurry
Very Blurry
Simple
Bob/progressive scan
Stretch each field to 2x height
60p
Super
No
Very blurry
Very blurry
Simple
Smart interlace
Bob during fast motion, otherwise do nothing and duplicate frames
60p
Super
No
Sharp
Very blurry
Complex
Line interpolation
Every blank line is filled by combining the lines above and below it
60p
Super
No
Slightly blurry
Slightly blurry
Simple
Motion compensation
Crop and scale each line based on speed of motion
60p
Super
No
Very slightly blurry
Very slightly blurry
Complex