This invention relates generally to television transmission and reception systems and, more particularly, to television transmission and reception systems utilizing compensation techniques that remove distortions interjected into a video signal during the transmission and reception process.
Secure transmission of video signals has become a matter of increasing importance with the growing popularity of video teleconferencing, cable TV and satellite TV transmissions, and with the introduction of direct-broadcast-satellite (DBS) transmission. Various encoding techniques have been developed that provide varying levels of security, with corresponding levels of complexity and cost. These range from relatively simple but easy to "break" sync-suppression techniques to relatively complex but difficult to "break" digital encryption techniques. One technique that provides a relatively secure video signal, with a modest amount of complexity and cost, is line spin scrambling.
Line spin scrambling is performed in an encoder by segmenting the active portion of each video line at a breakpoint determined by a pseudorandom number generator. The two segments of each video line are then interchanged, or "rotated," while the horizontal and vertical synchronization and blanking intervals are left intact. After transmission and reception of the video signal, the signal is unscrambled in a decoder by reversing the line spin scrambling applied to each video line in the encoder. The spin breakpoint of each scrambled video line is determined in the decoder by an identical pseudorandom number generator that is synchronized with the pseudorandom number generator in the encoder.
Although line spin scrambling offers many advantages, it has certain disadvantages. One of the disadvantages is that line tilt causes a distortion in the unscrambled video signal. Line tilt is generally a sawtooth-shaped voltage error that corrupts each video line during the transmission and reception process. The line tilt waveform is a linear charge ramp during the horizontal blanking interval and a linear discharge ramp during the active portion of the video line. Consequently, line tilt is correlated with the line rate of the video signal.
Line tilt corrupts the active portion of each video line whether the video line is scrambled with the line spin technique or not. However, the effect on a received picture is generally undetectable when a video signal has not been line-spin scrambled. This is because the amplitude and phase of the line tilt waveform are approximately the same for all video lines and, therefore, the effect across the received picture is constant in the vertical direction and is a gradual luminance variation in the horizontal direction. However, a video line that has been line-spin scrambled has the full amplitude of the line tilt applied at a single point, where the two segments are pieced back together during unscrambling. This causes a sharp luminance discontinuity at the randomly chosen spin breakpoint in each video line, resulting in a chaotic hashing of luminance striations in the received picture.
The above-referenced application discloses a line tilt compensation technique in which line tilt is removed from the active portions of a video signal by applying a linear complementary ramp, based on measurements of the amplitude of the line tilt, to the active portions of the video signal. Measurements of the line tilt amplitude are made by transmitting a constant-amplitude dummy video line at the bottom of each field of the video signal. At a receiver, the amplitude of the line tilt interjected into the active portion of each dummy video line during the transmission and reception process is measured by accumulating amplitude differentials between adjacent points sampled across the active portion of each dummy video line.
An NCTA technical paper entitled "Digital Techniques Cure Line Segmentation Scrambling Problems," by Gregory A. Baxes, presented at a Las Vegas Convention June 2-5, 1985, pp. 308-313, discloses a line tilt compensation technique in which the amplitude of the line tilt in each video line is measured by adding a reference level to the beginning and the end of the active portion of each video line prior to transmission. After transmission and reception, the amplitude differential between the two reference levels in each video line is measured and entered into a look-up table. The look-up table generates a linear complementary ramp that is summed with the active portion of the respective video line to remove the line tilt in that video line.
Both of these line tilt compensation techniques generate a simple complementary linear ramp. However, the line tilt waveform is frequently more complex than a simple linear ramp and, therefore, these techniques do not completely remove line tilt. Furthermore, both of these line tilt compensation techniques do not compensate for other types of line rate correlated noise, such as cross channel interference. Accordingly, there has been a need for an improved line tilt compensation technique. The present invention clearly fulfills this need.