ADVENT #10 — laserdisc

Little to say about Alice today except that you may have noticed that the letters in the earlier installments were drawn with respect to the viewport, and not the direction of the text. I rarely use the string drawing facility I built and made it until around day 4 in this series before it became a problem. A drawing language built around a write-once tape really should be called PostScript, but that name is already taken. That other PostScript is also built on a simple, powerful, archaic language. In LOGO, a turtle moves principally relative to its current position (forward, back, left, right). A design may be replicated by repeating it from a number of starting points. PostScript designs are done principally in absolute coordinates relative to some abstract viewport. A PostScript design is typically replicated by altering a coordinate transformation matrix maintained by the interpreter that transforms and translates the design.

One of the tricks PostScript made easy was printing two sheets of PostScript generated elsewhere on a single sheet of paper by rotating and scaling individual sheets into a 2-up viewport. Though notoriously complicated, processing and post-processing tasks like this were easy.

With the write-only Alice tape, it’s difficult to have things like a dynamic current transformation matrix. Even in the simple turtle case, the drawing in Alice is done with FFI calls to a turtle library that maintains dynamic state like current position and pen color. These are natural things to hold outside the system since they might just as easily be held by the current position and pen of a real physical system. I can create the appearance of top-level dynamic globals in Alice with some compiler work but like all forms of magic I can only cast so much per day. Instead, I rewrote the font portion that I needed for today’s installment to be parameterized by a gotoxy that can capture the coordinate system of the turtle at its current position. I guess that gives me the ability to render text along a path now, so I’ll now have to wrap this thing with a bow at the end of the series.

Today’s removable media is the LaserDisc. We profiled the MultiMediaCard last time, though it was unclear just what multi meant. Today, we have mixed media, mixed capitalization, and mixed success in the marketplace. The LaserDisc looks and feels much like an intermediate step in an evolution from wax cylinder audio recording to the Blu-Ray disc, slotting just between the vinyl LP and the Compact Disc. Like the LP, it was about a foot in diameter with material on either side. It captured a signal in the analog domain though with both video and audio, it could be quickly reproduced with a mechanical press, and it usually came in an attractive album cover with artwork and notes. Taken together, the disc and the package were a single work in mixed media. It far outshone (well, out-reflected) the LP in its use of a contactless red laser and photodetector to convert the mechanical signal to an electronic one. A laser disc could play for years without degradation. Maybe you knew somebody who had one of these, but probably not. They were a bigger hit in Japan. Mine still works. With technological improvements on the player side, later discs combined analog video with CD-style digital audio. Not only were the discs removable media, many needed to be removed, flipped, and replaced to get to the information on the other side halfway through movie playback.

The movie industry expectations for laserdisc were modest, which is to say that they regarded it as a low threat to their principal theater-based movie distribution system. This lead to the counterintuitive result that an obscure format had many titles available — often available nowhere else — and that those titles are regarded as some of the best produced until the DVD era. While modern digital picture quality eclipsed the discs by the DVD era, the laserdisc version often represents a cut that remained unavailable elsewhere until it became practical for fans to re-cut movies themselves. On laserdisc, for instance, Han will forever shoot first.

Laserdisc is not often thought of as a computer media but it served as one in two important ways. The first was as a computer-cued analog media. Laserdisc provided a video quality that was otherwise unavailable in arcade video games in the early 1980s, the most famous example of which is certainly Dragon’s Lair. That title impressed with smooth video and storytelling, but ultimately disappointed with a choose-your-own adventure feeling that didn’t seem worth plunking more money into after a few plays. Laserdisc was also used as part of the EditDroid prototype non-linear editing system built by George Lucas before workable pipelines existed to host high-quality video on computer workstations. These were both still fundamentally analog. Pioneer adapted the format into the mixed analog/digital LV-ROM for the BBC Domesday project to support the dissemination and use of contemporary interactive multimedia. While laserdiscs could be read without degradation, the discs themselves could be ticking time bombs. These days, even the players are rare. Several attempts have been made to emulate the Domesday project in the digital domain and we may soon see those emulators themselves emulated. Aperture cards may wind up lasting longer, having greater data density in the analog domain, and better surviving the death of the original readers. An aperture card could also be mechanically reproduced today with little investment, something impractical for laserdisc.

EditDroid interests me in particular. While the density and quality of laserdisc were impressive, seek performance was mediocre. EditDroid figured out that an array of players could improve seek performance by commanding several drives as part of a complex workload scheduler and accepting the first one to arrive at the desired cut. The same strategy works with non-linear execution editing for debugging. Seeking backwards in a running program is notoriously expensive, and seeking forward (which is to say executing) is rarely done at better than real time. A large number of program spindles, each able to execute (or execute in emulation) independently, can be used for the same purpose. EditDroid, with its parallelism and read-only discs, is one of the many technologies that influenced the design of Alice.