GB2262408A - Horizontal blanking for multiple rate scanning - Google Patents

Description

2262408 1 HORIZONTAL BLANKING FOR MULTIPLE RATE SCANNING This invention

relates to the field of synchronization systems for televisions and the like, and in particular, to the generation of horizontal blanking signals for multiple scan rate operation. For example, the horizontal blanking signals are generated at 2fH, where fH is the conventional horizontal scanning frequency.

Proper width and deflection/video timing are important in achieving a video signal that does not have any fold over. This is especially true in high end receivers that run at multiple horizontal frequencies (nfH), and lower overscans. The amount of overscan for multiple frequency scanning is approximately 5% to 7%, as compared to the 10% to 12% overscan which is common for conventional scanning rates. Accordingly, the need is greater for accurate timing of the horizontal blanking signal.

Typically, horizontal blanking is derived from a lower voltage pulse on a secondary winding of the high voltage flyback transformer. A problem with this scheme is that the rise time of the pulse is not fast enough to blank adequately the video in the horizontal yoke current retrace interval which is generated by a higher voltage pulse. Moreover, unless the pulse is heavily differentiated and then stretched to make the pulse wide enough, the timing of the beginning of blanking is later than needed. Unfortunately, heavy differentiation creates other problems in the form of false triggering of the blanking circuit when flyback ringing pulses become high enough to trigger the circuit.

One approach which is known to overcome these problems uses two capacitors arranged in a capacitive voltage divider arrangement. This removes the secondary ringing problem and gives better timing than the secondary winding approach. However, this approach is problematic in that at least one high voltage capacitor is needed in the divider.

Another approach is to generate a blanking pulse from timing signals that precede the retrace interval. This can be done by using a pair of oneshot monostable multivibrators. A first one of the one-shots is triggered by one of the horizontal synchronizing signals and defines an initial delay of nearly one whole horizontal line. A 2 RCA 86,646 second one-shot is triggered by the output of the first one-shot at the end of this delay, and defines the pulse width. However, there are problems associated one shots, such as false triggering, which results in improperly timed blanking.

A better solution in accordance with an inventive arrangement is particularly appropriate for a multiple horizontal frequency synchronizing circuit having a first phase locked loop (PLL) operating at 1fH and a second phase locked loop operating at 2fH. A synchronizing circuit having such first and second phase locked loops, as well as a circuit for dividing the output of a 32fH oscillator by 16 to obtain the 2fH signal, is described in U.S. Patent No. 5,043,813, issued August 27, 1991, and is also described in European Patent Application 91104749.6, published as EP 0 449 198 A2 on October 2, 1991.

The first phase locked loop includes an nfH oscillator, for example 32 N, and is synchronized with an incoming video signal. The second phase locked loop is synchronized with the horizontal deflection circuit. A 1fH to 2fH converter circuit, which can be implemented as a 32fH/16 frequency dividing counter, is responsive to the 32fH oscillator and synchronized by the IN output of the first phase locked loop. The frequency divider provides a 32fH/16 (i.e., 2fH) drive signal for the second phase locked loop by repetitively counting down the 32fH signal by a count of 16. In accordance with an inventive arrangement, the same synchronizing signal derived from the 32fH/16 divider provides a timing source for blanking fbr the RGB driver running at 2fH. Furthermore, the phase of the blanking signal is rendered adjustable in increments, by preloading the frequency dividing counter to a desired number that provides a frequency division count of other than 16.

FIGURE 1 is a block diagram of a horizontal synchronizing circuit and horizontal blanking generator according to an inventive arrangement, including two phase locked loops coupled through a frequency converter.

FIGURE 2 is block diagram of a digital circuit for implementing the frequency converter shown in FIGURE 1 as a 32fH/16 divider.

t 3 RCA 86,646 FIGURE 3 is a circuit schematic for implementing the horizontal bianking generator shown in FIGURE 1.

