New Developments in Nonlinear Editing
Fabricating High-Performance Nonlinear Video Editors from Off-The-Shelf Parts
By Clyde Tressler and Robert Lamm
In the past, the limitations of ISA, EISA and NuBus computers as well as the shortcomings of then-available digitizing cards and drives made it difficult to assemble a high-quality nonlinear editing system on an average personal computer. The data rate demands of digital video exceeded the capabilities of standard configurations of off-the-shelf equipment. Consequently, most nonlinear editing systems, such as Avid's Media Composer or the Data Translation Media 100, were sold as specific software/hardware combinations and highly optimized subsytems. The result was: pictures that showed compression artifacts and systems that crashed a lot. To top it off , the solutions were expensive and users could not choose editing software independently of hardware. This situation left many users stranded as various systems ceased development or went off in a different direction than the user did.
Several factors have changed this situation significantly:
One is the advent of the PCI-bus computer, now several years old, which can carry a theoretical data rate of 132 MB/sec data on its bus. Even in the real world the throughput is high enough to support multiple 21MB/sec uncompressed video signals. And this is the technology inside the home computer, not the specialized video black box.
Another factor is the new generation of video capture/playback cards, which can not only support 9MB/sec (almost Digital Betacam) quality, they can play back two simultaneous video streams and perform real-time effects (such as dissolves and keys) on them. These work with a variety of editing applications, from the pedestrian Premiere to the more premium applications like Avid MCXpress or Scitex SpherOus.
A third factor is the steady improvement in the capacity, speed, and economy of off-the-shelf AV-performance disk drives and the SCSI bus that talks to them. The new ultrawide bus can move data at 40mb/sec -twice that of the SCSI wide bus. SCSI ultrawide drives come in a 9 gig size, and, in our tests were able to sustain 9mb/sec data rate recording and playing back real video from a single drive. This is in the same league as a two- disk array of wide drives. So once again the baseline capability of ordinary equipment has moved past the point where its higher than the basic requirements of broadcast-quality video.
And finally, none of this would be possible without the availability of stand-alone software capable of exploiting all these hardware advances.
We experimented with one of these software packages, Avid MC- Xpress/NT 1.5, which was kindly provided to us as a beta release. It runs on a variety of video cards: we used it with the Truevision Targa 2000RTX, the highest-performace capture/playback card we could find. The system ran on a standard production-model Pentium PCI computer, the ACW 1000, which we got from ALS Computers in Brighton MA. This model comes standard with Ultrawide SCSI card, Windows/NT 4.0, etc.
Media was digitized onto a pair of Seagate Barracuda ST-19171W ultrawide SCSI drives, the fastest drives we could find on the open market, attached them to the Adaptek 2940UW SCSI card that came with the computer, and striped across them with Windows/NT 4.0.
We got very high performance. Based just on drive capacity, the MCXpress performance analyzer was able to get reads of 635KB/frame, writes of 563KB/frame, and suggested digitizing rates of 500KB/frame. This is considerably better than Digital Betacam, the current state-of-the-art production format, which has a data rate of about 360KB/frame. Unfortunately, the digitizing card isn't capable of actually working at this rate, when file size was compared with recording length, we discovered that recordings actually peaked out at 330KB/frame. Nevertheless, this is still a very high data rate, very close to Digital Betacam, and just slightly higher than the board spec of 300KB/frame.
The Targa 2000RTX offers dual-channel real-time effects, we tried these out with dual 300KB/frame streams and discovered that most of these (keys, dissolves, wipes) worked well. However, functions that required lots of math, such as picture-in-picture, weren't able to keep up and this showed up as one of the streams playing back in slow motion. But when the video streams were bumped down to 200KB/frame, all effects worked properly. Incidentally, we were unable to tell the difference between 200KB and 300KB/frame pictures, even with noisy Hi-8 video which normally generates lots of artifacts. This leads us to believe that most users will never even use the 300KB/frame digitizing rates offered by the system, 200KB/frame should be sufficient for transparent recordings.
