1. Field of the Invention
This disclosure is directed to a copy protection method and apparatus for use with (1) digital video recording, where it is desired to copy protect both an analog and digital video signal associated with a digital recording or playback apparatus and (2) any video material where the original source material is not copy protectable.
2. Description of the Prior Art
Various well known copy protection schemes for video signals include that disclosed in U.S. Pat. No. 4,631,603, John O. Ryan, Dec. 23, 1986 and assigned to Macrovision, incorporated by reference, directed to modifying an analog video signal to inhibit making of acceptable video recordings therefrom. This discloses adding a plurality of pulse pairs to the otherwise unused lines of a video signal vertical blanking interval, each pulse pair being a negative-going pulse followed closely by a positive-going pulse. The effect is to confuse AGC (automatic gain control circuitry) of a VCR (video cassette recorder) recording such a signal, so that the recorded signal is unviewable due to the presence of an excessively dark picture when the recorded signal is played back.
Another analog video protection scheme is disclosed in U.S. Pat. No. 4,914,694 issued Apr. 3, 1990, to Leonard, and assigned to Eidak Corp., incorporated by reference. The Eidak system (see Abstract) increases or decreases the length of each video field from the standard length, either by changing the time duration of the respective horizontal line intervals in each field while keeping a constant, standard number of lines per frame, or by changing the number of horizontal line intervals which constitute a frame while maintaining the standard duration of each line interval.
These video protection systems modify the video signal to be recorded (for instance on tape) or to be broadcast (for instance protected pay-per-view television programs) to make copying by ordinary VCRs difficult or impossible. When a video tape on which is recorded the copy protected video signal is played back for viewing using a VCR, the copy protection process is essentially transparent, i.e., it does not interfere with viewing. However, any attempt made to copy the video signal from the tape using a second VCR to record the output of the first (playback) VCR yields a picture degraded to some extent, depending on the efficacy of the particular copy protection system. These present video copy protection systems protect only analog video signals, which are the type of video signals broadcast and recorded using current consumer video technology.
Also well known are digital video tape recorders, although currently such digital video tape recorders (which both record and play back digitally) are available only for the professional market, due to their high cost. Such digital systems trade distortion-free performance for substantially higher bandwidth, i.e. substantially more information must be recorded per video frame. The advantage to the user of a digital recorder is that so long as the signals are recorded and played back in the digital domain, each successive generation of copies is without any significant reduction in picture quality, unlike the case with conventional analog recording technology. Current digital video recorders (not intended for consumer purposes) use the so-called "D-1", "D-2" or "D-3" video recording standards and require special video tape.