Digital Vertical Sync System

A digital vertical synchronization system has been designed and implemented in I2L. It is a two-mode system, operating with standard NTSC signals or with nonstandard signals from cable systems, video games, and the like. Mode recognition is automatic, but forced-mode operation is possible in isolated field cases where necessary. When receiving standard signals, the circuit includes a fixed modulus divider for deriving the vertical sync pulses from the phase-locked horizontal oscillator, an out-of-phase detection circuit, and digital integration to delay rephasing for several fields. In the nonstandard mode, the system functions much as a conventional vertical oscillator, with field-by-field synchronization, automatic retrace initiation in the absence of a sync pulse, and noise immunity for a time following retrace. The I2L technology permits linear circuits to be included on the chip. Among these are a monostable multivibrator, a voltage regulator, and a circuit which processes the integrated composite sync to provide more reliable phasing information. Another processing circuit converts the vertical drive pulse to a ramp suitable for driving the vertical power amplifier. Extensive field testing indicates that the system performs as well as conventional systems for any degraded signal and is substantially better for most.