When should I use 720p instead of 1080i? - AVS Forum
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post #1 of 9 Old 05-26-2010, 10:10 PM - Thread Starter
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My Sanyo HD1000 can do 720p of 1080i. Should I go for the "smoother" 720p or just record at 1080i?
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post #2 of 9 Old 05-27-2010, 03:37 AM
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Who's gonna watch your recordings? What looks better on their current system(s)?

From one POV, 720p is 1280x720. From another 1080i is 1920x540.

720p = 1280x720 = 921,600 pixels per frame
1080i = 1920x540 = 1,036,800 pixels per frame

Is there really much if any difference?

I tend to favor 720p, but I record in 1080p60. And my preference is currently dictated by my systems which favor 720p. My systems will eventually change. My originals wont. If you're rendering to DVD, either should look sufficient.
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post #3 of 9 Old 05-27-2010, 05:55 AM
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Quote:
Originally Posted by Shadow_7 View Post

Who's gonna watch your recordings? What looks better on their current system(s)?

From one POV, 720p is 1280x720. From another 1080i is 1920x540.

720p = 1280x720 = 921,600 pixels per frame
1080i = 1920x540 = 1,036,800 pixels per frame

Is there really much if any difference?

I tend to favor 720p, but I record in 1080p60. And my preference is currently dictated by my systems which favor 720p. My systems will eventually change. My originals wont. If you're rendering to DVD, either should look sufficient.

1080i is 1920 x 1080 = 2,073,600 pixels per frame. Your number is for the half-frame and the brain is combining 60 half-frames per second into a perceived 30 frames per second. The 60i frame contains over twice the number of pixels in total than the 720p one.

The 1080i picture contains much more detail and is what I would always use. The 720p might be useful for very fast motion or if you want to upload smaller files than 1080i to YouTube, for example.
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post #4 of 9 Old 05-27-2010, 06:30 AM
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Quote:
Originally Posted by GarageBoy View Post

My Sanyo HD1000 can do 720p of 1080i. Should I go for the "smoother" 720p or just record at 1080i?

If you want more detail, it's a no-brainer, go for 1080i. If it were between 1080i and 1080p, that's a whole other story, but you'll have far more detail in 1080i than 720p.
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post #5 of 9 Old 05-27-2010, 08:11 AM
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Shoot 720p if you want to do slow motion or freeze frame. Otherwise shoot 1080i
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post #6 of 9 Old 05-27-2010, 09:18 AM - Thread Starter
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post #7 of 9 Old 05-27-2010, 01:56 PM
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Quote:
Originally Posted by Tom Gull View Post

1080i is 1920 x 1080 = 2,073,600 pixels per frame.

Only when converted to p is it 2,073,600. The i stands for HALF frames. You get twice as many of them. And the content may be p in an i container. But the i is half frames. Some players default to stretching the half frames (bob filter) before combining two half images. Not all but some and it's a selectable option in some players. Also bear in mind that i might also mean 2x's the shutter speed which could make it unusable in low light conditions. So not really a cut and dry decision.
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post #8 of 9 Old 05-27-2010, 02:18 PM
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Quote:
Originally Posted by Shadow_7 View Post

Only when converted to p is it 2,073,600. The i stands for HALF frames. You get twice as many of them. And the content may be p in an i container. But the i is half frames. Some players default to stretching the half frames (bob filter) before combining two half images. Not all but some and it's a selectable option in some players. Also bear in mind that i might also mean 2x's the shutter speed which could make it unusable in low light conditions. So not really a cut and dry decision.

