APC Systems Used in Broadband Carrier Systems for Video Signal Transmission

From the viewpoint of better utilization of synchronization information in suppressed carrier VSB (vestigial sideband) television signals, various carrier regeneration and phase tracking systems are examined. Two APC (automatic phase control) systems with improved utilization of synchronous information are discussed further with emphasis placed on their precision phase tracking and carrier regeneration characteristics. Test models of both systems have obtained a carrier phase accuracy of ± 0.5 degree, which permits color television transmission over a chain of long haul links. Practical utilization of the first system, which employs an approximate (both sidebands) component is featured by the use of a variable parameter phase tracking control loop system which attains a stable, high sensitivity. To satisfy stability, sensitivity, and practical requirements, a second-order variable parameter astatic zero tracking system has been adopted. Response of the test equipment to a typical picture has been analyzed, and the theoretical results have been verified by experiment. If the sensitivity parameter is fixed, the first system becomes the phase tracking circuitry of the second system. This is a carrier reproducing system which uses time band filtering, sampling, and which must fully utilize trisynchronization (V- and H-synch and blanking) information to secure the start and continuance of sampling. Its carrier regeneration system features the use of a self-holding gate which synchronously gates by using the natural and horizontal synchronous periods and not regating for a certain fixed time period. The stationary and starting transient responses of this system are discussed and its reliable starting verified through experimentation. It has been found to be effective with an ECR (excess carrier ratio) of 0.5. A phase tracking method similar to that in the first system was used.