Abstract:
A signal treatment circuit treats an input signal containing line sync pulses used for displaying data on a screen. The circuit contains a phase locked loop to control horizontal sweeping according to active edges of line sync pulses, and a filter circuit that filters equalizing signals from the input signals and provides a filtered input signal to the phase locked loop.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to circuits for the treatment of line sync pulse signals used to display data on a screen. More particularly, the invention relates to a filter allowing the elimination of equalizing signals in a composite signal. An application to the field of monitors is described. 
     2. Discussion of the Related Art 
     To control the display of data on a monitor, and particularly to control the sweeping of an electron beam on a screen, synchronization signals are used. A frame is a collection of lines used to form an image on a screen of a monitor. 
     The synchronization signals are added to the useful signal, which represents the data to be displayed. Synchronization signals contain information allowing the determination of a start of a line (horizontal, or line sync) and the beginning of a frame (vertical, or frame sync). The synchronization signals are typically pulsed logic signals, largely defined by the polarity, frequency and duration of the pulses they contain. The polarity of the pulses may be positive or negative, according to which of the rising or falling edges are used. These signals are used by phase locked loops (PLLs). 
     FIGS. 1 a  and  1   b  illustrate such horizontal and vertical sync signals, H, V respectively. The signals represented are both of positive polarity and formed by pulses respectively labeled  101  and  102 . The line sync signals have a higher frequency, the frame sync pulses have a longer duration, and the active edges (rising—edges for positive polarity) of frame sync and line sync pulses are in phase. 
     Line and frame sync signals are either transmitted separately, or in the form of a single composite signal, which includes simultaneously the line and frame synchronization information. 
     FIG. 1 c  shows such a composite signal, C(H+V). This composite signal corresponds to the logical OR of the signals H in FIG. 1 a  and V in FIG. 1 b.  A drawback of this type of composite signal is the absence of edges during the frame sync pulse. 
     FIG. 1 d  shows a composite signal C including serration signals  103  which may be inserted into the composite signal to reduce this problem. The serration signals produce active edges in the composite signal during frame sync pulses. The active edges added by the serration signals typically have the same frequency and phase as the active edges of the line sync signal in the composite signal. 
     Finally, FIG. 1 e  illustrates a composite signal C containing equalizing signals  104 . For essentially historic reasons related to television, such equalizing signals may be inserted into the composite signal between the line sync pulses before and after the frame sync pulses, and between the serration signals during the frame sync pulses. They double the frequency of active edges when present. Typically, equalizing pulses appear five line sync pulses before a frame sync pulse, and they disappear five line sync pulses after the end of the frame sync pulse. 
     The presence of equalizing signals in a composite signal may disturb the functioning of a PLL which uses the composite signal to control the line sweep. There is a risk of locking the PLL onto a frequency double that-of the line sync pulses. 
     FIG. 2 schematically illustrates a PLL circuit used in monitors, in particular with regard to the line sync signals. The line sync signals are mainly treated by a PLL labeled PLL-H. This PLL includes a comparator  10 , a charge pump  11 , a capacitive filter  12 , a voltage controlled oscillator (VCO)  13 , and a phase adjustment means  14 . The comparator  10  compares an input signal (line sync H or composite signal C) with a reference pulse signal Sref. According to the result of this comparison, the comparator controls charge pump  11 . This pump  11  charges or discharges filter  12  so that a voltage Vref at the terminal of the filter  12  represents the result of the comparison. Oscillator  13  produces a triangular signal Vosch, the frequency of which is proportional to the voltage Vref. Signal Vosch is later transformed into pulse signal Sref by means  14 . The position of edges in signal Sref is determined by a phase adjustment signal Sadj. 
     Comparator  10  is preferably a phase frequency comparator, which avoids locking PLL-H on a multiple of the line sync frequency. Once the input H/C and reference Sref sync signals are in phase and at the same frequency, voltage Vref stabilizes. A detection means  15  will generate a signal DETVER, the state of this signal indicating whether the PLL is in lock with the frequency and phase of the input signal. 
     Typically, an input interface  17   a  and an input/output interface  18  allow the input synchronization signals to be provided to PLL-H and to a frame sync signal treatment means  16 . The input interface is for example a polarity detecting and signal cleaning interface, which provides positive polarity signals for use internally. 
     The interface  17   a  is connected to interface  18  by a circuit  17   b,  which is for example an integrator with integrated capacitor. 
