Abstract:
In a method and circuit for recovering a sync signal from an input sync signal passing through a cable to a display device, an average value of the input sync signal is obtained during a predetermined time period so as to obtain a sync threshold, which is compared with the input sync signal. A sync signal is output when the input sync signal is greater than the sync threshold.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a sync circuit for a display monitor. 
       BACKGROUND OF THE INVENTION 
       [0002]    Host devices such as digital computers and games employ display units such as LCDs or cathode ray tubes (CRT) by which the computer or game is monitored. It is commonly required to physically separate the display device from the host devices and this is most typically done using cables that are connected via mating plugs to respective sockets in the electronic device and display monitor. Normally these cables are in the order of four to six feet in length, enabling limited separation of a monitor from a computer or other host device. 
         [0003]    There are, however, situations where it is desirable to increase significantly the separation between a host device and display monitor. This may be by virtue of space limitations or because of environmental considerations, the latter sometimes including an inhospitable environment for a host device. 
         [0004]    Display monitors include a control circuit that includes horizontal and vertical scan synchronization circuits for controlling the raster scan sweep pattern of the electron beams generated by the display device. The horizontal scan synchronization circuit synchronizes each horizontal sweep of the electron beams with a horizontal sync signal in the received display signal, and generates a distinctive transient voltage signal at the beginning of each horizontal sweep of the electron beams. Likewise, the vertical scan circuit synchronizes each vertical sweep of the electron beams with a vertical sync signal in the received display signal, and generates a distinctive transient voltage signal at the beginning of each vertical sweep of the electron beams for redirecting the electron beams to the start of the display screen. 
         [0005]    There is a natural limit to the maximum separation between a raster scan display monitor and a host device that is achievable merely by extending the length of the connecting cable. This is due to the self-capacitance of the cable, which both attenuates and distorts the sync signals required to synchronize horizontal and vertical scanning The effect of attenuation is such that once the sync signals become too small to pass a threshold value, they will not be detected by the control circuit. The effect of distortion is such that the rise time of the sync signal increases, such that it takes longer to pass a given detection threshold. This can result in the sync signal being detected too late. In either case, the display goes blank. 
         [0006]      FIGS. 1   a  and  1   b  show graphical representations of normal and inverted sync pulses, respectively. The dotted lines depict recovered sync pulses after a short travel in a coaxial or CAT5 cable. The width of the recovered sync pulse is equal to the time between the sync pulse passing a slicing level as it rises and subsequently falls. Thus, when the sync pulse is perfectly square as shown in  FIGS. 1   a  and  1   b  such that its rise time is substantially zero, the width of the recovered sync pulse is the same as that of the input sync pulse fed to the cable. 
         [0007]      FIGS. 2 ,  3  and  4  show graphically the effect on the sync pulse caused by passing through cables of increasing length. Thus,  FIG. 2  shows that after passing through a cable of short length (up to 10 m) the recovered sync pulse is somewhat narrower than normal since its rise time is no longer zero and so by the time it passes the slicing level, the remaining pulse width is reduced. The CMOS slicing level is in the middle of the sync pulse so as to detect the sync pulse when its amplitude is half its maximum value. 
         [0008]      FIG. 3  shows that after traveling in a long cable (up to 60m), the sync pulse is both much more distorted and much reduced in amplitude. The distortion manifests itself in that the sync pulse now takes much longer to pass the threshold and the recovered sync pulse is very narrow as sync pulse only just reaches the slicing level. 
         [0009]      FIG. 4  shows that after traveling in a very long cable (up to 300m), the sync pulse is both very highly distorted and very much reduced in amplitude, so much so that it does not pass the slicing level and so cannot be recovered. 
         [0010]    It would clearly be desirable to permit longer cables to be used, while avoiding the above-mentioned drawbacks. 
       SUMMARY OF THE INVENTION 
       [0011]    It is therefore an object of this invention to provide a circuit that allows the length of cable connecting a host device and a raster scan display unit to be increased while still allowing the control circuit of the display unit to detect the sync signals. 
         [0012]    This object is realized in accordance with an aspect of the invention by a method for recovering a sync signal from an input sync signal passing through a cable to a display device, the method comprising: 
         [0013]    obtaining an average value of the input sync signal during a predetermined time period so as to obtain a sync threshold; 
         [0014]    comparing the input sync signal with the sync threshold; and 
         [0015]    outputting a sync signal when the input sync signal is greater than the sync threshold. 
         [0016]    According to another aspect of the invention, there is provided a circuit for recovering a sync signal from an input sync signal passing through a cable to a display device, the circuit comprising: 
         [0017]    integrator for obtaining an average value of the input sync signal during a predetermined time period so as to obtain a sync threshold that is automatically adapted to a polarity of the input sync signal, and 
         [0018]    a comparator for comparing the input sync signal with said sync threshold and outputting a sync signal when the input sync signal is greater than the sync threshold. 
         [0019]    The invention employs an automatic, signal-dependent sync recovering dynamic threshold to allow full width sync pulses to be recovered from any full, distorted or muted sync signals even when the cable connecting the display monitor to the host unit is as long as 300 m. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
           [0021]      FIGS. 1   1   a  and  1   b  show prior art graphical representations of normal and inverted sync pulses; 