We tried an experiment to see whether this was truly the case, this time with a D-Vision Online XED sytem with a SMPTE-259 serial digital input (so digital/analog conversions wouldn't contaminate the experiment). Using a D1 deck and special test tape provided by Wilson Chao of Cambridge Television Productions, as well as Mr. Chao's well-trained eye, we made a 200KB/frame recording and asked Mr. Chao to see if he could tell the recording from the original. He couldn't see any difference.
We also spent quite a bit of time playing with MC-Xpress and found it to be very convenient and capable. Digitizing and logging are very quick and easy to do, digitized media can be sorted in various folders, you can even storyboard the production in the bin, then drag all the clips onto the timeline at once. The program has a nice character generator program (Inscriber), a large array of digital effects, and even a collection of DVE-type effects such as exploding cubes, various flips and slides, etc. The new version supports multilayered effects editing, as well as batch digitizing, EDL import/export, and the real time effects that we tested. It imports and exports a wide variety of graphics and moving video file types, can do CD-ROM padding, and comes with a software MPEG encoder.
Some lessons learned...
We learned a lot about configuring and optimizing computers for video editing. Basically we found out that it's a lot easier than it used to be! PCI bus hardware is self-configuring, so there's no need to fiddle with jumpers and configuration software. Windows/NT has direct capture and disk striping built right into the operating system, this provides very high data rates and eliminates the need for separate RAID hardware or software.
In the past, highly technical drive parameter tweaks had a large effect on performance, but a lot of these optimizations sacrificed data integrity for the sake of speed: People were basically turning off error correction so that virtually- invisible flipped bits wouldn't turn into large stutters while the drive tried to reread them. However, this compromised the drive's ability to serve other applications, such as databases, which need absolute accuracy. Now that drives can handle data rates that are almost three times what most people will probably want, we feel that people should leave the disk defaults alone, and limit optimization to turning on write cache and the more limited optimizations that the configuration software that Avid provides does. (That's what we did.)
Turning write cache on, easily done with Adaptek EZ-SCSI SCSI Explorer, allows the computer to write data to the drive buffer without having to wait for the physical write to the disk to take place. The only reason drive manufacturer's usually don't turn this on as a default is because they're worried about write data in the buffer being lost in the event of a power failure.
The Avid configuration utility is a program that one can run from Windows anytime one adds a new drive.
Configuration Problems and Solutions
Most problems we encountered will sound familiar to any video person: Poor connections, unterminated lines, trying to do things improperly without reading the instructions and overheated eqquipment. We would advise anyone putting together an off-the- shelf system to make especially sure that his SCSI drives have been assigned unique ID numbers, are connected to the computer with a robust cable, are properly terminated and are housed in a well-ventilated case. These drives throw off a lot of heat and should be carefully mounted so that air can circulate around them and is drawn off quickly.
Be careful about hooking up non-media drives (such as CD-ROMs) to the same SCSI chain, since they may require you to use a narrow SCSI cable and not all the lines will be looped through it. But most of these are likely to be internal devices anyway, in which case they'll be on a separate internal SCSI chain anyway.
One can occasionally get interrupt conflicts when installing the hardware, this is easily solved by taking the conflicting cards out and installing them back in one-at-a-time. Windows/NT has a device interrogator one can use to check the interrupt values.
Finally, Avid MC-Xpress wants the video capture board to work with a gamma of 1.0. The board default is 2.2, and it's easy to forget to make this change if one doesn't read the Avid instructions. Mis-set gamma makes the pictures dark and murky.
Based on our research, we feel that we can state that off-the- shelf computer technology now exceeds the most critical professional image quality standards, offers features previously unavailable to the user except at a very high cost, and has become much easier to assemble. The current software offerings are quite satisfactory, and if the rumors and announcements we hear about future developments are true, will offer some really nice capabilities in the near future.
Clyde Tressler is a video editor and animator. He can be reached at (617) 646-6834, firstname.lastname@example.org.
Bob Lamm works at CYNC Corp., which integrates and sells nonlinear video editing systems. He can be reached at (617) 277- 4317, email@example.com.