Abstract:
A video interface circuit reduces loading losses when a video system component, such as a television receiver, is connected to a transmission line by amplifying the video signal with a high frequency amplifier and providing positive feedback of the amplified signal to the cable.

Description:
BACKGROUND OF THE INVENTION 
     This invention is directed generally to television signal distribution systems and particularly to systems using a cable to distribute multiple video signals to a plurality of video devices. This invention relates more particularly to apparatus and methods for connecting a video device to a single uninterrupted cable and reducing the disturbance caused by connecting the video device to the cable. 
     Most commercial signal distribution systems use a high-powered amplifier to drive signals down a single cable with signal taps used as drops to various outlets. To minimize impedance mismatches, the signal attenuation of the taps to each drop is high, on the order of 12 to 20 dB so that one or more very powerful and costly amplifiers are required to maintain the signal at an acceptable level. 
     Most residential cable TV distribution systems have a main cable that delivers video signals to the customer&#39;s home. Signal splitters are then used to connect “home run” cables between the main cable and video outlets. The result is sometimes referred to as a “star” network. Although the splitters supply the necessary impedance matching for each run, the total signal loss through the splitters often exceeds 12 dB, again requiring one or more expensive amplifiers. 
     There is a need in the art for a technique for allowing video devices to be connected directly to a single cable, thereby eliminating the signal losses associated with taps and/or splitters, while not disturbing the system&#39;s impedance match. 
     SUMMARY OF THE INVENTION 
     The present invention provides a video interface circuit that reduces loading losses when a video system component, such as a television receiver, is connected to a cable network. 
     The present invention includes an interface circuit connected to a signal tap in the cable for receiving video signals therefrom and producing an amplified video signal for delivery to the video device. An amplifier circuit and a passive positive feedback circuit are connected between the amplifier circuit output and the tap for introducing an amplified video signal into the cable network. The amplifier is capable of amplifying signal frequencies of up to 10 GHz or more so that the signal fed back into the cable does not cause signal distortion in the cable. 
     The interface circuit according to the present invention preferably comprises a first transistor circuit connected to the tap for receiving video signals therefrom. The invention preferably further includes a second transistor circuit arranged to receive signals from the first transistor circuit and produce an amplified video signal. The feedback circuit is connected between the second transistor circuit and the tap for introducing the amplified video signal into the transmission cable. 
     The feedback circuit preferably comprises a passive network connected between the amplifier output and the tap. The feedback circuit preferably provides a gain of 12 to 15 dB in the video signal that is introduced into the coaxial cable at the tap. 
     An appreciation of the objectives of the present invention and a more complete understanding of its structure and method of operation may be had by studying the following description of the preferred embodiment and by referring to the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a generalized block diagram of the video interface circuit according to the present invention; and 
     FIG. 2 is a schematic diagram of circuitry that forms a video interface circuit according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 and 2 illustrate the basic principles of the invention. The invention is described with reference to the particular circuitry of FIGS. 1 and 2 only for the purpose of facilitating explanation of the basic principles of the invention. The present invention is not limited in its application to the specific circuitry shown and described herein. 
     Referring to FIG. 1, a video signal transmission cable  10  permits bi-directional propagation of video signals. The video signal transmission cable  10  is preferably a coaxial cable of the type commonly used in video systems. The coaxial cable  10  has a central conductor  12  and a shielding conductor  14 , which is grounded. The shielding conductor  14  has a circular cross section. The center of the central conductor  12  is located at the center line of the shielding conductor  14 . The central conductor  12  generally is a solid wire, and the shielding conductor  14  is braided from small wires. 
     An interface circuit  11  for connecting a video device  15  to the video signal transmission cable  10  comprises an amplifier  16 , a feedback network  17  and a pair of resistors  18  and  19 . A conductor  22  connects the non-inverting input of the amplifier  16  to a signal tap  24  formed in the video signal transmission cable  10 . The feedback network  17  is connected between the non-inverting input and the output of the amplifier  16 . The output of the amplifier  16  is also connected to the video device and to a first terminal of the resistor  18 . The resistor  18  has a second terminal that is connected to the inverting input of the amplifier  16  and to a first terminal of a resistor  19 . The resistor  19  has a second terminal that is grounded. Thus it is seen that the inverting input of the amplifier  16  is connected to the junction of the resistors  18  and  19 . The video device  15  is connected to the junction of the amplifier  16  output, the resistor  18  and the feedback network  17 . 
     Referring to FIG. 2, there is shown a schematic diagram of a preferred embodiment of a cable television interface circuit  20  according to the present invention. A coupling capacitor  26  is connected to the conductor  22  to block the transmission of DC and low frequency signals and only allow the transmission of RF signals. Therefore, the capacitor  26  has a high impedance for DC and low frequency signals and a low impedance for signals of the RF video frequency. 
     A resistor  28  is connected between the capacitor  26  and to the base of a transistor  30 . The transistor  30  preferably is an NEC 85633 transistor or the equivalent. The basic requirement for the transistor  30  is that it be capable of operating with signal frequencies of up to 10 GHz or more. The resistor  28  reduces the Q of the input lead to the transistor  30  to suppress parasitic oscillations in the transistor  30 . 
     The emitter of the transistor  30  is connected to a capacitor  32  and to one terminal of a first emitter degeneration resistor  34 . The other terminal of the resistor  34  grounded. The capacitor  32  preferably has a capacitance of about 0.1 μF. The resistor  34  preferably has a resistance of about 39 Ω. 
