by Michael Buday and Steve Cohen

Go to 24P Part 1

The first part of this article introduced one of the most exciting developments in the world of high definition television, ‘24P’: the new, progressively scanned, HD video format that runs at 24 frames per second and boasts a screen resolution of 1080 lines. In production, 24P cameras will allow features to be shot on tape with excellent quality. In post-production, 24P allows editors to create a single master that can be easily converted to all the delivery formats that broadcasters and exhibitors will want: NTSC, PAL, all the HD formats, and theatrical projection.

Last month’s installment covered a bit of NTSC and HD history and focused on offline editing in a 24P environment. This part will cover the issues you’ll face after you lock your picture: prepping for online, the online itself and the myriad of potential delivery formats you’ll choose from.

24P Online Basics

24P was just a glimmer in the eyes of engineers at Sony just a couple of years ago. Though the idea of 24 fps HD video mastering seemed to offer the possibility of a universal mastering format, Sony believed that to make the system work, they’d have to develop 24P-specific versions of nearly every piece of gear that is seen in an HD online bay. But engineers at Laser Pacific here in Hollywood showed them that with some simple modifications, the existing HD gear could be made to work effectively. Sony and Laser jointly patented these modifications. As a result, 24P capabilities appeared at Laser Pacific first, using prototype VTR’s, switchers and DVE’s from Sony. Other post-production facilities followed their lead once the gear had been debugged and patent issues were resolved.

24P onlines are very similar to the standard definition onlines we’re all familiar with. But there are some significant ‘gotchas’ that you must be careful of. And since HD onlines are more expensive than NTSC, any problems you encounter are going to be significantly more costly. A good understanding of the process and a properly prepped EDL will inevitably save time and money.

In order to online in 24P, you’ve got to start with 24P originals, which means that you’ve shot your show on 24P video or you’ve shot your show on film and telecined your dailies to 24P. The dubs you’ll use for cutting in an Avid or other offline system will be 30 fps conversions from the 24P masters.

Know Your Delivery Format(s)!

Even in the world of Standard Definition TV, namely NTSC and PAL, video delivery was complicated and many versions of your show typically had to be generated. But the HD world makes delivery even more difficult. The HD broadcast standard includes 17 distinct formats, and standard consumer HD TVs must be capable of receiving and displaying all of them. Know your delivery requirements before you begin! For network delivery, you’ll most likely be required to supply interlaced masters in both 480i and 1080i formats. ABC has chosen 720P as their HD standard, but they will accept a 720P master that has been converted from 1080i.

Problems

The goal of onlining in 24P is to create a single master that will survive the test of time and can be easily converted into any delivery format required. In fact, Sony’s 24P decks automatically and simultaneously output video in three different formats: 24P, 1080i and 480i.

To produce PAL masters, a simple software switch allows the 24P deck to run at 25 fps and output 580i PAL directly with excellent image quality – but with a 4% speed increase. Standards conversion is also available with additional gear. This produces unaltered running times, but image quality suffers.

In practice, producing masters in various formats is rarely as simple as ‘pressing play’. Problems stem from two sources: aspect ratio differences between formats, and frame rate differences.

Aspect Ratio Issues

NTSC and PAL video are recorded in the 4:3 aspect ratio – which produces the familiar height and width of conventional television sets. But all the HD formats, including 24P, are recorded in the 16:9 aspect ratio. This is similar to, but slightly taller than, 1.85 (actually 1.77). On film, when we shoot for 1.85 theatrical projection, we always photograph the entire film frame and then crop to 1.85 in the projector. We use the rest of the filmed image (the top and bottom) when we transfer to tape. But 16:9 video is just that – there’s no top and bottom that can be used when converting from one format to another. This means that the 4:3 submaster that you make from your 24P or other HD online master will have to be created by cropping the HD image.

