GB2183122A

GB2183122A - Synchronizing signal generating apparatus for PAL system

Info

Publication number: GB2183122A
Application number: GB08627316A
Authority: GB
Inventors: Teruo Hieda
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1985-11-18
Filing date: 1986-11-14
Publication date: 1987-05-28
Also published as: DE3638868A1; JPS62117490A; DE3638868C2; GB2183122B; GB8627316D0

Abstract

Oscillator 1 develops a 13 &cirf& 375 MHz signal FHS for the horizontal transfer in solid-state imager 8. This signal is divided, 2, to provide the horizontal synch signal FH. It is further divided, 9, to provide the vertical synch signal FV. The output of a second oscillator 4 is divided, 5, to develop the colour sub- carrier FSC. The two oscillators are synchronised by comparing their frequency divided outputs, 3, 6, in a phase-locked loop. Modifications are shown in Figs. 2 and 3. <IMAGE>

Description

SPECIFICATION Synchronizing Signal Generating Apparatus for PAL System This invention relates to a reference signal generating apparatus for a color television camera and more particularly to a synchronizing signal generating apparatus for the PAL system.

The color subcarrier frequency fscl horizontal synchronizing frequency fH and vertical synchronizing frequency f, of the phase-alternation line system (hereinafter referred to as the PAL system) which is one of color television systems are standardized as follows: 625 fH= fv (1) 2 1135 1 fsc= fH+ fv (2) 4 2 It is, however, extremely difficult to obtain these frequency values fsc, fH and fv in a state of perfectly satisfying Formulas (1) and (2) above by frequency dividing the output of one and the same reference signal generator, because the least common multiple of these frequency values fH and fsc is about 11 G . Hz.The reason for which the frequency values fsc, fH and fv are thus standardized is as follows: In the PAL system, the color subcarrier of one of the color difference modulation axes is inverted once in every horizontal scanning period for the purpose of minimizing a color error resulting from the phase distortion of a transmission system. In the case of 1/2 interleaving arrangement, therefore, the phase of the color subcarrier of the inverted axis becomes uniform in every horizontal scanning period. During reproduction buy a monitor, therefore, dots come to vertically align to show conspicuous lines called dot disturbance. To solve this problem, the 1/2 interleaving arrangement is changed to a 1/4 interleaving arrangement to have the dots align in a chequered pattern.In addition to that, the dot position is arranged to be inverted for every field with phase inversion effected between one field and another by a fv12 offset.

For obtaining the frequency values fsc and fH, varied methods have been known including among others a method of using a frequency computing arrangement and a method of using a phase locked loop (hereinafter referred to as PLL) and a pulse removing circuit which is arranged to extract pulses for every horizontal and vertical periods as disclosed in Japanese Patent Publication No. SHO 60-27469. Simpler known methods for attaining this purpose include a method of performing synchronizing control with a PLL over 1/161 of a reference frequency which is 282 n times as high as the horizontal synchronizing frequency and 1/162 of a frequency which is n times as high as the frequency fsc (Japanese Patent Publication No. SHO 59-12224).

These conventional methods have various shortcomings. The method of using a frequency computing arrangement necessitates complex arrangement and complicated adjustment. The method of using the PLL and the pulse removing circuit for extracting pulses for every horizontal and vertical periods requires complex arrangement. As for the method of using the PLLfor performing synchronizing control over 1/161 of the reference frequency which is 282 n times as high as the horizontal synchronizing frequency and 1/162 of the frequency which is n times as high as the frequency fsc is applicable only to a case where the reference frequency for obtaining the horizontal frequency is integertimes as high as 282 fH=4.406250 MHz.

It is therefore a general object of this invention to provide a novel synchronizing signal generating apparatus and method for the PAL system.

It is a more specific object of one aspect of this invention to provide a novel synchronizing signal generating apparatus for the PAL system which is simply arranged and requires no adjustment work thereon.

