Horizontal scanning rate correction apparatus for beam index color cathode-ray tube

Horizontal scanning rate correction apparatus is provided for beam index color cathode-ray tubes of the type having a screen, an electron gun for projecting an electron beam upon the screen, a plurality of index elements positioned to be struck by the electron beam as it scans across the screen, a deflection device for causing the electron beam to repeatedly scan across the screen in a vertical succession of horizontal lines, and an index signal processing circuit for producing an index signal having a frequency determined by the frequency of the incidence of the electron beam upon the index elements as it scans horizontal lines and for controlling color switching circuitry which determines which of a plurality of color signals modulates the intensity of the electron beam. The horizontal scanning rate correction apparatus includes a memory circuit for storing correction values derived from the index signal processing circuit during horizontal scanning by the electron beam and representing the deviation of the horizontal scanning rate from a desired scanning rate at each of a plurality of horizontal sampling positions along a horizontal line. The apparatus further includes circuitry for reading the correction values from the memory circuit and for producing a corresponding signal which is supplied to the electron beam deflection device to substantially cancel deviations in the horizontal scanning rate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to apparatus for controlling the horizontal scanning 
rate in beam index color cathode-ray tubes. 
2. Description of the Prior Art 
Beam index color television receivers are well known in the prior art. They 
usually include a cathode-ray, 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 
strips. 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 apparatus which determines which of the three primary color 
signals modulates the intensity of the electron beam, so that the 
intensity of the electron beam is modulated by the primary color signal 
whose corresponding phosphor is being scanned by the beam at that moment. 
In beam index color television receivers it is important to prevent 
deviations in the horizontal scanning rate, because such deviations not 
only distort the shape of images projected upon the picture screen, as 
they do in other television receivers, but they also cause color 
misregistration. This misregistration results from the fact that the color 
switching apparatus which controls when each of the primary color signals 
modulates the electron beam does not respond instantaneously to the 
incidence of the electron beam upon the index stripes, but rather responds 
to the index signal with a delay that varies as a result of changes in the 
horizontal scanning rate of the beam and thus is difficult to compensate 
for. 
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 inserted between the photodetector 
which detects the index signal and the color switching apparatus. 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. The PLL circuit provides an input to the color switching 
apparatus which is more uniform in amplitude and frequency than the index 
signal. In addition, by insertion of a dividing circuit 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 picture 
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, 
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 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. In the prior art, various 
horizontal scanning rate correction methods have been used, but 
unfortunately none of them has been able to limit such fluctuation to the 
desired level of less than several tenths of one percent. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide horizontal 
scanning rate correction apparatus for beam index color cathode-ray tubes 
that avoids the above-described defects inherent in the prior art. 
Another object of the invention is to provide horizontal scanning rate 
correction apparatus for beam index color cathode ray tubes by which 
deviations in the horizontal scanning rate from a desired value can be 
substantially eliminated. 
A further object of the invention is to provide horizontal scanning rate 
correction apparatus which is simple in construction. 
Yet another object of the invention is to provide horizontal scanning rate 
correction apparatus which corrects for deviations in the horizontal 
scanning rate experienced by the individual cathode-ray tube in which such 
correction apparatus is used. 
A still further object of this invention is to provide a horizontal 
scanning rate correction apparatus for use in beam index color cathode-ray 
tubes of the type including a screen, means for projecting an electron 
beam upon that screen, a plurality of index elements positioned to be 
struck by the electron beam as it scans across the screen, a beam 
deflection device supplied with at least horizontal and vertical beam 
deflection signals for causing the electron beam to repeatedly scan across 
the screen in a vertical succession of horizontal lines, and an index 
signal processing circuit for producing an index signal of a frequency 
determined by the frequency of the incidence of the electron beam upon the 
index elements as it scans across the horizontal lines, and for 
controlling color switching circuitry which determines which of a 
plurality of color signals modulates the intensity or density of the 
electron beam. 