"From Wikipedia, the free encyclopedia

1080i is the shorthand name for a format of high-definition video modes. 1080 denotes the number of horizontal scan lines—also known as vertical resolution—and the letter i stands for interlaced. In the alternate format of high-definition video mode, known as 1080p, the p would stand for progressive scan.[1][2]

1080i is a high-definition television (HDTV) video mode. The term usually assumes a widescreen aspect ratio of 16:9, implying a horizontal resolution of 1920 pixels and a frame resolution of 1920×1080 or about 2.07 million pixels"

As to the interlacing affecting shutter speed and thus low light performance, you must not have been watching the last year and checking out videos of the Sony 5xx camcorders and others that flipped the hardware on the sensor chips to be behind the light-capturing elements instead of in front of them. They may be interlaced, but their low-light performance is significantly better than their predecessors (excellent, in other words).

And this extract from this article:

http://www.avsforum.com/avs-vb/showt...1#post18698256

"3. Why is 1080p theoretically better than 1080i?

1080i, the former king of the HDTV hill, actually boasts an identical 1,920x1,080 resolution, but conveys the images in an interlaced format (the i in 1080i). In a tube-based television, otherwise known as a CRT, 1080i sources get "painted" on the screen sequentially: the odd-numbered lines of resolution appear on your screen first, followed by the even-numbered lines--all within 1/30 of a second. Progressive-scan formats such as 480p, 720p, and 1080p convey all of the lines of resolution sequentially in a single pass, which makes for a smoother, cleaner image, especially with sports and other motion-intensive content
...
6. What happens when you feed a 1080i signal to a 720p TV?

The 1080i signal is scaled, or downconverted, to 720p...
.....
8. What happens when you feed a 1080i signal to a 1080p TV?

It's converted to 1080p with no resolution conversion. Instead, the 1080i signal is "de-interlaced" for display in 1080p. Some HDTVs do a better job of this de-interlacing process than others, but usually the artifacts caused by improper de-interlacing are difficult for most viewers to spot."
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post #9 of 9 Old 05-29-2010, 12:11 AM
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Quote:
Originally Posted by Shadow_7 View Post

Only when converted to p is it 2,073,600.

They are using the correct video terms: 1080/60i delivers 1920x1080 pixels per frame. A frame consists of two fields, each contributing half of the lines in the frame.

Quote:
Originally Posted by Shadow_7 View Post

The i stands for HALF frames.

The "i" standard for interlaced, which means you'll have two fields per frame, each frame is completed every 1/30th of a second, each field every 1/60th of a second.

Quote:
Originally Posted by Shadow_7 View Post

Some players default to stretching the half frames (bob filter) before combining two half images. Not all but some and it's a selectable option in some players.

For display on a television, there's really no issue... 1080/60i is supported quite well, and displays in full resolution (sometimes enhanced by various video processing techniques), and there's very little in the way of "interlacing artifacts" visible, even on modern progressive televisions.

It's a very different story on your PC. For one, the PC can't likely display interlaced video... some graphics cards actually do, some monitors do, but few even try. Secondly, your video is not usually synched to the monitor... it's just data dumped into graphics memory... that graphics frame buffer is the thing synched to the monitor. This isn't usually a problem, both because monitors are fast, and because the update itself is usually synched, at least via double-buffering, so it's rare, these days, to see video changing in the middle of a screen update.

But you will need to de-interlace in some way from 1080i, or you're going to see very obvious motion artifacts... the temporal distance between one field and the next when composited as a single progressive frame, viewed at the same time. Yeah, some players can Bob at 60p, but that's going to be ugly. There are other ways to do it, most involve some degree of lowering the resolution to smooth over the interlacing artifacts. With enough time, you can do motion estimation and adaptive smoothing to leave static things alone, do smoothing only on images in motion.

Quote:
Originally Posted by Shadow_7 View Post

Also bear in mind that i might also mean 2x's the shutter speed which could make it unusable in low light conditions. So not really a cut and dry decision.

It can, but you have to watch that anyway. It's not uncommon for video cameras to lock out very slow shutter speeds by default, going only to 1/60th for 60p, 60i, or 30p, and 1/48th for 24p. Unlocking this, of course, you can get perfectly useful video at 1/30th and 1/24th, respectively. And few (Sanyo's, for example) don't seem to have any real interlock between video mode and shutter speed.
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