Abstract:
A circuit for generating video synchronization timing signals includes a negative peak detector (FIG.  5 ) for following variations of a composite video signal (FIG.  1 ), rather than clamping the most negative voltage of the composite video signal. The negative peak detector provides a voltage level VTIP representative of the voltage at the synchronization tip of the composite video signal. A sample and hold circuit ( 700,702,704 ) is used to add an offset VSLICE to VTIP, VSLICE being a voltage level of the breezeway, color burst, or back porch segments of the composite video signal, or a combination of these segments. To prevent amplifier DC offset error voltages from affecting the perceived VSLICE level, an amplifier ( 800 ) can be connected in a first position TTIP as part of a negative peak detector to store VTIP on a capacitor, in a second position TH as part of a sample and hold circuit to store VREF on a capacitor, and in a third position TCOMP to compare VSLICE+VTIP measured from the capacitors with the composite video signal to generate the overall circuit output.

Description:
PRIORITY CLAIM 
     This application is a divisional of U.S. patent application Ser. No. 10/453,210, filed Jun. 3, 2003 (now U.S. Pat. No. 6,977,692), which application is a continuation of and claims priority to U.S. patent application Ser. No. 09/398,375, filed Sep. 17, 1999 (now U.S. Pat. No. 6,573,943). 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of video circuits. More particularly, this invention relates to a circuit for detecting synchronizing pulses embedded in composite waveforms of a video signal. 
     2. Description of the Related Art 
       FIG. 1  illustrates components of a composite video waveform. The composite video waveform contains: a horizontal sync pulse or sync tip used for receiver scan timing; a “breezeway” where the level is a reference for video intensity; a color burst which is a series of sinewaves at a very precise frequency and phase, used as a color reference; a back porch which is a level reference similar to the breezeway occurring after the color burst segment; and the picture occurring after the back porch, the picture being any possible signal up to a maximum level, and whose content is unpredictable to receiver electronics. 
     The video receiver systems must discover timing details from the sync tip. Unfortunately, the sync tip almost never has a known DC level. In fact, most composite signals are AC coupled and the average DC level varies unpredictably with picture content. 
     One method for providing a video signal timing reference is to use a circuit which uses the most negative going feature of the composite signal as a reference level. The composite video signal standard which is predominantly used in North America, the National Television Systems Committee (NTSC) standard, was designed to enable such a reference level to be set approximately 50 years ago. 
     A prior art circuit for setting a reference level at the most negative feature of a composite waveform is the clamping circuit shown in  FIG. 2 . The circuit includes a capacitor  200  having an input receiving the composite video signal input, and an output providing the composite video signal with its most negative voltage clamped to 0 volts. The circuit further includes a diode  202  and current sink  204  connecting the output of the capacitor  200  to ground. The diode  202  is assumed to be ideal so that it generates no DC offset. The current sink  204  provides a small pull down current I PULLDOWN  to discharge the capacitor  200  and allow the clamped output signal to follow the varying content of the composite input. 
     A clamped output signal from the circuit of  FIG. 2  is shown in  FIG. 3 . As shown in  FIG. 3 , the diode of  FIG. 2  forces the capacitor coupled composite video signal&#39;s most negative voltage, here the sync tip voltage (V TIP ), to ground level. Because the composite video signal provides transient currents, clamping may distort the composite signal and may be an undesirable method. 
     To provide a synchronization (sync) timing signal, the clamped output of the circuit of  FIG. 2  is provided to a first terminal of comparator  400  shown in  FIG. 4 , while a DC offset voltage is provided to the second terminal of comparator  400 . The sync timing signal is generated when the comparator output transitions. A DC voltage offset generator  402  provides the DC offset voltage at a desired “slice level” (V SLICE ), as shown in  FIG. 3 , so that the sync timing signal is generated on an edge of the sync pulse at the voltage V SLICE  approximately midway between the sync tip voltage level V TIP  and the breezeway voltage level. 
     SUMMARY OF THE INVENTION 
     The present invention provides a circuit for following variations of the composite video signal, rather than clamping the most negative voltage of the composite video signal. 
     The present invention includes a negative peak detector with an input receiving the composite video signal and an output coupled to a first input of a first amplifier the first amplifier functioning as a comparator. The second input of the comparator receives the composite signal, and the output of the comparator provides a synchronization timing signal. 
