Digital vertical sync separator

Abstract: An improved high-performance vertical sync separator (10) which can reliably operate during high frequency and non-standard video signal conditions and that utilizes a device-independent methodology. The synth separator (10) includes a signal separator circuit (18) having an input port (30) for receiving a composite signal (16). A measuring device (36) connected to the input port (30) measures a first and second characteristic of the composite signal. A processor (40) receives and compares the first and second characteristics, and recovers a vertical sync signal (20) from the composite signal (16).

Description: BACKGROUND OF THE INVENTION 1. Technical Field The present invention relates generally to devices and methods for processing synchronization signals used to coordinate scanning in video devices and is particularly, but not exclusively, useful as a digital signal recovery circuit that can separate a horizontal or a vertical synchronization signal from a high frequency and nonstandard input video signal. 2. Discussion A typical video signal contains a composite synchronization signal that coordinates the horizontal and vertical sweeping action of an image scanning beam. The scanning beam generates a video image by drawing a series of horizontal lines starting at the top of the image in a left to right direction. When the video image has been completely generated by the scanning beam, the scanning beam returns to the upper left-hand comer of the image, and the scanning process is repeated. The horizontal and vertical sweeping action allows the video image be displayed in raster-like fashion on a screen or other viewing surface. Embedded within the composite synchronization signal are horizontal and vertical synchronizing, or sync, pulses. These sync pulses occur, as the scanning beam is retraced, to effectively create horizontal and vertical blanking intervals. To properly blank the scanning beam, however, the vertical sync pulse must be separated from the horizontal sync pulse. The separation of composite synchronization signals into their component parts has long been performed in the video arts using a variety of analog and digital methods and devices. Many of these methods are inflexible, in that they rely on particular timing characteristics of components used to effect the separation. Other methods process the composite synchronization signal by comparing an input sync signal with a reference signal having a known standard format. Such a methodology cannot reliably separate the composite synchronization signal into its component parts when the input signal is of a non-standard format. Still other sync separation methodologies cannot operate when the vertical sync frequency exceeds a certain value. For example, many sync separation devices cannot reliably separate a vertical sync signal when the frequency of the vertical sync signal is higher than 150 Hz. In addition, the circuitry required to implement these sync separation devices often requires numerous components and complicated circuit design. Examples of digital circuits for separating out a vertical sync signal are to be found in US-A-4641189 and US-A-4459612. The present invention is a signal recovery circuit as defined in Claim 1, and a corresponding method defined in Claim 9. The invention may, in preferred embodiments, provide a vertical sync separator utilizing a device-independent methodology that minimizes the size and complexity of the associated separator circuitry; and an improved high-performance vertical sync separator which can reliably operate during high frequency and non-standard input signal conditions. From the subsequent detailed description taken in conjunction with the accompanying drawings and subjoined claims, other objects and advantages of the present invention will become apparent to those skilled in the art. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description and accompanying drawings, wherein: Figure 1 is a block diagram of a video signal processing system incorporating the vertical sync separator of the present invention; Figure 2 is a block diagram illustrating the vertical synch separator of the present invention in greater detail; Figure 3 is a typical composite horizontal sync signal; Figure 4 is a schematic diagram showing the digital components of the vertical synch separator of the present invention; Figure 4A shows the wave form of an edge detection circuit in the vertical sync separator of the present invention; Figure 5A shows a standard composite horizontal sync pulse; Figure 5B shows a vertical sync signal separated from the composite horizontal sync pulse of Figure 4A using a conventional sync separator; Figure 6A shows a non-standard composite horizontal sync pulse; Figure 6B shows a vertical sync signal separated from the composite horizontal sync pulse of Figure 5A using the sync separator of the present invention; Figure 6C shows a vertical sync signal separated from the composite horizontal sync pulse of Figure 5A using the modified sync separator of the present invention; and Figure 7 illustrates the signal outputs of the digital components of the vertical sync separator shown in Figures 3 and 4. