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Aaron Nowakowski
720 60p
on Mar 19, 2010 at 1:07:40 am

I know I've seen a number of postings regarding 30p and 60p, however I am still having trouble wrapping my head around what 60p really is...so I apologize for posting this subject again. I am just trying to keep myself educated.

I recently received some press junket footage that was shot in 720 60p. I posted it in Avid MC in a 720 60p project with no problem. I then, to my surprise, down converted the sequence and dropped it into a SD 29.97 sequence and it played out fine. It went to tape with no problems at all. My question is how is this possible? If my footage is shot in 60p and I dropped it into a 29.97i sequence, shouldn't my 60p footage play out at a different speed? You have double the frames than a SD sequence correct?

Second question: Why would you want to shoot in 60fps or 30fps? We use Sony EX3s and I would like to use this format if it is beneficial. I appreciate any explanations anyone can offer.

Big Thanks!



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Don Greening
Re: 720 60p
on Mar 19, 2010 at 8:02:18 am

[Aaron Nowakowski] " I then, to my surprise, down converted the sequence and dropped it into a SD 29.97 sequence and it played out fine."

That's because it's supposed to. 60p is built to play at normal speed in a 29.97 timeline. As with interlaced which plays 60 fields per second, 60p plays at 60 frames per second, or if you like, 2 complete pictures (like a photograph) every 30th of a second.

[Aaron Nowakowski] "Why would you want to shoot in 60fps or 30fps"

Shooting 60p has more temporal resolution which means with 60 complete pictures every second there's less motion blur for things such as sporting events or anything that moves fast. Progressive shooting is part of the "film look" that is all the rage in video production these days, along with 30p and 24p. It's strange the way people perceive film quality in different parts of the world. In North America people relate the interlaced look to television and news broadcast, the everyday realism. Anything but dramatic storytelling. In Japan, it's the exact opposite. There the viewer associates the video look as dramatic storytelling.

- Don



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Aaron Nowakowski
Re: 720 60p
on Mar 19, 2010 at 2:36:12 pm

So then what is physically happening when you over or undercrank? I always understood that as changing the amount of frames per second?

Thank you for the help Don!



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Don Greening
Re: 720 60p
on Mar 19, 2010 at 8:01:56 pm

[Aaron Nowakowski] "So then what is physically happening when you over or undercrank?"

Beats me. I suspect that there's a flag in the 720 60p stream that instructs an NLE to play the video at normal speed. Or there are flags in the overcrank stream but not in the 60p streams. The undercrank mode is pretty simple to figure out. If you remove frames from normal video and jam together whats left it's going to look speeded up.

- Don



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Aaron Nowakowski
Re: 720 60p
on Mar 19, 2010 at 8:16:20 pm

Thanks for taking the time to help out Don. It will be interesting to see if anyone else has any thoughts on this.



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john sharaf
Re: 720 60p
on Mar 19, 2010 at 8:32:30 pm

The Panasonic Varicam, as its name implies, was the first "variable speed" video camera, whiuch while imitating the variable speed function of a film camera did so in a slightly different manner.

The ability to "undercrank", essentially meaning to shoot at frames rates lower than normal, meant repeating frames, such that for example if you shoot at 10 frames per second each frame is repeated for six frames (as the camera always records 60 frames per second - called 60p).

At so called "normal speeds" of either 24 or 30 frames per second the camera duplicates frames either in a 2:3 cadence (like teleine) at 24p or by repeating every frame twice at 30fps. At all frame rates that require repeating frames the camera marks the first of each set of repeats with an "A' frame in the user bits. This way if you want to recreate an undercranked effect, or a 24 or 30 fps timeline, your software can "pull" only the discreet frames out of the recording and not the duplicates.

Overcranking is made possible by playing pack the frame rates higher than "normal" (either 24 or 30) at the normal frame rate. For example if you shoot at 60p and play back at 30p the slomo effect will be 2x. If played back at 24p the slomo effect is higher at 2.5x. It is also possible to double print the 60p and have somethink like (but not exactly the same as) 120fps. The cameras have the capacity to make that many frames but are unable to record them.