To solve the above-stated problems of the prior art devices, a synchronizing signal generating apparatus for the PAL system which embodies this invention comprises: a first oscillator arranged to generate a signal of a reference frequency of 13.375xn (MHz) for a frame synchronizing signal; a second oscillator arranged to generate a signal of a frequency 4 m times as high as a color subcarrier frequency; a first frequency divider arranged to frequency divide the output signal of the first oscillator into 1 451xm a second frequency divider which frequency divides the output signal of the second oscillator into 598in a phase comparator arranged to compare the phase of the output of the first frequency divider and that of the second frequency divider; and control means arranged to control the oscillation frequency of at least one of the first and second oscillators according to the output of the phase comparator.

With the embodiment arranged as described above, the frequency generated by the first oscillator is divided by 451 x m. The frequency generated by the second oscillator is divided by 598x n. The phases of the outputs of the frequency dividers thus obtained are compared with each other by the phase comparator. The oscillation frequency of at least one of the first and second oscillators is controlled on the basis of the output of the phase comparator to cause the first and second oscillators to oscillate in synchronism with each other.

The invention will now be described in greater detail, and certain specific embodiments thereof set out by way of example only, reference being made to the accompanying drawings, in which: Fig. 1 is a block diagram showing a first embodiment of this invention; Fig. 2 is a block diagram showing a second embodiment of this invention; and Fig. is a block diagram showing a third embodiment of this invention.

Use of a solid-state image sensor necessitates a reference oscillation frequency which differs from the conventional oscillation frequency for obtaining pulses required in driving the image sensor.

Assuming that a tri-color stripe filter is used, and that a video signal obtained from the solid-state image sensor includes 4 MHz of luminance zone, the horizontal transfer frequency fHs becomes: fHs=(4+a)x3 Assuming that a=0.5 MHz, fas*13.5 MHz.

In obtaining a certain relation between the horizontal synchronizing frequency and the color subcarrierfrequencyfsc by a synchronous oscillation arrangement, the PAL system specifies the relation as follows: 709379 fH=2500 fsc Therefore, this relation is not simply obtainable by a frequency dividing process. It is, therefore, extremely difficult to obtain the frequency values fH and fsc in exact conformity to the specification by using a generally conceivable frequency dividing ratio.In view of this, in accordance with this invention the above-stated relation is modified as shown below: 451 x4 fisc*598 fHs fHs=13.375 (MHz)=856 fH The value Hs shown above is selected to be extremely close to the above-stated horizontal transfer frequency of the solid-state image sensor and to be convenient. It is thus a feature of this invention that with the relation in question modified in close proximity into a relatively simple relation, the relation between the horizontal synchronizing frequency and the color subcarrier frequency is synchronously oscillated by means of a PLL. Embodiments of this invention is arranged as described below with reference to the accompanying drawings: Fig. 1 shows an embodiment of this invention in a block diagram.The illustration includes an oscillator 1; a 1/856 frequency divider 2; a 1/451 frequency divider 3; an oscillator 4; a 1/4 frequency divider 5; a 1/598 frequency divider 6; a phase comparator 7; a circuit 8 for driving a solid-state image sensor; a frequency divider 9 which is arranged to frequency divide the frequency fHS into [2/856x1/625]; and a low-pass filter 10 which is arranged to cut the high frequency band component of the output of the phase comparator 7. The oscillator 1 oscillates at the frequency fHS of 13.375 MHz. The output of the oscillator 1 is frequency divided by the frequency divider 2 into the frequency fH. The frequency fHS is supplied also to the 1/451 frequency divider 3. The output of the divider 3 is supplied to the phase comparator 7.

The oscillator 4 generates a frequency of 4fsc which is four timers as high as the frequency fsc. The frequency 4fsc is divided by the 1/4 frequency divider 5 into the frequency fsc. Further, the frequency 4fsc is divided also by the 1/598 frequency divider 6. The output of the divider 6 is supplied to the phase comparator 7. The comparator 7 compares the output phase of the divider 6 with that of the 1/451 frequency divider 3. The output of the phase comparator 7 then comes via the low-pass filter 10 to control the oscillation frequency of the oscillator 4.

