by Michael Buday
and Steve Cohen
Go to 24P Part 1
Last months installment covered a bit of NTSC and HD history and focused on offline editing in a 24P environment. This part will cover the issues youll face after you lock your picture: prepping for online, the online itself and the myriad of potential delivery formats youll 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, theyd 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 VTRs, switchers and DVEs 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 were 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, youve got to start with 24P originals, which means that youve shot your show on 24P video or youve shot your show on film and telecined your dailies to 24P. The dubs youll 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, youll 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.
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, Sonys 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 theres 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.
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 wont be transferred and, as far as video is concerned, dont exist. The final NTSC version of your show will represent a very significant cropping of the film frame. DPs 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 weve 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 youll 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:
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 cant 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 theyve 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?
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 youll 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 theres a second problem. To use your 30 fps NTSC material in the 24P online, youll have to convert the frame rate, which means potential motion artifacts. Here are some possibilities:
If the elements were originally shot on film:
If the elements were originally shot on video your best bet is to deal with NTSC material after youve created your 30 fps masters from your 24P online original. Heres how:
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 youll 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.
Whats in the Suite?
What will you see upon entering a 24P linear suite? Itll 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 youll be working in also has the following essential bits of additional gear:
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. Its 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 isnt 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 youve finished conforming. Then, its fairly easy to play both VTRs from an edit controller and manually "bump" them into sync. Youll 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 youll 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 youve done youre homework and prepped accordingly, your first 24P online should be as boring as any other conforming session!
Michael Buday is an offline and online editor as well as
a consultant to Sony Broadcast, Leitch, Inc. and JVC.
His HD online credits this year include
'Family Law', 'Judging Amy', 'James Brown Live at
the House of Blues' and 'Touched by an Angel'.
He can be reached via email
editor of the Guild Magazine. He is currently
cutting '15 Minutes' for New Line.
He can be reached via email