A DIGITAL 25 HZ OFFSET GENERATOR

Abstract: Non-orthogonality of the blanking region information caused by the 25 Hz offset in a digital PAL-encoded color tele­ vision signal is corrected by a dynamic offset circuit. To this end, a plurality of waveforms describing the envelopes of the banking, sync and burst are stored in a PROM (36), and during video signal processing are sequentially addressed at a 25 Hz rate. The resulting assembled output blanking information is orthogonal to the television scanning frequency.

Description: The invention relates to the generation of television blanking information and more particularly to the digital generation of PAL blanking region information via a dynamic offset circuit which makes the information appear orthogonal after digital to analog conversion. In a television studio, or when otherwise recovering digitally sampled video signals from a recording media or other noisy source such as a satelite receiver, it is necessary to re-insert new video blanking, sync and burst timing information. That is, in such video processes, a sync generator is used to provide video sync blanking and burst signals, in order to maintain the proper relationship of all synchronizing information relative to the active video signal. In a system employing the NTSC color television standard, it is relatively simple to maintain the phase relationship between the color subcarrier and the horizontal sync of the television signal because there is a direct relationship between the two signals. That is, one signal is generated directly from the other whereby a fixed phase relationship between the signals is readily reproduced. In the PAL standard, however, the relationship between the horizontal frequency and the color subcarrier frequency is more complex as shown by the relationship F sc = 1135/4 F h + 25, where F sc is the color subcarrier and F h the horizontal frequency. This relationship results from the 25 Hz offset which is used in the PAL standard. Stated more simply, in a standard rectangular television picture the horizontal blanking information such as sync and burst are orthogonally related to a vertical line along the left hand side of the picture. In an NTSC color television standard, since there is the fixed frequency relationship between horizontal frequency and the color subcarrier frequency, an orthogonal blanking region configuration readily is achieved. That is, the timing of all blanking region information begins exactly on the vertical line, regardless of whether the video signals are being processed in the analog or digital domain. Likewise, in the PAL standard, if the video signals are being processed in the analog domain an orthogonal blanking region configuration also readily is achieved. That is, since an analog signal is not sampled and inherently is a continuous signal, the sync blanking and burst edges are readily generated in an orthogonal configuration. However, problems arise when a PAL-encoded video signal is processed entirely in the digital domain, as further discussed below. Presently available time base correctors, (TBC's) digital video tape recorders (VTR's), and the like, typically process various portions of the video signal by analog means, particularly in the processing amplifier and D/A converter area. In such schemes, the video signal is put through a path which includes various complex digital processes culminating in digital-to-analog (D/A) conversion. The various timing signals however, are processed in a separate channel and are put through other analog processes unrelated to the digital video signal processes. Thus, when the video signal and the timing signals are recombined as required prior to D/A conversion, there are inherent instabilities in the timing between the blanking region information and the active video signal caused by drift, noise etc. However, notwithstanding the problem of instability, it is highly desirable in this generation of VTR's and associated TBC's that the video signal be processed entirely in the digital domain. Optimum video signal processing is achieved in the digital domain since the television picture is defined very accurately by the digital samples, and analog associated problems such as instability and signal drift inherently are overcome. As previously discussed, in a digital PAL system the color subcarrier and thus the sampling clock are offset from the horizontal scanning frequency by the frame scanning frequency of 25 Hz. Accordingly, when blanking region information is re-inserted, the samples cannot be taken along the vertical line of previous mention. As a result the blanking interval information is non-orthogonal relative to the rectangular television picture. It follows that the 25 Hz offset in a digital PAL system causes intolerable horizontally displaced steps in the blanking interval timing signals, which cause the generation of an undesirable family of blanking, sync and burst envelopes that do not represent the instantaneous timing of the original television signal. The present invention overcomes the disadvantages of processing video signals in the analog domain, while overcoming the problems of non-orthogonality of the blanking region information caused by the 25 Hz offset in a digital PAL-encoded color television