Appendix B: Achieving Frame Accuracy in a Non-Frame Accurate World

Keeping video signals synchronized is a time-honored process. Ideally, all routed video signals in a facility are raster time-aligned horizontally and vertically and with a known frame time code. The basic method used to synchronize a target video signal is to apply a correcting time shift and thereby align it with a provided master reference signal. In most cases, the vertical and horizontal timing of the target signal is altered to agree with the provided video house reference. There is usually a one or more frame delay (input to output delay of corrector) to accomplish the time alignment.

It is not difficult to imagine how this process works. The input signal needing correction is fed into a “frame synchronizer” (standalone box or internal device process). It is written to a digital frame memory (sometimes called an elastic buffer) at the input rate and timing. Next, a separate process reads from the same memory to create a new signal for output. The output timing is aligned to the provided master reference signal. As long as the input data rate and output data are equal on average, then the buffer will not overflow or underflow. There is a clean separation of the two processes and any output H/V timing relation may be produced independent of the input signal timing.

Frame syncs are applied to many real-world problems. For example, a received satellite video feed will not naturally be time-aligned with a facility’s internal timing reference, so the received signal must be frame synced to align it. Incidentally, most video facilities have a video reference signal that is distributed to all A/V gear. When the reference signal is used, all device A/V I/O can be aligned to the master reference signal. In some cases, the input frame rate and output frame rate are not precisely equal. Over time (sometimes many hours or days) the frame sync needs to duplicate or drop a frame to keep the buffer from underflowing or overflowing. If this is done only on occasion, then the frame jumps are rarely noticed. With GPS worldwide timing, frame buffers rarely exceed their limits. Also, the audio associated with an aligned video must be delayed or resampled for lip-sync agreement. An infrequent audio click may be detected due to the frame drop/add operation.

When signals are streamed across digital networks, they will invariably drift in time due to the introduced delay and jitter and may need to be frame corrected at some point in the workflow. Frame synchronizing is an important process in the real world of A/V systems. For more information on new techniques for A/V signal synchronizing, do a Web search for documents published by the “SMPTE EBU Joint Task Force on Timing & Synchronization.”