TI Describes Method to Achieve Variable Frame Rates in Cinemas

By Chris Chinnock
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Tim Ryan from TI’s DLP Cinema group gave a presentation on the way DLP cinema projectors can offer variable frame rates. The projectors can run at various frame rates already, but require time to switch rates. Filmmakers are thinking of creating content where the frame rate change from one frame to the next, but they can’t do this until a projection solution is available. However, the capability could also allow the mixing of trailers and advertisements in various frames rates as well.

It turns out that doing this in the DLP cinema projector is not that hard, but there are four key requirements that have to be met:

  • Foreknowledge of future frame rate
  • Frame rate communicated to the entire system deterministically and sufficiently in advance of the actual change
  • Projector must store a minimum of two complete sets of frame rate specific data
  • Projector must switch from using one set, to another set of data seamlessly.

The DCP format already has a frame rate information section which Ryan says can be modified without any standards changes to support variable frame rates. The idea is to attach the frame rate of each reel to this table and pass this information to the system electronics (produced by their partners).

Communication of this data is done in the ancillary data (metadata) section of SMPTE 291-1 by simply defining a new data packet, which Ryan says is easy to do. He thinks simply defining current frame rate and next frame rate for each reel is sufficient as outlined in the example below.

Ryan explained that in the latest series 3 DLP cinema projectors, TI has turned over even more of the electronics control to its partners with TI just responsible for “flipping mirrors”. One upgrade that TI made was to double the memory as a double buffer. That turns out to be exactly what is needed for variable frame rate operation. It allows the next frame with an alternative frame rate to be loaded into the buffer and “swapped” for the preceding frame. Since this occurs at the frame boundary, there is no disturbance to the playback.

In the Q&A, Ryan clarified that variable frame rates from 20 to 120 fps were possible, but this capability needs to be implemented by its partners (NEC, Christie and Barco). The issue of audio synchronization was also raised as currently this is pegged to the frame rate with a delay to allow for the video processing. How would this all work with variable frame rates someone asked? Ryan didn’t have an answer for that one, so work still needs to be done to have a practical solution. – CC