Abstract
A system for adding an auxiliary data channel to a standard NTSC television signal has been developed by taking advantage of the fact that for most scenes video voltage levels are very low in 2.5-MHz region. No use is made of the vertical or horizontal blanking intervals. A low-amplitude subcarrier is inserted at an odd multiple of one-half the line-scanning frequency after being biphase-modulated by a specially clocked data stream. The signal is essentially invisible on a standard receiver but when processed by an add-on decoder the original information modulated on the signal may be recovered and displayed on the receiver screen. A 21-kb/s data rate has been obtained with acceptable error rates. Applications of the add-on data communication system include: multilanguage subtitles, subtitles for the deaf, network communications and teaching aids.
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