23.976 fps

[Bruce Watson]

I've been looking and haven't found a coherent (at least to me, otherwise I wouldn't be making this post ;-) explanation of the telecine problem -- 24fps -> 23.976 fps (or 24/1.001 fps). How did we end up with such a seemingly odd ball frame rate? What's it's purpose? What's special about that 1001st frame?

I'm guessing it's got something to do with sync. But what?

[Graham Hickling]

From Wikipedia: The NTSC field refresh frequency in the black-and-white system originally exactly matched the nominal 60 Hz frequency of alternating current power used in the United States. Matching the field refresh rate to the power source avoided intermodulation (also called beating), which produces rolling bars on the screen. When color was later added to the system, the refresh frequency was shifted slightly downward to 59.94 Hz to eliminate stationary dot patterns in the difference frequency between the sound and color carriers.

From THE definitive explanation of 29.97 and 23.98 timecode - Gearslutz.com : Consequently, [many] video monitors only displayed 29.97fps. To allow film to be played at this rate, it needed to be referenced to video time instead of film time:

24fps / 60Hz = X / 59.94Hz (solve for X)
X = 23.976

[Bruce Watson]

I knew it had to be something arcane. Thanks for the pointer to Gearsluts, nice explanation.

[Ron Evans]

Even 24fps came from the lowest frame rate for adequate optical sound track performance to get the lowest cost for distribution film costs.

I think another confusion is that 60i is 30p transmitted as fields. It is in fact 60p transmitted as 60i i.e. half the vertical information missing from each frame. So the temporal motion is of a 60p frame rate not a 30p frame rate. Which is were I have a problem calling this 29.97fps or 29.97i !!! What's wrong with 60i ?
We are of course stuck with the past. 24fps of film was an economic decision as was the issues with sync to mains electricity clock rates around the world and the ability of early electronics to provide adequate guard bands etc. None of which apply today other than the complications of accounting for the past !!!!

Ron Evans

[Shaun Roemich]

Ah semantics.... this is a game I love.

Ron is right when he states that 29.97 is essentially 60P with half the vertical information missing. Where the 29.97fps comes in is that this is now recombined to form frames that require two halves (the fields) which are 1/60th of a second a part to be recombined which of course can only be done 1/30th (1/29.97 actually) of a second. In transmission is when it BECOMES 29.97. The individual fields do not exist for any practical purpose - it is only when they are recombined that we have something that can be displayed in a normal fashion.

It's almost like saying orange juice from concentrate is pure juice:
1. It started out as pure juice
2. We took ONLY the water away
3. We later added water

Unfortunately, while it certainly bears resemblance to pure juice, it IS "juice from concentrate".

[Ron Evans]

Well I guess my issue is 60i is never 30 or 29.97. It is transmitted as 60 fields for most broadcasts and for a CRT is displayed as fields 1/60 sec apart. Problems arise with flat panel displays which require a progressive display at 60hz. The display has a choice of how it creates a 60p display from a 60i input. The good ones interpolate full frames from the fields especially the newer 120hz and 240 hz displays. Cheaper models do a poorer job of creating these progressive frames that result in artifacts further complicated by whether the display is a 720 or 1080 display and what the input resolution happens to be. The really cheap and nasty way is to add the fields and display twice resulting in the most artifacts!!!
Can't wait for 60p for everything!!!

Ron Evans

[Shaun Roemich]

The problem with that logic though is that a single FIELD is non-displayable. A frame is the smallest displayable form and that is comprised of two fields and displayed at 29.97 so while I understand your point, the discussion of 60i NEVER being 29.97 is flawed.

And for the record, I refer to SD as 480i60, NOT as 480i29.97 (although in slates I state 29.97 DF, which is the valid Timecode description)

[Ron Evans]

An interlaced CRT displays 1 FIELD AT A TIME, 1/60sec apart whether it is SD or HD. Most broadcasts in SD or HD are interlaced. The signal is interlaced until it is decoded by the display. I agree that unlike a CRT a flat panel display needs a progressive image to display and thus has electronics to de-interlace and scale to the pixel count of the display from the input signal. In this regard it is unlikely to display a single field but not impossible.

The frame rate for a 60i interlaced video is 60 fps NOT 29.97. The convention is to call this 29.97i. The camera shot the video at 60fps it just happened to only record half the vertical resolution but the temporal motion is of a 60fps video viewed best on a CRT for which the whole system was designed many years ago.

Shaun I think we have had this discussion before and I don't want to repeat again.

Best regards
Ron Evans

[Shaun Roemich]

Originally Posted by Ron Evans

The frame rate for a 60i interlaced video is 60 fps NOT 29.97.

The FIELD rate is 60 FIELDS per second, not frames. The frame rate is in fact 29.97.