     If the horizontal and vertical synchronization signals are separate, interface  17   a  supplies line sync signals H to PLL-H, and interface  18  provides frame sync signals V to means  16 . 
     If the received signal is a composite signal, the input interface supplies the composite signal (rectified or not) to PLL-H and circuit  17   b.    
     FIG. 3 c  shows a frame sync signal TRAMEXT, produced by circuit  17   b,  by extraction from the signal received by interface  17   a,  if this signal is a composite signal. Frame sync signal TRAMEXT is delayed by an amount d. 
     Referring to FIG. 3 b,  a classic technique for producing an extracted frame sync signal is to control the charging and discharging of the integrated capacitor of circuit  17   b  according to received pulses. Then, the voltage at the terminals of the integrated capacitor may be compared to a reference voltage, this reference voltage being chosen so as to not be reached when the charging period corresponds to the duration of a line sync pulse. 
     Circuit  17   b  typically includes an edge detector, such as a latch, to control a switching circuit included in interface  18  to supply signal TRAMEXT to means  16  and externally. Signal TRAMEXT may be used to inhibit PLL-H to avoid drifting of this PLL during reception of frame sync signals, if the sync signal is a composite signal. 
     The use of a phase/frequency comparator  10  may be preferable to the use of a simple phase detector, because locking PLL-H into a multiple frequency when only line sync pulses are present is then avoided. A problem is that this type of comparator is intolerant to an absence of pulses (for example, during a frame sync pulse without serration pulses) and to the presence of parasitic or unwanted pulses such as equalizing pulses. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a circuit avoiding the above mentioned problems. In order to achieve this, in one embodiment, a windowing operation is used, centered on the active edges of the received line sync pulses. In this way, drift which could be caused by equalizing pulses is eliminated. 
     An embodiment of the invention includes a circuit for the treatment of signals including line sync pulses used to display data on a screen, the circuit including a PLL to control the line sweep of the screen according to the active edges of the sync pulses. The circuit may also include means for producing a windowing filter to filter the signal received by the PLL when in an active state so that active edges of this received signal which are out of phase with active edges of the line sync pulses are eliminated. 
     In one embodiment, this windowing is combined with a windowing created by a frame sync signal treatment circuit, so that the windowing is effected around and during the frame synchronization pulses. This may avoid locking onto a multiple of the line sync frequency, which could happen with permanent windowing, in the case of an instantaneous frequency rise to a multiple of the previous frequency. 
     A permanent windowing leads to a risk of loss of phase lock of the PLL if the received sync pulses drift in frequency. A windowing limited in the time domain using equalization signals allows rapid adaptation to such a phase drift. 
     In one embodiment, the windowing signal is obtained by the combination of a signal representing the line sync pulses and a signal representing the frame sync pulses, in such a way that the windowing signal is activated at a time before the beginning of each frame sync pulse, and such that the windowing signal be deactivated after the end of each frame sync pulse. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages and improvements according to the present invention will become apparent in the following description of certain non-limiting embodiments, with reference to the accompanying drawings, within which: 
     FIGS. 1 a  to  1   e  represent waveforms of known sync signals; 
     FIG. 2 schematically represents a known PLL; 
     FIGS. 3 a  to  3   c  represent waveforms of a composite type signal and a frame sync signal extracted from the composite signal; 
     FIG. 4 schematically represents a circuit according to an embodiment of the invention; 
     FIG. 5 represents circuitry included in an embodiment of the circuit of FIG. 4; 
     FIGS. 6 a,    6   b,    6   c,  and  6   d  represent waveforms of signals related to the creation of a windowing signal, as provided by an embodiment of the circuit of FIG. 5; 
     FIG. 7 represents circuitry included in an embodiment of the circuit of FIG. 4; 
     FIGS. 8 a,    8   b,    8   c,    8   d,    8   e  and  8   f  represent waveforms of signals related to the formation of a windowing signal by an embodiment of the circuit of FIG. 7; 
     FIG. 9 represents circuitry included in an embodiment of the circuit of FIG. 4; 
     FIGS. 10,  11  and  12  represent circuits allowing inhibition of the PLL of a circuit according to an embodiment of the invention, 
     FIG. 13 represents a modification which may be made to the circuit of FIG. 9; and 
     FIG. 14 represents a circuit allowing the production of a signal used by the circuit of FIG.  13 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 4 schematically illustrates a circuit according to an embodiment of the invention. Other than the features mentioned in FIG. 2, it includes a circuit  20  producing a windowing signal EWI from the signal Vosch produced by the oscillator  13  and the phase adjustment signal Sadj; a circuit  21  producing a windowing signal TRAMFIN from the frame synchronization signal treatment circuit, a circuit  22  allowing a windowing signal WS to be produced from signals EWI, TRAMFIN and DETVER; and an AND gate  23  allowing filtration of the received signal by PLL-H using signal WS to eliminate the equalizing pulses. 
