| 摘要 |
A method and circuits for checking for a pattern on a display is provided where the CRC of the pattern is calculated and compared to a CRC reference. A blanking signal is produced when the two patterns do not match. Video pixel data and a first seed value are processed in a CRC circuit generating a first CRC value. The first CRC reference is stored in a first memory that can be one 24 bit register, much smaller than the 2048 bit pattern storage memory conventionally required for the 32xl6x2 pattern. The comparison circuit compares the first CRC value and the first CRC reference, producing an unequal signal at the input of a control circuit if they do not match. The control circuit also receives a D/A blanking signal. Responsive to an unequal signal, the control circuit produces and latches the blanking signal at its output and the input of the D/A converter coupled to it. The D/A converter stops producing analog video output, and the screen goes blank. For interlaced images the CRC circuit generates a first even and odd CRC values. The CRC references also need to be based on even or odd interlaced frames of the protected pattern. The even and/or odd CRC value is compared to its respective CRC reference in the comparison circuit and the result passed to the control circuit. For additional security the CRC circuit can generate a second CRC value based on a second seed value and the video pixel data. The comparison circuit compares the second CRC value to a second CRC reference stored in a second memory. When the two do not match the unequal signal is produced at the output of the comparison circuit and is latched by the control circuit at the input of a D/A converter. Having two CRC values increases the security, because to keep the D/A converter from shutting down either the protected pattern or a different pattern that produces two identical CRC values has to be generated. |
