1080i - how many frames per second? - AVS Forum
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Old 01-08-2008, 01:12 AM - Thread Starter
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Question on 1080i:

I see 1080i described as 30 frames per second delivered 1 field every 1/60 of second. I understand the basics of interlaced versus progressive but isn't 1080i programming actually 60 frames per second but you only get 1/2 of each frame? To put it another way instead of:
A-odd
A-even
B-odd
B-even
C-odd
C-even

It is actually:
A-odd
B-even
C-odd
D-even
E-odd
F-even

I understand it is a moot point if objects are stationary between frames but isn't the problem with interlaced content the fact that 1/2 of each frame never even gets captured?

And if you can point me to a good discussion on 720p versus 1080i CONTENT I would be obliged.

-D
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Old 01-08-2008, 01:53 AM
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Broadcast 1080i is 30 interlaced frames per second, which is 60 fields (half-frames) per second. Two fields are needed for one complete frame however with video each field is captured 1/60 second apart so there could be movement between the two fields that make up a frame, that makes it tougher for de-interlacers used for progressive displays.

With 720p its 60 full frames per second, so no such issues as with interlaced video and the higher frame rate is great for fast motion sports.
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Old 01-08-2008, 03:47 AM
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Quote:
Originally Posted by Dom2u View Post

I understand it is a moot point if objects are stationary between frames but isn't the problem with interlaced content the fact that 1/2 of each frame never even gets captured?

Exactly. Interlacing is simply a method of data compression that gives up some motion detail (which is noticed less by the viewer) to achieve greater static detail (which is noticed more by the viewer) using the same bandwidth.

Most of the 1080i content you see, however, is not interlaced but telecined, which means it contains 24 progressive frames split into the 60 interlaced frames, which your display can piece together back to the full progressive frames without any data loss.
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Old 01-08-2008, 05:26 AM
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Quote:
Originally Posted by Dom2u View Post

I understand it is a moot point if objects are stationary between frames but isn't the problem with interlaced content the fact that 1/2 of each frame never even gets captured?

And if you can point me to a good discussion on 720p versus 1080i CONTENT I would be obliged.

-D

You really don't want to go there! You'd get a straighter answer if you asked for a good discussion on the Middle East!

It's not that half of the picture is never captured. The "missing" information is averaged into the lines above and below the scanned lined.

"CONTENT"? Real-life images are not sampled in any dimension, horizontal, vertical, or temporal.

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Originally Posted by Joxer View Post

With 720p its 60 full frames per second, so no such issues as with interlaced video and the higher frame rate is great for fast motion sports.

No, even 60 frames per second is too slow for sports. What's better, a blurry 1080x540 picture or a blurry 1280x720 picture. Doesn't make much difference. Progressive can match display panels better, but today's TVs are often 1920x1080. So the answer is -- 1080p/60!

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Originally Posted by nm88 View Post

Exactly. Interlacing is simply a method of data compression that gives up some motion detail (which is noticed less by the viewer) to achieve greater static detail (which is noticed more by the viewer) using the same bandwidth.

Interlacing is just a method of data reduction, and an analog one at that! Progressive scanning requires twice the bandwith, which is why progressive images have less "spatial resolution." They end up being about equal, and 720p and 1080i are BOTH high definition, but picking one over the other is a sure way to start a fight.
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Old 01-08-2008, 07:50 AM
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Quote:
Originally Posted by Dom2u View Post

I understand it is a moot point if objects are stationary between frames but isn't the problem with interlaced content the fact that 1/2 of each frame never even gets captured?

It partly depends on whether or not the material was originally shot at 60i. If it was originally shot at 30p or 24p, and converted to 60i by using 2-2 pulldown or 2-3 pulldown, respectively, then you might not lose anything, since a de-interlacer could weave the fields back together to reconstruct the original progressive frame. If it was originally shot at 60i, then a de-interlacer has to "invent" what was lost. Some de-interlacers are better than others.
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Old 01-08-2008, 07:59 AM
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http://en.wikipedia.org/wiki/1080i

30fps for NTSC/ATSC and 25fps for PAL/DVB.

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Old 01-08-2008, 08:01 AM
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Quote:
Originally Posted by Dom2u View Post

And if you can point me to a good discussion on 720p versus 1080i CONTENT I would be obliged.

There is little point in that discussion.