     In the following description, it is assumed that the received signal is a positive polarity, composite signal. Should the original signal have a negative polarity, a simple logic inversion of this signal will produce a positive polarity signal. 
     Also, it is assumed that the logic signals are active at the high state, and inactive at the low state. By analogy, the rising edges are called active edges, and the falling edges are called inactive edges. 
     FIG. 5 illustrates an embodiment of circuit  20 . This circuit has four inputs  2001 ,  2002 ,  2003 ,  2004 , and an output  2005 . Inputs  2001 ,  2002  receive signals Sadj and dV. These two signals are used to define signal EWI (illustrated in FIG. 6 c ). A voltage controlled voltage source  201  receives signals Sadj and dV, and produces two further voltages Sadj+dV, Sadj-dV. Input  2003  receives a triangular waveform Vosch (illustrated in FIG. 6 b ) from the oscillator  13 . Input  2004  receives a signal Qb (illustrated in FIG. 6 d ) which is active during the falling slope of signal Vosch. Signal Qb may be produced by a capacitor discharge detector, the triangular waveform Vosch produced by the oscillator being typically produced by charging and discharging a capacitor. 
     A comparator  202  may be provided, and compares signal Vosch on its non-inverting input with signal Sadj+dV at its inverting input. A further comparator  203  is also provided, and compares signal Vosch on its inverting input with and with signal Sadj-dV at its non-inverting input. Circuit  20  may also include a NOR gate  204  having inputs receiving output signals of comparators  202  and  203 , and signal Qb. Voltage Sadj used in the circuit of FIG. 5 is the signal used to detect the active edges of line sync pulses in the circuit  14 . A pulse signal EWI is produced, whose frequency is identical to that of the line sync pulses, of positive polarity, and where the pulses are centered on the active edge of line sync pulses. The operation of circuit  20  will now be described with reference to FIGS. 6 a  to  6   d.    
     FIG. 6 a  illustrates a line sync signal, as also shown in FIG. 1 a.    
     FIG. 6 b  shows the triangular waveform of Vosch, which includes a succession of positive ramps, where the signal voltage increases from a minimum value Vmin to a maximum value Vmax, and negative ramps, where the voltage descends from VVmax to Vmin. The positive ramps have a duration substantially equal to the period between successive active edges of the line sync signal. Thus, the edges of the sync signal correspond to points in time when the positive ramps reach a voltage Sadj, which lies between Vmin and Vmax. 
     FIG. 6 c  illustrates the windowing signal EWI. While the voltage of the ramp is less than Sadj-dV, comparator  203  produces an active signal. Signal EWI is therefore inactive. When the positive ramp voltage reaches the value Sadj-dV, the output of comparator  203  becomes inactive. Signal EWI becomes active, and the signal produced by comparator  202 , and Qb both remain inactive. Later, signal Vosch reaches the value Sadj+dV. The signal produced by comparator  202  becomes active. Signal EWI returns to its inactive state. Once the maximum voltage Vmax is reached, signal Vosch begins to decrease towards Vmin. During this time, signal Qb is active, which ensures that signal EWI remains inactive as Vosch decreases from Sadj+dV to Sadj−dV. Thus, pulses are produced on signal EWI, extending to either side of the active edges of line sync pulses. 
     FIG. 7 illustrates an embodiment of circuit  21 . This embodiment of circuit  21  includes five inputs and an output. Inputs  2101 ,  2102 ,  2103 ,  2104 ,  2105  receive voltages Vseuill, Voscv, Vseuil 2 , Vseuil 3  and Vid, respectively. An output  2106  provides a logic signal TRAMFIN. The values of voltages Vseuil 1 , Vseuil 2 , Vseuil 3  are chosen to lie between voltages Vminv and Vref. Vseuil 2  takes the lowest of the three values, and Vseuill takes the highest value. 
     Three comparators  211 ,  212 ,  213  are also shown in FIG.  7 . Comparator  211  receives Vseuill on a non-inverting input and Voscv on an inverting input. Comparator  212  receives Voscv on a inverting input and Vseuil 2  on a non-inverting input. Comparator  213  receives Voscv on a non-inverting input and Vseuil 3  on an inverting input. 
