1. Field of the Invention
This invention relates to apparatus for controlling the horizontal scanning rate in cathode-ray tubes, and more particularly is directed to such an apparatus which is adapted for use with a beam index color cathode ray tube.
2. Description of the Prior Art
In cathode-ray tubes in which an electron beam is caused to repeatedly scan across the screen in a vertical succession of horizontal lines, it is important to control the rate at which the electron beam travels across each horizontal line. It is common for the picture information within a video signal to be timed in such a manner that it will be projected with the proper shape upon the screen of a cathode-ray tube only if the electron beam travels across each fraction of a horizontal line at a specified rate. Deviations of the horizontal scanning rate from such a specified rate cause distortions in the shape of the projected image and are thus undesirable.
Deviations in the horizontal scanning rate are particularly undesirable in beam index color television receivers because such deviations may also cause color misregistration in such receivers.
Beam index color television receivers are well known in the prior art, and usually include a cathode-ray tube, or picture tube having an electron gun which emits a single electron beam and a phosphor screen having a repeating pattern of red, green and blue primary color phosphor stripes extending vertically upon the screen. The beam index picture tube also has a plurality of vertical index phosphor stripes spaced across its phosphor screen in a known relationship to the spacing of the color phosphor stripes. When the electron beam horizontally scans the screen, a photodetector generates an index signal in response to the light emitted each time an index stripe is struck by the electron beam. This index signal is used to achieve color registration by controlling the color switching circuit which determines when the three primary color signals respectively modulate the intensity of the electron beam so that, at any moment, the intensity of the electron beam is modulated by the primary color signal whose corresponding color phosphor stripe is then being scanned by the beam.
In such beam index color television receivers deviations in the horizontal scanning rate cause color misregistation because there is an inherent delay in the response of the color switching circuit to changes in the horizontal scanning rate of the beam and it is difficult to compensate therefor. This delay results from the fact that it is common for index signal processing circuitry, for example, comprised of a bandpass filter and a phase-locked loop (PLL) circuit, to be interposed between a photodetector which detects the index signal and the color switching circuit. The bandpass filter removes unwanted noise from the index signal in preparation for the application of that signal to the input of the PLL circuit which provides an input to the color switching circuit of greater uniformity, in amplitude and frequency, than the index signal. In addition, by insertion of a frequency divider in the feedback loop of the PLL circuit, the latter can be made to produce an output frequency which is a predetermined multiple of the frequency of the index signal. The last feature is important since in most beam index color cathode ray tubes the number of color phosphor stripes is not equal to, but instead is an integral multiple of, the number of index stripes.
Unfortunately, the delay inherently associated with the above-described index signal processing circuitry, particularly with the PLL circuit, varies as a function of the frequency of the index signal, which in turn varies in proportion to the horizontal scanning rate. For this reason, a variation in the scanning rate is not immediately or precisely reflected in a corresponding change in the rate of color switching so that deviations in the horizontal scanning rate adversely affect color registration.
Deviations in the horizontal scanning rate are also disadvantageous in beam index color television receivers because they make it more difficult for the PLL circuit to correctly follow and lock onto the instantaneous frequency of the index signal, as is necessary for proper color registration. In order to cause the PLL circuit to properly follow the frequency of an index signal when the horizontal scanning rate deviates, it is necessary to increase the minimal signal strength of the index signal. This requires that the minimal intensity of the electron beam be increased, which, in turn, has the undesirable result of increasing the luminance of the darkest areas that can be projected on the picture tube, and, thus, of decreasing the contrast of the produced image.
For all of the above reasons, it is desirable to limit the maximum fluctuations in the horizontal scanning rate of beam index color television receivers to less than several tenths of one percent. The prior art contains various proposed schemes for correction of the horizontal scanning rate, but unfortunately none of them has been able to limit the scanning rate fluctuation to the desired level of less than several tenths of one percent.
In the copending application Ser. No. 99,911, filed Dec. 3, 1979, and assigned to the same assignee as this application, there is disclosed a horizontal scanning rate correction apparatus having a memory for storing corrections values representing deviations of the horizontal scanning rate from a desired scanning rate at a plurality of respective horizontal sampling positions along a plurality of horizontal lines, and circuitry for reading the correction values from the memory as a video signal is being displayed and for producing corresponding signals supplied to a horizontal deflection device to substantially cancel the unwanted deviations in the horizontal scanning rate. The apparatus disclosed in such copending application is designed for use with an index beam color cathode-ray tube in which a PLL circuit including a voltage-controlled oscillator receives the index signal as an input and produces an output having a frequency equal to the frequency at which color phosphors are being scanned. In such a PLL circuit the control voltage fed to the voltage-controlled oscillator is proportional to the frequency of the index signal, and it is from this control voltage that the correction values stored in the memory circuitry are derived.
The foregoing scanning rate correction apparatus can accurately cancel deviations occurring in a particular horizontal scanning line so long as the correction values accurately correspond to the deviation of the horizontal scanning rate along that line and as long as enough correction values are stored in the memory to accurately indicate variations in the scanning rate which occur along its length. For example, it has been found that, if accurate correction values are recorded for 32 sampling positions along a given horzontal line, the apparatus will be able to cancel deviations in the horizontal rate along the line sufficiently to substantially remove the above-mentioned undesirable effects of deviations in the horizontal scanning rate.
Such horizontal scanning rate correction apparatus would produce ideal results if correction values were stored for each horizontal line of the video field. Unfortunately, this would require a very large and expensive memory capacity, since there are 236 lines in the effective picture area of the video field of an NTSC video signal. For example, if 32 8-bit correction values were stored for each of 236 lines, a memory capacity of 60,416 bits would be required.
It would be possible to reduce the memory capacity required of such a horizontal scanning rate correction apparatus by storing correction values for less than the total number of horizontal lines in the effective picture portion of each field, for example, for every 16th horizontal line, and then substituting correction values from a nearby line for which values have been stored when scanning a horizontal line for which correction values have not been stored. Unfortunately, such a method of correcting for deviations in the horizontal scanning rate often produces poor results. If a group of horizontal lines at various vertical positions all have their scanning rate corrected according to correction values stored for only one of such lines, no compensation is provided for the difference in the horizontal scanning rates occurring as a result of the different vertical positions. Furthermore, a relatively large difference will exist between the scanning rate corrections made to a first group of horizontal lines corrected according to a set of correction values specific for one of those lines and that made to a next group of horizontal lines corrected according to a set of correction values specific for a line in such next group. Such large difference between the scanning rate corrections is likely to create a distorted picture in which the groups of horizontal lines corrected according to different correction values appear as horizontal bands.