Linearity correction circuit for television receiver

In a horizontal deflection output circuit adapted for correction of horizontal linearity in a television receiver or the like, and which comprises a switching transistor, a damper diode, a horizontal deflecting coil and an S correcting capacitor connected in series to the horizontal deflecting coil; the terminal voltage of the S correcting capacitor is modulated by a sawtooth signal produced by a linear distortion correcting circuit synchronously with a horizontal periodic signal. The sawtooth signal produced by the switching transistor and fed to the horizontal deflecting coil is applied to the S correcting capacitor which feeds an S correcting current to the horizontal deflecting coil, so as to correct the image distortion caused particularly by the resistance component of the horizontal deflecting coil.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a horizontal deflection output circuit for 
use in a television receiver or the like and, more particularly, is 
directed to a horizontal output circuit adapted for correction of 
horizontal linearity. 
2. Description of the Prior Art 
FIG. 1 shows a conventional horizontal output circuit employed for driving 
a horizontal deflecting coil in a cathode-ray tube (CRT) of a television 
receiver, and which includes a switching transistor T turned on and off 
synchronously with a horizontal periodic signal or drive pulse, a damper 
diode D, a resonance capacitor C.sub.R, a horizontal deflecting coil Ly, 
an S correcting capacitor C.sub.S, and a choke coil FBT normally 
constituted by a primary winding of a flyback transformer. 
As is well known, such horizontal output circuit operates so that, when the 
switching transistor T is in its on-state, a sawtooth current is allowed 
to flow in the horizontal deflecting coil Ly. On the other hand, when the 
switching transistor T is turned off, the electromagnetic energy 
accumulated in the horizontal deflecting coil Ly is transferred to the 
resonance capacitor C.sub.R due to the resonance phenomenon arising from 
such coil in combination with the capacitor C.sub.R, and the resultant 
oscillation serves to invert the current in the horizontal deflecting 
coil. Then, the inverted current is decreased gradually via the damper 
diode D while charging the S correcting capacitor C.sub.S. 
Accordingly, when the switching transistor T is turned on and off 
synchronously with the horizontal drive pulse, an alternating sawtooth 
current is made to flow in the horizontal deflecting coil Ly, and the 
resulting magnetic field causes an electron beam to scan in the horizontal 
direction in the CRT. 
However, in such conventional horizontal output circuit, two factors are 
usually present which deteriorate the linearity of the horizontal 
scanning. The first factor is the difference between the center of 
curvature of the CRT fluorescent screen and the center of deflection of 
the electron beam, and such difference results in a raster distortion by 
which the image contracts at the center of the screen and expands at the 
opposite sides thereof. 
The second factor is the deviation of the deflecting current from a 
straight line due to the series resistance existing in the output circuit 
(principally, the resistance of the deflecting coil Ly), whereby the 
deflecting current has a saturation curve with linear distortion such that 
the image expands at the left side of the screen and contracts at the 
right side of the screen. 
In order to eliminate the raster distortion resulting from the first factor 
described above, it has been customary heretofore to form the deflecting 
current, for example, as indicated by the solid line 1 in FIG. 2(a), by 
utilizing the resonance of the S correcting capacitor so as to contract 
the image at the left and right hand sides relative to the center of the 
image. Further, for elimination of the linearity distortion caused by the 
second factor described above, it has been known to connect a saturable 
reactor having the inductance characteristic shown in FIG. 2(b) in series 
with the horizontal deflecting coil Ly so that the deflecting current has 
an overall characteristic represented by the curve 2, on FIG. 2(a), 
thereby emphasizing the contraction rate at the left side of the image 
while reducing the contraction rate at the right side thereof. Thus, the 
two kinds of horizontal distortion of the image are substantially 
eliminated by the conventional techniques mentioned above. 
However, it is difficult to achieve complete correction of the linearity 
distortion by the provision of a saturable reactor (HLC), and different 
types of television receivers may require reactors with different 
characteristics leading to the further disadvantage of considerable 
variations due to the temperature characteristics, whereby it is extremely 
difficult to attain fine linearity correction. Accordingly, particularly 
with dimensional increases of the television screen, the beam scanning 
speed is not maintained constant at all horizontal positions across the 
image. In such case, when a caption is superimposed on the image by a 
projector so as to run horizontally across the displayed image, there may 
be a variation in the size of the characters running across the image, 
leading to difficulty in reading the caption. 
OBJECT AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
improved horizontal output circuit which is capable of enhancing the 
linearity while eliminating the necessity of providing a saturable 
reactor. 
In a horizontal deflection output circuit according to an aspect of the 
present invention, and in which a sawtooth signal produced by a switching 
transistor is fed to a horizontal deflecting coil; the sawtooth signal is 
applied to an S correcting capacitor which applies an S correction to the 
horizontal deflecting current, so as to correct the image distortion 
caused particularly by the resistance component of the horizontal 
deflecting coil, and the terminal voltage of the S correcting capacitor is 
modulated by a sawtooth signal provided from a linear distortion 
correcting circuit synchronously with a horizontal periodic signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 3 of the drawings, it will be seen that, in 
accordance with the general concept of the present invention, a 
conventional horizontal deflection output circuit of the type previously 
described with reference to FIG. 1 and similarly having a switching 
transistor T, a damper diode D, a resonance capacitor C.sub.R, a 
horizontal deflecting coil L.sub.y, an S correcting capacitor C.sub.S and 
a primary coil FBT of a flyback transformer, is further provided with a 
linearity distortion correcting circuit 10 which feeds a smooth sawtooth 
signal of the horizontal period to the S correcting capacitor C.sub.S. 
