Abstract:
A video signal input selector is subject to cross coupling between selectable video signals. The input selector comprises a video amplifier coupled to amplify a video signal selected from said selectable video signals. A controllable switch has first and second elements, the first element receives the selected video and is coupled to signal ground. The second element receives the selected video and is coupled to the amplifier. During a first condition the first element is controlled to be open circuit and the second element is controlled to be short circuit coupling the selected video signal to the video amplifier. In a second condition the first element is controlled to short circuit the selected video signal to the signal ground and the second element is controlled to be open circuit substantially inhibiting the selected video signal from coupling to the video amplifier.

Description:
[0001]    This invention relates to the field of signal source selection and in particular to the reduction of crosstalk between signals coupled for source selection.  
         BACKGROUND OF THE INVENTION  
         [0002]    In typical analog video signal source selectors, for example switching matrices or vision switchers, the physical circuit layout is given as much attention as the circuit design of the switching or cross point elements. For example, on printed conductor boards, a ground plane may be interspersed between signal carrying conductors. Similarly grounded conductive layers may be used separate signals likely to contain frequencies having a greater susceptibility to cross coupling to adjacent signal conductors. Often coaxial cable conductors are employed or printed conductors arranged to emulate transmission line characteristics in order to inhibit unwanted coupling of signal energy into adjacent circuits. In a video display device such preventative crosstalk prevention methods may be precluded by virtue of physical space, display layout and product cost. Thus, a video display device which can be capable of receiving multiple baseband analog signal inputs in both standard and high definition TV signal formats from such sources as digital or analog recorders, a satellite TV receiver, DVD player, or computer, may be subject to undesirable and unwanted picture impairment resulting from signal cross coupling.  
           [0003]    Component signal formats often comprise a luminance signal with red and blue color difference signals, frequently expressed as Y Pr Pb, and for a digitally derived signal of standard definition, can contain a luminance signal spectra extending to about 5 MHz. Similarly a luminance signal derived from a high definition source can result in a luminance signal spectra extending to 20 MHz and beyond. Typically the color difference signals are constrained to have significantly less bandwidth, typically half or a quarter that of the luminance component and hence these signals pose a less severe crosstalk threat.  
           [0004]    Furthermore multiple input signals can occur in both standard and high definition TV signal formats which are asynchronous one with the other, or expressed another way, the plurality of input signals have differing timebase frequencies. Hence, crosstalk susceptible signals will be more obviously displayed as a consequence of the relative motion between the synchronizing signals of the selected signal and the timebase of the unwanted cross coupled component.  
           [0005]    In the exemplary switching matrices and vision switchers mentioned previously, input signals are often synchronized or standards-converted to achieve a common timebase frequency. In addition, such synchronized or standards converted signals are usually coupled within the matrix or switcher to enable simultaneous signal selection at multiple destinations. Thus it can be appreciated that the various physical coupling methods described earlier, directed to the prevention of unwanted signal coupling are an essential prerequisite. Hence any residual cross coupling can only exist as a consequence of crosstalk across the signal source selector or switch to produce an unwanted signal at the signal processing destination. Such cross coupling can often occur at the signal source selector by virtue of parasitic capacitance present between open circuit or non-selected switching contacts or elements. Thus signal source selectors frequently have the form of a series shunt combination of switching elements. In this arrangement a series switch element couples or inhibits coupling of the wanted signal to the destination. The shunt switch element is turned on when the series switch element is open thus effectively grounding the output of the series switch and removing any unwanted signals coupled via the parasitic capacitance of the open circuit series switch. A series shunt switching combination is controlled in an inverse manner such that when one switching element is on the other is off, in this way any unwanted crosstalk signals are largely prevented for contaminating the wanted signal source selection.  
           [0006]    However, as mentioned previously, the preventative crosstalk techniques employed in video matrices although desirable are generally precluded from use in a video display device by cost and space considerations. As a consequence a video display device capable of selecting between multiple viewing sources, some asynchronous, in both standard and high definition formats can result in severe signal crosstalk with wanted picture degradation. Furthermore, although the series shunt switching arrangement described previously provides a remedy for signal coupling or leakage across the series switch element, any cross coupling occurring prior to the series shunt switching elements is largely unremedied, with the consequence that the selected picture remains impaired.  
         SUMMARY OF THE INVENTION  
         [0007]    Cross coupling in a video selector between unwanted and wanted signals is obviated by an inventive switching arrangement. A video amplifier is coupled to amplify a selected video signal. A controllable switch has first and second elements, the first element receives the selected video signal and is coupled to a signal ground. The second element receives the selected video signal and couples to the video amplifier. In a first condition the first element is controlled to be open circuit and the second element is controlled to be short circuit controllably coupling the selected video signal to the video amplifier. In a second condition the first element is controlled to be short circuit coupling the selected video signal to the signal ground and the second element is controlled to be open circuit substantially inhibiting coupling to the video amplifier. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0008]    [0008]FIG. 1 is a simplified block diagram of a video signal selector for a display apparatus including inventive arrangements. 
