Whether a dark ride, a theater or a show, an amusement park combines many elements that must work together. Synchronizing the sensations of a ride to the concert-style sound and illusions worthy of the greatest magicians requires precision. The role of "Show Controller" is to ensure, among other things, events are triggered in a timely fashion. A reference clock must keep sounds, images and other devices in synchronism.
This article deals specifically with this clock.
A common reference:
To work together, show subsystems must share the same time reference. Current technology allows sharing clock signal of infinite precision to be shared by different devices. Yet the film, television and attractions rely on a relatively low resolution technology, for at least three reasons: it's convenient, "backward-compatible", and sufficient.
The clock signal commonly used to synchronize the attraction components is called "time code". This rustic standard comes from the film, where each image is assigned a specific address, which will be used for editing, and to synchronize images and sound.
This standard was developed in the 60s by the Society of Motion Picture and Television Engineers. It has become so common that it has taken the name of the organization that developed it. It is common to hear the technicians talk about "SMPTE" to describe the time-code itself.
As this is a standard designed for the image, we can deduce that its resolution will be ... an image. Depending on the frame rate (the number of images per second, we'll come back to that in a moment) time code, the resolution will be roughly 1/25 or 1/30 of a second.
Is that enough? Yes, at least for images and visual effects. Our retina keeps for a short while an imprint of the images it is exposed to. This limitation allows us to mistake the succession of still images recorded on film for a fluid motion .
What is the right number of frames per second for an attraction? The options are 24 images / second, the historical rate of film, 25 frames per second, the frame rate of the video in Europe and in countries using the PAL video system, or 30 frames per second for television in USA and other territories using NTSC. In practice, for obscure historical reasons, these rates are slightly different, such as 29.97 frames / second for example.
In an industry dominated by Disney, Americans set the tone. Most major attractions are using 30 frames/second rate. When they were still used in parks productions, cameras and movie projectors were modified to operate at 30 frames instead of 24 in regular theaters.
This de facto standard works well. American show control devices makers optimize and test their products primarily for this frame rate. Also, more frames per second means less flicker.
A central device, which we call the time code generator produces a clock signal that is distributed to all the subsystems of our show (sound, image, light, effects, animations ...). This central clock may be a dedicated device, or, as in our case, the circuitry integrated in the audio player we use. It becomes the "master" of the show as far as clocking is concerned. Once decoded and displayed, the time-codes looks pretty much like and old quartz clock, except that the last digits represent images. For example, if the selected rate is 30 frames, the value after 1 second and 29 frames is ... 2 seconds. Few movies or shows that last more than 24 hours. In attractions, we look after hours, minutes, seconds and frames. For example, 1h 06min, 15s and 14 images is a timecode value, which is written: 1.06.15:14.
The real beauty of the timecode is its simplicity. It is easy to distribute and check. A simple speaker is enough to verify his presence, and even without specialized equipment, a little practice is enough to decode a value by looking at a frame recorded on an oscilloscope.
Traditionally, the initial value of the show is not 0 hours, 0 minutes, 0 seconds and 0 picture, but one hour. Why is that? The various elements of the show (sound, video, lighting ...) need a few seconds to "lock-in" the time code. The time code generator is set to start a few seconds before one hour. Subsets use those few seconds to synchronize and be ready for the start of the show which takes place at 1.00.00: 00. If the show is to be presented in several versions, or more segments, it is customary to use 2h, 3h and so on as the starting value of each segment.
Mainly for security reasons, the control of physical effects is almost always assigned to a PLC (Programmable Logic Contoller-). These devices from the industrial world are obviously unfamiliar with time-code. Fortunately, Tejix has teamed up with a leading automation manufacturer to develop a technique that allows the PLC to decode timecode. With a little imagination, it was possible to use of-the-shelf equipment to "oversample" the time code signal so it can be decoded by the controller. Simple and effective.
Most lighting consoles used in concerts do not know about time-code. Some models offer this feature, but it is often overlooked by lighting operators. Leaving such a console to run the show is unthinkable, both economically and practically (leave free access to dozens of buttons is perhaps not the best idea). In general, concert-type lighting consoles are used with the time-code of the attraction for programming, then the result is transferred to a DMX recorder such as the LightCue from Alcorn McBride, which accurately chase SMPTE.
In an attraction, the time code is generated by a specific device that acts as a « time master » . Most often, it is a dedicated time code generator. Alcorn McBride A/V players we use in our designs incorporate a SMPTE generator. Given the modest speed and simplicity of the data structure, it is customary to use audio distributors to feed the different show devices with SMPTE signal (lighting, effects, animatronics ...). To avoid any pollution, outputs of the time-code distributor are often isolated by transformers.
Is SMPTE indispensable?
SMPTE has its origins in a time when sounds and images were on film and tape. Wealth of imagination was needed to make one follow the other.
Today, the systems that make up an attraction are built around a high precision clock, making drifts virtually impossible in the few minutes of an attraction.
However, in its master function, the time code generator becomes « the Justice of the Peace » : it ensures that the start of all show elements occur within the same frame. Subsequently, the role of reference removes any doubt about the temporal integrity of the show.
And we can still look at these little clocks on the various devices, and remember the days where keeping a show in sync was really complicated...