Descrambling system for broadcast television signals scrambled by active line rotation

摘要

A descrambler for descrambling a signal scrambled by active line rotation (i.e. by interchanging two portions of a line, Fig 1) has a pair of line stores 8 connected to a descrambling control input 13 (receiving data on the cut point between the two portions) to effect the line rotation. A tilt measurement circuit 3 measures line tilt, namely any overall change in video amplitude level across a line, and compensates by generating 4 a converse ramp signal which is added 5 to the video signal. The ramp signal is generated to ten-bit accuracy whereas the video signal has only eight bits. The two least significant bits are accumulated 6, 7 on a sample-by-sample basis to effect error feedback. Thus the visibility of image artefacts due to contouring is reduced. IMAGE