     In one embodiment, the present invention further provides buffering at the input and output of the negative peak detector. Buffering is provided to the input with a second amplifier having a noninvertng input receiving the composite video signal. The inverting input of the second amplifier is connected to a first terminal of a diode of the peak detector and also to a current source in the negative peak detector. The output of the second amplifier is connected to the second end of the diode of the negative peak detector. The second amplifier serves to buffer the composite video signal from the current source. Buffering at the output of the negative peak detector is provided by a third amplifier connected in a voltage follower configuration between the output of the negative peak detector and the comparator. 
     In one embodiment, the present invention also includes a voltage slice level offset generator connecting the output of the negative peak detector to the comparator. The slice level offset generator includes a sample and hold circuit and a resistor divider. The sample and hold circuit is configured to sample the composite video signal during the breezeway segment, color burst segment, or back porch segments of the composite video signal, or any combination of the segments. The output of the sample and hold circuit then provides a sample of these segments V REF  to a first end terminal of the resistor divider. The second end terminal of the resistor divider is driven by the buffered output of the negative peak detector which provides a synchronization tip voltage signal V TIP , and the center terminal of the resistor divider is provided to the first input of the comparator. The comparator output can then provide a timing signal transitioning at a point V SLICE  on the composite signal halfway between V TIP  and V REF . 
     In another embodiment in accordance with the present invention, circuitry is configured to reduce amplifier DC offset which can cause errors in a perceived V SLICE  level. The circuitry includes a first amplifier which receives the composite video signal and is connectable by switches in one of three positions T COMP , T TIP  and T H . In the T COMP  position the first amplifier acts as a comparator with no feedback to compare the value V SLICE +V TIP  with the composite video signal. V SLICE  is set between V TIP  and V REF  based on values stored on capacitors in the circuit. The T COMP  position is used prior to the negative going synchronization tip edge of the composite video signal. After the negative going edge of the synchronization tip, the circuit is set in the T TIP  position. In the T TIP  position, the output of the first amplifier is disconnected from providing the synchronization timing output, and is connected to provide buffering for a negative peak detector to store T TIP  on a capacitor. After the synchronization tip, during the breezeway, color burst or back porch segments of the composite video signal, or during a desired combination of these segments the circuit is connected in the T H  position. In the T H  position, the first amplifier forms part of a sample and hold circuit for storing a value V REF  on a capacitor. After the desired period for T H , the circuit is again connected in the T COMP  position for detection of the next negative going synchronization tip edge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details of the present invention are explained with the help of the attached drawings in which: 
         FIG. 1  shows a composite video signal; 
         FIG. 2  shows a prior art circuit for clamping the most negative voltage of a composite video signal; 
         FIG. 3  shows a clamped composite video signal output from the circuit of  FIG. 2 ; 
         FIG. 4  shows circuitry used with the circuitry of  FIG. 2  to generate a synchronization timing signal; 
         FIG. 5  shows a negative peak detector according to the present invention for providing a voltage reference at the sync tip voltage level of a composite video signal; 
         FIG. 6  shows the negative peak detector of  FIG. 5  with buffering at its input and output, and with additional circuitry to generate a sync timing signal; 
         FIG. 7  shows the video synchronization signal generating circuit of  FIG. 6  with components for a slice level offset generator providing an adaptive V SLICE  value; and 
         FIG. 8  shows a video synchronization signal generating circuit enabling elimination of amplifier DC offset voltages. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides a circuit for establishing a sync tip baseline without clamping the most negative portion of the composite signal to a known value. Instead of clamping the sync tip, the negative peak detector follows variations in the composite signal. The circuit of the present invention includes a negative peak detector as shown in  FIG. 5 . 
     As shown in  FIG. 5 , the negative peak detector is a rectifier including a p-n type bipolar diode  500 , or other rectifying element, with the composite video signal provided to the n terminal and the sync tip level output provided at the p terminal. The negative peak detector also includes a weak current source  502  and a capacitor  504  connecting the p terminal of diode  500  to ground. The current source  502  functions to charge the capacitor  504 . The output of the negative peak detector is a reference voltage which tracks the sync tip voltage level V TIP . 
       FIG. 6  shows the negative peak detector of  FIG. 5  with buffering at its input and output, and with additional circuitry to generate a sync timing signal. Components carried over from  FIGS. 5 to 6 , as well as components carried over in subsequent drawings, are similarly labeled. 