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, Figure 1 is a block diagram of a video system incorporating a vertical synch separator according to the present invention. The system 10 includes a sync stripper 12 that receives a video carrier signal 14 and strips a composite sync signal 16 from the video carrier signal. The sync stripper outputs the composite sync signal 16 to a vertical sync separator 18 of the present invention. The sync separator 18 separates a vertical sync signal 20 from the composite sync signal and outputs the vertical sync signal to a raster timing generator 22. The raster timing generator 22 subsequently determines sync priority to blank the video signal displayed on a video display device 24, as is well known in the art. It should be appreciated that, depending upon the particular design of the video system, a sync stripper 12 may not be necessary if the composite sync signal is input separately from the video carrier signal. Also, the vertical sync separator of the present invention may be operative to separate the vertical sync signal from a composite horizontal sync signal or a composite horizontal sync signal embedded in a video signal. Referring to Figure 2, a block diagram of the components of the sync separator 18 is shown at 28. The composite sync signal 16 is input into the sync separator at separator input port 30. The input port 30 feeds the signal into an edge detection circuit or signal detection device 32 that detects a falling edge of the composite sync signal, as shown at 34 in Figure 3. The falling edge of the composite sync signal indicates the beginning of a horizontal pulse period, designated Np, and the beginning reference point for measurement of the horizontal sync signal pulse width, designated Nw. Both the signal period Np and the signal pulse width Nw are essential in separating the vertical sync signal from the composite sync signal, as will be explained in detail below. The edge detection circuit 32 outputs a signal indicative of the sensed sync signal falling edge to a signal measuring circuit 36. As will be described below, the signal measuring circuit 36 measures the latched pulse width Nw latch, the latched period Np latch , and the present pulse width Nw in terms of clock pulses. The signal measuring circuit outputs both current and latched composite sync signal pulse width and period data to a signal processor 40. The processor 40, which is preferably a sync signal comparator that is programmed in Altera Hardware Description Language (AHDL), outputs the vertical synchronization signal based on data input from the edge-detection circuit and the signal measuring circuit. The output vertical sync signal is then utilized for effective blanking of the beam of energy scanned in the vertical direction during generation of a video image. Referring to Figure 4, a detailed schematic diagram of the edge detection circuit 32, signal measuring circuit 36 and signal comparator 40 is shown at 60. The edge detection circuit 32 includes a pair of D-type flip-flops 64a, 64b that latch each falling edge of the composite sync signal pulse. Signals are clocked into both flip-flop inputs by a system clock signal 66, which preferably has a frequency of around 25 MHz. The composite sync signal is clocked into the flip-flop 64a at an input 68. In response to a sync signal pulse falling edge, the flip-flop 64a outputs a logical "0" output, with an associated one clock cycle delay, to the input of the flip-flop 64b, thereby causing the flip-flop 64b to generate a logical "0" output pulse signal. The output signal of the flip-flop 64a is also input to a NOT gate 70. The NOT gate 70 inverts the logical "0" output signal from the flip-flop 64a and inputs the inverted signal to a first input of an AND gate 72. The signal generated by the flip-flop 64b has an associated two clock cycle delay, and is output to a second input of the AND gate 72. As a result, the AND gate 72 outputs a logical "1" signal indicative of the composite sync signal falling edge based on the logical state of the inverted signal from flip-flop 64a and the non-inverted signal input from flip-flop 64b. More particularly, the AND gate output signal is based on the wave form shown at 76 in Figure 4A. As indicated above, a logical "1" output with one system clock width from the AND gate indicates the occurrence of a composite sync signal falling edge. The logical "1" signal output from the AND gate indicating a composite sync signal falling edge is clocked into the signal measuring circuit 34, and, more specifically, into a third flip-flop 84a where it is delayed one system clock. The delayed signal is used as an enable signal for the first counter 86. The output signal is also inverted by a NOT gate 87 to clean a second counter 88. In addition, the output signal from the third flip-flop 84a is fed to the fourth flip-flop 84b in the signal measuring circuit 34. After another system clock delay by 84b the falling edge signal at the output of AND gate 72 is inverted by a NOT gate 89 to clean a first counter 86. The first counter 86, which includes two interconnected 8count counters 90a, 90b, is used to measure the period, Np, of the composite sync pulse from a first detected falling edge to a second detected falling edge. The second counter 88 also includes two 8count counters 92a, 92b and is used to measure the pulse width Nw of the sync signal from a signal falling edge to a signal rising edge. Both counters output signal measurement data to the comparator sync 40. Preferably each counter is an up-down carry-look-ahead 8 bit counter, that functions as described by the truth table shown below in Table 1. Inputs Outputs CLK CLRN SETN LDN DNUP GN QH QG..QB QA X L H X X X L L.L L X H L X X X h g..b a ┌ H H L X X h g..b a ┌ H H H H L Count Down ┌ H H H L L Count Up ┌ H H H X H Hold Count Inputs outputs DNUP LDN* GN OH QG..QB QA COUT L H L H H..H H H H H L L L.L L H Others L The comparator sync processes the measurement data from both counters 86, 88 along with the AND gate output signal, to output the vertical sync signal at 94 based on logic programmed into the sync comparator, as will now be described. As shown in Figure 5A, for a composite horizontal sync signal 100, it is a technical requirement that the composite, or horizontal, sync signal width normally is less than 10% of the signal period. However, during the period of a vertical sync signal, the width is usually larger than 90% of the horizontal period. Therefore, the vertical sync signal can be detected based on this characteristic, assuming the input composite sync