           [0022]      FIGS. 2 ,  3  and  4  show graphically the effect on the sync pulse caused by passing through cables of increasing length in prior art configurations; 
           [0023]      FIG. 5  shows schematically an electronic circuit according to an embodiment of the invention for recovering horizontal and vertical sync signals regardless of cable length; and 
           [0024]      FIG. 6  is a graphical representation of the recovered sync pulses obtained using the circuit of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0025]      FIG. 5  shows schematically an electronic circuit  10  according to an embodiment of the invention for recovering horizontal and vertical sync signals regardless of cable length. Identical circuits may be used for recovering both horizontal and vertical sync signals, so that in the following description reference to “sync pulse” may be taken to imply either the horizontal or the vertical sync signal. 
         [0026]    The circuit comprises an input IN for receiving a TTL sync pulse directly or an analog input sync signal SYNC IN  via a first resistor R 1  that may optionally be switched into the circuit by means of a selector switch SW. In the case of an analog sync signal that requires a 75Ω load, the input sync signal is 75Ω terminated to ground via the switch SW. In one embodiment reduced to practice, the values of the resistors R 1 , R 2  and R 3  were respectively 90.9Ω, 100Ω and 392Ω such that the combined resistance of R 1  connected across the series connection of R 2  and R 3  is approximately 75 ohms. 
         [0027]    The input sync signal SYNC IN  passes through a voltage divider comprising a second resistor R 2  having a first end coupled to the input IN and a second end coupled to the first end of a third resistor R 3  whose second end is connected to GND. At the junction of the second and third resistors R 2  and R 3 , the input sync signal is separated into two paths. 
         [0028]    One path leads the signal via a fourth resistor R 4  into the positive input of an OP-AMP U 1 . The input is protected against over-voltage by first and second diodes D 2  and D 3  connected back-to-back in opposite polarities that operate as a clamping diode, which may be realized by an integrated circuit such as BAV99. The clamping diode clamps the input sync signal to VCC and VEE in case of over voltage. 
         [0029]    The second path conveys the input sync signal via a fifth resistor R 5  to a third rectifier diode D 3  such as FDLL4148 that charges a capacitor C 1  with its voltage. A sixth resistor R 6  serves as a bleeder resistor to maintain the average voltage of the sync signal and eliminate the possibility that the capacitor C 1  will become over charged. 
         [0030]    The DC voltage at the junction of the capacitor C 1  and the sixth resistor R 6  is fed to the negative input of the OP-AMP U 1  and is used as a reference voltage for the input sync pulse fed to the positive input of the OP-AMP. The OP-AMP U 1  acts as a threshold comparator for comparing the level of the input sync signal at its positive input with the reference slicing voltage at its negative input, which constitutes a voltage threshold. If the input sync signal exceeds the threshold voltage, the output recovered sync is now full TTL level (5 V peak-peak) and is sent to the respective sync control circuits of the display monitor. If desired, up to four receivers display monitors can be fed with the recovered sync signal each via a respective output resistor such as R 7  and R 8 . 
         [0031]    If the sync pulse is positive going as shown in  FIG. 6   a , the average voltage generated at the junction of the capacitor C 1  and the sixth resistor R 6  is low, —due to the low duty cycle of the sync pulses and the ratio between the charging resistor R 5  and the bleeding resistor R 6 — so, that the slicing threshold is approximately 700 mV above ground level. 
         [0032]    If the sync pulse is a negative going pulse as shown in  FIG. 6   b , the average voltage generated at the junction of the capacitor C 1  and the sixth resistor R 6  is high, and the slicing threshold is approximately 700 mV below the highest sync level. 
         [0033]    The result is that regardless of the polarity or shape of the input sync signal, once its level rises above 700 mV, the circuit  10  dynamically detects and recreates a full width and full TTL level sync at the output. 
         [0034]    It should be noted that the threshold of 700 mV is selected as a compromise between the desirability of increasing the length of cable over which the sync signals can be recovered, while reducing sensitivity of the control system to noise. Thus, the smaller the threshold, the longer is the cable that may be used while allowing recovery but the greater the risk that noise will be interpreted as a sync signal. The threshold of 700 mV is thus merely an example of a signal level that is of sufficient amplitude to allow signal synchronization while being distinguishable from a predetermined upper noise level. Different types of display monitor employ sync signals of different amplitudes and as display devices evolve it is, in any case, likely that sync signals of lower amplitude will be feasible and that better noise suppression may be utilized such that the threshold may be reduced below 700 mV. 
         [0035]    For the sake of completeness, it should be noted that in an actual embodiment reduced to practice, the values of the components were as follows: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component 
                 Value 
               
               
                   
                   
               
             
             
               
                   
                 R1 
                 90.9Ω 
               
               
                   
                 R2 
                 100Ω 
               
               
                   
                 R3 
                 392Ω 
               
               
                   
                 R4 
                 1KΩ 
               
               
                   
                 R5 
                 10KΩ 
               
               
                   
                 R6 
                 100KΩ 
               
               
                   
                 R7, R8 
                 51Ω 
               
               
                   
                 C1 
                 l μF 
               
               
                   
                   
               
             
          
         
       
     
         [0036]    It will be appreciated that these values are given by way of example only and changes can be made to the circuit without departing from the scope of the invention as changed. 
         [0037]    It will also be understood that the invention is applicable to all types of raster scan display, such as cathode ray tube (CRT), LCD or Plasma.