This application relates to video input circuits and, more particularly, to video input circuits for a video hard copy controller of the type that converts and formats video signals into digital signals for application to a hard copy generating device.
Prior art video input circuits have included a video amplifier circuit having input terminals for receiving a composite video signal comprised of a video data component and a synchronization component, control terminals for receiving a gain control signal to control the gain of the amplifier circuit, and output terminals at which an amplified composite video signal is developed.
In the past, a combination automatic gain control and d-c restorer circuit was also employed to generate the gain control signal and to restore the tip extremity of the synchronization component to a reference potential, such as a d-c common.
A major problem with the above video input circuits arises from the fact that the amplified composite video signal at the output of the video amplifier was first started to be d-c restored and then, during the d-c restoration process, the automatic gain control (AGC) circuit was enabled. Although d-c restoration started before the AGC circuit was enabled and the AGC circuit completed its first operational iteration after d-c restoration was terminated, yet there was a significant period of time during which the d-c restoration and AGC operations overlapped. This posed significant conflicting effects with unreliable results. Also, since the tips of the synchronization component were restored to the d-c reference common, there was substantial likelihood of unwanted ground noise influence.
It would be desirable to provide video input circuits of the general type above described wherein the undesirable overlap of d-c restoration and AGC operations is eliminated. It would further be desirable if such video input circuits could include a simpler and more efficient d-c restorer circuit that could reduce ground noise.