Thursday 15 May 2008

24fps film or HD out to SD PAL

24fps film sources mastered on Digital Intermediate systems like iQ or Resolve can be transferred to HDCam-SR or D-5 masters as 1080p24. These are recorded at, and playback at 24fps. 


When going out to SD PAL to a DigiBeta/Beta or even to a DVD recorder, these film/HD systems behave differently. Systems including but not restricted to - Quantel iQ, DaVinci Resolve, Sony HDCAM-SR VTR SRW-5500, Panasonic D-5 VTR AJ-HD3700.  All these play out 24 fps as 25 fps to be compatible with SD-PAL. That is, they play the source, at 104% speed, or 4% faster. 


Is there a way to prevent this? I haven't found it yet - with any of the above machines.  The method we use (of transferring a 24fps source to a 25fps tape master) without changing speed is to capture the HD 24fps into an Avid or FCP system and then playing it back at what is called 24+1 OR 24@25 repeat. Both Avid and FCP systems have the capability of playing a 24fps timeline to a 25fps tape or monitor. 


This transfer incurs no losses as Avid and FCP systems are now capable of doing uncompressed video. SDI in SDI out. HD or SD. So these are lossless as far as video and audio quality goes. With Avid/FCP systems and this method makes audio sound right but includes an ever-so-slight once per sec freeze in the video. Those who know can see it but most people cannot see this video motion artifact. 


Incidentally, 24fps to NTSC playback or layoff to tape is NOT affected by this anomaly. Because there is a age-old method of making 24fps into 30 fps by way of pulldown frames. PAL has NO such method. (That I know of). In fact editors and post professionals in NTSC countries are often not even aware that this issue exists in the PAL world.


Here in India, we make 24fps film into 25fps DigiBeta for Indian Television as 24+1 using Avid or FCP systems. Or, for film sources, via telecine which also has the capability of transferring 24fps to 25fps without changing speed. These systems repeat one frame each second so 24 frames per second plays out as 25 frames per second. Actually they operate on the field level by repeating one field every 12 frames. 


In India, our song and music professionals do not accept the 104% faster sound. They don't seem to be bothered by the small video motion artifact. So even Indian TV accepts the slight freeze in video that 24+1 creates. Look at any Hindi movie on Indian TV and you'll see it in long pan or moving scenes. 


On the other hand our 24+1 tapes have not been accepted by UK and German (amongst others) telecast authorities. They insist on clean video and couldn't care less about our songs playing faster. The one per sec video freeze doesn't go down well with them. So for telecast in UK/EU we do the '24 as 25' method as provided by iQ/Resolve or HDCam-SR/D-5 systems. 4% faster sound. 


So two different methods with two different results. 24+1 has a slight motion artifact but correct audio and 24 as 25 has clean video but fast sound. For one feature, two years back, I used Apple Compressor to do a smooth 24 to 25 conversion. That looked good and the freeze was gone. The new Apple Compressor does an even better job. 


But this conversion takes time. And with a 3 hour feature to master on an unreal schedule, time is the last thing on has. So I wonder, is there some other method of converting 24fps HD to 25fps DigiBeta without introducing imperceptible video motion artifacts and still holding sound speed and doing it all in real time?. 


Update Sep 2011. 
When I first posted this, we had Dual and Quad G5 Mac systems which took 10 or more times real time to do this smooth conversion.
Today, with the newer i7 MacBook Pro and iMac systems and 8-core and 12-core Macpro systems, its now fast enough that the 24-25 smooth motion interpolated correction is now only slightly over real time. A 2.5 hr feature in SD can be corrected in about 3-4 hours.
What about HD? You may not need the conversion in HD. HD can play 24fps from tape.
Unless you are delivering a HD file for telecast on HD channels in India. They accept 25fps even for HD. 

7 comments:

  1. Hi,

    Do you actually get 24p on HD ? I think it's 23.98 fps ?


    And I would use After Effects to convert from 24p to 25p.

    ReplyDelete
  2. If you mean 24p HD on HDCam-SR or D-5, then yes it is 24fps. We telecine film to 24p HD silent, then take the 24fps sound that ProTools exports as 24fps, and lay it on the same tape and they are in sync even for 2 hour lengths.

    HDCam-SR and D-5 play 24p as 23.98 fps when going out to NTSC 29.97 and as 25fps when going out to PAL 25.

    So the down-convert NTSC from 24p HD is of the same speed, but in PAL its 4% fast.

    ReplyDelete
  3. "So I wonder, is there some other method of converting 24fps HD to 25fps DigiBeta without introducing imperceptible video motion artifacts and still holding sound speed and doing it all in real time?. "

    Sir I have been looking for an answer to this question as well for sometime
    a google on the subject yields some answers
    http://en.wikipedia.org/wiki/Telecine#2:2:2:2:2:2:2:2:2:2:2:3_pulldown
    which would result in less noticable motion artifact every 13th frame but seems very practical
    and possibly can work realtime.
    24frames progressive video will result in 48 fields
    25 frames = 50 fields
    so instead of doing a full frame jump we repeat a field every 13th frame.


    Motion estimation to generate the extra frame would be the second answer which is what apple compressor does(is what i know)
    but will take really a lot of time

    ReplyDelete
  4. I remember reading somewhere that Avid Film Composer would do a 24-25 conversion by repeating a field every 12 frames. That's exactly what you're referring to. And it was available in 1994.

    I'm not sure how it is with Mojo.

    Smooth conversion from 24 to 25 fps is possible and even looks rather good in Compressor. But its not real time.

    ReplyDelete
  5. I wouldn't call a repeat field every half-second an 'imperceptible' artefact. It's readily noticeable, particularly on pans or other lateral movement. As you've pointed out, shows mastered this way will fail QC by major broadcasters in Europe (and elsewhere).

    As you mentioned, the solution for everyone else is 24@25, which is how Hollywood (and, indeed, Bollywood) movies are mastered for DVD in Europe.

    If you want real-time conversion without a speed-change, check out Alchemist from Snell & Wilcox with the Film Tools upgrade. This can handle smooth format conversions with a variety of framerates, including automatic pulldown removal and cadence detection for mixed format sources.

    I've never done 24fps to 25 with Alchemist myself, so I'd want to get some tests done to see the results. I must say though, as an online editor I'd be apprehensive about doing a conversion which left only one 'pure' (ie. original) frame for every second of footage. That's a lot of interpolation for a master tape.

    ReplyDelete
  6. how to convert 24fps to 25fps in finql cut pro
    my clip record on canon 5d mark II .
    plz solve my problem

    ReplyDelete
  7. Monty you can conform 24 to 25 in Cinematools for a 4% speed up, or do 24+1 in a 24@25 timeline.

    ReplyDelete

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