system. The video signal and the timing information may be processed entirely in the digital domain, which is a decided advantage, for example, in a time base corrector, a digital VTR, etc. The invention digitally generates the blanking region information via a non-orthogonal circuit while processing the information with the same clock that processes the video data. To this end, a dynamic offset circuit is provided which makes the blanking region information appear orthogonal when the subsequent process of digital-to-analog conversion is performed, whereby the blanking interval timing signals of successive television frames or pictures are precisely synchronized. It is known from US-A-4535357 and EP-A-0130588, which discloses features corresponding to those in the pre-characterising part of claim 1, to generate a waveform in a line blanking interval of a television picture signal which is composed of digital samples occurring at a rate such that there is a progression from line to line in the relative phase of the samples and a reference phase position at the start of a scanning line. In other words, a line drawn through the positions of corresponding samples of a succession of lines is non-orthogonal to the axes of the television picture. It is known from for example FR-A-2520178 that there is a variety of television systems differing in the scanning frequencies and also in the relationship between the color sub-carrier frequency and the line scanning frequency. In several of these systems there is a frequency offset which will, if the sampling frequency is for example chosen as a multiple of the sub-carrier frequency, give rise to the said phase progression. According to one aspect the invention provides apparatus for generating a waveform in a line blanking interval of a television picture signal which is composed of digital samples occurring at a rate such that there is a progression from line to line in the relative phase of the samples and a reference phase position at the start of a scanning line of the television picture, characterised by: means for storing a multiplicity of sets of signals, each set comprising a succession of samples defining a respectively phased version of the waveform; means for reading out samples in a selected set at a rate corresponding to the sampling rate; and means for changing the selection of the set at intervals in accordance with the rate of the line-to-line phase progression, such that the phasing of the version selected for a line at least partly compensates for the said phase progression. The said progression is normally cyclic and the said sets are each associated with a particular one of a multiplicity of points spaced apart over at least a part of a cycle of the said progression and each set defines the waveform with a respective phase compensating for a respective value of the said relative phase. The said part may be a quarter of a cycle and the points maybe evenly spaced. The means for storing may comprise a read-only memory and the means for reading out the samples and changing the selection comprise binary counters which are clocked respectively at the sampling rate and at a rate which is related to the rate of the line to line phase progression and the number of said points in said cycle. The invention also provides in another aspect a method of generating a waveform in a line blanking interval of a television picture signal which is composed of digital samples occuring at a rate such that there is a progression from line to line in the relative phase of the samples and a reference phase position at the start of a scanning line of the television picture, characterised by: storing a multiplicity of sets of signals, each set comprising a succession of samples defining a respectively phased version of the waveform; reading out samples in a selected set at a rate corresponding to the sampling rate; and changing the selection of the set at intervals in accordance with the rate of the line-to-line phase progression, such that the phasing of the version selected for a line at least partly compensates for the said phase progression. The waveform may describe a timing envelope in a PAL-encoded blanking interval, and usually the sampling rate would be a multiple of a color sub-carrier frequency equal to [(1135/4)f H + 25] wherein f H is the line scanning frequency. The samples in the sets may represent gain coefficients which are multiplied by a signal that represents the peak magnitude of the waveform. Figure 1 is a graph pictorially representing a television picture on which is shown the 25Hz offset inherent in a PAL system. FIGURE 2 is a graph depicting a plurality of waveforms descriptive of blanking interval envelopes and the relative sampling instants in a PAL system which define storable gain numbers. FIGURE 3 is a block diagram of the invention offset generator. FIGURES 4A, 4B, 4C are a schematic diagram depicting an implementation of the binary counter and shift register of the circuit of FIGURE 1. FIGURE 5 is a schematic diagram depicting an implementation of the PROM of the circuit of FIGURE 1. In FIGURE 1, the numeral 12 refers to a television picture (for example, a frame of video) of conventional rectangular configuration, wherein a vertical line 14 represents generally the start