P vs I


http://www.tech-notes.tv/Jim/Articles/24_Frame.html January 29, 1997


what’s the big hubbub over bandwidth to display 1080p then? same amount of information…

This whole interlaced vs progressive thing is left over from the early computer displays where the progressive displays eliminated flicker. The discussion has a whole lot more meaning if you are talking about a crt display where the picture is actually scanned at 60hz versus being scanned at much higher rates. Also the progressive elimination of motion blur is real when compared at the same resolution and the early computer graphic games would update the video memory at much faster frame rates that the crt scan rates. Once something like the progressive vs interlaced gets started, there are a lot of people that will jump on the bandwagon. It is like propaganda. Mix a bit of truth in with fiction or implications and it all sounds like it is true.

So you are right if you are speaking of film transfers where the two fields of an interlaced frame is from the same film frame, then the resulting frame after 1/30 sec would be the same. However if the frame is built from a HD interlaced camera source there would be a difference in the two fields.

CRTs can not update at 1/30 sec progressive without flicker. By using interlaced scanning the even lines will die down to about 70% when the odd lines get scanned, but since the two fields are interlaced, the overall brightness of the screen is maintained. Your brain will actually average out the two brightnesses. In order for the screen to be refreshed progressively it needs to be done in 1/60 sec and that is why there is the 1080i format - for CRTs. Notice there is not a 720i format? Also notice that on CRT based TVs that will receive a 720p format (most will not) they convert to 540p or 1080i for display. This is due to the non-ability of consumer grade CRT based displays to update faster than about 540p. When fixed pixel displays are applied to all of this a lot of the issues that are aimed at CRTs do not apply.

Bottom line is the broadcasters do not transmit 1080p at 30 fps because the CRT based TVs can not use it and that is still the majority of the TVs in use. There is no 1080p at 60 fps format like there is a 720p at 60 fps format.


It is difficult to compare the two on horizontal lines alone.

Let’s look at some numbers. At 1920 x 540 pixels per scan that is a total of 1,036,800 pixels. The two scans ends up with 1920 x 1080 or 2,073,600 pixels. Now you compare that with 1280 x 720 which is 921,600 pixels. Even each 540 lines that make up the half frame has more pixels than the 720p format.

The only case for 720p is the motion blur case, which is valid when you compare 480i with 480p when both of them have the same number of horizontal pixels. It is much less valid when you compare 1080i against 720p, but we humans tend to discount this. Consider the motion blur of interlaced scan vs the pixel straddling blur of object edges occuring between pixels. This can and is as bad or worse as the motion blur and it happens in both directions. The more pixels the less this blur is.



I knew I had run across this subject before. In this excerpt from http://www.dvddemystified.com/dvdfaq.html#1.1 the situation is the DVD is recorded as interlaced video, but when 24fps film conversions are recorded, they are recorded as two interlaced field pairs at 48 hz from the same film frame. When the DVD player puts the two fields back together, the original frame is reconstructed same as if it were recorded as a progressive frame at 24fps.

The entire article is well worth reading.

[1.40] What’s a progressive DVD player?

A progressive-scan DVD player converts the interlaced (480i) video from DVD into progressive (480p) format for connection to a progressive-scan display (31.5 kHz or higher). Progressive players work with all standard DVD titles, but look best with film source. The result is a significant increase in perceived vertical resolution for a more detailed and film-like picture. Since computers use progressive-scan monitors, DVD PCs are by definition progressive-scan players, although quality varies quite a bit (see 4.1 and 2.12).

There’s enormous confusion about whether DVD video is progressive or interlaced. Here’s the one true answer: Progressive-source video (such as from film) is usually encoded on DVD as interlaced field pairs that can be reinterleaved by a progressive player to recreate the original progressive video. See 3.8 for further explanation of interlaced and progressive scanning.

You must use a progressive-scan display in order to get the full benefit of a progressive-scan player. However, all progressive players also include interlaced outputs, so you can use one with a standard TV until you upgrade to a progressive TV. (You may have to use a switch on the back of the player to set it to interlaced output.)

Toshiba developed the first progressive-scan player (SD5109, $800) in mid 1998, but didn’t release it until fall of 1999 because of copy protection concerns. Panasonic also released a progressive-scan player (DVD-H1000, $3000) in fall of 1999. Many manufacturers have released progressive models since then at progressively cheaper prices (pun intended). It’s also possible to buy an external line multiplier to convert the output of a standard DVD player to progressive scanning.

Converting interlaced DVD video to progressive video involves much more than putting film frames back together. There are essentially three ways to convert from interlaced to progressive:
1- reinterleaving (also called weave). If the original video is from a progressive source, such as film, the two fields can be recombined into a single frame.
2- Line doubling (also called bob). If the original video is from an interlaced source, simply combining two fields will cause motion artifacts (the effect is reminiscent of a zipper), so each line of a single field is repeated twice to form a frame. Better line doublers use interpolation to produce new lines that are a combination of the lines above and below. The term line doubler is vague, since cheap line doublers only bob, while expensive line doublers (those that contain digital signal processors) can also weave.
3- Field-adaptive deinterlacing, which examines individual pixels across three or more fields and selectively weaves or bobs regions of the picture as appropriate. Chips to do this used to cost $10,000 and up, but the feature is now appearing in consumer DVD players.
4- And there’s also a fourth way, called motion-adaptive deinterlacing, which examines MPEG-2 motion vectors or does massive image processing to identify moving objects in order to selectively weave or bob regions of the picture as appropriate. Most systems that do this well cost $50,000 and up (aside from the cool but defunct Chromatic Mpact2 chip).