     Buffering at the input of the negative peak detector is provided by an operational amplifier  600 . Amplifier  600  has a noninverting input receiving the composite video signal, an inverting input connected to the p terminal of the diode  500 , and an output connected to the n terminal of diode  500 . The amplifier  600  forms an operational rectifier with the diode  500  and, thus, buffers current drawn from the current source  502  from the composite video signal. The amplifier  600  also eliminates diode offset voltage errors with diode  500  not being ideal. 
     Buffering at the output of the negative peak detector is provided by operational amplifier  602 . The amplifier  602  is connected as a voltage follower with a noninverting input connected to the p terminal of the diode  500 , and its output and inverting input connected together. The amplifier  602  buffers the sync tip level voltage V TIP  on the capacitor  504  from any load. 
     An amplifier  606  configured as a comparator, and a slice level generator  604  are included with the amplifiers  600  and  602  and negative peak detector to provide a synchronous timing signal. The composite video signal is provided to the noninverting input of the comparator  606 . The output of amplifier  602  is provided through a slice level offset generator  604  to the inverting input of the comparator  606 . Synchronization timing signals are produced at the output of comparator  606 . 
     In one embodiment, the slice level offset generator  604  generates a fixed DC voltage V SLICE  which is added to V TIP  at the output of amplifier  602 . The output of the comparator  606  will then transition when the composite video signal transitions through the voltage level V TIP +V SLICE . 
     Because the composite video signal has a varying amplitude, it maybe desirable to have an adaptive rather than a constant V SLICE  value.  FIG. 7  shows specific components for an embodiment of the slice level generator  604  which can provide such an adaptive V SLICE  value. 
     The offset slice generator  604  of  FIG. 7  and includes a sample and hold circuit made up of amplifier  700 , switch  702  and capacitor  704 . The noninverting input of amplifier  700  receives the composite video signal, while the inverting input of the amplifier is connected by the switch  702  to its output. The noninverting input of amplifier  700  is also connected to capacitor  704 . The switch is selectively switched by a signal T H  which is timed to close the switch during the breezeway, color burst or back porch segments of the composite video signal, or during any combination of the breezeway, colorburst or backporch segments. The capacitor  704 , thus, stores a voltage V REF  equal to the sampled voltage of the breezeway, color burst, or back porch segments, or the desired combination of these segments. 
     The voltage held by the capacitor  704  is buffered by operational amplifier  706  and applied to a resistor divider made up of resistors  708  and  710 . The operational amplifier  706  is connected in a voltage follower configuration with its noninverting input connected to the inverting input of the amplifier  700 , and its inverting input connected to its output. The output of the amplifier  706  is connected to a first terminal of resistor  708 , while the output of amplifier  602  is connected to the first terminal of resistor  710 . The second end of resistors  708  and  710  are both connected to the noninverting input of comparator  606 . 
     With resistors  708  and  710  having equal values, the output of the comparator  606  will transition when the composite video signal passes through V TIP +V SLICE  with V SLICE  being halfway between V TIP  and V REF . The present invention can, thus, provides an adaptive V SLICE  value. 
     With the circuit of  FIG. 7 , the offset of all four amplifiers  600 ,  602 ,  700  and  706 , and the comparator  606  can add to cause undesirable errors in the desired signal V TIP +V SLICE . Errors in the value for V TIP +V SLICE  show as timing errors since the input to the circuit of  FIG. 7  does not have a large slew rate, and even non-functionality if the errors accumulate large compared to undersized input signals. 
       FIG. 8  shows an embodiment of a video synchronization signal generating circuit in accordance with the present invention which enables elimination of amplifier DC offset voltages. 
     The circuit of  FIG. 8  includes an amplifier  800  and switches  802  and  804  to selectively connect the amplifier  800  to different components. The noninverting (+) input of the amplifier  800  receives the composite video signal input. The switch  802  connects the output of the amplifier  800  to one of three terminals labeled T H , T COMP  and T TIP . The switch  804  connects the inverting (−) input of the amplifier  800  to one of three terminals labeled T H , T COMP  and T TIP  up to correspond with the connections for switch  802 . The switches  802  and  804  are controlled to switch together to a terminal with a common label. 
