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. 

Thursday 8 May 2008

Golden Eye - the scanner

In a small corner of the South Hall at NAB there was this Swedish company that has years experience doing analysis of film for auto crash testing and defence. Those films where you see cars with dummy humans crashed into walls. And slomo films of missiles leaving their silos.

These films need to be analyzed and data used to re-design things for safety or destruction. Whatever. And this company does just that.

Now they've used that expertise in designing a scanner and film analysis system that scans using capstan transport but providing the steadiness of pin-registered scanning. At least that's how I understood it. Its called Golden Eye.

The scanner is quite small, almost like a Cintel diTTo. Table-top. And there's a Win PC to analyze the film and save it as .DPX or a variety of formats. Scans can be 4k or 2k and there's even a swap option. Its a line scanner so many in-between resolutions are also supported.

The light source is a halogen lamp, what we call a 'cup halogen' lamp in India. Its a halogen bulb with a reflector and its an ordinary light source used in showrooms. My German friend even remarked that its a 5 dollar lamp. Light is conveyed to the film via a 'light pipe' so film doesn't get the heat. And a complex system of current control and filters ensures that the colours and intensity stays constant across the life of the lamp. Very ingenious and simple.

The stated purpose of this scanner is for archiving. And should be too. This transport is best suited for old film that has sprocket damage. In fact I think you probably don't even need sprockets.

Other nifty features. There are two 'cameras'. One scans the frame and the other, the entire film. So keycodes are read and need no complex alignment. And these two cameras help the software stabilize the film and ensure a steady frame. The other surprising feature is that the scanner is natively 35mm or 16mm ready with no complex replacement of wheels or lamp house.

Speed is 12fps at 2k and 3fps at 4k. And maybe they could get faster at HD or SD. So its a good healthy speed without the need for very high speed storage that 2k real time needs. 2k at 12 fps should need 180 MB/sec capable meaning HD-Ready storage should do. That's for 2k full aperture. If you're doing 3-perf, 1:1.85 or 'scope, you'll have smaller files so even lower data rates. Even 4k at 3fps amounts to about that much data rate.

The software can ingest EDLs and even ALEs as a source for scan lists. And they told me they could even work out a software feature that pre-reads the film so sorted negs can be read against cut lists and scanned by keycode. That would be a killer feature.

I think this scanner needs a look-at if one is into film restoration, even VFX. And since they can output Quicktime, I'd even try it out for dailies.

There's even a smaller version that's a bit larger than a Digi Beta deck! But its slower and does 1000 ft loads only.