Time in Television Systems

Craig, Donald

Clocks are fundamental to the transmission of television - the when of the picture top and left hand side is as important for display as the image content, and maintaining audio lip sync has always been a battle. In overlapping periods since the modern television era began in 1941 there have been a variety of different clock synchronization and time labeling schemes. Real or imagined compatibility constraints during architectural transitions, as well as confusion of time the label versus time the physical property have led to the evolution of remarkably byzantine clocks, with repeating decimal fractional frame rates, discontinuous timelines, and complex counting rhythms.

Beginning with monochrome transmissions, through the analog NTSC color system and into the digital ATSC system of today, this presentation explores the origins of the television systemís clocks, and views from the safety of hindsight unintended consequences of some innocent design optimizations. Clock system technical constraints included the collapse of logically independent layers for needed efficiency, synchronization of electro-mechanical as well as purely electronic systems, and transmission paths ranging from short range coaxial cable to long range variable length microwave RF. As a commercial and quasi-governmental enterprise television also has had to accommodate the eccentricities of civil timekeeping, offering content to a schedule synchronized with wall clock time across multiple time zones.

Does it all need to be this complicated? A transition to television distribution over general purpose computer networks, coupled with the vastly improved performance ratio of low cost modern digital circuitry should eliminate the technical requirement for specialized clocks. The adoption of a new IEEE/SMPTE timing reference based on an epoch and an atomic time scale will provide an elegant mechanism for general purpose support of higher picture frame rates and audio sample rates. It remains for humans to abandon the old clocks, and finally formalize the relationship with wall clock time in a way that doesnít unduly complicate system engineering.

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