In accordance with an aspect of this invention, a horizontal scanning rate 
correction apparatus for a beam index color cathode-ray tube, as 
aforesaid, includes memory means for storing correction values which 
represent the deviation of the horizontal scanning rate of the electron 
beam from a desired scanning rate at each of a plurality of selected 
horizontal positions along at least one of the horizontal lines scanned 
across the screen by the electron beam, as determined by the horizontal 
and vertical beam deflection signals, reading means for reading the 
correction values out from the memory means and for producing a deflection 
correction signal in accordance therewith, and means for supplying the 
deflection correction signal to the beam deflection device so that the 
deviation of the horizontal scanning rate is substantially cancelled. 
According to another aspect of the invention, the index signal processing 
circuit includes a phase-locked loop which has a phase comparator 
receiving the index signal as one input, a voltage-controlled oscillator, 
means connected to the output of the phase comparator for supplying a 
control voltage to the voltage-controlled oscillator, and means for 
supplying to a second input of the phase comparator a second input signal 
having its frequency controlled by the output of the voltage-controlled 
oscillator. According to this aspect of the invention, the correction 
values are derived from the control voltage supplied to the 
voltage-controlled oscillator. 
According to yet another aspect of the invention, the beam deflection 
device used in the beam index color cathode ray tube includes horizontal 
and vertical deflection coils for receiving the horizontal and vertical 
beam deflection signals, respectively, and a separate correction coil 
placed on a yoke separate from the horizontal and vertical deflection 
coils for receiving the deflection correction signal. 
The above, and other objects, features, and advantages of the invention, 
will be apparent in the following detailed description of illustrative 
embodiments of the invention which is to be read in connection with the 
accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring first to FIG. 1, it will be seen that a beam index color 
cathode-ray or picture tube 10 to which this invention may be applied is 
provided with an electron gun 11 that emits a single electron beam, the 
intensity or density of which is modulated by a signal applied to a grid 
electron 12 forming part of electron gun 11. Picture tube 10 also includes 
a phosphor screen 13 composed of a repeating pattern of red, green and 
blue primary color phosphor stripes R, G, B (FIG. 2) extending vertically 
upon the inner surface of the glass face-plate or panel 14 of picture tube 
10. A thin metallic layer 15, for example, of vacuum deposited aluminum, 
covers the inner surface of screen 13 and is transparent to electrons of 
the electron beam while being effective to reflect toward the viewer the 
light emitted by the color phosphor stripes. Picture tube 10 also has a 
plurality of vertical index phosphor stripes I spaced across layer 15 on 
the inside surface of phosphor screen 13 in a known relationship to the 
spacing of color phosphor stripes R, G and B. A beam deflection device 16 
is supplied with horizontal and vertical beam deflection signals for 
causing the electron beam emitted by electron gun 11 to repeatedly scan 
across screen 13 in a vertical succession of horizontal lines. 
An index signal processing circuit is associated with tube 10 and shown to 
be comprised of a photodetector 20, a band-pass filter 21 and a 
phase-locked loop (PLL) circuit 22. Such PLL circuit 22 includes a 
phase-comparator 23, a low-pass filter 24, a voltage-controlled oscillator 
25, and a frequency divider 26. The index signal processing circuit is 
used for controlling color switching circuitry comprised of a mode set 
pulse generator 30, a gate pulse generator 31, and a gate circuit 32, 
which determines when each of a plurality of color signals E.sub.R, 
E.sub.G or E.sub.B is supplied to grid 12 so as to modulate the intensity 
or density of the electron beam. 
When the electron beam emitted by electron gun 11 horizontally scans screen 
13, photodetector 20 provided at the funnel-shaped portion of picture tube 
10 generates an index signal in response to the light emitted each time 
that an index stripe I is struck by the electron beam. The output signal 
from photodetector 20 is applied to bandpass filter 21 for the purpose of 
removing from the index signal certain signal components, such as, those 
generated during the flyback period, which have frequencies different than 
the frequency with which the electron beam strikes index stripes I during 
horizontal scanning intervals. The resulting index signal produced at the 
output of band-pass filter 21 has a frequency determined by the distance 
between index phosphor stripes I and the scanning speed of the electron 
beam. The index stripes I are spaced across the screen 13 so that the 
basic distance between index stripes I remains constant along a given 
horizontal line scanned by the electron beam and thus so that the 
frequency of the index signal varies in proportion to the horizontal 
scanning rate of the beam. The index signal from the output of bandpass 
filter 21 is applied to one input of phase comparator 23 in PLL circuit 
22. The variable output voltage of phase comparator 23 is applied, through 
low-pass filter 24 which removes unwanted noise therefrom, to the control 
input of voltage-controlled oscillator 25 which has a control frequency N 
times the normal frequency of the index signal. The output from 
voltage-controlled oscillator 25 is supplied to the input of frequency 
divider 26 which divides the frequency of the output from 
voltage-controlled oscillator 25 by N, where N is an integral value 
representing the number of color phosphor stripes R, G, B between adjacent 
index phosphor stripes I. The output of frequency divider 26 is supplied 
to a second input of phase comparator 23 for phase comparison with the 
index signal derived from the output of bandpass filter 21. 