A 4:3, NTSC, TV frame superimposed over a 16:9, HD frame. The 4:3 image is normally cropped from the HD master and the result is that the sides of the image are lost. Panning and scanning can be used to help show important parts of the image.

Because most broadcasts still occur in 4:3, one issue you’ll face is how to view your work during offline editing. Should you cut at 16:9, knowing that most of the world is going to see your work cropped, or should you cut at 4:3 and know that there are parts of the image you can’t see? Depending on how you make your offline dubs and how you set up your offline system, you can work either way and both entail compromises. In the end, there’s no substitute for running your cut show in both aspect ratios and checking for problems before you lock picture.

Cinematographers confront the same problem – the fact that there is no way to create images that look perfect in two aspect ratios simultaneously. One or the other has to represent a ‘good enough’ compromise. When we shoot for 1.85 theatrical projection, for example, we make the 1.85 frame look great and we protect the taller TV frame (essentially the full film frame), assuming that whatever is in there will be okay and available when transferred to conventional video. But if telecine and mastering are done in HD (24P or 1080i) the top and bottom of the film frame won’t be transferred and, as far as video is concerned, don’t exist. The final NTSC version of your show will represent a very significant cropping of the film frame. DP’s must take this into account when shooting and their viewfinder reticle should be adjusted to account for it.

Frame Rate Issues

24P runs at 24 fps (actually 23.976). Conventional and most HD video formats run at 30 fps (actually 29.97). For years we’ve known how to convert 24 discrete (progressive) film frames to 30 interlaced video frames (60 fields). We do it every day in telecine by duplicating fields and the result is smooth-running video with minimal motion artifacts. But converting the other way, from 60 fields to 24 frames, is much harder because fields must be eliminated. Motion problems are almost impossible to eliminate. Smooth pans become jerky; title crawls become jumpy. Video to film conversions always confront this problem and get around it in various ways, but the results are never perfect.

Getting Started: Generate a 24 fps EDL

To begin the online process you’ll need to create an EDL from the offline system that will work in a 24 fps environment. All the rules that apply to creating a good EDL for a standard definition online apply to 24P. Pay careful attention to the following:

• 24P uses only non-drop-frame timecode. All your daily reels and your EDL should be non-drop.
  
  
• Motion effects work like film. Slowing down video shot at 60 fields per second is very different than slowing down video or film shot at 24 frames per second. Just like film opticals, the only effects that will look good are whole-number multiples or fractions of your frame rate – namely double prints (50%), triple prints (33%), etc. Hopefully, any slo-mo sequences were anticipated beforehand and over-cranked during production. However, if your project was shot in the HDCAM 24P format, there is currently no way to over-crank the camera. For now, the only solution for HDCAM 24P projects is to shoot scenes that require over-cranking on film, and then transfer to 24P tape.
  
  
• If your project was edited on Symphony or another Avid system that supports a 24 fps EDL (make sure you check!), you can output a true 24 fps EDL directly from EDL Manager. For all other editing systems, you’ll have to output a 30 fps, non-drop frame EDL, which will later be converted back to 24 fps via "matchback" and cheated frames. Lists generated by Film Composers that don’t support 24P will require a double matchback to create a 24 fps EDL.
  
  

Graphic and Title Elements

Ask yourself how graphics and titles will be handled. 16:9 safe or 4:3 safe? Obviously, you can make life easy by making all titles fall within the 4:3 safe area, but this means that they can’t use the sides of the widescreen 16:9 format. If you have the time and budget, it will be better to create title elements for both aspect ratios and then separately add them to the HD, NTSC and PAL masters after they’ve been created from the 24P online.

Incorporating NTSC Material into a 24P Master

24P is a new standard and for now, most of the video in the world is in other formats. What happens if you need to cut conventional (NTSC, 4:3, interlaced) video into your 24P master?

Using NTSC materials in an HD online. The original NTSC material is incorporated by blowing it up to fill the 16:9 frame-width and then cropping it top and bottom. But when you make a standard-definition dub of the completed show, this material will be cropped again, producing an even bigger blow-up.

