Abstract: A video dot clock generator includes a phase-locked loop (PLL) which includes a voltage controlled oscillator, a frequency divider, a phase comparator and a loop filter. The voltage controlled oscillator (VCO) is programmable to provide multiple frequency ranges for a given range of control voltages applied to the oscillator. The programming affects both the frequency range and the gain of the VCO. The phase comparator includes circuitry which simulates a predetermined minimum phase error which, when compensated for, substantially eliminates jitter in the dot clock signal. The frequency divider used in the PLL and a similar frequency divided used to generate the reference signals for the phase comparator are programmable via an internal memory which also holds programmable control signals for the VCO. The memory, in turn, may be programmed by the user to achieve desired frequency and loop again characteristics for a given application.

Abstract: A teleconferencing audio-graphics system using control signals transmitted by a transmitting station to control the operation of an overhead transparency projector and a motor driven roll mechanism that moves the transparency graphic copy output provided by a graphics copy output device over the transparency receiving platen of the projector. The graphics copy output device provide the transparency copy output in response to electronic input signals received from the transmitting station.

Abstract: A phase locked oscillator comprising a phase comparator, filter and voltage controlled oscillator coupled in conventional manner to form the forward path of the phase locked loop and a counter and logic circuit coupled to form the feedback path. An input pulse stream is applied to a monostable flip flop which responds to each input pulse to produce a pulse of predetermined length that is applied to one input of the phase comparator. The counter is coupled to receive the oscillator output pulses for counting repetitively to a predetermined count. The logic circuit responds to first and second counts in the predetermined count for respectively generating and terminating a reference pulse in response to each input pulse. The reference pulse is applied to the other input of the phase comparator which in turn provides an output signal indicative of the time displacement between each input pulse and its associated reference pulse to control the oscillator to run at a harmonic frequency of the input pulse stream.