As a result of the foregoing arrangement of PLL circuit 22, the output of 
the voltage-controlled oscillator 25 will vary in frequency until the two 
input signals supplied to phase comparator 23 are of the same frequency. 
As a result, the output of voltge-controlled oscillator 25 will have a 
frequency which is N times as great as the frequency of the index signal 
and three times as great as the so-called triplet frequency at which the 
repeating pattern of red, green and blue color phosphor stripes R, G, B 
are scanned by the electron beam, so that one pulse is generated by 
voltage-controlled oscillator 25 for each of the color phosphor stripes 
being scanned. 
The frequency generated by voltage-controlled oscillator 25 varies in 
proportion to the control voltage which is supplied to its input by 
low-pass filter 24. Each time the frequency of the index signal supplied 
to the input of phase comparator 23 is varied, PLL circuit 22 tends toward 
a new equilibrium at which the frequency of the two inputs to phase 
comparator 23 is equal but at which the phase of the two inputs differs as 
a function of the frequency of the index signal. At each disturbance of 
such equilibrium, the resulting change in the phase difference between the 
signal supplied by the frequency divider 26 and the index signal supplied 
by bandpass filter 21 causes phase comparator 23 to generate an output 
voltage which, when applied through low-pass filter 24, causes 
voltage-controlled oscillator 25 to suitably vary its output frequency and 
phase for restoring the equilibrium at comparator 23. Thus, it can be seen 
that the voltage supplied by phase comparator 23 through low pass filter 
24 to the input of voltage-controlled oscillator 25 varies with changes in 
the frequency of the index signal. 
The output of PLL circuit 22 is applied to gate pulse generator 31, which, 
for example, may include a ring counter (not shown). Gate pulse generator 
31 responds to each output pulse from voltage-controlled oscillator 25 by 
producing either a red, a green, or a blue gating pulse, in response to 
the count of its ring counter. The resulting repeated sequence of red, 
green and blue gating pulses are 120 out of phase from each other and are 
supplied to respective control inputs of gate circuit 32. In response to 
these repeated sequences of gating pulses, suitable gates (not shown) in 
gate circuit 32 sequentially select either a red, a green or a blue 
primary color signal, E.sub.R, E.sub.G, and E.sub.B, respectively, and 
supply it through the R contact of mode switch circuit 33 to the input of 
drive circuit 34 which, in turn, provides the selected color signal to 
grid 12 so that it can modulate the intensity or density of the electron 
beam projected upon screen 13. 
The index signal from bandpass filter 21 is also applied to mode set pulse 
generator 30 which is used to determine the phase relationship between the 
red, green and blue gating pulses from generator 31, and the scanning of 
the three primary color phosphors R, G, B. Such mode set pulse generator 
is provided where the phase relationship between the index signal and the 
color phosphor stripes R, G, B is not constant, for example in cathode-ray 
tubes in which the index stripes I are usually separated by only two color 
phosphor stripes, as is shown in FIG. 2, rather than by a full set of all 
three color phosphor stripes. Mode set pulse generators are known in the 
prior art, and they operate, for example, by determining the position of 
the electron beam upon screen 13 by counting the pulses of the index 
signal. 
When mode set pulse generator 30 determines that a specified phase 
relationship exists between the index signal and the scanning of color 
phosphors by the beam, it generates a mode set pulse which is applied to 
gate pulse generator 31. The mode set pulse causes the ring counter within 
gate pulse generator 31 to be set so that the gating pulses thereafter 
generated are in phase with the colors of the phosphors then being 
scanned. 