FIGURES 4(a) and 4(b) are comparative timing diagrams. FIGURE 4(a) is the signal at the junction of resistors R5 and R6 in 5 FIGURE 3, and FIGURE 4(b) is line 214 of the Philips video pattern.

FIGURES 5(a) and 5(b) are comparative timing diagrams. FIGURE 5(a) is the 2fH yoke current, and FIGURE 5(b) is line 214 of the Philips video pattern.

A horizontal synchronizing circuit 10 for 2fH scanning utilizing two phase locked loops is shown in FIGURE 1. A one-chip processor 12 provides IF, video, chroma and deflection functions. A phase locked loop 14 in the one-chip processor generates a VH output by dividing a 32fH clock signal, from a voltage controlled oscillator 13, by 32. The 1N output is synchronized with the horizontal synchronizing component of an incoming video signal, due to phase locked loop 14. A divide-by-16 circuit 16, forming a VH to 2fH frequency converter, provides a 2N output by dividing the 32N oscillator output by a nominal count of 16. The 1N output is used to synchronize the divide-by-16 circuit.

The phase of the 32fH/16 timing signal generated by the frequency converter can be adjusted relative to the synchronizing component of the incoming video signal. This is accomplished by preloading a starting number into the frequency dividing counting circuit 16, that counts down the 32N pulses. The number can be supplied by a microprocessor (not shown), for conveniently adjusting the phase, for example, in 2 gsec steps. Such a phase adjusting system is described in European Patent Application No. 91104520.1, published as EP 0 449 1130 A2 on October 2, 1991.

The 32fH/16 timing signal synchronizes a second phase locked loop 18 with an output deflection stage 20. Phase locked loop 18 operates at 2fH and generates a 2fH scan synchronizing signal, synchronized with the 32fH/16 timing signal. Flyback pulses at the 2fH rate are coupled as an input to a ramp generator 22. The ramp generator is AC coupled to the flyback input of the second phase locked loop 18 by capacitor C. A variable resistance 24 can provide a further fine phase adjustment, for example from 0 to 2 gsee, by 4 RCA 86,646 slightly changing the DC offset to the phase comparator in the second phase locked loop. The 32fH/16 timing signal also provides an input to a horizontal blanking generator 17, which includes an inverter/driver circuit 19.

A digital circuit for implementing the divide-by-16 circuit 16 is shown in FIGURE 2. The 1N and 32N signals are buffered by inverters 26 and 28 respectively. The buffered 1IN signal is an coupled to the D input of a first D-type flip/flop 30. The Q output of flip/flop 30 is an input to a second D-type flip/flop 32 and another inverter 34. The Q output of flip/flop 32 and the output of inverter 34 are inputs to NAND gate 36, the output of which controls the load (LDN) input of a counter 38 for loading the starting count from signals on a bus coupled to the processor. In the drawings, input terminal names ending with "N" generally indicate a signal which is a logical NOT input.

This processed VH rate signal, which is delayed by one 32fH clock cycle and is one 32fH clock cycle wide, loads the bus data, gP BUSO, gP BUS1, gP BUS2 and gP BUS3, into the counter 38. The 32f1A signal, buffered by inverter 28, is the clock input for the flip/flops 30 and 32 and for the counter 38. The QO and Q1 outputs of the counter 38 are inputs to a NAND gate 40. The 02 and 03 outputs of counter 38 are inputs to a NOR gate 42. The outputs of NAND gate 40 and NOR gate 42 are inputs to a NAND gate 44. The output of NAND gate 44 is the 32fH/16, or 2fH, signal which drives the second phase locked loop. The relative phase of the 32fH/16, or 2fH, timing signal output of counter 38 is determined by the starting number loaded in from the microprocessor. According to the embodiment shown, this phase can be expressed in Boolean terms as:

(00 - Q1)' (Q2 + Q3)]', where: indicates a logical AND; + indicates a logical OR; and, indicates a logical NOT, or signal inversion.