of the blanking intervals. In particular, line 14 represents the horizontal scanning frequency, and lines 16 represent the PAL television standard color subcarrier frequency (Fsc) and thus the sampling points of a 4Fsc sampling frequency, used in the description herein by way of example only. As may be seen, the proportions of the picture, lines and sampling points are exaggerated for purposes of description. At a time represented by a point 18, the 50% point of the blanking interval waveform corresponds to a zero crossing of the sampling phase. In the PAL system, due to the 25 Hz offset, subsequent point 20 also represents the 50% point of the waveform corresponding to a zero crossing of the sampling phase. As may be seen at point 20, and subsequent point 22, etc., the sampling points lie successively further from the vertical line 14 due to the 100 Hz offset between the sampling frequency and the scanning frequency. Thus in a digital PAL system, it is not possible to re-insert blanking along the vertical line 14 and therefore, the blanking interval information is not orthogonal. Referring to FIGURE 2, in accordance with the invention, a sufficient number of waveforms to describe a sampling clock cycle are stored, each with a slightly different phase value, as represented by the waveforms 24, 26, 28 etc., in FIGURE 2. In this example, the sampling frequency is 4Fsc whereby it is necessary to store only 16 waveforms which describe one quadrant of the Fsc cycle, and then repeat the quadrant four times, delaying the start address of each quadrant by one clock pulse each 4Fsc cycle. It may be seen that at the point 20, the envelope 26 appears to lead the envelope 24 of point 18 by one sample time, while the envelope 28 of point 22 appears to lead the envelope 26 of point 20 by one sample time. Thus if the 16 stored envelopes are sequentially read from memory at a (64 x 25) Hz rate, the blanking region will be offset in a direction which compensates for the PAL 25 Hz offset. As a result, the blanking interval information is assembled orthogonally to the television scanning frequency when the new blanking information is re-inserted in the video signal. Since the offset between Fsc and F H occurs at one Fsc cycle per frame, and a frame frequency is 25 Hz, one cycle per frame equals 25 Hz offset. Since 4Fsc is the sampling frequency herein, and since 16 waveforms are stored in memory, if the plurality of waveforms are repeated four times during a picture, the stepping process is performed 64 times per picture. With a PAL subcarrier of 4.43 MHz, the resulting blanking signal timing error is of the order of 3.5 nanoseconds which is well beyond the resolution requirement of the horizontal blanking interval waveforms. Referring to FIGURE 3, a programmable read-only-memory (PROM) 36 is loaded with the gain numbers of the set of 16 waveforms of preselected shape and successive phase differences. The number of waveforms is sufficient to describe one quadrant of the color subcarrier cycle. If desired, sufficient waveforms may be stored for a full subcarrier cycle. By way of example only, the gain numbers corresponding to the waveforms are herein selected to define a sine squared edge although any edge shape may be stored and addressed in accordance with the invention. The waveforms may have uniform or non-uniform phase differences, wherein a non-uniform phase configuration is addressed at a complementary non-uniform rate to provide a uniform data output. The PROM 36 is addressed at 4Fsc via a 3-bit envelope address as further described below. A binary counter 40 is clocked by clock signal of a frequency equal to 64 x 25 Hz and generates a binary word (of 7-bits) which represents the instantaneous phase of the 25 Hz waveform. Four LSB's of the counter 40 address the PROM 36 via a bus 42. Two MSB's, which represent the quadrant of the Fsc cycle and are used to control the phase of the start time for selecting the respective sets of waveforms, are fed via a bus 46 to a presettable binary counter 44 herein configured as a shift register. The counter 44 is clocked by a 4Fsc clock on a line 48, and generates in turn, the address signal at 4Fsc on an envelope shaping bus 50 coupled to the PROM 36. The gain numbers are sequentially addressed and are supplied as orthogonal blanking information to a multiplier 54 via a bus 52. The binary words representing the gain numbers comprise multiplying coefficients for modifying the gain of steady state switching waveforms that overlap the blanking envelope regions and which represent the peak magnitudes of the blanking, sync and burst signals. The multiplier 54 also receives the digital video signal, as well as blanking, sync and burst signals to be inserted in the video signal, via a 10-bit bus 56. As mentioned, the multiplying coefficients received from the PROM 36 are multiplied by numbers representing the overlapping signals for the blanking, sync and burst in the original video signal to generate precisely shaped, digital edges to be inserted into the video in place of the original blanking, sync and burst. The multiplier 54 is clocked by the 4Fsc clock on line 60 and supplies the recombined video signal and blanking interval information on a 12-bit output video bus 58. It is to be understood that the