There are inevitable delays between the time that a particular index stripe 
I is struck by the electron beam and the time that the color signal, 
E.sub.R, E.sub.G or E.sub.B, associated with a particular primary color is 
supplied to grid 12 in response to the resulting pulse of the index 
signal. For example, there are time delays introduced by the operation of 
bandpass filter 21 and PLL circuit 22. In order to maintain proper color 
registration of the image produced upon screen 13 it is necessary that 
such delays be compensated for so that the operation of gate circuit 32 
can be accurately synchronized with the actual scanning position of the 
electron beam, and so that primary color signals E.sub.R, E.sub.G, and 
E.sub.B modulate the electron beam as that beam scans the corresponding 
color phosphor stripes R, G, and B, respectively. If such time delays are 
constant they can easily be compensated for by properly choosing the 
timing parameters of the components in the feedback loop constituted by 
photodetector 20, bandpass filter 21, PLL circuit 22, gate pulse generator 
31, gate circuit 32, drive circuit 34 and picture tube 10. Unfortunately, 
the delay associated with that feedback loop, particularly the delay 
associated with the phase difference between the two inputs of phase 
comparator 23, varies as a function of the frequency of the index signal. 
For this reason, deviations in the horizontal scanning rate of the 
electron beam upon screen 13 make it difficult to maintain proper color 
registration. 
In accordance with the present invention, apparatus is provided for 
substantially eliminating the deviations in the horizontal scanning rate 
which tend to cause color misregistration in beam index color cathode-ray 
tubes such as the one described above. Such apparatus includes a memory 40 
for storing correction values which represent the deviation of the 
horizontal scanning rate from a desired scanning rate at each of a 
plurality of horizontal sampling positions along at least one sampled 
horizontal scanning line. The correction values are obtained from the 
index signal processing circuit, comprised of photodetector 20, bandpass 
filter 21 and PLL circuit 22, when the electron beam is made to scan the 
sampled horizontal line in accordance with the horizontal and vertical 
beam deflection signals. The apparatus also includes a reading means, 
comprised of a digital-to-analog converter 50, a drive amplifier 51, and 
an adjustably fixed voltage source 52, for reading correction values from 
memory 40 and for producing a deflection correction signal in accordance 
with the correction values that are read. The apparatus further includes 
means, for example, constituted by a wire 53 connected to the output of 
drive amplifier 51, for supplying the deflection correction signal to the 
beam deflection device 16 so that deviation of the horizontal scanning 
rate is substantially cancelled. In the embodiment of the invention shown 
in FIG. 1, the apparatus according to the invention also includes writing 
means, for example, comprised of a low-pass filter 60 and an 
analog-to-digital converter 61, for obtaining correction values from the 
index signal processing circuit and for writing those correction values in 
memory 40. 
In the embodiment of the invention shown in FIG. 1, low-pass filter 60 has 
its input connected to the output of low-pass filter 24 so as to receive 
the control voltage supplied from low-pass filter 24 to voltage-controlled 
oscillator 25. The output of low-pass filter 60 is connected to the input 
of analog-to-digital converter 61, and the digital output of 
analog-to-digital converter 61 is supplied to the data input of memory 40, 
which may be comprised of random-access memory capable of writing, 
storing, and reading a plurality of digital values each at a respective 
memory address. The data output of memory 40 is connected to the input of 
digital-to-analog converter 50, and the output of digital-to-analog 
converter 50 is connected to drive amplifier 51, which may be a 
differential amplifier having a second input connected to adjustably fixed 
voltage source 52. The output of drive amplifier 51 is supplied to a 
deflection correction device 70, which constitutes part of the beam 
deflection device 16, and which may be desirably formed, as shown in FIG. 
5, by a pair of ferrite cores 71 and 72 of a semi-annular shape which are 
placed in horizontally opposing positions from each other around the usual 
fourth grid of the electron gun in the neck portion of the picture tube 
10. A correction coil 73 having two ends 74 and 75 is wrapped around the 
cores 71 and 72, and the deflection correction signal is supplied between 
ends 74 and 75 so that a magnetic field will be induced in the vertical 
direction, as indicated by the arrows on FIG. 5, to horizontally deflect 
the electron beam. 
