SMPTE Timecode - Discontinuous Timecode, and Flywheel Processing

Discontinuous Timecode, and Flywheel Processing

Timecodes are generated as a continuous stream of sequential data values. In some applications 'wall clock' time is used, in others the time encoded is a notional time. After making a series of recordings, or after crude editing, recorded timecodes may consist of discontinuous segments.

In general it is not possible to know the linear timecode (LTC) of the current frame until the frame has already gone by, by which time it is too late to make an edit. Practical systems watch the ascending sequence of the timecode, and infer the time of the current frame from that.

As timecodes in analog systems are prone to bit-errors and drop-outs, most timecode processing devices check for internal consistency in the sequence of timecode values, and use simple error correction schemes to correct for short error bursts. Thus, a boundary between discontinuous timecode ranges cannot be determined exactly until several subsequent frames or discontinuous sequences of them have passed.

For this reason, most videotape editing attempts to keep the timecode of the recorded material continuous, so that multiple edits may be repeatedly over-recorded onto the same piece of videotape.

Although it would be possible in all-digital systems to eliminate the need for the flywheel algorithm by adding a frame delay to allow the timecode to be decoded prior to the processing of the frame, this is not done in most practical systems because

• it would introduce an unnecessary frame delay in the signal processing path, and
• there would still be a need to compensate for timecode errors in signals derived from analog video or audio systems.