bus sizes are herein specified for purposes of description only and that other size buses may be used throughout the system. The components of FIGURE 3 are depicted further in the schematic diagrams of FIGURES 4, 5, wherein like components are similarly numbered. In FIGURE 4A, the binary counter 40 is formed of three IC's 62, 64, 66, which are presettable binary counters which count the inverted reference H pulses provided by system timing on a line 68. Counters 62 and 64 are clocked at 64 x 25 Hz and provide the addressing. A pair of D-type flip-flips 70, 72 are clocked by reference vertical (V) pulses on a line 74, and the inverted reference horizontal (H) pulses on the line 68, respectively. The true output of flip-flop 70 is coupled to the input of flip-flop 72, and the not-true output of the latter is coupled back to the clear input of the former. The preset inputs of flip-flops 70, 72 are coupled to +5 volts. The true output of the flip-flop 72 is fed to the B input of the IC 62, and also to a pair of NOR gates 76, 78. NOR gate 78 is coupled to the load inputs of the IC's 62, 64 and NOR gate 76 is coupled to the load input of IC 66. The carry output of IC 64 is coupled to the NOR gate 78 and to the enable input of IC 66. The carry output of IC 62 is fed to the enable input of IC 64. PAL (+) and 25 Hz OFFSET (-) logic signals on lines 80, 82 respectively, are added to the pins 14 of the IC's 62, 64, 66. The IC's are preloaded via their preset inputs to provide the binary weighted addresses. Pins 13, 14 of IC 64 and pins 12, 13, 14 of IC 66 provide a 5-bit binary word as the output from the binary counter 40, which is coupled to respective A inputs of an adder 84 via bus 86. Another binary word is supplied to the B inputs of adder 84 via a latch 88, and corresponds to a desired fixed phase for use in a NTSC system wherein the counter 40 is disable since there is no offset problem. Thus, the word is used to phase the counter output and is supplied by a control data signal on a bus 90 under microprocessor control and stored by latch 88. In response to the 25 Hz OFFSET and PAL (+) signals on lines 82, 80 the present offset generator operates in the PAL standard, and the binary counter 40 divides down the reference H pulse by 625 to provide a 25 Hz offset signal to the adder 84 on the bus 86. In an NTSC standard mode, since there is no offset problem, the binary counter 40 is disabled and the preset binary word of previous mention is supplied to the adder 84 via the latch 88 to provide a fixed address to the PROM 36 to select a corresponding preselected waveform from memory. Since this feature is not relevant to the invention, it is not discussed further herein. Suffice it to say that the latch 88 provides means for phasing the binary counter 40. The adder 84 supplies a PROM address signal corresponding to the four LSB's (AM3-AM6) on the address bus 42, via a set of exclusive OR gates. The latter gates also are supplied with an inverted signal derived from composite blanking, composite sync and burst gate/video signals further discussed in FIGURE 4B. The PROM address signal addresses the gain numbers corresponding to the 16 waveforms stored in the PROM 36 (FIGURE 5) as previously discussed in FIGURE 3 and shown below in FIGURE 5. The adder 84 also supplies a quadrant select signal in the form of a 3-bit binary word corresponding to the three MSB's (AM⊘-AM2) on the bus 46 of previous mention. Two bits of the bus 46 actually provide the quadrant select signal which represents the quadrant of the 4Fsc clock cycle and which control the phase of the start time for selecting the different quadrants. The third MSB of bus 46 is used in the event the adder 84 overloads. Referring to FIGURE 4B, the bus 46 is coupled to the preset inputs of the binary counter 44, which includes first and second presettable binary counters 94, 96 configured as shift registers. The counter 94 counts the 4Fsc clocks (period 70 nanoseconds) when the signal at pin 7 enables the counting. The 25 Hz offset data from the adder 84 presets the count of counter 94, which then runs to its count and generates a signal on pin 15 and an inverted version on a line 98. The position of the pulses from pin 15 is determined by the preset numbers supplied via bus 46 to the load inputs of the counter 94, which reflect accordingly, the 25 Hz offset. The signal clocks a D-type flip-flop 100, and is coupled to a load input of the counter 94 via exclusive OR gates 102, as well as to the enable pin 7 via the flip-flop 100 and an exclusive OR gate 104. The gates 102 and 104 also are coupled to a line 106 which supplies a signal which marks the boundary of each transition in the blanking interval of the video signal. More particularly, input signals COMP BLANKING, COMP SYNC and BURST GATE/VID are supplied via lines 108, 110 and 112 respectively from the system sync generator (not shown). The timing of these three signals is generally known and indicate the start and stop times of the respective signal portions of the composite video signal. These signals are buffered and combined by inverters and a NAND gate 114. The resulting boundary marking signal on the line 106 is fed to the input of the flip-flop 100 and to the exclusive OR gates 102, 104, whereby each of the transitions of the