The horizontal correction apparatus is controlled by a control circuit 80 
which receives, as its inputs, suitably separated or generated vertical 
and horizontal video synchronization signals, P.sub.V and P.sub.H, 
respectively. Control circuit 80 is connected to analog-to-digital 
converter 61 and digital-to-analog converter 50 so as to supply clocking 
signals to those converters for determining the times at which they make 
their respective conversions. Control circuit 80 is also connected to 
memory 40 so as to apply thereto WRITE, READ and address signals for 
controlling whether values from converter 61 are written in memory 40 or 
whether values are read from memory 40 and supplied to converter 50. The 
address signals determine at which, or from which, memory address such 
digital values are written or read. In addition, control circuit 80 is 
connected to mode switch 33, which has two fixed contacts R and W 
selectively engaged by a movable contact under the influence of control 
circuit 80 for electrically connecting either of the two fixed contacts to 
the input of drive circuit 34. Fixed contact R is connected to the output 
of gate circuit 32, and fixed contact W is connected to a means for 
supplying a constant signal, such as, a variably fixed voltage source 90. 
Although mode switch 33 is shown in FIG. 1 as a mechanical switch, it will 
be obvious to those skilled in the art that a solid-state switching device 
could be used in its place. 
In operation, the apparatus of FIG. 1 functions in one or the other of two 
modes, that is, a WRITE mode, in which correction values are derived from 
the control voltage supplied to voltage-controlled oscillator 25 and are 
recorded in memory 40, and a READ mode, in which correction values are 
read from memory 40, and in which a deflection correction signal is 
produced in accordance with the read correction values and is supplied to 
deflection correction device 70, so that deviations in the horizontal 
scanning rate are substantially cancelled. Mode switch 33 is effective to 
change over the apparatus of FIG. 1 between the WRITE and the READ modes, 
and, as in the embodiment of the invention being described with reference 
to FIG. 1, mode switch 33 may be operated automatically by control circuit 
80, so that, whenever the television receiver including the circuitry of 
FIG. 1 is initially turned on, the apparatus will be set to operate in the 
WRITE mode until all of the correction values have been written, and then 
control circuit 80 causes the apparatus to be changed over to the READ 
mode in which the apparatus functions to reproduce video signals on screen 
13. 
When mode switch 33 is positioned to establish the WRITE mode by the 
operation of control circuit 80, the input of drive circuit 34 is 
connected to fixed contact W. As a result, fixed voltage source 90 is 
connected through drive circuit 34 to grid 12 so as to make the current in 
the electron beam projected upon screen 13 constant. During the WRITE mode 
this constant intensity electron beam is caused to scan across the screen 
13 under the influence of the horizontal and vertical beam deflection 
signals supplied to horizontal and vertical deflection coils 92, which 
constitute parts of beam deflection device 16 separate from deflection 
correction device 70. As a result, an index signal is detected by 
photodetector 20, filtered by bandpass filter 21 and supplied to PLL 
circuit 22. This index signal has a frequency that varies in response to 
any changes in the scanning speed of the electron beam across screen 13 
under the influence of horizontal and vertical deflection coils 92. 
In response to this index signal, the PLL circuit 22 produces a control 
voltage at the output of low-pass filter 24 which varies in proportion to 
changes in the frequency of the index signal and, thus, in proportion to 
changes in the horizontal scanning rate. This control voltage is supplied 
through low-pass filter 60 to the input of analog-to-digital converter 61. 
Control circuit 80 supplies clock pulses to analog-to-digital converter 61 
and address and WRITE signals to memory 40 in synchronism with the 
vertical and horizontal synchronizing signals P.sub.V and P.sub.H, so that 
the control voltage filtered by low-pass filter 60 is converted into a 
digital value by analog-to-digital converter 61 during the scanning by the 
electron beam of each of a plurality of horizontal sampling positions 
along a sampled horizontal line and recorded in memory 40 at an address 
corresponding to each of those horizontal sampling positions. 
The function of low-pass filter 60 is to remove from the control voltage 
supplied to analog-to-digital converter 61 those variations that have a 
high frequency relative to the frequency at which converter 61 samples the 
control voltage and converts it into digital values, so that the resulting 
correction values stored in memory 40 represent the average value of the 
control voltage at the time that each such sampling is made. 
