Abstract:
An electronic system providing a video signal to an output terminal intended to be connected to a receiver having one input impedance out of two input impedances, the electronic system including an adaptable amplifier providing the video signal and capable of operating according to one operation configuration out of two operation configurations, each operation configuration being adapted to one of the two input impedances of the receiver; circuitry for detecting characteristic portions of the video signal; and control and measurement circuitry capable of measuring a signal representative of the current provided to the output terminal by the electronic system during each detected characteristic portion, and of having the adaptable amplifier adopt one of the two operation configurations based on the comparison of the representative measured signal with thresholds.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the adaptation of an electronic system, called a source, transmitting an analog video signal, generally a variable voltage, to another electronic circuit, called a receiver, via transmit means. 
     The present invention relates to the transmission of a video signal from a source which for example corresponds to a reader of a DVD-type video disk (Digital Versatile Disk), to a camera or to a decoder box (Set Top Box) and a receiver, which for example corresponds to a display screen or to a video recording system. The transmit means connecting the source to the receiver may correspond to a cable. For the video signal received by the receiver to be as little deformed as possible, it is necessary for the source impedance and the receiver impedance to be equal to the characteristic impedance of the transmit means. It can then be said that the connection is adapted. The characteristic impedance of a cable most used for the transmission of a video signal is 75 ohms 
     2. Discussion of the Related Art 
     Different international standards, for example, standard EIA, define the features of the video signals used for such transmissions. Currently, to perform a transmission with the best possible quality while respecting the existing standards, the video signal comprises a non-zero D.C. component which is transmitted to the receiver. Such a connection is designated as DC and a receiver capable of receiving a video signal with a non-zero D.C. component is called a DC receiver. 
       FIG. 1  schematically shows a source  10  of a video signal S OUT  connected to a DC receiver  12  by a cable  14 . Source  10  comprises an output stage  16  comprising a generator  18  receiving a video signal S VIDEO  and providing a video signal S OUT . Generator  18  is connected to a source of a reference voltage  19 , generally the ground of source  10 . A resistor  20  is provided between the output of generator  18  and an output terminal O of source  10 . Cable  14  is connected between terminal O and an input terminal I of receiver  12 . DC receiver  12  comprises a resistor  22  connected between terminal I and a source of a reference voltage  24 , generally the ground of receiver  12 . To obtain an adapted connection, resistors  20  and  22  have the same value as the characteristic impedance of cable  14 . 
     There exist certain standards, for example, Japanese standards, which require that the video signal transmitted over the cable to comprise no D.C. component and which, for this purpose, provide for the receiver to comprise a capacitive element in series with a resistive element to eliminate the D.C. component of the video signal provided by the source. Such a connection is known as an AC connection and a receiver capable of receiving a video signal with a zero D.C. component is called an AC receiver. 
       FIG. 2  shows a diagram similar to  FIG. 1  in the case of an A.C. connection. AC receiver  12  comprises a capacitor  26  series-assembled between terminal I and resistor  22 . 
     The receiver to which the video signal source can be connected has an input impedance which may thus be purely resistive or comprise a resistive component and a capacitive component. In the case of a DC receiver, the source must be able to supply current while in the case of an AC receiver, the source must be able to both supply and absorb current. 
       FIG. 3  shows a conventional example of embodiment of a video signal source capable of being connected to a DC receiver or to an AC receiver. Output stage  16  comprises a circuit of emitter follower type comprising a differential amplifier  25  having its positive terminal (+) receiving video signal S VIDEO  and having its negative terminal (−) connected to a node E. A resistor R g1  is provided between node E and a source of a reference voltage V REF . A resistor R g2  is provided between node E and a node F. The output of amplifier  25  drives the base of an NPN-type bipolar transistor T buf  having its collector connected via a resistor R buf  to a source of a reference voltage  27 , for example, the positive supply of source  10 , and having its emitter connected to node F. Resistor  20  is arranged between nodes F and O. A current generator  28  is arranged between node F and ground  19 . Source  10  is likely to absorb and supply current and can thus be connected to an AC receiver or to a DC receiver. However, such a source  10  has the disadvantage of a strong consumption since current generator  28  supplies current uselessly when it is connected to a DC receiver. 
