Video Switch Circuit

This invention relates to electronic switching circuits and more particularly to a circuit for selecting between alternative sources of video signals to be processed by a television receiver.

With the emergence of the television receiver as a vehicle for receiving, demodulating, and reproducing more than merely the broadcaster's signals, it has become necessary to provide circuitry for interfacing the receiver with peripheral equipments. Examples of such equipments are video tape and video disc players and home video games. Although these equipments can be made to provide a modulated RF signal that can be coupled to the receiver through its antenna terminals, enhanced picture quality can be achieved by utilizing video signals coupled into the receiver's video processing circuitry. Clearly when more than one video source is available, means for selecting the desired source must be provided. This invention is directed to a simple and cost-effective video switch for selecting one of a plurality of available sources of video signals.

The invention is a video switch comprising diode means having inputs that may be coupled to at least two sources of video signals. A bias source coupled to the output of the diode means applies a bias voltage to the diode means. A control circuit is coupled to the input of the diode means and selectively applies control voltages of various amplitudes to those inputs, thereby determining, in conjunction with the bias source, which of said video signals is coupled to the output of the diode means.

The subject video switch represents a relatively simple and cost-effective interface between the video processing portion of a television receiver and alternate source of video signals, such as video discs and tape recorders. Applying these peripheral sources of video signals directly to the receivers video circuit, rather than to the receiver antenna terminals, which would entail a modulation and demodulation process, has been found to result in improved picture quality.

The sole drawing is a schematic diagram of the subject video switch.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawing.

Referring now to the drawing, a first source of video signals (not shown) is coupled to Video Input #1. That input is coupled through a resistor R1 to ground and through a capacitor C1 to a first input of diode means 1 at the anode of D1. Similarly, a second source of video signals (also not shown) is coupled through as resistor R2 to ground and through a capacitor C2 to a second input of diode means 1 at the cathode of diode D2. The cathode of D1 and the anode of D2 form the output of the diode means. The output of the diode means is coupled to a bias source 2 formed by the series-connected resistors R5 and R6. R5 and R6 are coupled between a 24-volt supply and ground and are chosen so that a bias voltage equal to approximately one-half the difference of those voltages, twelve volts, appears at this junction and is coupled to the output of the diode means.

The output of the diode means is also coupled through a capacitor C3 to an output buffer 3. The buffer may be optional or modified depending on the specific circuitry with which the video switch is used. In the shown embodiment, the buffer includes an NPN transistor Q1 having a collector coupled to the 24-volt supply and an emitter coupled through resistor R10 to ground. The base of Q1 is coupled through resistor R9 to C3 and to one end of a parallel-connected resistor R8 and diode D3. The other end of R8 and D3 is coupled to a bias voltage appropriate to the characteristics of the video signals encountered and the requirements of Q1.

Control means 4, including a ganged DPDT switch, SW1, determines which of the two video signals appears at the output of the video switch. Although for the purposes of convenience the switch is illustrated in a conventional mechanical configuration, that illustration is deemed generic and substitution for it by conventional electronic or hybrid, configurations, as described below, in no way departs from this invention. SW1 has a first pole (a) coupled through resistor R3 to the anode of D1 and through a resistor R4 to the cathode of D2. A first terminal (b) associated with pole (a) is coupled to a first control voltage, that is, the 24-volt supply. Similarly a second terminal (c) is coupled to a second control voltage, ground. SW1 also includes a second pole (d) coupled through resistor R7 to the output of the diode means. Third and fourth terminals (e) and (f), associated with pole (d), are coupled to ground and the 24-volt supply, respectively.

Operation is as follows. With SW1 in a first position as shown in the drawing (pole (a) to terminal (b), pole (d) to terminal (e)), 24 volts will appear at the junction of R3 and R4. Because the bias supply maintains a voltage of approximately 12 volts at the output of the diode means, D1 will be forward biased (conducting) and D2 will be reverse biased (non-conducting). As a result the signal from the first video source will be coupled to the output of the diode means and the signal from the second video source isolated from it. The appropriate 12-volt reverse bias voltage across D2 tends to reduce its junction capacitance and therefore minimizes the coupling of signals from the second video to the output of the diode means. Conversely with SW1 in a second or alternate position (pole (a) coupled to terminal (c) and ground; pole (d) coupled to terminal (f) and the 24-volt supply), D1 will be reverse biased (non-conducting) and D2 will be forward biased (conducting). As a result the signal from the second video source will be coupled to the output of the diode means and the signal from the second video source isolated from it.

To those with ordinary skill in the art, it is apparent that the upper section of SW1 (that is, pole (a) and terminals (b) and (c)) is by itself sufficient to switch D1 and D2 and therefore video sources. However, when done in this manner it can be shown that the DC voltage at the output of the diode means will be approximately two diode drops (about 1.4 volts) higher when the control switch is in the first position than when it is in the second position. By arranging the second section (pole (d) and terminals (e) and (f)) as described above, the DC voltage at the output of the diode means remains substantially independent of the video source chosen. While not necessarily essential in other types of equipments, it is generally desirable to maintain a constant DC input to the DC-coupled video channel of a television receiver. Variations at the DC will be manifest as variations in the overall brightness of the television image.

Various modifications can be made without departing from the scope of the subject invention. For example, the second section of SW1, comprising pole (d) and terminals (e) and (f), can be replaced by the circuitry shown in FIG. 2. The replacement circuitry includes an NPN transistor Q2 having a collector connected to the end of R7 that would otherwise be connected to pole (d) and an emitter connected to ground. The base of Q2 is coupled through resistor R11 to pole (d) and through resistor R12 to ground. Because Q2 will be effectively saturated when SW1 is in the first position and effectively cutoff when SW1 is in the second position, operation will be substantialy equivalent to that obtained with the inclusion of the second section of the DPDT switch.

Furthermore, although the bias voltage applied to buffer 3 may, in some equipments, be a simple DC supply, the DC-coupled video channel of high-quality television receivers is sufficiently sensitive to the DC component of the demodulated video signal so that a clamping circuit may be required to restore the DC component of the video signal applied to the base of Q1. DC restoration or clamping circuits are well known to circuit designers skilled in the art and the particular circuit decided upon can be applied to Q1 through R8 and D3. Because the DC voltage at the output of the diode means is maintained substantially independent of the video signal source chosen, the switching transient often associated with the DC restoration circuitry will be minimized.

Finally, although the subject invention has for the purposes of explication been described with particular reference to the switching and selection of television video signals, it is clear that the circuit described herein has widespread application in other types of electronic equipment.

Accordingly, while there has been shown and described what are at present considered the preferred embodiments of the invention, it wil be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

The subject video switch is useful in equipments to which are selectively applied signals from a plurality of sources.