Why is it necessary to multiply frame rates?

admin_exported

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I was just wondering why a 24hz signal, for example, has to be multiplied to 48hz or 72hz, or a 25hz signal multiplied to 50hz before being displayed on a tv screen.Why can't they just be displayed as they are, ie. at 24hz or 25hz?

If anyone could answer this or find out for me I'd very much appreciate it because this is something that I just don't understand.
 
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Anonymous

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I've just decided to simplify my question a little. Why can't a 24fps signal just be shown on a television at 24hz? After all, isn't that how thay are shown at the cinema, frame for frame?
 
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Anonymous

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Because although the image doesn't alter you'll notice the screen flashing. At a higher refresh rate you won't. Bit like a car at low revs in 5th gear at 20 mph or in 2nd gear at very high revs at 20 mph. It'll be smoother at the higher refresh rate. That's why.
 

D.J.KRIME

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A film at the cinema is shown at 24fps when traditionally on 35mm (not sure if this is still the case with a digital cinema projection system?) BLU-RAY/HD-DVD at 24fps but how can this be a true 24fps because PAL is displayed at 25 FPS and NTSC at 30 FPS? This is why a NTSC dvd will have a shorter running time over the same film on a PAL dvd.
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Anonymous

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How does THAT work? The audio would get totally out of sync?
 

D.J.KRIME

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[quote user="Will Harris"]How does THAT work? The audio would get totally out of sync?[/quote]

the audio is slightly slower on a PAL disc compared to a NTSC one as well. For example in one hour a PAL dvd will display 90,000 frames as where NTSC will display 108,000 frames being 1.2% more, so there for every 1 hour of movie in PAL the NTSC version will run 1.2% faster hence the differnce in running times. But which is at the correct speed is another issue as that is dependant on if the film was originally mastered onto dvd in PAL or NTSC.

Films on DVD are measured by running times so there will be a differnce on the region 1 disc compared to the region 2 this leads to confusion as to if the film is Cut by the BBFC over here, as where a Film at the cinema will have exactly the same running time everywhere in the world (unless it is cut) due to the fact that at the cinema a film is shown at 24fps the world over and being on 35mm are not in either PAL or NTSC as these systems are purely tv related and only come into play when the origanal 35MM negative is transfered to dvd etc, this is why films at the cinema are not mesured by a running time but buy the actual lenght of thr 35MM negative in feet and inches.
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Anonymous

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Hmmm, actually that works out at 20% more frames not 1.2% I think you'd notice it. I'd be curious to know how this is gotten around in actual practise. Perhaps that's why TV's have variable refresh rates of 60Hz (double NTSC frame rate) and 50Hz, (almost double PAL frame rates).

Could this be it?
 

D.J.KRIME

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[quote user="Will Harris"]Hmmm, actually that works out at 20% more frames not 1.2% I think you'd notice it.[/quote]

No Will it is 1.2% see 108,000 divided by 90,000 = 1.2% which is hardly noticeible when watching a movie. If you dont believe me get 2 copys of the same movie on dvd 1 NTSC and 1 PAL, put them into your dvd player and start the movie. If you press the info button to display the films running time you will see that the NTSC disc is slightly shorter even tho the actual film on both discs are identical.

If you dont believe me ask Mr Everhard!
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Anonymous

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Hmmm, Kriminals often make good accountants. But not in this one's case! Yes the product of your division is 1.2 but not 1.2% It's actually 120% which shows a 20% increase with PAL is it?. Have a very good look at it and you'll see I'm right. Now, once that's done what do you think of my Hz refresh rate theory?
 

D.J.KRIME

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Right here is 1 movie to give a example to you.

Star Wars Episode IV has a Cinema running time of 124 mins and 35 seconds

A NTSC DVD running time of 123 mins (dont have exact seconds to hand)

and a PAL DVD running time of 119 mins and 35 seconds

The differnce between NTSC and PAL does not equatre to 20%
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Come on Andrew tell Will I am right.
 