Take, as an example, a TV series that has switched from onlining and delivering standard NTSC (480i) to onlining on 24P. The show is shot on film, but many of the elements that are normally used (opening titles, bumpers, teasers, stock, etc.) exist only on 480i videotape in a 4:3 aspect ratio.

Delivery includes a 16:9 HD master and a full-frame 4:3 NTSC master. How do you deal with old NTSC video? If you blow it up to HD you’ll have to crop it top and bottom to create the HD 16:9 image. But if you do that, you will have "boxed yourself in" when it comes time to create a 4:3 master. Any material that was blown-up or stretched will have to be cropped again to 4:3. That creates a double blow up on your NTSC master.

The only solution is to create the 24P master with slugs for the NTSC elements and add them to the HD and NTSC masters separately.

But there’s a second problem. To use your 30 fps NTSC material in the 24P online, you’ll have to convert the frame rate, which means potential motion artifacts. Here are some possibilities:

If the elements were originally shot on film:

• Re-telecine the elements to 24P (assuming that they still exist). This offers the best picture quality and control over framing and aspect ratio during retransfer. However, this could be a costly proposition for heavily layered visual effects material, since it will have to be re-composited.
  
  
• Remove the pulldown. If the 480i material was originally telecined from cut film, it is possible to remove the pulldown and recreate the original progressive frames for 24P. However, if it was edited or composited on videotape, chances are very high that a continuous "3:2 cadence" was not maintained from cut to cut, which will make the removal of 3:2 difficult or impossible.
  
  

If the elements were originally shot on video your best bet is to deal with NTSC material after you’ve created your 30 fps masters from your 24P online original. Here’s how:

• Using all native 24P source material, create a clean "24P-EM" (edit master) minus titles and graphics by auto-conforming your project in 24P. If you have 24P versions of your title sequences, bumpers, etc., cut them in at this point. Leave slugs for NTSC material.
  
  
• Using the "EM", perform tape-to-tape color correction to create a "24P-CTM" (color timed master).
  
  
• Using the "24P-CTM", add any additional 24P titles or graphics in a second, 24P online session. If any of these new elements are NOT 4:3 safe, be sure to retain "clean" versions of any underlying material.
  
  
• Convert your "24P-CTM" to 1080i. Call this the "60i-CTM". Up-convert your 30 fps NTSC material to 30 fps 1080i. In a 1080i edit session, cut in the up-converted material (this will be much easier if the up-conversion preserves your original source time-codes).
  
  
• Down-convert your "60i-CTM" to 480i (using the same time-code). Call this the "480i-CTM". In a 480i edit session, add your NTSC material using the EDL from the 1080i edit session. Since source and record time-codes will be the same, this should go quickly.
  
  

The Online: Linear vs. Non-Linear?

In theory, the advantages of non-linear online editing in an HD world are the same as they were in old-fashioned NTSC: fast auto-assembly and editorial flexibility. But speedy assembly has to be balanced against the extra time that you’ll need to digitize your master tapes into the nonlinear system. And editorial flexibility has a dark side, too. If changes are easy to make, changes are going to be made, and changes in online have time and cost ramifications for the whole finishing process.In practice, HD nonlinear onlines are going to take more time than they did in NTSC because the bandwidth requirements of HD are so much greater. Uncompressed Standard Definition TV (480i) requires a bandwidth of about 270 megabits per second (Mb/s). By comparison, uncompressed high definition TV (1080i) requires about 1.3 gigabits per second (Gb/s), or about 5 times as much. HDTV also gobbles up drive space at about 5 times the rate of SDTV. The result is that nonlinear editing systems that could play two or three streams of uncompressed standard definition TV in real time may be capable of playing only one stream of HD. Visual effects that were easily accomplished in standard definition boxes may put you into rendering hell in HD, and because HD frames are so much bigger, renders will be very slow.