boundaries is converted to a respective pulse on the signal on line 98 by the edge detector circuit formed of the IC's 100, 102, 104 and 94 of previous mention. In accordance with the invention, the preset inputs fed to counter 94 determine the position of each pulse, i.e., provides a one clock cycle delay that occurs in the four quadrant of 25 Hz, which provides shifting the blanking, the sync and burst envelopes in quantized steps at the 25 Hz offset rate. That is, the preset numbers loaded into the counter 94 delay the start address, i.e., control the phase of the start time, for selecting the sets of stored waveforms by one clock pulse each 4Fsc cycle. The shifted boundary pulses on line 98 are used to signal when each sine squared edge (corresponding to the stored waveforms which, for example, are sine squared curves) is to be formed subsequently. The pulse signal is used to begin addressing the PROM 36 whereby the gain numbers corresponding to the waveforms appear at the PROM output corrected for the 25 Hz offset. However, the respective addresses for accessing the PROM 36 must be provided. Thus the pulse signals on line 98 are fed to the counter 96 which counts 4Fsc clock cycles starting from a preset number each time a pulse on line 98 loads the preset count. The resulting signals A⊘-A3 on a bus 116 correspond to the binary outputs of the counter 96 and comprise the addresses which perform the envelope shaping. The A⊘ signal is a one for one count of the 4Fsc clocks on clock line 118. Signal A1 is a binary division of the signal A⊘ by two, signal A2 is a binary division by 2 of the signal A1, and signal A3 is a binary division by two of the signal A2. A signal A4 also is provided which makes a high to low transition on a 4Fsc transition following a terminal count of 16. The signals A⊘-A4 are supplied via bus 116 is a PROM 120 which provides various horizontal timing signals. Three more signals A5, A6, and A7 are generated in FIGURE 4C to serve as address signals for the PROM 120. The signal A5 is low when the gain bits generated in the system envelope generator (not shown) are to be increasing from 0 to 1.0 and is high when the gain bits are going from 1.0 to 0. The signal A6 is a wide sync signal which is in a logic 1 state starting from a time before the start of the sine squared edge of the leading edge of the horizontal sync pulse is to be formed and remaining in the logic 1 state until a time after the sine squared trailing edge of the horizontal sync pulse is to be formed. The signal A7 is a wide burst signal which makes a transition to a logic 1 state starting at a time before the sine squared leading edge of the burst envelope is to be formed and which remains in the logic one state until a time after the sine squared trailing edge of the burst envelope is to be formed. Since the signals A5-A7 are not per se relevant to the invention, they are not described in further detail herein. The signals from PROM 120 are re-clocked via a latch 122 and the 4Fsc clock on a line 124. The lines AM⊘-AM2 of the latch 122 provide the envelope shaping signal on the address bus 50 (herein labelled an envelope shaping bus) of previous mention in FIGURE 3 and are fed, along with the various horizontal timing signals, to the circuits of FIGURE 5. In FIGURE 5, the PROM 36 includes a sync envelope PROM 124, a 525 blanking envelope PROM 126 and a 625 blanking envelope PROM 128. Input signals include the PROM address bus 42, the envelope shaping bus 50, a wide blanking line 130, a wide burst line 132 and a narrow blanking line 133, all supplied by the PROM 120 and latch 122 of FIGURE 4C. A SECAM (+) logic signal, a 625/525 logic signal and a SECAM bottle enable(-) logic signal also are supplied on respective lines 134, 136 and 138. The gain numbers which define the desired edge shape for the sync pulse edges and the edges of the burst envelope are stored in the PROM 124. The gain numbers for the blanking edges is stored in PROM 126 for a 525 line NTSC standard video blanking pulse. The PROM 128 is used to store the gain numbers defining the desired edge shape for the blanking pulse used in the 625 line PAL standard blanking pulse. The plurality of stored gain numbers defining each edge are selected by the AM⊘-AM2 address bits on the bus 50. These address bits cycle through the addresses to select the gain numbers 1-8 sequentially for values 0 to 1.0₁₀ or from 1.0₁₀ to 0 depending upon the state of signal A5 in FIGURE 4C. The output gain numbers are placed on the bus 52 and re-clocked through a latch 140 clocked by the 4Fsc clock and are then coupled to the X input port of the multiplier 54 of previous mention in FIGURE 3. Gates 142, 144 and 146 combine the WIDE BLANKING, SECAM BOTTLE EN (-) and SECAM (+) signals on respective lines 130, 138 and 134 respectively, to enable the PROM 128 (or 126 in NTSC) when blanking edges are to be formed, and the PROM 124 when sync or burst edges are to be formed. The recombined video signal and new blanking interval information is fed to a downstream D/A converter (not shown), the output of which is a video signal whose blanking interval regions are orthogonal to the picture and in conformance with the television standard, whereupon the correction of the PAL 25 Hz offset may be observed in the picture.