In the embodiment of the invention shown in FIG. 1, memory 40 is comprised 
of random-access memory (RAM). Random-access memory normally loses the 
values stored in it when its power is shut off. For this reason, control 
circuit 80 causes the apparatus of FIG. 1 to be briefly set to its WRITE 
mode each time it is turned on, so that correction values will be stored 
in memory 40 before the subsequent operation of the apparatus in the READ 
mode. 
After a complete set of correction values has been stored in memory 40, 
control circuit 80 causes the apparatus of FIG. 1 to be changed over to 
the READ mode in which mode switch 33 connects the input of the drive 
circuit 34 through contact R to the output of the gate circuit 32. Thus, 
in the READ mode, color signals E.sub.R, E.sub.G, and E.sub.B are 
sequentially supplied through drive circuit 34 to grid 12 in the manner 
described above, so that color video images are reproduced upon screen 13. 
In addition, in the READ mode, control circuit 80 causes the correction 
values stored at various addresses in memory 40 to be read and applied to 
digital-to-analog converter 50 for conversion to a corresponding analog 
voltage. This analog voltage or signal is then applied through drive 
amplifier 51 to deflection correction device 70 to cancel unwanted 
deviations in the horizontal scanning rate. Control circuit 80 receives 
the vertical and horizontal synchronizing signals P.sub.V and P.sub.H, so 
that the address and READ signals which it supplies to memory 40 and the 
clock signals which it supplies to digital-to-analog converter 50 are 
properly synchronized with the scanning motion of the electron beam and so 
that the deflection correction signal supplied by drive amplifier 51 to 
correction coil 73 is derived from a correction value stored at an address 
in memory 40 corresponding to the current scanning position of the 
electron beam and representing the previously determined deviation of the 
horizontal scanning rate at that scanning position. 
It will be appreciated from the foregoing that the apparatus shown in FIG. 
1 is designed so that the deflection correction signal supplied to coil 73 
generates a magnetic field sufficient to substantially cancel unwanted 
deviations in the horizontal scanning rate. The correction values stored 
in memory 40 vary in proportion to the frequency of the index signal at 
various horizontal sampling positions when the apparatus was last operated 
in the WRITE mode. When the apparatus is operated in the READ mode, 
correction values are read from memory 40, converted to proportionally 
corresponding analog voltages, and supplied to the input of drive 
amplifier 51. Since drive amplifier 51 is a differential amplifier which 
has a second input voltage supplied to it from adjustably fixed voltage 
source 52, which voltage is adjusted to equal the output of 
digital-to-analog converter 50 that would result upon the reading of a 
correction value from memory 40 representing the desired horizontal 
scanning rate, the voltage of the deflection correction signal produced by 
drive amplifier 51 varies in proportion to the horizontal scanning rate 
error, that is, in proportion to the difference between a desired 
horizontal scanning rate and the horizontal scanning rate at the time that 
the correction value currently being read was recorded. 
The voltage of the deflection correction signal is integrated by the 
inductance of coil 73 to produce a current in that coil and a resulting 
magnetic field which are both proportional to the integral of the 
horizontal scanning rate error. The horizontal scanning rate of the 
electron beam is altered by an amount substantially proportional to the 
rate of change of the magnetic field created by the flow of the current in 
coil 73. As a result, the horizontal scanning rate is altered by an amount 
substantially proportional to the differential of the integral of the 
horizontal scanning rate error, that is, by an amount substantially 
proportional to the horizontal scanning rate error itself. Thus, by 
connecting the deflection correction signal to coil 73 with the proper 
polarity, it is possible to substantially cancel the horizontal scanning 
rate error of picture tube 10. 
As discussed above, the apparatus shown in FIG. 1 has a WRITE mode in which 
new correction values can easily be recorded in memory 40, for example, 
when the apparatus is first turned on. In FIG. 3, in which components 
similar to those of FIG. 1 are similarly labeled and identified by the 
same reference numerals, an embodiment of the present invention is 
disclosed which is identical to that of FIG. 1 except for the fact that it 
does not provide means for operation in a WRITE mode. In the apparatus of 
FIG. 3, memory 40a is comprised of read-only memory (ROM) and correction 
values are written in that memory 40a by means of circuitry external to 
the apparatus, for example, circuitry at the factory where the apparatus 
is manufactured, or at a repair shop where such apparatus might be 
serviced. Although the apparatus of FIG. 3 has the disadvantage of not 
being able to have its correction values renewed as frequently, or as 
easily, as the apparatus shown in FIG. 1, it has the offsetting advantage 
of not requiring the writing means required by the apparatus of FIG. 1, 
such as is constituted by mode switch 33, low-pass filter 60, 
analog-to-digital converter 61, and that portion of control circuit 80 
which relates to the writing of correction values. 