The oscillator 4, the 1/598 frequency divider 6 and the phase comparator 7 jointly form a PLL which causes the two oscillators 4 and 1 to synchronously oscillate.

The frequency fsc which is thus obtained can be expressed as follows: 598 4fsc= fHs 451 598 = x13.375MHz 451 =17.734479 MHz This shows an error of 4 Hz from the value 4fsc=17.734475 MHz specified by the PAL system standards. However, this is a negligible error well within an allowable range and thus presents no problem for practical applications.

In the embodiment, the PLL arrangement is not applied to the oscillator 1 which generates the frequency fHs for driving the solid-state image sensor but is applied to the other oscillator 4 which generates the frequency 4fsc. This arrangement ensures highly stable oscillation of the frequency fHs. Therefore, the solid-state image sensor can be stably driven. This is a highly advantageous feature.

Further, in the embodiment shown in Fig. 1,the use of the oscillator 4 may be replaced with a modification which is arranged to prepare the oscillator 1 in the form of a voltage controlled oscillator (VCO for short) and to apply the output of the phase comparator 7 to the oscillator 1 via the low-pass filter 10. The arrangement of this modification is as shown in Fig. 2.

While the frequency of the oscillator 1 of this specific embodiment is set at 13.375 MHz and that of another oscillator 4 at 4fsc, each of these values of course may be replaced with some value which is integer times as high as the set value. In the case of such a modification, the frequency dividing ratio of each of the frequency dividers 3 and 6 corresponding to these oscillators is also multiplied by an integer as applicable. Further, the same advantageous effect is also attainable by changing the oscillation frequency of the oscillator 4 to 2fsc, the frequency dividing ratio of the frequency divider 6 to 299 and that of the frequency divider 5 to 2 as shown in Fig. 3 at an oscillator 4', a frequency divider 6' and another frequency divider 5'. This modification permits simplification of the arrangement of the oscillator 4'.

In accordance with this invention, as described in the foregoing, the frequency offset of the color subcarrier of the PAL system can be effected with ease; and the arrangement of the synchronizing signal generating device for the PAL system can be simplified. Further, the frequency of 13.375 MHz generated by the oscillator 1 according to this invention is highly suitable for use as a reference frequency in driving the solid-state image sensor.

Claims

1. A synchronizing signal generator for the PAL system, comprising: a) a first oscillator arranged to generate a signal of a frequency which is a reference frequency 13.375xn MHz (n: an integer); b) a second oscillator arranged to generate a signal of a frequency which is 2m (m: an integer) times as high as a color subcarrier frequency; c) a first frequency divider arranged to frequency divide the output of said first oscillator into 1 451xn d) a second frequency divider arranged to frequency divide the output of said second oscillator into 299xm e) a phase comparator arranged to compare the phases of the outputs of said first and second frequency dividers with each other; and f) control means for controlling the oscillation frequency of either one of said first and second oscillators on the basis of the result of phase comparison made by said phase comparator.

2. A generator according to claim 1, wherein said first oscillator is a voltage-controlled oscillator having the oscillation frequency thereof controlled by said control means.

3. A generator according to claim 1, wherein said second oscillator is a voltage-controlled oscillator having the oscillation frequency thereof controlled by said control means.

4. A generator according to claim 1, further comprising: g) a third frequency divider arranged to frequency divide the oscillation frequency of said first oscillator into 1/856.

5. A generator according to claim 1, further comprising: g) a fourth frequency divider arranged to frequency divide the oscillation frequency of said first oscillator into 2 1 x 856 625

6. A generator according to claim 1, further comprising: g) a fifth frequency divider arranged to frequency divide the oscillation frequency of said second oscillator into 2m and to produce the output thereof as a color subcarrier frequency.

7. A signal generating apparatus for generating a signal determined by a relation 1135 1 fsc= fH+ fv 4 2 which obtains among a color subcarrier frequency fsc, a horizontal synchronizing frequency fH and a field frequencyfv, comprising: a) first means for generating a signal of a frequency of 2mxf5c (m: a positive integer), said signal being arranged to have a variable frequency; b) second means for frequency dividing the frequency of said signal generated by said first means by 299xm; c) third means for generating a signal of a frequency which is equal to the frequency of the signal generated by said second means; and d) control means arranged to phase compare the signals generated by said second and third means and to control the frequency of said signal generated by said first means in such a manner as to have the phases of signals of said second and third means coincide with each other.

