This invention relates to a television camera system and more particularly to a system including means for reducing the number of transmission channels required in communicating video information and timing information from the camera to a remotely located camera control unit.
In a conventional television system camera, the camera head communicates with a remote camera control unit by means of a suitable transmission cable. In the past, transmission cables have been used which have separate conductors for each of the signals which must be communicated between the camera head and the camera control unit. In view of the very large number of these signals, the multiconductor cables were both bulky and expensive.
Current efforts have been directed toward combining as many of the signals as possible so that the composite signal resulting from this combination of signals could then be communicated over a transmission cable having a smaller number of conductors. Preferably, a single composite signal will be generated so that only a single length of coaxial or triaxial cable will be required. Frequency division multiplexing has been used effectively in this regard, but requires the allocation of a separate frequency channel for each signal which must be independently transmitted. Since the attenuation characteristics of coaxial and triaxial cables increases with the frequency of the signal being transmitted thereon, it is desirable to reduce the bandwidth requirement of the signal by reducing the number of frequency channels required. It is therefore desirable to provide a means of combining signals which not only does not require a separate conductor for each of the signals, but also would not require the allocation of a separate frequency channel in a frequency division multiplexing scheme.
Normally, a camera head provides a video signal which is made up of regularly occurring horizontal blanking intervals of fixed duration separated by video information intervals. Each blanking interval is divided into two slots with the first typically containing a white pulse of fixed duration such as 1.5 microseconds. This white pulse will have a signal level which indicates the maximum excursion of the video signal during the video information interval. This white pulse is followed by a black level signal for the duration of the horizontal blanking interval, wherein the balck level indicates the minimum signal which will occur during the video information interval. In addition to the black and white level information, the camera additionally provides a separate camera timing signal which occurs in synchronism with the horizontal blanking interval of the video signal. These camera timing pulses must be communicated to the camera control unit for purposes of synchronizing the operation of each camera head with the standard timing of the control unit involved therewith.
It is therefore a primary object of the present invention to conserve the transmission channel requirements in a television camera system by combining a camera video signal with a modified camera timing signal to form a composite signal which may be communicated over a single channel, with provisions being made at the camera control unit for reconstructing the original signals.
It is a further object of the present invention to provide such a system as discussed above where the composite signal includes video information as well as information representative of maximum white and black levels, together with timing information representative of the beginning and ending of the camera timing pulses.
It is still a further object of the present invention to provide a system for accurately recovering both the camera timing signal and the video signal from the composite signal.
The present invention contemplates the provision of a television camera system wherein a camera employs means for providing a video signal as well as a camera timing signal and in which the video signal includes regularly occurring horizontal blanking intervals of fixed duration separated by video information intervals. It is contemplated that each blanking interval is divided into two successive time slots wherein the signal levels in these time slots represent the maximum and minimum magnitudes of the video information signal. Moreover, it is contemplated that the camera timing signal includes regularly occurring timing pulses each of the same duration as, and synchronized with, a corresponding horizontal blanking interval.
In accordance with the present invention, circuitry is made responsive to each camera timing pulse for purposes of providing a modified horizontal timing pulse having its leading edge delayed by a fixed amount from that of the camera timing pulse and its trailing edge corresponding with that of the camera timing pulse. This camera timing pulse has a signal level outside of the limits established by the white and black signal levels. The video signal is additively combined with these horizontal timing pulses to provide a composite signal for transmission from the camera head to the camera control unit through a single transmission path.
In accordance with another aspect of the present invention, circuitry is provided at the camera control unit for separating the modified horizontal timing pulses from the composite signal. A phase locked loop is provided for synchronizing one phase of a sixteen phase oscillator with the modified timing pulses. Selected phases of the oscillator are logically combined with the modified timing pulses and the composite signal so as to reconstruct the video signal and unmodified horizontal timing signals therefrom.