     
    
     DETAILED DESCRIPTION  
       [0009]    A video input selector for a multimedia display device is depicted in simplified form in FIG. 1. FIG. 1 includes selector  5 , which forms part of integrated circuit  6 , allows user selection by means of I 2 C control bus, between a composite encoded video signal Vc, for example, a NTSC or PAL encoded signal, and a luminance signal component Yc input from a signal source which provides separate luminance and chrominance signal components, for example S-Video, coupled via connector J 2 . Switch  4  couples the composite encoded video signal, (NTSC or PAL), to a comb filter within IC  6  that removes color subcarrier signals and provides separation of luminance and chrominance (chroma) components from the composite signal Vc. Switch  3  selects the luminance video component Yc input from an S-video signal input at connector J 2 . Since the luminance component Yc represents a separated or derived luminance signal it does not require comb filtering to remove any color subcarrier signal, thus it is coupled within IC  6  to a point following comb filter processing. Integrated circuit  6 , for example an F2PIP/Comb fitter, generates both a chrominance signal (chroma) and a luminance signal (Yo) responsive to selection controlled by the I 2 C control bus. Signal Yo is coupled via a wire conductor to provide an input signal that is coupled via printed conductor track  24 A to a series connected resistor R 3  and via printed conductor track  24 B to selector switch  1  of IC  2 , for example CMOS type 74HC4053. The chrominance signal, chroma, from block  6  is coupled directly to video processor integrated circuit IC  1 , for example Toshiba type TA1276, for demodulation to produce color difference signal components. The demodulated color difference signals for example I/Q or R-Y/B-Y are output from video processor IC  1  and coupled to selector switch  52  of IC  3  which facilitates selection between demodulated color difference signals and component input color difference signals, Pr/Pb from an external signal source.  
         [0010]    An external component signal, for example denoted as Y Pr Pb and originating from, for example, a VCR, DVD player, satellite or terrestrial digital television DTV receiver can be applied to input connector J 1 . As described, depending on the signal source the external luminance signal Yext can contain signal frequency components in the order of 5 MHz for signals from a DVD player, and in excess of 20 MHz for DTV signal derived from a high definition television signal source. Signals Y, Pr, Pb present at connector J 1  are terminated by resistors R 6 , R 7 , and R 8  respectively and coupled directly to circuit block  10 . External color difference component signals Pr and Pb are coupled, via protection circuitry not shown but similar to that shown for luminance signal Yext, to switch S 2 A of integrated circuit IC  3 , for example CMOS type 74HC4053, for selection as described previously. The external luminance component signal Yext is coupled via printed conductor track  22 A where it is divided and applied to a pair protection networks formed by a series connected resistor R 1  (R 2 ) and capacitor C 3  (C 4 ) and terminated by a Zener diode D 1  (D 2 ) which is connected to ground. Zener diodes D 1  and D 2  provide asymmetrical voltage amplitude protection and have different breakdown voltages in correspondence with expected operating signal amplitudes.  
         [0011]    Luminance signal Yext is AC coupled by capacitor C 3  and supplied as signal Yext 1  to selector switch S 2 B of integrated circuit IC  3  for coupling to video processor IC  1  as signal Y 2   in . Similarly signal Yext is AC coupled by capacitor C 4  and supplied as signal Yext 2  to switch S 1  of integrated circuit IC  2  for coupling as a synchronizing signal to integrated circuit T 4  (not shown).  
         [0012]    Stray or parasitic capacitance can exist between printed conductor tracks  22 B and  22 BB respectively, and track  24 B. These stray capacitances are depicted as capacitors Cs 1  and Cs 2  shown with dashed lines. It can be appreciated that when a luminance signal Yext is present on connector J 1 , high frequency energy present in signals Yext 1  and Yext 2  will be coupled via stray capacitors Cs 1  and Cs 2  to on conductor  24 B and luminance signal Yo. Similarly luminance signal Yo will be coupled onto conductor tracks  22 B and  22 BB. However, this reciprocal cross coupling is avoided because signal Yo is inhibited when viewing external component signals, as will be explained.  
         [0013]    When an external component signal is selected for viewing, by means of the I 2 C bus, switches S 1  and S 2 A/B of circuit block  10  change from the position depicted in FIG. 1 to permit coupling of luminance signal Yext to video processor IC  1  and to provide a delayed signal for synchronizing integrated circuit T 4  (not shown). Furthermore selecting the external component signal also results in switches  3 ,  4 , and  5  within the FPIP/Comb filter IC  6  assuming a condition which removes both chrominance signal (chroma), and luminance signal (Yo) from the output of block  6 . Thus, no internally processed luminance or chrominance signals are coupled to selector  10 , and printed conductors tracks  24 A and  24 B. Thus unwanted cross coupling between internal luminance signal Yo and external luminance signals Yext 1 /Yext 2  is prevented.  
