How to convert 25i/p fps to 24p fps with superior results to standard Adobe methods

(Can also be used converting 30 fps to 25 fps)

25i/p fps to 24p fps (or reverse for 24 to 25)

It can done in several ways.

One method involves slowing the film down, but this results in a pitch change to the audio and when the audio is corrected by software, it is noticable music and singing.

Another method involves dropping the 25th frame but the result is jerky motion.

Twixter can 'invent' pixels which looks good, but problematic on some images.

In interlaced material, (all stations broadcast in interlace but play a lot of progressive material) Adobe and other software software programs just use the first field from each frame and drop every 12th field and last field of each second. This produces much smoother motion than above and the sound is unchanged but results in a halving of resolution.

But there is a complicated process involving using ALL fields. It is complicated as field reversal happens every half a second producing an alternate softing of the image

Full Resolution 25 i or p to 24p

The method

Dub a 25i (or 25p if using progressive clips) to a 50p clip. Take this clip and dub 2 clips, one upper field first and one lower field first to 2 24i clips. Put both these clips in a 24i timeline (one top of the other 50% opacity)
Problem is that after every 12 frames the picture alternates to soft because the field order has changed, as one field was dropped to make the speed change. To overcome this open a 24i lower field dominance timeline and reverse the upper field dominate clip and cut out the soft sections of each clip in a 12 frame pattern and make a new 24i (lower field first) master. Now you have a great 24i fps master.

Problem is that it is interlaced and if you make a progressive master all software programs just use one field, so you are back where you started.

To make a full resolution master, (true progressive) open this interlace sub master as a smart object in photoshop and use the interpolate filter to make 1 each of an odd and even field progressive dub. If you 'duplicate' instead of 'interpolate' the field order, the result is more alising on the edges. Open these two dubs in a new 24p time line and do a 50% mix. Re dub to to 24p and presto!.

You then need to check if there is a shot change (edit) in the reversed field sections of each 12 frame section as there will be 2 fields with 2 different shots. These need to be intercut these with a true progressive frame from the original 25i timeline. (by the same method, but you can skip every 12 frame group edit)

When working in the new 24p timeline, make sure you interperate the 25p master to 24fps otherwise the edit program will try and convert it to fit. It doesn't matter which frame you add, either the end of the first shot or the start of the second

These speed changes are the result of an enormous amount of trial and error and if you would like to show your appreciation please donate to:

Account Name: Graeme Beck Pty Ltd
BSB: 032087
Account: 198430
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30fps to 25fps

The White Paper

How to convert t 30fps to 25i/p in smooth motion with the reverse 3:2 'pull down' method.

The result is smooth motion and lip sync without creating additional pixels, artifacts or any sound pitch changes.

30p is best converted to 25fps via a reverse of the standard 2:3 'pull down,' to a 2:3 'pull up.' No pixels are recreated and the result is smooth motion, lip sync and without any pitch change.

Standard 2:3 'pull down' as used in transferring film to Television in the United States and Japan.

A variation of the above process is used where 30p is 'pull up' to 25i/p resulting in 2 progressive frames followed by 3 interlaced frames. When viewed on an interlaced monitor the results are vastly superior to any other method of conversion. The motion is very smooth. To view on a progressive monitor you have to follow another complicated step in converting these files into true progressive which is explained in the following section on the 25 to 24 fps conversion.

(1) The 3:2 'Pull up' method

The actual process is a little tricky, but in Premiere CS5 it is as follows.

Open a 60i timeline and import your 30p files into the timeline. 'Upper field' if going to dub to HD and lower field if going dub straight to SD. (Syncing of rushes and any grading is best performed in a 30p timeline and then cut and pasting the clips into a 60i timeline)

Right click each clip in timeline and select 'interlace consecutive frames.' Dub to whatever codec required in a 25i file. No need to change any file interpretations.

This method gives the smoothest motion of 2 progressive frames followed by 3 interlaced frames. It is chalk and cheese difference when viewed on an interlaced monitor.

For HD, Mpeg 2 gives a good universal conversion format and at a constant bit rate of 38mbs comes in just under the file size of the original clip. We can not see any loss in comparing the files. File size is not a problem in a Windows system but with Macs, file size needs to be kept under 4? gigs.

The 30p to 25i/p process in HD or SD both take a little over 4x real time on an 8 core i7 processor. Multi processors have little or no effect on the speed of the conversions. Going from a new Mpeg 25i HD timeline to a SD or DV file is quick and we can see no difference in a file struck direct from the original 30p HD timeline.

Electricfilms offer this service.

(2) Quicker variation of above

A quicker variation of the above where the computer processing is just about real time can be performed, although the results aren't quite as good as (1) when view on an interlaced monitor they are superior to a frame based blend.

Put the 30p files in a 60i (upper or lower field timeline does not make a difference and there is no need to change any file interpretations) and right click each clip in timeline and select 'interlace consecutive frames.' Dub to 30i HD (Mpeg 2 50 mbs gives good results).

Open a 30i timeline and import new file. Dub to a 25i timeline in SD or HD. Again, Mpeg 2 is a good universally accepted codec.

This result is a quicker process than (1) and results in every frame being interlaced. When viewed on an interlaced monitor is still superior to a frame based (blended) conversion but not quite as smooth as (1).

When viewed on a progressive monitor, (1) & (2) look the same as a blended frame based conversion.

Other Conversion methods

(3) Progressive to progressive - blended frames
Most computer programs do this method. We find the process a little jerky especially when viewed on an interlaced monitor. The fifth frame usually jumps.

(4) Progressive to progressive - pixel tracking
Twixter and After Effects have a great pixel tracking program where pixel vectors are created to guess intermediate frames but artifacts can occur with soft images. Fantastic results for super slow motion and Electric films offer this service.

(5) Slow Motion
Another method is to let the 30p material run slower at 25p. While this is a bonus for some images in slow motion, sound is 20% slower.


30p - The ideal speed
Even after the anticipate software release from Canon enabling the camera to shoot in 24p and 25p, on some jobs I would still shoot in 30p and do a 25i/p conversion. (2 progressive frames followed by 3 interlaced frames)
I find the 24p or 25p (progressive frame) a little staccato looking, especially when viewed on a big screen and the 50 field interlace look a little bit too fast.
30p (progressive) is an ideal speed.

A good work flow is to make your programmes in 30p for delivery to the Web, DVD and NTSC television.

A smooth motion PAL 25i/p master via the above 3:2 pull up method (1) can be made for PAL television.

Contact: Graeme Beck

m: 0411 82 7799

p: (02) 9980 7788

e: graeme @ director.net.au

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