The apparatus disclosed in FIGS. 1 and 3 will have enough information to 
accurately compensate for deviations in the horizontal scanning speed of a 
given horizontal line if the control voltage E.sub.CV, shown graphically 
at the bottom of FIG. 6, supplied to voltage-controlled oscillator 25 is 
converted by analog-to-digital converter 61 and recorded in memory 40 for 
each of 32 sampling positions indicated at P.sub.0, P.sub.1, P.sub.2 . . . 
P.sub.30 and P.sub.31 on FIG. 6, along that given horizontal line. In 
order to achieve the most accurate correction of deviations in the 
horizontal scanning rate, it would be desirable to record correction 
values for each of such 32 sampling positions along each of the over 200 
horizontal lines within the effective picture or image area 100 of a video 
field. Unfortunately, this would require memory 40 to have a very large 
storage capacity, which would result in an undesirable increase in cost. 
To avoid such difficulties, it is possible to suitably program control 
circuit 80 to cause the storage of 32 correction values, one for each of 
32 horizontal sampling positions, P.sub.0 -P.sub.31, on each of 16 
predetermined horizontal sampling lines, L.sub.0, L.sub.1, L.sub.2 . . . 
L.sub.14, and L.sub.15. These 16 sampling lines, L.sub.0 -L.sub.15, could 
be spaced at every 16th horizontal line throughout a given video field so 
that, as shown in FIG. 6, 14 of the sampling lines would lie within the 
effective picture portion 100 of a video field, and two lines L.sub.0 and 
L.sub.15, would lie in portions of the raster scan occurring, 
respectively, before and after the effective picture portion 100, that is, 
above and below the image area. The storage of correction values for only 
one in each 16 horizontal lines greatly reduces the capacity required of 
memory 40. 
In the case in which memory 40 has correction values stored for only the 16 
sampling lines L.sub.0 -L.sub.15, it is necessary to provide a way for 
supplying a deflection correction signal to deflection correction means 70 
during the reproduction of the 15 horizontal lines which occur between 
adjacent sampling lines. The simplest way of doing this is by having 
control circuit 80 suitably programmed to read from memory 40 the same 32 
correction values during the scanning of each of those 15 horizontal lines 
that would be read from memory 40 at corresponding time periods during the 
scanning of the immediately preceding sampling line. Although this 
technique is very simple, the correction it makes in horizontal scanning 
speed may be less than perfect, as illustrated in FIG. 7. 
The solid line 200 in FIG. 7 represents the changes in horizontal scanning 
rate that occur at a given horizontal location as a function of changes in 
vertical position within a video field. If these changes in horizontal 
scanning rate are recorded only for the predetermined sampling lines, such 
as the lines L.sub.0 -L.sub.4 shown in FIG. 7, and if the correction 
values recorded for each of these predetermined sampling lines are used as 
the correction values for each of the subsequent 15 horizontal lines, then 
the deflection correction signal supplied to deflection correction means 
70 will have a discontinuous function as shown by broken line 202 in FIG. 
7. The resulting discontinuities in the deflection correction signal may 
give rise to the appearance of horizontal bands in the image produced by 
picture tube 10. To avoid these problems, it is desirable to suitably 
program control circuit 80 so that, when one of the 15 lines between 
adjacent sampling lines is being reproduced, the corresponding correction 
value from the sampling line immediately preceding, and the correction 
value from the sampling line immediately after, the currently scanned 
horizontal line will be read from memory 40 and the applied correction 
deflection signal will be determined by interpolating between these two 
correction values on the basis of the relative vertical distance of the 
currently scanned horizontal line from the immediately preceding, and 
immediately subsequent sampling lines. If such interpolation is linear, 
the correction deflection signal will vary as a function of vertical 
position in the manner shown by the chain line 204 of FIG. 7. Thus, by use 
of such interpolation it is possible to produce an apparatus which can 
effect substantially accurate correction of deviations in the horizontal 
scanning rate throughout all of the video field without requiring a huge 
storage capacity in memory 40. 