     An OR logic gate  214  is depicted in FIG. 7, and receives as inputs the signals produced by comparators  212  and  213 , in addition to signal Vid. A NAND-type RS latch  215  is provided, and receives the signal produced by gate  214  on reset input R, the, signal produced by comparator  211  on set input S and the output Q is connected to circuit output  2106 . 
     FIGS. 8 a  to  8   f  illustrate signals produced or received by an embodiment of circuit  21 . 
     FIG. 8 a  shows a frame sync pulse signal. 
     FIG. 8 b  shows signal Voscvw produced by the frame sync pulse treatment circuit  16 . Signal Voscv includes a succession of positive ramps (voltage rising between a minimum voltage Vminv and a maximum voltage Vmaxv) and negative ramps (voltage decreasing from Vmaxv to Vminv). The active edges of the frame sync pulses generally occur at a time when the positive ramps reach a voltage Vrefv. The negative ramps have a fixed duration, for example 250 μs. 
     FIG. 8 e  shows signal Vid, which is active while Voscv is decreasing, and which is inactive otherwise. 
     The frame sync treatment circuit implemented in the TDA9103 product of SGS-THOMSON Microelectronics may be used, for example. 
     In one instance, the signal Voscv may be increasing, and may have a value greater than that of Vseui 13 . In such an instance, the signal produced by comparator  213  is active, and so the signal (illustrated in FIG. 8 d ) produced by gate  214  is also active. When signal Voscv reaches value Vseuil 1 , signal produced by comparator  211  becomes inactive. Signal TRAMFIN (illustrated in FIG. 8 f ) then passes to the active state. 
     While signal Voscv is falling, signal Vid being active, the signal produced by gate  214  remains active. However, the signal produced by comparator  211  becomes inactive when Voscv reaches the value Vseui 11 . Once signal Voscv has reached Vminv, it begins to increase again. Once it reaches the value of Vseuil 2 , the signal produced by gate  214  becomes inactive, which causes TRAMFIN to become inactive, until the Voscv again reaches the value Vseuil 1 . When value Vseui 13  is reached later, the signal produced by the comparator  213  becomes inactive, which causes the signal produced by gate  214  to become active. Latch  215  is ready to be set again. 
     A signal TRAMPIN is thus produced which is active during a period from the occurrence of an active edge of a frame sync pulse, and ends after the end of the corresponding frame sync pulse. Preferably, the values of Vseui 11 , referenced to Vrefv, and Vseui 12 , referenced to Vminv, may be chosen so that TRAMFIN is active in the time periods where equalization pulses may occur. 
     The frequency of line sync pulses is classically between 15 and 150 kHz, and threshold values may be chosen so that this whole frequency spectrum is covered. This is realized by choosing Vseuill so that Vrefv-Vseuill corresponds in time to a period of 333 μs (i.e. five periods of the lowest possible line sync frequency), and by choosing Vseui 12  so that the time period corresponding to Vseui 12 -Vminv plus 250 μs is longer than the maximum duration of a line sync pulse (e.g. 700 μs) plus 333 μs, and calculated at the maximum frame sync pulse frequency. 
     FIG. 9 illustrates an embodiment of circuit  22 . This embodiment has inputs  221 ,  222 ,  223  receiving the signals TRAMFIN, DETVER and EWI respectively. This embodiment produces a signal WS at an output  228 . This embodiment of circuit  22  also includes an AND gate  224  receiving signals TRAMFIN and DETVER on inputs, and having an output connected to an input of a D-type latch  226 , which also receives a clock signal CKL. An inverter  225  may be provided to supply signal /EWI, the inverse of EWI. Also, the circuit may include a NAND gate  227  having inputs connected to /EWI and to the output of D-type latch  226 . 
     If PLL-H is out of lock, signal DETVER is inactive. The output signals of gate  224  and latch  226  are therefore inactive. The output signal of gate  227  is thus active. The output of the AND gate  224  is therefore identical to the sync signal provided by interface  17 . No windowing need be performed on this signal. This is justified by the fact that the PLL is searching for the phase and frequency of received line sync pulses. 
     If PLL-H is in lock, and the frame sync pulse is relatively far away with respect to time, DETVER is active, and TRAMFIN inactive. The outputs of gate  224  and latch  226  are therefore inactive. The output of gate  227  is active. This is identical to the previous case, where no windowing is performed on the sync signal provided by interface  17 . As the frame sync signal is far off, only the line sync signals are present. 