     SUMMARY OF THE INVENTION 
     The present invention provides a video signal source which is capable of being connected, according to an adapted connection, to a receiver having an input impedance which is purely resistive or comprises a resistive component and a capacitive component and which has a reduced power consumption whatever the nature of the receiver to which it is connected. 
     Another object of the present invention is to provide a video signal source of simple design. 
     The present invention provides an electronic system providing a video signal to an output terminal intended to be connected to a receiver having one input impedance out of two input impedances. The electronic system comprises an adaptable amplifier providing the video signal and capable of operating according to one operation configuration out of two operation configurations, each operation configuration being adapted to one of the two input impedances of the receiver; means for detecting characteristic portions of the video signal; and control and measurement means capable of measuring a signal representative of the current provided to the output terminal by the electronic system during each detected characteristic portion, and of having the adaptable amplifier adopt one of the two operation configurations based on the comparison of the representative measured signal with thresholds. 
     According to an embodiment of the present invention, the adaptable amplifier provides the video signal in the form of a succession of cycles, each cycle starting with a pulse, said characteristic portions corresponding to said pulses. 
     According to an embodiment of the present invention, the adaptable amplifier comprises a current generator connected to the output terminal, said control means being capable of deactivating the current generator when the sum of the current provided by the current generator and of the current provided to the output terminal is greater than a first current during one of the characteristic portions, and of activating the current generator when the current provided to the output terminal is smaller than a second current during one of the characteristic portions, the second current being smaller than the first current. 
     According to an embodiment of the present invention, the system provides a given number of output signals to said given number of output terminals, each connected to a receiver having one input impedance out of two input impedances, the electronic system comprising said given number of adaptable amplifiers, each providing one of said given number of video signals, each amplifier being capable of operating according to one operation configuration out of two operation configurations, each operation configuration being adapted to one of the two input impedances of the receiver; means for detecting characteristic portions of a video signal out of said number of video signals; and said number of control means, each control means being capable of measuring a signal representative of the current provided by one of the amplifiers adaptable to the associated output terminal during each detected characteristic portion and of having said adaptable amplifier adopt one of the two operation configurations based on the comparison of the representative measured signal with thresholds. 
     According to an embodiment of the present invention, the adaptable amplifier comprises a differential amplifier having a first input receiving an input video signal and having a second input connected to a node, said node being connected to a source of a reference voltage via a first resistor and to the output terminal via a second resistor, the output of the differential amplifier being connected to the control terminal of a first transistor having a first main terminal connected to the output terminal and having a second main terminal connected to a source of a first reference voltage via a third resistor, the current generator comprising a second transistor having a first main terminal connected to the output terminal and having a second main terminal connected to a source of a second reference voltage. 
     According to an embodiment of the present invention, the current generator comprises a switch arranged between the control terminal of the second transistor and the source of the second reference voltage. 
     According to an embodiment of the present invention, the control means comprises third and fourth transistors having their control terminals connected in common to a first main terminal of the third transistor, a second main terminal of the third transistor being connected to the source of the first reference voltage via a fourth resistor, the first main terminal of the third transistor being connected to the source of the second reference voltage via a fifth resistor, a first main terminal of the fourth transistor being connected to the second main terminal of the first transistor via a sixth resistor, a second main terminal of the fourth transistor being connected to the source of the second reference voltage via a seventh resistor, the representative signal being the voltage across the seventh resistor. 
     According to an embodiment of the present invention, the control means comprise a hysteresis comparator receiving the representative measured signal and being capable of turning on the switch when the representative signal is greater than a first voltage and of turning off the switch when the representative signal is smaller than a second voltage smaller than the first voltage. 
     According to an embodiment of the present invention, the control means comprise means for storing the representative signal connected to the hysteresis comparator; and a switch controlled by the detection unit and arranged between the storage means and the first main terminal of the fourth transistor. 