     When the switches  802  and  804  are set to the T COMP  connections, the amplifier  800  acts as a comparator with no feedback to compare the value V SLICE +V TIP  with the composite video signal. The switches  802  and  804  are preferably set to the T COMP  positions during a majority of the composite video signal prior to the negative synchronization tip edge. 
     With the switches  802  and  804  in the T COMP  position, the (−) input of the amplifier  800  is connected to the output of amplifier  602 . Like the amplifier  602  in  FIGS. 6 and 7 , the amplifier  602  in  FIG. 8  is connected in a voltage follower configuration with its (+) input connected to a negative peak detector formed by diode  500 , capacitor  504  and current source  502 . The diode  500  is disconnected from the output of the amplifier  800  when the switches  802  and  804  are connected in the T COMP  position, but the capacitor  504  will store the voltage value V TIP  of the synchronization tip. The value V TIP  is measured and stored by capacitor  504  after the negative edge of the synchronization tip signal with the switches  802  and  804  connected to the T TIP  connections, as discussed in detail to follow. 
     The output of the amplifier  602  is connected to the (−) input of the amplifier  800  through a resistor  710  when the switches  802  and  804  are in the T COMP  position. Also, the (−) input of the amplifier  800  is connected to the output of amplifier  706  through resistor  708 . The amplifier  706  is connected in a voltage follower configuration similar to the amplifier  706  of  FIG. 7 . The capacitor  704  is charged up to a voltage V REF  equal to the sampled voltage of the breezeway, color burst, or back porch segments, or the desired combination of these segments. The capacitor  704  is charged up to V REF  when the switches  802  and  804  are in the T H  position as described in more detail to follow. 
     With resistors  708  and  710  having equal values, the output of the amplifier  800  will transition when the composite video signal passes through V TIP +V SLICE , V SLICE  being half way between V TIP  and V REF . Further, any offset in amplifier  800  is stored in capacitors  504  and  704  and is superimposed on the V SLICE  value by amplifiers  602  and  706  and fed back to the (−) input of amplifier  800 . Functioning as a comparator, the offset of the amplifier  800  is now cancelled with respect to the input. Any offset of amplifiers  602  and  706  will be reduced by the voltage gain of amplifier  800  with respect to the input of the synchronous detector circuit. 
     After the output of the synchronous detector circuit signals the negative edge of the synchronization tip, the switches  802  and  804  are set to T TIP . The amplifier  800  then is disconnected from providing the sync timing output signal and acts only as a buffer for a negative peak detector. The amplifier  800  has a (+) input receiving the composite video signal as in  FIGS. 6 and 7 , but instead of having the (−) input connected directly to the p terminal of the diode  500 , the (−) input is buffered from the diode  500  through the voltage follower amplifier  602 . As connected, the amplifier  800  will reduce the offset of diode  500  as well as the offset of amplifier  602 . 
     The switches  802  and  804  are left in the T TIP  position until the capacitor  504  charges up to the synchronization tip voltage level V TIP . After a time period for the capacitor  504  to adequately charge, the switches  802  and  804  are set to T COMP  again, waiting for the positive going edge of the synchronization tip. 
     After the positive-going edge of the synchronous tip is perceived, the switches  802  and  804  are set to the T H  connections. As with the circuit of  FIG. 7 , the time period for T H  can be during the breezeway, color burst or back porch segments of the composite video signal, or during any combination of the breezeway, color burst or back porch segments. The switches  802  and  804  are connected to the T H  connections during the desired segments and returned to the T COMP  connections afterward. 
     With the switches  802  and  804  connected in the T H  position, the amplifier  800  functions similar to the amplifier  700  of  FIG. 7  with capacitor  704  to form a sample and hold circuit. Rather than being connected directly to the capacitor  604  like the (−) input of amplifier  700  of  FIG. 7 , the (−) input of the amplifier  800  is connected to the capacitor  704  through the voltage follower amplifier  706 . The offset of amplifier  706  is reduced by this loop. 
     With the switches  802  and  804  connected in the T H  position, the capacitor  704  will charge up to and store a voltage V REF  equal to the sampled voltage of the breezeway, color burst, or back porch segments, or the desired combination of these segments. After the desired segments, the switches  802  and  804  will be placed in the T COMP  positions until the negative going edge of the synchronization tip is detected again. 
     Although the invention has been described above with particularity, this was merely to teach one of ordinary skill in the art how to make and use the invention. Many modifications will fall within the scope of the invention, as that scope is defined by the claims which follow.