     Finally, if PLL-H is in lock, at a time around the frame signal, DETVER is active, and TRAMFIN- becomes active. So, the output of gate  224  becomes active, and the output of the D-type latch  226  becomes active at the next clocking pulse on signal CKL. Therefore, signal WS may be identical to the EWI signal received by the circuit  22 . Only the edges of line sync pulses and serration pulses are taken into consideration by PLL-H. The clock signal CKL is preferably valid on the active edge of the EWI signal, in order to avoid accidental start-up. 
     As described above, inhibition of the PLL during frame sync pulses is classically performed, to avoid a drift in the voltage Vref. If the serration pulses ate absent from the received signal, e.g. if the received signal is a composite signal such as H+V, it may be desirable to include such an inhibition function, to avoid creation of parasitic edges at the input of the PLL corresponding to edges of the EWI signal. If the state of the received signal is active at the same time as the windowing, the output of AND gate  23  will correspond to signal EWI. A phase difference will then be introduced into the loop, the active edges of EWI being in phase lead compared to the active edges of the line sync pulses. 
     Otherwise, when serration pulses are present, there is also a risk of producing parasitic pulses if the window EWI is too large, and the signal EWI becomes active before the received signal has returned to the inactive state. 
     In this way, an inhibition of the loop may be preferable in any case during the presence of frame sync pulses. 
     To achieve this, the charge pump may be disconnected from the filter, for example by placing a CMOS analog switch between these PLL elements. Such a switch could be controlled according to the extracted frame synchronization pulse, if a composite synchronization signal is used. If the synchronizations are differentiated, there is no need to inhibit PLL-H, and TRAMEXT is inactive. 
     FIG. 10 shows an example of control circuit  24  which may be used to control PLL inhibition. This circuit comprises three inputs  241 ,  242 ,  243  receiving signals TRAMEXT, EWI, DETVER, respectively, and an output  244  providing a control signal INHIB 1 . Circuit  24  also includes a NAND gate  245 , an inverter  246 , an AND gate  247 , a counter  248 , a NOR type RS latch  249  and an AND gate  250   
     Gate  245  receives as inputs the signals EWI, and the inverse of TRAMEXT, as produced by inverter  246 , the output of gate  245  is used to time the counter  248 . In this example, the counting is performed on the falling edge of the output signal of gate  245 . The output of inverter  246  is also used to operate the counter  248 . Gate  247  is used as a comparator. Selected positive and inverse outputs of counter  248  are connected to inputs of gate  247 , so that gate  247  produces an active signal when the counter reaches a certain desired value. The latch  249  receives signal TRAMEXT on its set input S, and the output of gate  247  on its reset input R. Gate  250  receives signal DETVER and the signal produced by the inverted output /Q of the latch as inputs, and has an output connected to the circuit output  244 . 
     If the PLL is not in phase lock, signal DETVER is inactive. Therefore, signal INHIB 1  is inactive. The pump is then connected to the filter, and the loop is in a phase and frequency search mode, on the signal received from interface  17 . 
     If the PLL is in phase lock, signal DETVER is active. Therefore, the state of signal INHIB 1  is determined by the state of the output of latch  249 . An active edge of signal TRAMEXT causes the output of latch  249  to become active. Signal INHIBl becomes active, which causes isolation of the filter from the charge pump. When the signal TRAMEXT becomes inactive again, counter  248  begins to count the descending edges of EWI. Once the counter reaches the desired value set by the connections of the inputs of gate  247 , the reset input R of latch  249  becomes active. The output of latch  249  becomes inactive, and the inhibition signal INHIB 1  becomes inactive. The described use of counter  248  allows a delay to be created between the end of the frame sync pulse and the end of PLL inhibition. 
     As an option, the two line sync pulses following the end of a frame sync pulse may be ignored. 
     It can be seen that if the line and frame sync signals are separate, the signal received by the PLL contains only line sync pulses. It is then unnecessary to inhibit the PLL as described above with reference to circuit  24 . 
     Additionally, if a composite sync signal without serration pulses is used, no equalizing pulses will be present. It is then unnecessary to use signal WS to perform windowing on the input. This windowing could even be harmful if the previously described PLL inhibition is being used. If the filter capacitor leaks its charge, voltage Vref will reduce, which will cause the PLL oscillator to drift. Such drifting will introduce a time offset into signal EWI. It could then happen that the active edges of the line sync pulses appear when EWI is at the inactive state. The signal received by the comparator will then be permanently inactive. 