     The present invention also provides a method for adapting an electronic system providing a video signal to an output terminal intended to be connected to a receiver having one input impedance out of two input impedances. The method comprises the steps of providing an adaptable amplifier providing the video signal and capable of operating according to one operation configuration out of two operation configurations, each operation configuration being adapted to one of the two input impedances of the receiver; detecting characteristic portions of the video signal; measuring a signal representative of the current provided by the electronic system to the output terminal during each detected characteristic portion; and having the adaptable amplifier adopt one of the two operation configurations based on the comparison of the representative measured signal with thresholds. 
     The foregoing objects, features, and advantages of the present invention, as well as others, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2 , previously described, schematically show a conventional video signal source respectively connected to a DC receiver and to an AC receiver; 
         FIG. 3 , previously described, shows a conventional example of a source likely to be indifferently connected to a DC receiver or to an AC receiver; 
         FIG. 4  shows an example of a composite video signal; 
         FIG. 5  schematically illustrates the operating principle of an example of embodiment of a video signal source according to the present invention; 
         FIG. 6  shows a more detailed embodiment of the source of  FIG. 5 ; 
         FIG. 7  shows the relation between a characteristic voltage and current used in the present invention; 
         FIG. 8  shows a variation of an element of the source of  FIG. 6 ; 
         FIG. 9  shows a variation of video signals likely to be provided by a video signal source; and 
         FIG. 10  shows an alternative embodiment of a video signal source according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the drawings, the same reference numerals designate identical elements or similar elements exerting identical functions. Further, in the following description, the base of a bipolar transistor and the gate of a MOS transistor are designated as the control terminal of a transistor, and the emitter or the collector of a bipolar transistor and the drain and the source of a MOS transistor are designated as the main terminal of a transistor. 
     The present invention provides having the video signal source automatically detect the nature of the receiver to which it is connected so that the source adapts to the receiver to maintain a small power consumption. 
     A possibility to distinguish the nature of the receiver is to measure the average current provided by the source when it is connected to the receiver. Indeed, the average current is substantially zero for an AC receiver and is generally not zero for a DC receiver (the average value of video signal S OUT  being generally different from 0). However, video signal S OUT  is a very irregular signal and the determination of an average value of the current representative of the nature of the receiver would require acquiring samples over a significant time period. Such a method for detecting the nature of the receiver would then be little reactive and would have a significant power consumption cost. 
     The present invention provides performing an automatic detection of the nature of the receiver by measurement of a signal representative of the current provided by the source to the receiver at specific times for which the video signal provided by the source is constant and keeps the same value at the different measurement times. The current measured at such times thus has a substantially constant value which will depend on the nature of the receiver. According to the measured current, the source adopts one operation characteristic out of two possible operation characteristics, one being adapted to a DC receiver and the other to an AC receiver. As an example, the source comprises a current generator which is deactivated when a DC receiver is detected and which is activated when an AC receiver is detected. 
       FIG. 4  shows a conventional example of video signal S VIDEO  received by the output stage of a video signal source. Such a signal is called a composite video signal or CVBS signal (for Chroma Video Blanking Synchro). Signal S VIDEO  is a cyclic signal for which duration T C  of a cycle, for example, of 64 μs, corresponds to the duration of the scanning of a line of a screen and of the fly-back to the next line. A cycle starts with a pulse  40  of duration T I , for example, of 4. 7 μs. When receiver  12  is a display screen, pulses  40  are used to provide synchronization signals to control the screen scanning. For this reason, pulses  40  are generally called synchronization pulses. For each cycle, pulse  40  is successively followed by a stage of constant level  42 , representative of the “black” level of the image, with a portion of variable level  44 , which corresponds to the actual information content of a line in the image, that is, to the luminance and to the chrominance. Variable portion  44  is followed by a stage  46  of the black level which closes the cycle. A current measurement is performed for each cycle during start-of-cycle pulse  40  or during stages  42 ,  46 . In practice, pulses  40  being easy to detect, an example of embodiment of the present invention provides detecting a signal representative of the current provided by the source during the pulses of composite video signal S VIDEO . 