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Anonymous

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Then your frame rates is wrong unless the refresh rate can adjust to take it into account. If we're just talking about the maths, going from 90 to 108 is a 20% change you can see that with your own eyes. If the question is why does that only equate to around a 1% playing time difference, then we have to look at how many frames the screen can show, and that would equate to the different refresh rates taking up the difference.

For instance, if you have a 30 frame per second film and a 60Hz screen refresh rate you can show each frame twice within a second. Or interlaced, each frame once per second in full. If you have a 25 frame per second film (or 24 but speed it up to 25 frames) then at 50Hz you can show each frame twice per second. Or, interlaced, each frame in full, once, as it takes two frames to refresh the image fully.

Does this make sense to anyone else? LOL
 

D.J.KRIME

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If you look at the Star Wars running times I have quoted, the real differnce is between the Cinema running time or 124.35 at 24FPS and the NTSc running time of 123(+sec) at 30 FPS being a differnce of about 1 min. Now the PAL dvd was encoded from a NTSC master not the original negative so is speed adjusted using a TBC (time base corrector) to adjust the 30 FPS to 25 FPS. If the PAL disc was encoded directly for the master in the same way as the NTSC has been then the differnce between the PAL disc and the Cinema original would not be as big due to the fact there is only 1 FPS differnce between PAL and 24 FPS at the cinema.

Now I remember this for a fact, on DVD Compare if you look up the film IRREVERSIBLE you'll find the American disc has a run time of 93m 33s which is exactly the same as the UK PAL release. "Wait,", I hear you say, "...this will only support the your argument." But it won't because in the notes at the bottom it states the American print was taken from a PAL master that hasn't been speed corrected.
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Anonymous

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No, I don't doubt you're right about run times and sources etc and actually this is very interesting. I'm curious about how they convert 30 to 25 and sort out the speed step in audio which IS a change of 20% regardless of what you do with screen refresh rates. 20% is enough that you'd hear it, so perhaps they add a frame (copy of the last of every 5) from PAL to NTSC so for every 5 frames they add one in and make it up to 6. Then in NTSC to PAL, for every 6 frames they take one out. They would transpose the audio onto the new mix. This would satisfy the timing requirement and I doubt you'd notice it on screen.

Does that work?
 

D.J.KRIME

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The differnce between the PAL and NTSC Star Wars disc is roughly 4 mins, if you worked on your 20% that would equate to 12 mins differnce per hour (20% of a hour is 12 mins) so maybe we are both a bit out with the exact percentages. But there is a definate differnce, maybe this is why a PAL tv can handle NTSC as where most NTSC models do not support PAL? Some dvd players have a PAL60 mode for tv's that dont support a native NTSC signal.

I rember a interview with Tom Selleck about Magnun PI and he said he hated watching it in the UK in PAL as he sounded so high pitched due to the NTSC to PAL conversion making the program run faster. Its a bit like when you adjust the pitch control on a turntable to speed it up i suppose.
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Anonymous

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Mate, as we're on the same team and I think we're just misinterpreting each other in the main, I'm just going to read Andrew's article. At some point we should arrange a gathering somewhere. I'm going to try and make it to the Novotel, but we'll have to see. I have a course that day. Though as Thaiman has no photo up and neither do you, if either of you are there and I do make it, grab me if you recognise me!
 
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Anonymous

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Guys, this is a relatively easy one.

Movies the world over are shown at a frame rate of 24 fps - that is 24 images are projected onto the cinema screen every second. Movie film is a very high resolution format, far higher in resolution than the DVD format. Accordingly, the movie itself is not the limiting factor in deciding between PAL and NTSC format DVDs, as the same source material is usually used to create both the PAL and the NTSC versions of a particular DVD. Therefore, the limiting factor in resolution is the DVD itself, with PAL inherently higher in resolution than NTSC.