     The collector of the transistor  30  is connected to a first terminal of resistor  36 . A second terminal of the resistor  36  is connected to a junction  42 . A first terminal of a second emitter degeneration resistor  38 , a capacitor  40  and a +12 volt power source are also connected to the junction  42 . Thus the second terminal of the resistor  36  is connected to the resistor  38 , the capacitor  40  and the +12 volt power source. The resistors  36  and  38  preferably have resistances of about 1 K Ω and 39 Ω, respectively. The capacitor  40  preferably has a capacitance of about 0.1 μF, which means that the junction of the resistors  36  and  38  is effectively at ground potential for the high frequency video signals. 
     Therefore, the emitter degeneration resistors  34  and  38  are connected in parallel for all AC frequencies of interest. The effective impedance seen by the emitter of the transistor  30  is about 19.5 Ω. The impedance of the emitter of the transistor  30  is about 5 Ω. which means that the combined impedance of the emitter and the resistors  34  and  38  is about 25 Ω. The amplifier has an effective impedance of about 2500 Ω for which the present invention compensates. 
     The collector of the transistor  30  is also connected to the base of a transistor  44 , which also is preferably is an NEC 85633 transistor or the equivalent. The collector of the transistor  44  is connected to one end of a resistor  46 . The other end of the resistor  46  is connected to the resistor  38  at a junction  47 . The capacitor  32  is connected between the emitter of the transistor  30  and the junction  47  of the two resistors  38  and  46 . The capacitor  32  thus functions as an AC short circuit between the junction  47  and the emitter of the transistor  30 . A resistor  48  is connected bet ween the emitter of the transistor  44  and ground. The transistor  44  inverts the signal at its collector and produces an output across the resistors  38  and  46 . The resistors  38  and  46  preferably have a resistance ratio of 3:1, which results in a voltage division of 4:1. 
     A capacitor  50  is also connected between the emitter of the transistor  44  and ground. The resistors  46  and  48  preferably have resistance values of about 62 Ω and 82 Ω respectively. The capacitor  50  preferably has a capacitance of about 0.1 μF. so that the emitter of the transistor  44  is effectively grounded for the frequencies of the video signals. 
     The emitter of the transistor  44  is additionally connected to a first end of a resistor  52 . The other end of the resistor  52  is connected to a resistor  54  and to a capacitor  56 . The resistor  54  and the capacitor  56  are connected together in parallel between the resistor  54  and an inductor  58 . A first terminal of a capacitor  60  is connected to the inductor  58  so that the capacitor  60  and the resistor  58  are in series. The other terminal of the capacitor  60  is connected to the collector of the transistor  44  and to a first terminal of a capacitor  62 . The other terminal of the capacitor  62  is connected to a video device  63  that is arranged to receive video signals from the coaxial cable  10 . Both of the resistors  52  and  54  preferably have resistance values of about 510 Ω. The inductor  58  preferably has an inductance of about 560 nH. The capacitor  60  preferably has a capacitance of about 1000 pF, and the capacitor  62  preferably has a capacitance of about 1000 pF. 
     A first terminal of a resistor  64  is connected between the capacitor  26  and the resistor  28  at a junction  70 . The second terminal of the resistor  64  is connected to a junction  72  where the resistor  52  is connected to the parallel-connected resistor  54  and capacitor  56 . A resistor  76  has a first terminal that is connected to the junction  72  and hence to the resistor  64 . The other terminal of the resistor  76  is grounded. The resistors  64  and  76  preferably have resistances of about 1 KΩ and 510 Ω, respectively. 
     The interface circuit  20  may have resistance, inductance and capacitance values chosen to make the video device and interface circuit  20  transparent to the cable  10 . To accomplish this desired result, the interface circuit  20  generates a negative impedance to compensate for loading losses caused by connecting the video interface circuit  20  to the cable  10 . 
     Cable losses can cause limitations on the length of cable that may be used. These losses may be compensated for by having a negative input impedance at the tap that will give an overcompensation, which may be used to offset part of the cable line losses. The present invention may also have resistance and capacitance values chosen to compensate for both interface loading losses and cable losses, which makes it possible to maintain adequate signal strength on a single uninterrupted bus of up to 0.25 mile long without requiring additional amplification. 
     The video signal from the cable  10  passes though the capacitor  26  and resistor  28  to the base of the transistor  30 . The video signal is output at the emitter of the transistor  30 , which is connected to the resistor  46  at the junction  47 . The video signal output to the video device  63  is taken at the collector of the transistor  44 . An amplified video signal also appears at the emitter of the transistor  44 . This amplified video signal is fed back to the coaxial cable  10  through the resistors  52  and  64  and the capacitor  26 . In a preferred embodiment of the invention, the amplifier provides 12 dB of non-inverted gain. It has been found that a 12 dB gain, which corresponds to a voltage gain of 4, provides reasonable values of signal overload tolerance and noise in the network. 
     The structures and methods disclosed herein illustrate the principles of the present invention. The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects as exemplary and illustrative rather than restrictive. Therefore, the appended claims rather than the foregoing description define the scope of the invention. All modifications to the embodiments described herein that come within the meaning and range of equivalence of the claims are embraced within the scope of the invention.