       FIG. 5  illustrates the operation principle of an example of embodiment of a video signal source  50  according to the present invention. The output stage comprises an adaptable amplifier  52  which receives video signal S VIDEO  and which provides a video signal S OUT  and a current I c  to receiver  12 . The output stage comprises a unit  54  for detecting the synchronization pulses contained in video signal S VIDEO  which provides, on each detection of a pulse  40 , a control signal S 1  to a current measurement and comparison unit  56 . For each detected pulse, unit  56  determines a signal representative of current I c  provided by source  50  and compares the determined value with thresholds. According to the result of the comparison, unit  56  provides a control signal S 2  to amplifier  52  which adopts an operation characteristic adapted to a DC receiver or to an AC receiver. 
       FIG. 6  shows a more detailed example of embodiment of the output stage of source  50  of  FIG. 5 . The elements common with output stage  16  shown in  FIG. 3  are designated with the same references. In particular, it shows differential amplifier  25 , power transistor T buf  assembled as an emitter follower and resistors R g1  and R g2 . In the present example of embodiment, current generator  28  is formed of an NPN-type bipolar transistor T s  having its collector connected to node F and having its emitter connected to ground  19 . The base of transistor T s  is connected to a circuit for providing a bias signal, not shown, and to the drain of an N-type MOS transistor  58  having its source connected to ground  19 . The gate of transistor  58  receives signal S 2 . The current measurement and comparison unit comprises a current measurement circuit  60  which, in the present example of embodiment, comprises a pair of PNP-type bipolar transistors T 1 , T 2  having their bases connected in common to the collector of transistor T 1 . The emitter of transistor T 1  is connected via a resistor R e  to the source of reference voltage  27  and the collector of transistor T 1  is connected via a resistor R I0  to ground  19 . The emitter of transistor T 2  is connected via a resistor R e  to the collector of power transistor T buf  and the collector of transistor T 2  is connected via a resistor R S  to ground  19 . The voltage across resistor R S  is noted V S . The current measurement unit comprises a sampling and comparison unit  65  which comprises a controllable switch  66  having a terminal connected to the collector of transistor T 2  and having its other terminal connected to a node G. A capacitor  68  is provided between node G and ground  19 . The voltage across capacitor  68  drives a hysteresis comparator  70  which provides signal S 2 . Switch  66  is controlled by signal S 1  provided by synchronization pulse detection unit  54  which receives video signal S VIDEO . Synchronization pulse detection unit  54  is an element conventionally used, especially by a receiver corresponding to a display screen, and will not be described any further in the present description. 
     The operation of the output stage according to the present example of embodiment will now be described. Voltage V S  is representative of current I col  received by the collector of transistor T buf , itself substantially equal to the current I buf  provided by the emitter of transistor T buf . Current I buf  is equal to the sum of current I c  provided by source  50  to load  12  and of current I s  absorbed by current generator  28 . On each pulse of video signal S VIDEO , pulse detection unit  54  provides a signal S 1  which turns on switch  66 . Voltage V S  is then applied across capacitor  68 . Based on the comparison of voltage V S  with threshold voltages, it is possible to determine whether the receiver connected to source  50  is an AC receiver or a DC receiver and to block or turn on transistor  58 , which respectively activates or cancels current I s  via transistor T s . 
       FIG. 7  illustrates variation curve  72  of voltage V S  according to current I col . It should be noted that curve  72  comprises a substantially linear central portion having an extent defined by the values of resistors R e , R s , and R I0 . 