Cameras since the Varicam have copied these techniques and now there are models that can write 1920x1080 at rates as high as 60p.

Hope this helps!

JS





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Enrique Orozco
Re: 720 60p
on Mar 20, 2010 at 12:44:31 am

..I use 720 60p in my EX3 when shooting sports or some action clips....to me is an option to overcrank ... "slowing" the frame rate of the 60p clips in Vegas gives me nice "slo-mo" results....

this is a little example, what do you think ?? (2nd. part of the video...):







...my 2 cents...

Enrique Orozco R.
http://www.dvideostudio.com >iDEA DigitalVideoStudio


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Aaron Nowakowski
Re: 720 60p
on Mar 22, 2010 at 2:41:16 am

Thanks for sharing Enrique, the overcranking looks clean.



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Aaron Nowakowski
Re: 720 60p
on Apr 18, 2010 at 5:07:21 pm

Thanks John for taking the time to respond to my post. Another question for you. What would be the benefits of shooting say an interview at 720 60p? From watching dateline or 60 Minute interviews it appears that they are shot in a progressive frame rate. It would seem to me that by shooting something at 30fps it would be more 'broadcast friendly' and take to SD easier. Is 30fps a common use for such an application?

Thanks John!



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john sharaf
Re: 720 60p
on Apr 18, 2010 at 6:18:32 pm

The "benefit" of shooting an interview at 60p is that it has a "live tv" look, as opposed to the more "feature" quality that shooting at 30p (as 60 Minutes does) has.

Frame rate is a very subtle and subjective variable. In general 60p and 60i yield a live look and are used exclusively for sports and any live programming (like the six o'clock news). 30p is used for feature programing and especially for any material intended for internet or web use, because the repeated and fewer frames per second are most friendly to high compression rates that are used for this sort of distribution.

By having a progressive look rather than interlaced or "live" shows that are shot at 30p are different enough in temporal effect as to look special and even "film like" with less stuttering than actual 24fps, if not for the additional expense of film and processing film cameras would probably have standardized on 30fps when they transitioned from the silent era.

24p on the other hand exactly imitates the look of film (when transferred to video via telecine) and in addition, makes the material easy to translate to actual film for distribution in theaters and/or to 25fps for use in PAL countries, where 25p is broadcast by repeating frames as 50i and approximates the 30p look here in NTSC land.

JS





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David Burch
Re: 720 60p
on Apr 18, 2010 at 8:11:34 pm

Don't forget pulldown. IMO, footage shot at 24p with a 2:3 pulldown looks much better than footage shot at 30p, especially when seen on an interlaced monitor. Since broadcast is an interlaced medium, any footage shot at 24p needs a 2:3 pulldown added before it can be submitted. For this reason, 24p still results in a far better "film" look than 30p. IMO, any time I see 30p on the air it looks cheap, like they are cutting corners. Really, I can see no benefit of 30p over 30i in terms of bandwidth, as an interlaced and progressive frame both have the same number of pixels. However, for web distribution 24p does have the distinct advantage of having 20% less frames per second, and 24p looks just fine on a progressive scan computer monitor. Honestly, I really have a hard time understanding the desire for 30p in any situation.


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john sharaf
Re: 720 60p
on Apr 18, 2010 at 9:04:02 pm

David,

Just a few fact checks:

Broadcast is not an "interlaced medium". There are progressive standards including 720/60p that are broadcast.

For the web, 24fps material has the telecine built in so that frames are repeated in the familiar 2:3 cadence. The only time that 24 is used in its native frame rate is in editing in a 24p timeline, essentially to save storage space while retaining a film look (although the telecine is applied to monitoring otherwise there would be a prominent flicker effect) and for DVD's for essentially the same reason (that more material can fit on limited storage real estate) where the player applies the telecine cadence to the attached display device.

You're right in that there is no benefit of "30p over 30i" because there is no such thing as 30i.