signal is negative going. By using two 12-bit counters operating under system clock (25 MHz), the sync pulse width, Nw, and period, Np, can be measured. Nw latch and Np latch are the latched pulse width and period at the presently occurring falling edge of horizontal sync. The vertical sync signal is then separated as shown at 102 in Figure 4B through implementation of the following: IF Nw latch < ½ Np latch V_Sync=Vcc; ELSE V_Sync=GND; where Vcc and GND refer to +5V and the ground of the system, respectively. However, there are some non-standard composite sync signals that have the wave form shown at 110 in Figure 6A. In such cases, the delay of vertical sync separated by using the previous method is greater than the delay that can be handled by the above logic. The above implemented logic would result in the vertical sync signal shown at 112 in Figure 6B. The signal 112 would cause the raster timing generator to incorrectly blank the video display device. To handle this kind of situation, the above logic must be modified as follows: IF (Nw latch < ½ Np latch ) or (Nw < ½ Np latch ) V_Sync=Vcc; ELSE V_Sync=GND; The resulting separated vertical sync signal is shown at 114 in Figure 6C. The associated signal delay is much smaller than with previous vertical sync signal separators. More specifically, the delay is always less than one horizontal line and is guaranteed. Operation of the vertical sync separator will now be described in relation to the signals output from the sync separator components, as shown graphically at 120 in Figure 7. Specifically, the composite sync signal SYNC is docked into the edge detection circuit 32 by system clock signal (sync_clk). Upon detecting a falling edge, the processor latches both the composite sync signal period Np and width Nw. After Np is latched, a clear signal /CLRN1 of the first counter is asserted, and the counter is cleared. After that, the first counter starts to count the system clock numbers until a counter enable signal /GN1 goes high at the end of one horizontal period. The /CLRN1 signal is generated by delaying and inverting the /GN1 signal. The output of the second flip-flop 64b is fed to the second counter as an enable signal. Therefore, the second counter can count the system dock numbers of the sync pulse width, Nw. Nw is latched in the same way as the first counter 90, i.e. Nw is latched at the falling edge of the next horizontal sync. After Nw is latched, the second counter is cleared by asserting a /CLRN2 signal, which is generated by inverting the counter enable signal /GN1 of the first counter 90. After the second counter is cleared, the second counter counts the system clock numbers of the sync pulse width and the procedure is repeated. The counter output signals are latched at the falling edge of the composite sync signal. These two latched signals, d_latch (which is Np latch ) and w_latch (which is Nw latch ) are compared by the comparator sync 40. The comparator sync subsequently outputs the separated vertical sync signal V_SYNC to the raster timing generator. The vertical sync separator of the present invention has been implemented using a part of an ALTERA FPGA, Model No. 10K20RC208-4. The vertical sync separator of the present invention has been tested on a Hughes JVC Model M200 video projector using a Model 100 System/RTG board. The video signal generator used in the testing was a QUANTUM DATA Model 801 GF generator, which can generate over one hundred video sources. The vertical sync separator of the present invention successfully separated the vertical sync signal from both a composite horizontal sync signal and a composite sync signal on a green video channel (SOG), with an associated delay of less than one horizontal line. Since no loop exists between the vertical sync signal separation and sync priority circuitry when this sync separator is used in conjunction with a Model 100 RTG board, the sync selection circuit is simplified dramatically compared to the presently used sync selection circuits in Hughes JVC Model M200/M220 projectors. The new digital sync priority circuitry, including the digital vertical sync separator of the present invention, exhibits improved performance, because all input sync signals are treated equally. Upon reading of the foregoing description, it should be appreciated that the vertical sync separator of the present invention exhibits improved performance when compared to existing vertical sync separators that are implemented in analog methods. The vertical sync separator of the present invention successfully separates the vertical sync signal from any standard or non-standard composite sync signal through implementation of the above-described logic which compares three sets of data, Nw, Nw latch , and Np latch . Only when either Nw or Nw latch are detected as being less than half of Np latch is a vertical sync signal detected. Also, it should be appreciated that the vertical sync separator of the present invention operates at a high frequency, including frequencies beyond 150 Hz, and therefore is applicable to a wide variety of video systems, including high definition television, computer and VCR applications. The vertical sync separator can also be combined with digital serration removal and even/odd field detection to yield a digital sync process. The sync separator of the present invention is completely digital, therefore allowing the separator to be implemented in a cost-effective manner. The sync separator of the present invention also is cost-effective in that it does not require the horizontal sync signal to be converted to voltage before the vertical sync signal is extracted. It is understood that many modifications and variations of the present invention are possible in light of the above teachings. Therefore, any other embodiments which fall within the scope of the present invention are intended to be covered by the following claims.