     An example of determination of the threshold voltages used by comparator  70  will now be described. When source  50  is connected to a DC receiver, theoretical value I cth  of current I c  provided to the DC receiver is determined, assuming that current generator  28  is deactivated, from the value of the voltage provided by the source on occurrence of a pulse of signal S VIDEO . As an example, on occurrence of a pulse of signal S VIDEO , current I cth  is on the order of 2 mA. When source  50  is connected to an AC receiver, theoretical value I sth  of current I s  to be provided by current generator  28  is defined by the negative minimum value that the voltage across the resistor of the receiver in series with the input capacitor of the receiver can reach. As an example, current I sth  is on the order of 8 mA. 
     When current generator  28  is deactivated, that is, when MOS transistor  58  is on, if current I c , that is, I col , decreases below I cth , this means that the receiver is not of DC type but of AC type. Current generator  28  being deactivated, the presence of an AC or DC receiver can thus be determined by comparing current I col  with a minimum threshold I col1 , for example, of 1 mA. This amounts to comparing voltage V S  with a threshold voltage V 1 . If V S  is greater than V 1 , this means that source  50  is connected to an AC receiver. MOS transistor  58  is then off, which activates current generator  28 . Current I s  is then present. 
     When current generator  28  is activated, if current I col  is greater than theoretical current I sth  provided by current generator  28 , this means that the receiver is not of AC type but of DC type. Current generator  28  being activated, the presence of an AC or DC receiver can thus be determined by comparing current I col  with a maximum threshold I col2 , for example, on the order of 9 mA. This amounts to comparing voltage V S  with a threshold voltage V 2 . If V S  is smaller than V 2 , this means that source  50  is connected to a DC receiver. MOS transistor  58  is then turned on, which deactivates current generator  28 . Current I s  cancels. 
     Threshold voltages V 1  and V 2  are determined from curve  72  of  FIG. 7 . As an example, with I col1  equal to 1 mA and I col2  equal to 9 mA, resistors R e , R s , and R I0  may be selected so that threshold voltages V 1  and V 2  are respectively on the order of 4 V and 1.5 V with R buf  on the order of some ten ohms. 
       FIG. 8  shows an alternative embodiment of current detection unit  60  in which an additional transistor T 3  having its base connected to the collector of transistor T 1 , having its collector connected to ground  19 , and having its emitter connected to the bases of transistors T 1  and T 2 , is provided. Such a variation enables improving the temperature stability of variation curve  72  of voltage V S  according to current I col . 
     In certain applications, the output stage of the video signal source provides different video signals on different outputs. Such signals for example are Y/C-type signals (also called S-video signal) comprising a luminance signal (signal Y) and a chrominance signal (signal C). The source outputs may be connected to receivers of different natures. Thereby, a current measurement must be performed at the level of each output of the source. 
       FIG. 9  shows an example of variation of signals Y and C. Generally, only luminance signal Y comprises synchronization pulses  74 . Chrominance signal C comprises no synchronization pulses, but only stages of constant levels between two cycles. 
       FIG. 10  shows an example of embodiment of an output stage according to the present invention of a source  76  capable of providing two video signals S OUT  and S OUT′  to two receivers  12  and  12 ′, possibly of different natures. As an example, signals S OUT  and S OUT′  are respectively provided from signals Y and C. The present invention provides for the circuits for providing S OUT  and S OUT′  to each comprise an adaptable amplifier  52 ,  52 ′ receiving a control signal S 2 , S 2 ′ provided by a current measurement and comparison unit  56 ,  56 ′, as described previously in relation with  FIG. 5 . Since signals Y and C are synchronous and only signal Y comprises synchronization pulses  74 , the output stage of source  76  comprises a single synchronization detection unit  54  which provides the same control signal S 1  to current measurement and comparison units  56 ,  56 ′. 
     More generally, in the case of complex video signals formed of several signals, for example, YUV-type video signals (also called Y—Pr—Pb or Y-Cb-Cr signals), the present invention provides using a single synchronization pulse detection unit which controls each current measurement and comparison unit associated with each circuit for providing a component of the video signal. 
     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, in the previously-described examples of embodiment, the source of reference voltage  27  corresponds to the positive power supply. However, such a source  27  may correspond to a negative power supply, the polarity of the bipolar transistors being then inverted. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.