At the time of this writing, there are very few non-linear systems currently on the market capable of conforming a 24P HD project. Quantel (Editbox Chaser) and Discreet Logic (Fire/Smoke) have just begun shipping 24P HD versions of their editing products. At the ITS show in July, there were promising beta versions of 24P HD editors from Softimage (DS) and Pinnacle (Targa 3000/Cine card running Final Cut Pro). While some of these systems may be available by the time you read this, ask yourself if you want to be the first one on your block to use them. Be afraid, at least for now.

What’s in the Suite?

What will you see upon entering a 24P linear suite? It’ll look much the same as a standard definition suite, except for video and audio monitoring. The HD video monitor will be of a 16:9 aspect ratio and capable of displaying images at resolutions up to 1920 x1080 pixels. As for audio, most HD suites have been upgraded to handle at least six channels, if not eight. All the other gear – edit controller, switcher, digital effects boxes, audio mixer, color corrector, character generator – will look familiar, but all should have been upgraded or replaced to handle the 24P HD format.

Before you book a 24P online session, make sure the room you’ll be working in also has the following essential bits of additional gear:

• A ‘graticule’ generator:
  Used for temporarily keying alignment markers over the image to allow you to see "title safe" and "action safe" areas for both 4:3 and 16:9 images. This is an obvious necessity for projects that will be delivered in different aspect ratios. It can also be used to display standard or non-standard theatrical markers for 1:85, 2:35, etc.
  
  
• A standard-definition monitor:
  If a standard-definition master is to be generated, make sure that an interlaced, NTSC monitor is in the suite with you in addition to the HD monitor. The ability to see the HD, 16:9 and SD, 4:3 versions of your project simultaneously is invaluable. This is especially important when making color-timing adjustments, since NTSC does not operate in the same "color space" as HD.
  
  
• 1080i capability:
  Be sure the suite you’ve booked is capable of switching from 24P to 1080i and editing in that format if necessary. Depending on your requirements, you may need to edit some elements in their native interlaced format.
  
  
• A true HD character generator:
  Some HD facilities have not made the commitment to purchase a "true" HD character generator. Instead, they up-convert titles using a standard NTSC character generator. While this approach may look acceptable, true HD titles look much better, especially if your project will be transferred to film.
  
  

What to Do with the Digital Cut

Most online editors are fond of copying the digital cut of a project to the online edit master prior to the start of conforming. It’s a great way to check that each shot in the EDL is being recorded as it was intended. Another favorite technique is to slave the digital cut to the online master so that they play together as the online progresses. Unfortunately, neither method will work in 24P because the digital cut isn’t running at the same frame rate as the bay. The digital cut runs at 30 fps while the edit session runs at 24.

The only solution available at this time is to wait till you’ve finished conforming. Then, it’s fairly easy to play both VTR’s from an edit controller and manually "bump" them into sync. You’ll need two monitors however: an HD, progressive scan monitor for the 24P master and a standard definition interlaced monitor for the digital cut.

Determining Actual Running Time

Because 24P supports only non-drop timecode, the actual clock time of the edited master will be different than the timecode on the tape. However, the online editor should be able to provide you with an accurate running time, if desired. If your online facility uses a Sony BVE-9100 edit controller (the most popular linear editor for 24P), the ‘Real Duration’ button is available for this purpose (it has to be assigned through the ‘key-mapping’ menu).

Wrapping It Up

A year ago, very few people knew about the 24P format. But interest in it, both for production and post-production, is beginning to ramp up in a very big way. Chances are high that in the near future you’ll find yourself face to face with 24P-based materials, either in offline or online. Despite considerable confusion surrounding 24P technology and the endless DTV formats, knowing exactly what you need before entering an online suite is sure to save you considerable time and money. If you’ve done you’re homework and prepped accordingly, your first 24P online should be as boring as any other conforming session!