Finally, it is generally accepted that the motion is smoother at 30p vs 24p because of the repeating of each frame twice, rather than 2 times and then three times and two times again. As I said, if it were not for the economics of using 25% more film and processing, film would look better in the cinema at 30 frames per second rather than 24.

Showscan was a system that was shot and projected film at 60 frames per second, and other than looking like a crystal sharp television picture 40' tall it was more than spectacular, but obviously the cost of the additional film stock, processing, printing, shipping, not to mention new projectors, etc. doomed it just for the same reasons that 24fps won out over 30fps in the conversion from silent to sound film.

JS








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Aaron Nowakowski
Re: 720 60p
on Apr 18, 2010 at 9:36:37 pm

This was REALLY helpful. I appreciate everyone taking the time to educate. Thanks again!



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David Burch
Re: 720 60p
on Apr 19, 2010 at 12:13:16 am

I was not aware that broadcast specs included 60p...if so I stand corrected on that aspect :) I do know that the stations around here only broadcast in 1080i or SD, and that has been the extent of my experience with broadcast.

By 30i I was implying 29.97 interlaced; I'm well aware there's no such thing as 30i :) My point was that I can see no advantages from a compression or bandwidth standpoint by shooting progressive over interlaced given the same amount of frames per second. By shooting 30p, you are essentially trading temporal resolution for image resolution.

I disagree completely with your claim that 30p looks smoother than 24p with a 3:2 pulldown, and here's why: 30p uses no pulldown to play back in an interlaced format. Essentially, it is displayed as a progressive segmented frame. The result is a progressive-looking image on an interlaced screen, because there is no temporal difference between fields of the same frame. The 3:2 pulldown required for 24p, however, creates the illusion of higher temporal resolution because half the resulting frames are a combination of two temporally different source frames. In other words, 2 out of four frames contain fields that are temporally different, creating a smoother look. The cadence is as follows:

AA BB BA AB BB AA BB BA... ad infinitum.

The 3rd and 4th frames use fields that are temporally separated. Compare this with 30p:

AA BB AA BB AA...

None of the frames are temporally segmented, so the image appears to have less temporal resolution than the 2:3 pulldown of 24p, meaning the motion looks less smooth overall.

IMO, the only time 30p will look smoother than 24p is when both are viewed back to back in their native progressive format. I have to disagree with your claim about web video. Web video is almost always progressive, and rarely uses a full 30fps. The frame rate can be nearly anything, but most video I have seen is typically either 12 or 15 fps, in order to save on bandwidth. If you want to put something online at its full frame rate, 24p is an option, as is 30p. There would be no advantage to adding a pulldown to 24p for web video, as interlaced material generally gets de-interlaced anyway. But then, perhaps we are talking about different forms of delivery? I am essentially referring to things such as YouTube and Apple TV, both of which support progressive, 24fps video (to my knowledge).

One last thing; it is true that DVD players apply the pulldown when outputting to an interlaced monitor, but most DVD players these days support progressive scan output, which ONLY works when 24p material is being played back. As far as I know DVDs do not support a true 30p format, but since every frame in 30p contains no temporal segmenting it really doesn't matter. An interlaced monitor will look progressive when displaying 30p, and a progressive monitor will de-interlace the video perfectly. Progressive scan DVD players, however, are capable of sending a true progressive (or possibly PSF) image to a screen that is capable of displaying it (LCD monitors, for instance). This is another advantage to shooting 24p, besides the fact that the limited bandwidth of DVD compression produces better results.


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David Burch
Re: 720 60p
on Apr 17, 2010 at 1:46:46 am

As far as I know, shooting "overcrank" flags the footage as whatever speed your camera is set at. For instance, say you're shooting 720p24, and overcranking to 60fps. The file that is being created is a true 24p file; it's simply being recorded onto the card faster than real time. I don't know this for sure, but i believe that the advantage of shooting overcrank as opposed to simply shooting at 60p and slowing down in post is compression. Remember, the XDCAM-EX format is 35 mbps, regardless of resolution or framerate. Theoretically, overcranked footage plays back at the same bit rate as footage recorded at the normal rate. Therefore, the camera has to process data much faster than 35 mbps when overcranking, which explains why you can only overcrank to 60fps while shooting 720p (and why Sony doesn't recommend using non SxS media when overcranking).

