Interlocking is the practice of running a single print between multiple projectors at the same time. This was a common practice in multiplex venues using platter systems, and it allowed theater operators to maximize profits by scheduling additional screenings of a film that was performing well without requesting additional prints from the distributor.
To interlock the projectors, the film would run from a feed platter to the first projector, then run through a series of guide rollers to the second projector, with a loop accumulator (usually a swing arm that pressed down on the film) between the projectors to maintain optimal tension. From the second projector, it would continue to additional screens or run to a takeup platter.
When using purely mechanical projectors, each projector was run on a fixed-RPM motor wired on the same leg of the electrical system to keep them running at the exact same speed. The start times were synchronized through a centralized automation system that fired commands to start each projector simultaneously. This works well for projectors that use timing belts, such as Simplexes and Kinotons, but projectors that are driven by V belts, such as Centuries, would fall out of sync as the belts wore. Some electronically controlled projectors, such as the Kinoton E series, can be programmed to run in perfect unison without these systems in place.
In some cases, projectors were interlocked without adequate synchronization. In this situation, a movable loop accumulator could be placed between each projector, allowing the tension to rise and fall within a tolerated range to compensate for out-of-sync motors. Some projectors, such as the Simplex XL, have manual brakes that can be used to slow down the faster motor to relieve tension on the loop accumulator. This was not good for the equipment, and was usually done for temporary or improvised interlocks.
- Interlocked projectors were started remotely, and were often started on a timer. Without a projectionist present at the start of the show, any problem that occurred would go unnoticed.
- A convoluted film path increased the risk of print contamination and accidental physical contact with the print. While guide rollers were usually mounted along walls, the film path often crossed walkways or doorways, so without barriers and signage someone could walk into the film as it was playing.
- Misaligned guide rollers could lead to scratching.
- Guide rollers could be placed in inconvenient locations (ex., mounted overhead on top of sprinkler pipes) that made it difficult to clean them or check their alignment.
- The film path was complicated and the projectionist was often not given adequate time to check their work, so it was easier to misthread.
- Poorly calibrated tension could damage film and equipment. When initially setting up an interlocking system it was advisable to use acetate film to calibrate the tension, since it would tear instead of damaging the equipment. In some cases, polyester under excess tension could even topple a projector pedestal. If the film was not under enough tension the failsafes could trip, stopping all of the projectors and spilling film onto the floor. If this occurred in the middle of a show, it would also lead to impedance scratches.
- Interlocking was particularly dangerous if the film was not in good condition. One bad splice or film break would lead to cascading failures.