| 摘要 |
Digital information is encoded in the video portion of a television signal such that "n" (n is an integer) bits of the digital information are carried by each successive frame in a group of frames. The digital information is encoded by modulating a carrier signal, using, for example, either amplitude shift keying (ASK) or frequency shift keying (FSK), and the modulated carrier is then added to the video signal selectively, only in portions of the television program that (a) are not likely to be perceptible by a viewer, and (b) are of sufficient intensity to transmit the data. The video signal including the encoded digital data, may be transmitted, in real time, to a television receiver, or recorded for later playback. In either event, at a receiver, when the video signal containing the encoded information is displayed on a screen, the image is sensed or "viewed" by a photo detector, the output of which is applied to a band pass filter having a center frequency that corresponds to the oscillator frequency(s) in the encoder. The filter output is thresholded or FM detected, so that the receiver can recover the encoded data. By appropriately including checks bits and synchronization patterns, the bit stream output from the decoder can be synchronized, and the original encoded data recovered. Advantageously, the frequency of the oscillator in the ASK or FSK encoder is chosen so that the energy added to the video signal is in a portion of the frequency spectrum that otherwise contains little energy. This makes the added digital data imperceptible to a viewer. If desired, the same data may be repetitively encoded in several groups of frames, so as to increase the redundancy and thus the reliability of the overall system.
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