Claims: Apparatus for generating a waveform in a line blanking interval of a television picture signal which is composed of digital samples occuring at a rate such that there is a progression from line to line in the relative phase of the samples and a reference phase position at the start of a scanning line of the television picture, characterised by: means (36) for storing a multiplicity of sets of signals, each set comprising a succession of samples defining a respectively phased version of the waveform; means (40) for reading out samples in a selected set at a rate corresponding to the sampling rate; and means (44) for changing the selection of the set at intervals in accordance with the rate of the line-to-line phase progression, such that the phasing of the version selected for a line at least partly compensates for the said phase progression. Apparatus according to claim 1 wherein the said progression is cyclic and the said sets are each associated with a particular one of a multiplicity of points spaced apart over at least a part of a cycle of the said progression and each set defines the waveform with a respective phase compensating for a respective value of the said relative phase. Apparatus according to claim 2 wherein the said part is a quarter of a cycle. Apparatus according to claim 2 or 3 wherein the points are evenly spaced. Apparatus according to any of claims 2 to 4 wherein the means for storing comprises a read-only memory and the means (40, 44) for reading out the samples and changing the selection comprise binary counters which are clocked respectively at the sampling rate and at a rate which is related to the rate of the line to line phase progression and the number of said points in said cycle. Apparatus according to any foregoing claim wherein the waveform describes a timing envelope in a PAL-encoded blanking interval. Apparatus according to any foregoing claim wherein the sampling rate is a multiple of a color sub-carrier frequency equal to [(1135/4)f H + 25] wherein f H is the line scanning frequency. Apparatus according to any foregoing claim wherein the samples in the sets represent gain coefficients and means (54) are provided for multiplying a read-out sample with a signal that represents the peak magnitude of the waveform. A method of generating a waveform in a line blanking interval of a television picture signal which is composed of digital samples occuring at a rate such that there is a progression from line to line in the relative phase of the samples and a reference phase position at the start of a scanning line of the television picture, characterised by: storing a multiplicity of sets of signals, each set comprising a succession of samples defining a respectively phased version of the waveform; reading out samples in a selected set at a rate corresponding to the sampling rate; and changing the selection of the set at intervals in accordance with the rate of the line-to-line phase progression, such that the phasing of the version selected for a line at least partly compensates for the said phase progression. A method according to claim 9 wherein the said progression is cyclic and the said sets are each associated with a particular one of a multiplicity of points spaced apart over at least a part of a cycle of the said progression and each set defines the waveform with a respective phase compensating for a respective value of the said relative phase. A method according to claim 10 wherein the said part is a quarter of a cycle. A method according to claim 10 or 11 wherein the points are evenly spaced. A method according to any of claims 9 to 12 wherein the waveform describes a timing envelope in a PAL-encoded blanking interval. A method according to any of claims 9 to 13 wherein the sampling rate is a multiple of a color sub-carrier frequency equal to [(1135/4)f H + 25] wherein f H is the line scanning frequency. A method according to any of claims 9 to 14 wherein the samples in the sets represent gain coefficients which are multiplied by a signal that represents the peak magnitude of the waveform.

Inventors: MORRISON, ERIC FRASER
Applicants: AMPEX CORPORATION
Application year: 1986
Main classification: H04N9
IPC codes: H04N9