Due to the high number of variables involved in HDTV production and distribution, both 720p & 1080i can look great or lousy. In theory, 720p should be better for fast action, while 1080i should look better for detail, but in the real world this is not always the case.

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Old 01-08-2008, 08:42 AM
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Quote:
Originally Posted by hphase View Post

No, even 60 frames per second is too slow for sports. What's better, a blurry 1080x540 picture or a blurry 1280x720 picture. Doesn't make much difference. Progressive can match display panels better, but today's TVs are often 1920x1080. So the answer is -- 1080p/60!

Assume 1080x540 was a type for 1920x540 - or are you referring to so-called "HD Lite"? (Which I thought was 1440x1080 or 1280x540?)
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Old 01-08-2008, 09:38 AM
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Originally Posted by sneals2000 View Post

Assume 1080x540 was a type for 1920x540 - or are you referring to so-called "HD Lite"? (Which I thought was 1440x1080 or 1280x540?)

You got me! I meant 1920. Thanks for pointing it out.
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Old 01-08-2008, 01:03 PM - Thread Starter
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Quote:
Originally Posted by Joxer View Post

Broadcast 1080i is 30 interlaced frames per second, which is 60 fields (half-frames) per second. Two fields are needed for one complete frame however with video each field is captured 1/60 second apart so there could be movement between the two fields that make up a frame, that makes it tougher for de-interlacers used for progressive displays.

With 720p its 60 full frames per second, so no such issues as with interlaced video and the higher frame rate is great for fast motion sports.

Since each field capture is 1/60 of a second apart doesn't that mean field 1 is NOT part of the same frame as field 2?

I guess my question is on semantics...
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Old 01-08-2008, 01:08 PM - Thread Starter
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Quote:
Originally Posted by Ken H View Post

There is little point in that discussion.

Due to the high number of variables involved in HDTV production and distribution, both 720p & 1080i can look great or lousy. In theory, 720p should be better for fast action, while 1080i should look better for detail, but in the real world this is not always the case.

I have seen some discussions on 1080i vs 720P on other forums. People seemed to be fighting the next Holy War over it. What I appreciate about AVSforums is that the people here seem better informed and more reasonable.

You are probably right about other variables being more important than 720p vs 1080i. Football on ESPN (720P) looks better to me than on CBS (1080i). But CBS looks better than Football on Fox (720p).

I do look for interlaced artifacts during motion and can see them. But if I sit far enough away I don't have a problem.

-Dom
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Old 01-08-2008, 01:14 PM - Thread Starter
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Quote:
Originally Posted by Erik Garci View Post

It partly depends on whether or not the material was originally shot at 60i. If it was originally shot at 30p or 24p, and converted to 60i by using 2-2 pulldown or 2-3 pulldown, respectively, then you might not lose anything, since a de-interlacer could weave the fields back together to reconstruct the original progressive frame. If it was originally shot at 60i, then a de-interlacer has to "invent" what was lost. Some de-interlacers are better than others.

This is the answer I was looking for. So for sports (which I am most interested in) 720p should be "Technically" better since it was probably shot at 60i on a 1080i broadcast.

I do understand for movies the point is moot - but I did not know that material could have been shot 1080p30 and then broadcasted as 1080i60. Good de-interlacers must be able to look for 2-2 pulldown along with 2-3 pulldown then.

Thanks Erik,
-Dom
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Old 01-08-2008, 04:07 PM
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Quote:
Originally Posted by Dom2u View Post

I do understand for movies the point is moot - but I did not know that material could have been shot 1080p30 and then broadcasted as 1080i60. Good de-interlacers must be able to look for 2-2 pulldown along with 2-3 pulldown then.

My understanding is that 1080/30p is used far less frequently than 60i and 24p. 2:2 pulldown detection is more difficult than 3:2 (no duplicate fields as a clue to whether something is 2:2 or natively interlaced)
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Old 01-08-2008, 04:58 PM
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Quote:
Originally Posted by Dom2u View Post

Since each field capture is 1/60 of a second apart doesn't that mean field 1 is NOT part of the same frame as field 2?

Yes exactly. Thats why de-interlacing interlaced video is a complex issue in that case. Many deinterlacers use motion compensation to use the parts from both fields for higher vertical resolution on the static parts of the frame, while losing some vertical resolution on the moving parts of the frame, since the fields have movement in that 1/60sec difference.
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