On the other hand, if you shoot footage at 60fps, the bit rate is still 35 mbps, but the codec has to encode many more frames per second with this same amount of data, resulting in more compression per frame. While you can actually achieve decent results by slowing down 60p footage to 24 fps, the overall quality will be better if you overcrank while shooting.


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David Burch
Re: 720 60p
on Apr 17, 2010 at 1:50:29 am

Another thing about 60p is that it can be used to create a true 29.97 interlacing. For this reason I use it when I know my final product must be standard definition, 29.97 interlaced. Being a progressive format, it downscales much better than, say, 1080i, and doesn't result in a juttery look like 30p does. Personally, I can't stand 30p...if I want to shoot progressive I'll shoot 24p. That way, I can emulate a true film look, and non-progressive monitors get the benefit of a 3:2 pulldown, just like any movie. Also, DVD compression works much better with true 24p footage, which can be important when encoding content that is 2+ hours on a single-layer DVD.


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Rafael Amador
Re: 720 60p
on Apr 17, 2010 at 3:55:02 am

Overcrank and undercrank is just a "Conforming" in camera.
rafael

http://www.nagavideo.com


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Aaron Nowakowski
Re: 720 60p
on Apr 18, 2010 at 4:56:37 pm

Thanks David,

What sparked this question for me is I receive press junket footage in 720 60p and was wondering what the benefit was to that format. I was also confused as to why I could take that 60p timeline and drop it in a SD 29.97 seq without any playback issues (using Avid Media Comp). Does Avid pullout the excessive frames in order to conform to that timeline? Second question you mentioned you prefer 24p, that 30p gives you too much jitter. In experimenting on my own, I've noticed 24p gives me just as much jitter if not more. Is there something I am not doing properly?



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David Burch
Re: 720 60p
on Apr 18, 2010 at 8:03:22 pm

Try following your subjects as much as possible, keeping the depth of field small, and avoiding medium-speed pans. These are all shooting techniques that can keep the "jittery" look down. Yes, 24p viewed natively will have just as much "jitter" as 30p, but so does film. The difference here is that 24p can have a 2:3 pulldown added that significantly reduces this. This is what happens when you play a movie from a DVD player on a non-progressive screen. The player is essentially repeating a field on every other frame, resulting in 2 progressive frames followed by 2 interlaced frames. This is not possible to do with 30p, and as a result 30p will look far worse on an interlaced screen.

As for 60p, I assume what Avid is doing is using the frames to create true interlaced 29.97 SD video. Remember, NTSC 29.97 is actually 59.94 fields per second. In other words, for every frame there are two exposures that are temporally separated and interlaced together. This is what gives 29.97 interlaced footage its smooth look, and represents the difference between 29.97i and 30p. 30p only has one exposure for every frame, so while the image will be sharper while objects in the screen are in motion the motion itself is less smooth. Another way of saying this is that 30p has less temporal resolution than 29.97i.

60p gives you the benefit of both temporal resolution and image resolution. Every frame you see has only one exposure, but there are twice as many frames per second, so the image is just as smooth as 29.97i yet just as sharp as 30p. This is the real advantage of 720p60, as the vertical resolution you have per frame is actually greater than in 1080i.

Back to your question about Avid, what is happening is that it is taking two frames and interlacing them together to create a single frame that is indistinguishable from one that was shot interlaced to begin with. This is why 60p makes a great format for downconversion; the result is a clean, true interlaced image that benefits from the source being progressive (and therefore easier to downconvert cleanly).

In other words, Avid is not "pulling out" any frames; it is using all of them to create 29.97i, which it could not do if you started with 30p.


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