Where the most significant bit is QO and the counter counts down, the output of NAND gate 44 is true (low) at a binary count of 0000, 0100 or 1100 (corresponding to decimal 0, 4 or 12 respectively). Accordingly, this circuit provides a phase variation RCA 86,646 of one to eight clock cycles at 32fH, namely between 12 and 5 (binary 1100 to 0101). For purposes of the illustrated embodiment, the phase variation required is small. It is also possible to use a gating arrangement (e.g., with a NOR gate in place of NAND gate 40 for counting from 15 to zero) to obtain up to 16 cycles of phase variation. Generally, the amount of phase variation necessary is that required to generate a blanking pulse that is earlier and wider than otherwise possible, and that accurately tracks from the beginning of the scan.

The synchronizing pulses of the output 32fH/16, or 2fH, timing signal may be moved in 2 gsec steps throughout the 1N video period by changing the data loaded into the counter 38. The output 2N sync pulse is an active low TTL level pulse 6 jusec wide. The 32N/1 6 synchronizing pulse and a 2fH flyback derived ramp determine the phasing of the 2fH scan to the incoming VH video, which is clocked out at a 2fH rate by the appropriate digital signal processing, thereby synchronizing, at the picture tube, the 2f1A video and the 2fH scan. Finer phase control of 0 to 2 gsec can be obtained by slightly changing the DC offset to the phase comparator where the ramp is AC-coupled, as explained above. Finer phase control may be implemented by changing the slope of the ramp or introducing a small variable resistor in series with the ramp generator capacitor.

Since the 32fH/16 timing signal pulse is approximately 6 gsec wide and a typical 2fH flyback retrace interval is 5.7 gsec, an integrated flyback pulse from which the ramp is generated can be delayed up to approximately 200 gsec from the leading edge of the 32fH/16 timing signal pulse. Horizontal blanking would begin too late. If the horizontal. blanking is triggered from this same pulse of the 32fH/16 timing signal, according to the invention, it will start shortly before the retrace interval and be slightly wider than the retrace interval, thus providing correct blanking timing and width.

The comparative timing diagrams of FIGURES 4 and 5 illustrate how the 32fH/16 timing signal pulse can be used to generate a blanking signal. FIGURE 4(a) illustrates the signal at the junction of resistors R5 and %6 in FIGURE 4, and FIGURE (4(b) illustrates line 214 of the Philips video pattern. The blanking interval of FIGURE 6 RCA 86,646 4(a) can be seen to begin just prior to the portion of the video signal which needs to be blanked, and terminates before blanking active video for the next line. FIGURE 5(a) illustrates how the 2fH yoke current and FIGURE 5(b) illustrates how the same line 214 of the 5 Philips video pattern fall just within the blanking interval.

Care must be taken, if the horizontal blanking generator 17 utilizes an external inverting transistor, to minimize storage time effects and ensure that the blanking pulse ends at the proper time and does not blank active video. A suitable circuit 19 for an inverting and driving stage for a blanking circuit according to an inventive aspect is shown in FIGURE 3. The inverter/driver includes transistor Q1. The 32fH/16 timing signal is AC coupled to the inverter th rough capacitor C1. Resistors R1, R2 and R3 offer a high enough impedance to avoid any significant loading of the signal.

Diode CR1 controls the saturation of transistor Q1 to minimize storage time and output width. The choice of resistor R4 controls the slicing level of the trailing edge of the pulse, and the capacitor C2 provides a fixed fast start of the leading edge. The ramp at the emitter of transistor Q1 is generated during the pulse and lowers the slice level for the turn off point, thereby maintaining proper blanking width.

Resistors R5 and R6 form a voltage divider to provide interfacing to the buffer transistor Q2, which is configured as an emitter follower. The horizontal and vertical blanking signals are combined at the junction of diodes CR2 and CR3, which is also the base of transistor Q2. The output of transistor Q3 is a composite blanking signal.