         [0014]    When viewing an internally derived signal, unwanted cross coupling between an external luminance signal Yext 1 /Yext 2  and internal luminance signal Yo is advantageously prevented by an inventive arrangement shown in block  16  which will now be described.  
         [0015]    Selection between an external component signal and the various internally derived signals is controlled by addressing the various switching elements via the I 2 C control bus. Video processor IC  1  receives the I 2 C control bus and generates switch control signal  1  which is coupled to switches S 1  and S 2 A/B which provide a series switching function to select between internal and external luminance and color difference signals. In addition control signal  1  is coupled to an inventive shunt switch shown in block  16  and applied to the base terminal of an NPN transistor Q 1  via resistor R 5 . The base terminal of transistor Q 1  is also coupled to ground via resistor R 4  thus providing a potential divider for control signal  1 . The collector of transistor Q 1  is connected to the base terminal of an NPN transistor Q 2  and to a power supply, for example +9v, via resistor R 3 . The emitter terminals of transistors Q 1  and Q 2  are connected to ground. The collector of transistor Q 2  is connected to a pair of capacitors C 1  and C 2 . Capacitor C 2  is connected to the junction of capacitor C 3 , zener diode D 1  and an input of switch S 2  at conductor  22 B. Similarly capacitor C 1  is connected to the junction of capacitor C 4 , Zener diode D 2  and an input of switch S 1 B at conductor  22 BB.  
         [0016]    Operation of inventive shunt switch  16  will now be explained. When an external component signal is selected control  1 , at  20 , assumes a positive voltage value of approximately 5 volts or greater, and conversely when the internally derived signals are selected control  1  assumes a low or substantially zero voltage value. Thus with external components selected, the positive voltage of control  1  causes transistor Q 1  to turn on and assume a saturated state. With transistor Q 1  saturated, transistor Q 2  receive no base current and is held off with the collector terminal assuming a high impedance. Thus, capacitors C 1  and C 2  are effectively connected in series between nominally identical, AC coupled signals Yext 1  and Yext 2  present on conductors  22 B and  22 BB respectively. As described previously, unwanted cross coupling of internal signal Yo via stray capacitors Cs 1  and Cs 2  into external luminance signals Yext 1 /Yext 2  is prevented within block  5 .  
         [0017]    When internally derived components are selected for display, switch control  1  has a nominally zero voltage value which turns transistor Q 1  off causing the collector terminal to become a high impedance. With transistor Q 1  off resistor R 3  supplies current from the positive supply to the base of transistor Q 2  which is sufficient to cause saturation. With transistor Q 2  saturated, a very low impedance is formed between the transistor collector and emitter terminals, which effectively connects the junction of capacitors C 1  and C 2  to ground at the emitter of transistor Q 2 . Thus capacitors C 1  and C 2  are controlled or switched to form AC signal attenuators that reduce the amplitude of external luminance signals Yext 1  and Yext 2  present on conductors  22 B and  22 BB. Capacitor C 2  forms a shunt path for signal Yext 1  with attenuation occurring across resistor R 1  and capacitor C 3  which attenuates the amplitude of signal Yext 1  by about 30 dB at 5 MHz and about 41 dB at 20 MHz. Similarly for capacitor C 1  which forms a shunt path for signal Yext 2  with attenuation occurring across resistor R 2  and capacitor C 4  yielding a similar attenuation of signal Yext 2 . The AC signal attenuation that result from grounding capacitors C 1  and C 2  at the collector of transistor Q 2  may also be considered to form selectable or switched lowpass filters having a corner frequency of about 165 KHz. Furthermore, if the impedance to ground at the junction of capacitor C 4 , Zener diode D 2  and an input of switch S 1  is defined by a high value resistor, for example 10 kilo ohm, connected in parallel or across switched capacitor C 1 , the selectable lowpass filter can be considered a selectable or switchable bandpass filter. With the exemplary 10 kilo ohm resistor in parallel with capacitor C 1 , a low frequency −3 dB point of about 150 Hz and a high frequency −3 dB point of about 160 KHz result which can advantageously provide filtering to remove power line and high frequency noise components present on conductor  22 BB.  
         [0018]    When internally derived components are selected for viewing any external luminance component signal input to the display will advantageously be attenuated and severely lowpass filtered at the input to selector switches  1  and  2 . However this frequency dependent attenuation is controllably removed when an external component signal is selected for viewing. Thus high amplitude, high frequency signals present in an external luminance signal are advantageously attenuated to a level within the module at which cross coupling between conductors, and/or across open circuit switching elements, is substantially eliminated and rendered invisible.