8. An apparatus according to claim 7, further comprising: e) means for generating frequencies fH and fv from the signal generated by said third means.

9. An apparatus according to claim 8, wherein said third means includes: a) an oscillator arranged to generate a signal of a frequency which is n (n: a positive integer) times as high as the frequency of the signal generated by said second means; and b) frequency dividing means for frequency dividing the output of said oscillator into 1/n.

10. An apparatus according to claim 9, wherein the frequency of the signal generated by said oscillator is integer times as high as said frequency fH and is n times as high as the frequency of the signal generated by said second means.

11. An apparatus according to claim 9, wherein said oscillator is arranged to generate a signal of a frequency which is integer-times as high as said frequency fv and is n times as high as the frequency of the signal generated by said second means.

12. An apparatus according to claim 7, wherein said control means includes: a) phase comparison means for comparing the phases of the signal generated by said second and third means with each other; b) a low-pass filter arranged to cut the high frequency band component of the output of said phase comparison means; and c) means for applying the output of said low-pass filter to said first means.

13. An apparatus according to claim 7, wherein said apparatus is arranged to produce driving pulses for driving an image sensor; and the signal generated by said third means is produced as said driving pulses.

14. An apparatus according to claim 7, wherein said second means is arranged to generate a signal of a frequency which is integer times as high as 13.375 MHz.

15. A signal generating apparatus for generating a signal determined by a relation 1135 1 fsc fH+ fv 4 2 which obtains among a color subcarrier frequency fsc, a horizontal synchronizing frequency fH and a field frequency fvt comprising: a) first means for generating a signal of a frequency of 13.375xn MHz (n: an integer), said signal being arranged to have a variable frequency; b) second means for frequency dividing the frequency of said signal generated by said first means by 451 x n; c) third means for generating a signal of a frequency which is equal to the frequency of the signal generated by said second means; and d) control means arranged to phase compare the signals generated by said second and third means and to control the frequency of the signal generated by said first means in such a manner as to have the phases of signals of said second and third means coincide with each other.

16. An apparatus according to claim 15, further comprising: e) means for generating the frequency fsc from the output of said third means.

17. An apparatus according to claim 16, wherein said third means includes: a) an oscillator arranged to generate a signal of a frequency which is n (n: a positive integer) times as high as the frequency of the signal generated by said second means; and b) frequency dividing means for frequency dividing the output of said oscillator into 1/n.

18. An apparatus according to claim 17, wherein said oscillator is arranged to generate a signal of a frequency which is integer times as high as the frequency fsc and is I times as high as the frequency of the signal generated by said second means.

19. An apparatus according to claim 17, wherein said control means includes: a) phase comparison means for comparing the phases of the signal generated by said second and third means with each other; b) a low-pass filter arranged to cut the high frequency band component of the output of said phase comparison means; and c) means tor applying the output of said low-pass filter to said first means.

20. An apparatus according to claim 17, wherein said apparatus is arranged to produce driving pulses for driving an image sensor; and said driving pulses are produced on the basis of the output of said first means.

21. A method of synchronizing two oscillators providing synchronizing signals for a PAL system, in which method a first oscillator is arranged to generate a signal of a frequency (13.375xn) times as high as a reference frequency, where n is an integer, a second oscillator is arranged to generate a signal of a frequency 2m times as high as a color subcarrierfrequency,where mis an integer, the outputs of the first and second oscillators are respectively divided by 1/(451 x n) and 1/(299 x m), the divided frequency outputs are compared in a phase comparator and the result of the phase comparison is used to control the frequency of oscillation of one of the oscillators.

22. Synchronizing signal generating apparatus for a PAL system substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.

23. A method of synchronizing two oscillators providing synchronizing signals for a PAL system, which method is substantially as hereinbefore described.