Therefore, once the video delays have been established through the video processing channel and the phasing has been set by digital data (from the microprocessor or by hard wired jumpers) as well as the DC offset of the second phase locked loop, the blanking will be perfectly timed with the video.

7 RCA 86,646

Claims (17)

CLAIMS:

1. An apparatus, having:

a first phase locked loop (14) operable at fH and synchronized to a horizontal synchronizing component of a video signal; an fH to nfH converter (16) for deriving an nfH timing signal from outputs of said first phase locked loop (14), where n is an integer; and, a second phase locked loop (18), synchronized with said nfH timing signal, for generating an nfH scan synchronizing signal for a deflection stage (20) operating at nfH; characterized by:

means (17) responsive to said nfH timing signal and operable to generate a blanking signal for disabling an electron beam of during horizontal retrace intervals of said deflection stage.

2. The apparatus according to claim 1, characterized by an mfH oscillator (13) forming part of said first phase locked loop (14) and providing one of said outputs (32fH) to which said fH to nfH converter is responsive, where m is an integer multiple of n.

3. The apparatus according to claim 2, characterized in that m equals 32 and n equals 2.

4. The apparatus according to claim 1, characterized in that n equals 2.

5. The apparatus according to claim 1, wherein said fH to nfH converter (16) is characterized by a counter (38) for dividing a clock signal (32fH), said horizontal blanking signal having pulses with widths defined by integer multiples of periods of said clock signal.

6. The apparatus according to claim 1, further characterized by means (CR2, CR3, Q2) for combining said horizontal blanking signal with a vertical blanking signal to develop a composite blanking signal.

7. A horizontal blanking signal generator, having:

a first phase locked loop (14) coupled to a video signal having a horizontal synchronizing component at a frequency fH and including an oscillator (13) generating a signal at mfH signal; a frequency divider (16) for converting said MfH signal to an nfH timing signal by dividing said mfH signal; 8 RCA 86,646 a second phase locked loop (18), synchronized with said nfH timing signal, for generating an nfH scan synchronizing signal for a deflection stage (20) operating at nfH; and, means (pP BUS) for successively supplying said frequency divider (16) with starting numbers selected to control a phase relationship of said nfH timing signal and said MfH signal; characterized by:

means (17) responsive to said 2fH timing signal for generating video signal horizontal blanking pulses.

8. The signal generator according to claim 7, further characterized by means for modifying said nfH timing signal in at least one of phase and pulse width.

9. The signal generator according to claim 7, characterized in that n equals 2.

10. The signal generator according to claim 7, characterized in that m equals 32 and n equals 2.

11. A horizontal deflection system, having:

means (14,16) for generating an nfH timing signal synchronously with an fH horizontal synchronizing component in a video signal, where nfH is a higher frequency than fH; first means (18) responsive to said nfH timing signal for generating an nfH scan synchronizing signal synchronously with said nfH timing signal; and, a horizontal deflection stage (20) operable at nfH and responsive to said nfH scan synchronizing signal; characterized by:

second means (17) responsive to said nfH timing signal for generating horizontal blanking pulses.

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12. The system according to claim 11, further characterized by means (CR2, CR3, 02) for combining said horizontal blanking pulses with vertical blanking pulses to form a composite blanking signal.

13. The system according to claim 11, wherein said means for generating said nfH timing signal is characterized by:

a first phase locked loop (14); and, a frequency divider (16).

9 RCA 86,646

14. The system according to claim 13, wherein said first means responsive to said nfH timing signal for generating said nfH scan synchronizing signal is characterized by a second phase locked loop (18).

15. The system according to claim 11, wherein said means for generating said nfH timing signal is characterized by:

means (13) for generating a clock signal at MfH, where nlfH is a higher frequency than nfH; and, means (38) for dividing said rnfH clock signal to generate said nfH timing signal.

16. The system of claim 11, wherein said second means responsive to said nfH timing signal is characterized by a driver/inverter (19).

17. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.

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