     Moreover, if serration pulses are present, and with relatively narrow EWI pulses, it may be desirable to not inhibit the PLL, as these serration pulses are of the same frequency and phase as the line sync pulses. Windowing may then be used to eliminate any equalizing pulses that may be present. 
     The above issues may be accounted for by generating an inhibition signal INHIB from previously described signal INHIB 1 , and another signal INHIB 2 , representing the presence or absence of serration pulses. In one embodiment, the PLL is only inhibited if serration pulses are not present. 
     FIG. 11 shows a circuit which may be used to produce signal INHIB 2 . Circuit  26  includes four inputs  261 ,  262 ,  263 ,  264  and an output  265 . These four inputs receive signals Hext, EWI, TRAMEXT and TRAMFIN respectively, where Hext is the inverse of a signal Hext, provided at the output of AND gate  23  and received by the PLL. Input  262  receives signal EWI. Input  263  receives signal TRZMEXT. Input  264  receives signal TRAMFIN. Output  265  supplies signal INHIB 2 . 
     Circuit  26  includes a three-input AND gate  266 . The inputs of circuit  26  are respectively connected to circuit inputs  261 ,  262 ,  263 . The circuit also includes a two-input NAND gate  267 , whose inputs are respectively connected to the output of gate  266  and to circuit output  265 . A NAND RS latch  268  is included in the circuit, whose set input S is connected to the output of gate  267 , and whose reset input R is connected to the circuit input  264 . An inverting output /Q of this latch is also connected to the circuit output  265 . 
     FIG. 12 shows that signal INHIB may be produced by an AND gate  27  receiving INHIB 1  and INHIB 2  as input signals. 
     Initially, INHIB 2  is active. If there are no serration pulses, signal /Hext remains inactive during reception of a frame sync pulse, so that the output of gate  266  is high (active). Thus, INHIB 2  remains unchanged and signal INHIB is identical to INHIB 1 , and in this situation the PLL is inhibited. If serration pulses are present, signal /Hext becomes active while EWI is active. A pulse is produced at the output of gate  266 . This causes signal INHIB 2  to become inactive, and INHIB is then inactive. There is no inhibition of the PLL. Gate  267  allows the set signal S applied to latch  268  to become active again, to avoid problems with the latch when a falling edge of TRAMFIN arrives. 
     Considering the above, circuit  26  may usefully be modified by adding a validation signal. 
     FIG. 13 shows circuitry which may be added to circuit  22  to receive a VALID signal on input  229 . An AND gate  230  receives signals TRAMFIN and VALID, and the output of this gate is connected to the input  221  described in reference to FIG.  9 . TRAMFIN may be propagated by AND gate  230  only when VALID is active, and an inactive output is produced by AND gate  230  to input  221  if VALID is inactive. 
     FIG. 14 shows circuitry  28  which may be used to generate the signal VALID. The circuitry has four inputs  281 ,  282 ,  283 ,  284 , receiving signals TRAMEXT, EWI, TRAMFIN, and INHIB 2  respectively. The VALID signal is provided at an output  285 . A NAND gate  290  has inputs connected to circuit inputs  281  and  282 , and provides a signal to an edge sensitive clock input of a counter  286 . Circuit input  283  is connected to a start input of this counter. Selected positive and inverse outputs of the counter  286  are supplied to an AND gate  287 , which supplies an active signal when a certain, predetermined count is reached. A NAND gate  288  has an input connected to circuit input  284 , and a further input connected to an output of AND gate  287 . An RS latch  289  is also provided, having a reset input R connected to circuit input  283 , a set input S connected to an output of gate  288 , and an inverting output /Q supplying the signal VALID to circuit output  285 . 
     If serration pulses are present, signal INHIB 2  becomes inactive while TRAMEXT is active. The output of gate  288  will thus remain active, and so also will VALID. The windowing operation previously described is operative. 
     If there are no serration pulses, signal INHIB 2  remains active while TRAMEXT is active. When the counter  286  reaches the predetermined value, the output of gate  287  becomes active, the output of gate  288  becomes inactive, and this sets latch  289 , causing signal VALID to become inactive. The windowing operation is inhibited. 
     The invention may typically be used in a monitor, classically comprising a screen and horizontal and vertical deflection devices. The deflection is controlled using signals Voscv and Vosch, suitably amplified, and a flyback signal additionally used in controlling horizontal deflection. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.