In cathode-ray tubes of the type in which the horizontal scanning rate 
along all horizontal lines is substantially similar except for a factor 
associated with the vertical position of each horizontal line, a single 
horizontal line, for example, at the center of the video field, may be 
sampled and have its corresponding correction values stored in memory 40. 
In such a case, apparatus such as that shown in FIG. 4 can be used to 
produce a deflection correction signal. This apparatus reads each of the 
correction values corresponding to the sampled line from memory 40 during 
the scanning of each horizontal line and converts those correction values 
into a corresponding analog voltage in digital-to-analog converter 50. 
This analog voltage is supplied to drive amplifier 51 which produces a 
deflection correction signal that varies in proportion to the horizontal 
scanning rate error of the sampled line. The deflection correction signal 
is supplied to the input of a means for multiplying it, in the form of a 
gain control amplifier 94. A gain control signal is suitably applied to 
gain control amplifier 94 to multiply the deflection correction signal by 
a coefficient that varies as a function of the vertical position of the 
horizontal line currently being scanned to compensate the deflection 
correction signal for changes in the horizontal scanning rate associated 
with changes in that vertical position. For example, the gain control 
signal supplied to amplifier 94 could be one which varies parabolically 
with the vertical position of the horizontal scanning line, such as the 
signal E.sub.vert shown in FIG. 4. The signal from amplifier 94 is then 
applied to deflection correction coil 73 to correct for deviations in 
horizontal scanning rate along each of the horizontal lines of the video 
signal reproduced upon screen 13. 
By way of summary, it will be appreciated that, in the above described 
apparatus in accordance with this invention, correction values derived 
from the index signal processing circuit used in conjunction with a beam 
index color cathode-ray tube and representing the deviation of the 
horizontal scanning rate of an electron beam across the screen of that 
cathode ray tube are stored and subsequently read out from a memory to 
correct for those deviations when a video image is reproduced. Such 
apparatus not only compensates for deviations in the horizontal scanning 
rate which can be predicted from the design of the cathode-ray tube, but 
it also compensates for deviations in the horizontal scanning rate which 
may result from the unpredictable irregularity of circuit components 
within a given individual cathode-ray tube with which it is used. As a 
result, the horizontal scanning rate of such cathode-ray tubes can be kept 
substantially constant, preventing distortion of the shapes of the images 
reproduced and preventing color misregistration. 
The substantially constant frequency of the index signal made possible by 
this invention enables PLL circuit 22 to maintain synchronism with the 
index signal, even when that signal is weak, for example, due to a low 
beam current associated with the reproduction of a dark image area upon 
screen of the picture tube. As a result, this invention makes it possible 
to lower the minimum electron beam current while still maintaining 
synchronization of the PLL circuit, allowing low luminance portions of a 
video signal to be darker when reproduced, and, thus, improving the 
contrast of the resulting picture tube image. 
It will be apparent that with appropriate changes in the drive amplifier 51 
the deflection correction means used with this invention could be of an 
electrostatic type. Furthermore, it will be apparent that the deflection 
correction means could make use of the main horizontal deflection coil 92 
of the cathode ray tube for achieving its desired effect. For example, a 
saturable reactor may be employed which has the deflection correction 
signal supplied to its primary winding and which has its secondary winding 
connected in series with the horizontal deflection winding of the cathode 
ray tube so that the deflection correction signal can be used to vary the 
magnitude of the horizontal beam deflection signal which would normally be 
applied to the horizontal deflection winding. 
Furthermore, it will be apparent that the horizontal scanning rate 
correction apparatus according to this invention can be used with 
cathode-ray tubes in video apparatus other than television receivers, such 
as, for example, in color computer terminals. 
Having described specific preferred embodiments of the invention with 
reference to the accompanying drawings, it is to be understood that the 
invention is not limited to those precise embodiments, and that various 
changes and modifications may be effected therein by one skilled in the 
art without departing from the scope or spirit of the invention as defined 
in the appended claims.