Claims: A signal recovery circuit for recovering a vertical sync signal from a digital video composite sync signal (16), comprising: an input port (30) for receiving the video composite sync signal that includes a series of digital pulses; a signal detection device (32) connected to the input port for detecting a periodic parameter of the composite sync signal; a signal measuring device (36) connected to the signal detection device for measuring first and second characteristics of the composite sync signal after the signal detection device detects the periodic parameter; and a processor (40) connected to the signal detection device and the signal measuring device for comparing the first characteristic to the second characteristic to recover as an output signal the vertical sync signal (20) from the video composite sync signal, wherein the signal measuring device (36) is configured to measure the first characteristic by determining a pulse width (N W ) of a first pulse contained within the series of digital pulses, and wherein the signal measuring device measures the second characteristic by determining a period (N P ) of the composite sync signal from the temporal distance between the leading edge (34) of the first pulse and the leading edge of a second pulse contained within the series of digital pulses. The circuit according to Claim 1, wherein the composite sync signal (16) and the output signal (20) comprise a series of digital pulses each having a leading edge, a trailing edge, and a pulse width. The circuit according to Claim 2, wherein the processor is configured to generate the output signal by generating an output pulse when the pulse width (N W ) of the first pulse is less than half the period (N P ) of the composite sync signal. The circuit according to Claim 2, wherein the signal measuring device is configured to measure a third characteristic of the composite sync signal by determining an instantaneous value of the pulse width of the second pulse, the instantaneous value being determined by the time elapsed from the detection of the leading edge (34) of the second pulse and prior to the detection of the trailing edge of the second pulse. The circuit according to Claim 4, wherein the processor (40) is configured to generate the output signal when either the pulse width of the first pulse or the instantaneous value of the pulse width of the second pulse are less than half the period of the composite sync signal. The circuit according to claim 2, wherein the signal detection device (32) is configured to receive the composite sync signal from the input port, detect the leading edge of a pulse contained within the series of digital pulses, and generate control signals for controlling the measuring device (36) and the processor (40). The circuit according to any preceding claim, wherein the signal measuring device (36) and the processor (40) are configured to operate collectively to repeatedly update and compare the first characteristic with the second characteristic to recover the output signal even when the composite sync signal changes or deviates from a standard format. The circuit according to any preceding claim, wherein the composite sync signal is a synchronization signal used to coordinate a blanking operation of a beam of energy which is scanned both in a horizontal and a vertical direction to generate a video image, and wherein the recovered output signal is a vertical synchronization signal (20). A method for recovering a vertical sync signal from a digital video composite sync signal, comprising the steps of: detecting a periodic parameter of a composite sync signal (16) that includes a series of digital pulses; measuring a first characteristic of the composite sync signal; measuring a second characteristic of the composite sync signal; comparing the first characteristic to the second characteristic; and recovering an output signal (20) from the composite sync signal based on the step of comparing the first characteristic to the second characteristic wherein the step of measuring the first characteristic comprises measuring a pulse width (N w ) of a first pulse contained within the series of digital pulses, and the step of measuring the second characteristic comprises determining a period (N P ) of the composite sync signal by measuring the temporal distance between the leading edge of the first pulse and the leading edge of a subsequent second pulse contained within the series of digital pulses. The method according to Claim 9, wherein the composite sync signal and the output signal are digital signals comprising a series of digital pulses, each of the digital pulses having a leading edge, a trailing edge, and a pulse width. The method according to Claim 9 or 10, wherein the composite sync signal (16) is a synchronization signal that coordinates a blanking operation of a beam of energy scanned in a horizontal and a vertical direction to generate a video image, and wherein the step of comparing the first and second characteristics is performed to recover a vertical synchronization signal from the composite sync signal to synchronize the blanking of the beam of energy with the vertical scanning motion of the beam. The method according to Claim 9, 10 or 11, further comprising the step of generating an output pulse when the pulse width of the first pulse is less than half the period of the composite sync signal. The method according to Claim 9, 10, 11 or 12, further comprising the steps of: measuring a third characteristic of the composite signal by determining an instantaneous value of the pulse width of the second pulse; measuring the time elapsed from the detection of the leading edge of the second pulse and prior to the detection of the trailing edge of the second pulse; and generating an output pulse when either the pulse width of the first pulse or the instantaneous value of the pulse width of the second pulse are less than half the value of the period of the composite signal. The method according to any of Claims 9 to 13, wherein the steps of measuring and comparing the first and second characteristics are repeatedly performed such that the output signal can be recovered even when the composite sync signal changes or deviates from a standard format.

Inventors: XIE GUOXIN, XIE, GUOXIN
Applicants: VICTOR COMPANY OF JAPAN, VICTOR COMPANY OF JAPAN, LIMITED
Application year: 1998
Main classification: H04N5
IPC codes: H04N5