There are three common kinds of deinterlacing systems:
1- Integrated. This is usually best, where the deinterlacer is integrated with the MPEG-2 decoder so that it can read MPEG-2 flags and analyze the encoded video to determine when to bob and when to weave. Most DVD computers use this method.
2- Internal. The digital video from the MPEG-2 decoder is passed to a separate deinterlacing chip. The disadvantage is that MPEG-2 flags and motion vectors may no longer available to help the deinterlacer determine the original format and cadence. (Some internal chips receive the repeat_first_field and top_field_first flags passed from the decoder, but not the progressive_scan flag.)
3- External. Analog video from the DVD player is passed to a separate deinterlacer (line multiplier) or to a display with a built-in deinterlacer. In this case, the video quality is slightly degraded from being converted to analog, back to digital, and often back again to analog. However, for high-end projection systems, a separate line multiplier (which scales the video and interpolates to a variety of scanning rates) may achieve the best results.

Most progressive DVD players use an internal deinterlacing chip, usually from Genesis/Faroudja. Some use MPEG decoders with integrated deinterlacing. Some, such as Toshiba’s “Super Digital Progressive” players and Panasonic’s progressive-scan player add 4:4:4 chroma oversampling, which provides a slight quality boost from DVD’s native 4:2:0 format. Add-on internal deinterlacers such as the Cinematrix and MSB Progressive Plus are available to convert existing players to progressive-scan output. Faroudja, Silicon Image (DVDO), and Videon (Omega) line multipliers are examples of external deinterlacers.

A progressive DVD player has to determine whether the video should be line-doubled (bobbed) or reinterleaved (weaved). When reinterleaving film-source video, the player also has to deal with the difference between film frame rate (24 Hz) and TV frame rate (30 Hz). Since the 2-3 pulldown trick can’t be used to spread film frames across video fields, there are worse motion artifacts than with interleaved video. However, the increase in resolvable detail more than makes up for it. Advanced progressive players such as the Princeton PVD-5000 and DVD computers can get around the problem by displaying at multiples of 24 Hz such as 72 Hz, 96 Hz, and so on.

A progressive player also has to deal with problems such as video that doesn’t have clean cadence (as when it’s edited after being converted to interlaced video, when bad fields are removed during encoding, when the video is speed-shifted to match the audio track, and so on). Another problem is that many DVDs are encoded with incorrect MPEG-2 flags, so the reinterleaver has to recognize and deal with pathological cases. In some instances it’s practically impossible to determine if a sequence is 30-frame interlaced video or 30-frame progressive video. For example, the documentary on Apollo 13 is interlaced video encoded as if it were progressive. Other examples of improper encoding are Titanic, Austin Powers, Fargo, More Tales of the City, the Galaxy Quest theatrical trailer, and The Big Lebowski making-of featurette.

One problem is that many TVs with progressive input don’t allow the aspect ratio to be changed — they assume all progressive-scan input is anamorphic. When a non-anamorphic (4:3) picture is sent to these TVs they distort it by stretching it out! Before you buy a DTV, make sure that it allows aspect ratio adjustment on progressive input. Or get a player with an aspect ratio control option that “windowboxes” 4:3 video into a 16:9 rectangle by squeezing it horizontally and adding black bars on the side. Because of the added scaling step this degrades picture quality, but at least it gets around the problem.

Just as early DVD computers did a poor job of progressive-scan display of DVDs, the first generations of progressive consumer players are also a bit disappointing. But as techniques improve, and as DVD producers become more aware of the steps they must take to ensure good progressive display, and as more progressive displays appear in homes, the experience will undoubtedly improve, bringing home theaters closer to real theaters.

For more on progressive video and DVD, see part 5 and player ratings in the excellent DVD Benchmark series at Secrets of Home Theater and High Fidelity.


Also consider we watch movies at 24 frames per second all of the time, so the frames per second of 30 vs 60 is somewhat unimportant except for phosphor based displays where the persistance is timed basically for a 30 per second frame rate (2 1/60 sec passes) to reduce flicker. On fixed pixel displays the actual display update rate from the internal picture memory is whatever is required to eliminate flicker and may be much higher than 30hz. On these type displays interlaced video that was created from a progressive source will not matter to the display. Since the frame is completed (bobbed, weaved or whatever) at the rate of 30fps, the resulting display on a fixed pixel display is essecientally 1920 x 1080p @ 30fps and is at least as good as a movie presentation at 24fps. Going up to 60fps will not improve anything if the original movie was shot at 24fps, because many of the extra frames are just duplicate information.

The only situation where the 720p @ 60 hz will improve the picture is when the source is also shot at 720p @60hz, such as sports and the increased resolution of 1080i offsets that. The two formats are generally considered equal, although a case for 1080i being better for film transfers could be made due to the higher resolution.

Paper on making frames out of “thin air”: Temporal resolution enhancement in compressed video sequences

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