The general rule of thumb when considering the time differences between PAL and NTSC formats is a simple one. Assuming no cuts have been made and the features are identical, the NTSC format runs 4% longer than PAL.

When converting NTSC to PAL, two things need to be accomplished. Firstly, 480 lines of resolution have to be up-converted to 576 lines of resolution, and 30 images per second have to be down converted to 25 images per second.

The resolution up-conversion does not actually add any real picture information to the image because you cannot create real picture information where none existed before. It does, however, make the picture viewable on a PAL display often resulting in a superficially better-looking image, however, if both transfers have been taken from the same source and the conversion process has been correctly applied, the differences between the two are undistinguishable to the naked eye.

The frame rate conversion actually results in a loss of temporal resolution, as PAL has a lower frame rate than NTSC.

Some people have claimed that PAL sources sound speeded up and claim to be able to tell the difference between a PAL and NTSC print. I don't believe that - the human ear is simply not that sensitive.

When converting PAL to NTSC, the converse situation applies with 576 lines of resolution being down-converted to 480 lines of resolution, and frames need to be inserted to go from the 25 frames per second of PAL to the 30 frames per second of NTSC.

Once again, the resultant image is of less actual resolution than the original image, as information is discarded spatially and made up temporally.

Complicating this issue is the fact that more and more programming is being created in high definition and these high definition formats can usually be converted down to PAL or NTSC equally well, with both formats having their respective disadvantages.

It has been established that a PAL DVD has 20% more resolution than an NTSC DVD. This does not necessarily translate into a superior image.

The DVD format relies on a video compression format (MPEG-2) to allow a reasonable length of programming to fit onto a single DVD. The longer the programming, the higher the compression ratio needs to be, and the more likely it is that visible compression artefacts will be present. If an additional 20% of resolution needs to be compressed, then this can potentially result in a lesser quality image if the programming is over-compressed. Fortunately, many PAL DVDs are being produced as dual layer discs, whereas their NTSC counterparts are being produced as single layer discs, thus providing the necessary room for both versions to look their best.

Movies on PAL DVDs play back 4% faster than their NTSC counterparts. The great majority of people will never notice this but strangely for a small minority it's an intolerable artefact.

In conclusion, PAL is a higher resolution format for DVD than NTSC. All else being equal, a PAL DVD of a movie should look significantly better than the equivalent NTSC DVD. If a PAL version of a movie DVD is not 16x9 enhanced and the NTSC version is 16x9 enhanced, then the NTSC version will be the preferred version, all else being equal.

For video-based material it's generally better for the DVD to remain in the same format as the source material. Do your homework before purchasing a DVD! Check as many resources as you can in order to determine which version of a particular DVD is the most appropriate one to purchase.

Hope this helps.
 
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Anonymous

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[quote user="ToSellBeNastier"]The resolution up-conversion does not actually add any real picture information
to the image because you cannot create real picture information where none
existed before.[/quote] Not heard of interpolating then?
 
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Anonymous

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richardjlarby

Perhaps you'd like to enlighten me instead of simply stating something without offering anything to back it up? I said the resolution up-conversion does not actually add any real picture information to the image because you cannot create real picture information where none existed before. That's "real" picture information. As far as I'm aware, Interpolation is a method of constructing new data points within the range of a discrete set of known data points. Perhaps you can edify me, here?
 
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Anonymous

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Touche !
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Grt post ToSell, good read very informative. You know where its at
 

FuzzyinLondon

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Just to point out that when you go to the cinema, you are almost always watching a 24fps film projected at 48fps. Modern projectors are set up to display each frame twice to reduce flicker that would otherwise be perceptible to the human eye. In reality 24fps is too slow. Essentially this method is similar to 48hz or 2:2 pulldown on a television. Modern tvs that can correctly process the 24hz signal from Blu-Ray/HD-DVD will display the signal at a multiple of 24(Usually 72hz, 96hz or 120hz) to reduce flicker and make the picture seem smoother.
 

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