And I'm sure we have had this discussion before.

Again, I understand the POINT you are trying to make but the language you are using to explain it is incorrect.

[Brian Drysdale]

The convention is that it takes 2 fields to make up one frame.

576i - Wikipedia, the free encyclopedia

Interlace - Wikipedia, the free encyclopedia

As the latter points out: "A field is an image that contains only half of the lines needed to make a complete picture." This would seem to be being used to differentiate between having a pure complete frame system i.e. progressive. Although the complete interlace frame does have temporal information that can be seen if there's movement.

[Ron Evans]

The CONVENTION is 29.97 because one needs 2 fields to make a frame and thus 60 fields would make 30 (29.97) frames. However these are not real frames just like the fields are not complete frames. The 60 fields do not belong to these 30( 29.97) frames they belong to 60 frames that were taken 1/60 apart with only half the information being recorded. The temporal motion is at 60 fps not 30fps.

In North America the refresh rate for CRT and flat panel is 60hz for most displays. A CRT displays a field every 1/60 second first Odd then Even( the lines one sees if up close to a CRT) they don't over lap they interlace. Our eyes brain perceive the motion as if a full frame is displayed at 60fps because the image is in fact changing every 1/60 sec.

In the 120hz or 240hz LCD this interlaced information is used to interpolate a 120fps or 240fps image resulting in a very smooth image. A 60hz flat panel has to find another way of deinterlacing and the various manufacturers use different methods to do this and display a 60p image required for the display.

None of these displays a 29.97fps image accept by named convention. Most CRT display 60i, depending on flat panel the display rate varies and refresh rate is normally 60 or higher.

I will accept the convention but it is just that, it is not a real display frame rate.

Ron Evans

[Brian Drysdale]

Perhaps it best to keep to the conventions, otherwise confusion arises. If you factor in motion picture frames, which in PAL each would have two fields, the image temporal difference no longer applies.

[Ron Evans]

24p motion picture frame rate over television is yet another issue that is confusing!!! Your choice of 2:3 or 3:2 convention but 24 p cannot be shown on a 60hz display without pulldown. So film on 60hz or 50hz TV is not 24p it has a cadence which includes the pulldown so is not at all like film in the cinema from a 24fps projector with either a 3 or 5 blade shutter. One needs a 120hz or 240 hz or 72 hz display to show true 24p. Refresh needs to be a multiple of 24 for accurate display. Matters not whether one is in North America or Europe. Will it play ...of course, but it has the appropriate pulldown cadence or speed change. If it is transmitted/broadcast then the cadence is embedded and cannot be removed. Its what unfortunately people view as the film look on TV and wonder why film at the cinema looks different!!!!

For someone who has been around for a long time the difference between convention and reality is not a problem.But for someone new to the environment it can be very confusing.

Ron Evans

[Ron Evans]

Originally Posted by Shaun Roemich

Perhaps you should contact the NTSC and SMPTE with your opinions and see if they will reconsider their formats that have been in place since the onset of colour television.

As I have said before I accept the convention. Don't confuse this with reality. The temporal frame rate of 60i is 60 fps NOT 29.97. 29.97 is a convention not a display frame rate. That is all I am saying. When these standards were developed the main display was CRT with a refresh rate of 60hz one field at a time. There is a different display every 1/60 sec as perceived by a viewer its 60fps. Accurately displaying what the camera took capturing one field every 1/60 second and transmitting as a field to maximize use of bandwidth. Temporal motion was of a frame rate of 60fps and that is still the case with all current 60i cameras. There is a big difference shooting at 60P and 30p in motion that I am sure you are well accustomed to in your work. Displayed on my Sony 240 hz display there is no difference between 720P60 or 1080i from my camera. The display pretty much makes them both look the same. The temporal motion is the same. The still images from a frame are very different since there is a full frame from the 720P60 video so the still from the interlaced video lacks vertical resolution. 30P is very different introducing a judder not quite as bad as film 24fps but close and to me unacceptable.
This whole discussion revolves around my view that an image rate is what matters as apposed the technical issue of whether what is captured is a full frame or a field. Further if one is talking about fields one cannot blindly add fields without regard for the time difference of capture. 60 fields captured 1/60sec apart do not make a 30fps frame rate!!!! I accept that 2 fields make a frame and to allow for the colour signal etc that convention states that 60i to be 29.97frames.
Ron Evans

[Shaun Roemich]

Originally Posted by Ron Evans

60 fields captured 1/60sec apart do not make a 30fps frame rate!!!!

Actually, they do. They don't make a 30P frame, they make an interlace frame that plays back at 29.97 fps. I don't know what else to say.

Again, I'm not suggesting that 29.97 interlaced looks anything like 30P, nor does it look like 60P - 60i looks like 60i, for better or worse.