Television Applications and Transmission of Digital Data in the Vertical Blanking Interval

Abstract

It has become evident that the small piece of spectrum, known as the vertical blanking interval in the baseband video signal, can be almost as important as the video program itself. By design, the first 21 lines in the video signal were left blank to account for retrace tolerances in television receivers. The state-of-the-art is such that now lines 14 - 21 can be used to send ancillary signals and data without being visible on the television screen. Professional test signals have been transmitted for a number of years to give a signature of the transmission medium. Only in recent years, however, has data been transmitted for both housekeeping functions as well as the generation of revenue. This paper will explore the various ways these few lines are being utilized to transmit large amounts of data of all kinds. Applications by both the broadcast and cable industries will be outlined where their application varies. Finally, future trends and considerations will be discussed.