Microcomputer-controlled one-step-back automatic frequency control tuning system

An electronic, microcomputer-controlled television tuning system capable of acquiring channels offset by as much as 3.25 MHz from the designated frequency while also being able to accurately tune in channels operating at the designated frequency. After synthesizing the correct channel frequency, the microcomputer looks at AFC discriminator information from which local oscillator tuning voltage direction is determined. To achieve automatic fine tuning the tuning voltage is then stepped by the microcomputer in small frequency increments which vary from 28 KHz for channel 2 to 90 KHz for channel W in the direction of the desired frequency until the center video carrier frequency is passed. The tuning voltage is then returned to the preceding step to which frequency the receiver is then locked. With essentially all broadcasting stations operating at, or very close to, the FCC-designated frequency, highly accurate tuning to the selected channel is thus permitted. Video carrier verification is accomplished by microcomputer analysis of AFC discriminator and vertical sync states.