When the switching transistor T is turned on and off by drive pulses 
P.sub.H [FIG. 4(a)], a deflecting current i.sub.L is made to flow in the 
deflecting coil L.sub.y as indicated by the solid line in FIG. 4(d) so 
that the terminal voltage E.sub.C of the S correcting capacitor C.sub.S 
which is charged and discharged by such deflecting current i.sub.L has the 
parabolic waveform shown in FIG. 4(b). Accordingly, the current i.sub.L 
flowing in the deflecting coil L.sub.y becomes the dotted-line sawtooth 
wave shown on FIG. 4(d) which contracts at both a trace start point (left 
side of image) and a trace end point (right side of image), thereby 
correcting the raster distortion. 
However, there still remains the contraction in the right-hand image 
portion relative to the expansion in the left-hand image portion which 
results from the linearity distortion due to the aforementioned resistive 
component of the deflecting coil. In accordance with the present 
invention, a sawtooth correction signal e.sub.s [FIG. 4(c)] is fed from 
the linearity distortion correcting circuit 10 to the S correcting 
capacitor C.sub.S at a junction point between the latter and the 
deflection coil L.sub.y. As a result of the foregoing, the terminal 
voltage E.sub.C of the S correcting capacitor C.sub.S is modified, as 
represented by the dotted line on FIG. 4(b), so that the deflecting 
current i.sub.L is being fed from the S correcting capacitor C.sub.S is 
also modified to have the waveform represented by the one-dot chain line 
on FIG. 4(d), and in which the gradient is emphasized at the trace end 
point (right-hand image portion), whereby the right-hand image portion is 
expanded relative to the left-hand image portion. 
As a result of the above operation, the linearity distortion caused by 
saturation is corrected simultaneously with correction of the raster 
distortion. 
Referring now to FIG. 5, it will be seen that a horizontal deflection 
output circuit according to the embodiment of this invention there shown 
is generally similar to that described above with reference to FIG. 3, 
with the elements of the circuit shown in FIG. 5 which correspond to those 
described with reference to FIG. 3 being identified by the same reference 
symbols and numerals. However, in the specific embodiment of the invention 
illustrated by FIG. 5, the linearity distortion correcting circuit 10 is 
shown to comprise a direct-current blocking capacitor C.sub.1, a choke 
coil L.sub.1 and a switch S.sub.1. The capacitor C.sub.1 and choke coil 
L.sub.1 are connected in series with a secondary winding of the flyback 
transformer FBT to a junction point between the deflecting coil L.sub.Y 
and the S correcting capacitor C.sub.S. The switch S.sub.1 is connected in 
parallel with the choke coil L.sub.1 and is controlled by a drive circuit 
20 so as to be turned on or closed only during a flyback or retrace 
interval of each horizontal period. 
In the operation of the horizontal deflection output circuit shown on FIG. 
5, a flyback pulse e.sub.1 [FIG. 6(b)] of a predetermined negative level 
is supplied from the secondary winding of the flyback transformer FBT to 
the linearity distortion correcting circuit 10. As the switching 
transistor T is turned on and off in response to horizontal drive pulses 
P.sub.H [FIG. 4(a)] applied to its base electrode, a sawtooth current is 
made to flow through the horizontal deflecting coil L.sub.Y, so that the 
terminal voltage of the S correcting capacitor C.sub.S is varied, as 
indicated at e.sub.2 in FIG. 6(a). The negative flyback pulse e.sub.1 
generated during the retrace interval, as shown in FIG. 6(b), is fed 
through the switch S.sub.1 which, as earlier noted, is controlled by the 
drive circuit 20 so as to be turned on or closed only during such retrace 
or flyback interval. Closing of the switch S.sub.1 causing discharging of 
the S correcting capacitor C.sub.S by means of the discharge current 
i.sub.2 flowing through the switch S.sub.1, and which serves to lower the 
terminal voltage of the S correcting capacitor C.sub.S. Since the switch 
S.sub.1 is controlled by the drive circuit 20 so as to be turned off or 
open during the trace interval, that is, between successive flyback 
intervals, the S correcting capacitor C.sub.S is gradually charged by the 
charge current i.sub.1 [FIG. 6(c)] passing through the choke coil L.sub.1. 
As a result of the foregoing, the terminal voltage e.sub.2 [FIG. 6(a)] of 
the S correcting capacitor C.sub.S is modulated or modified as indicated 
by the dotted line and, therefore, the current fed from the S correcting 
capacitor C.sub.S to the horizontal deflecting coil L.sub.Y is adapted or 
varied for correcting the linearity distortion simultaneously with the 
correction of the raster distortion. 
The waveforms of the correcting charge and discharge currents i.sub.1 and 
i.sub.2 [FIGS. 6(c) and 6(d)] can be changed by adjusting the peak value 
of the negative flyback pulse e.sub.1 or the inductance of the choke coil 
L.sub.1. Therefore, the desired correction can be easily and accurately 
attained for improving the linearity of television receivers that may have 
different deflection characteristics. 
Referring now to FIG. 7, it will be seen that, in accordance with another 
embodiment of this invention, the linearity distortion correcting circuit 
10 of FIG. 5 may be replaced by a similarly functioning linearity 
distortion correcting circuit 10A in which a switching transistor T.sub.1 
is used in place of the switch S.sub.1 of the earlier described 
embodiment. More specifically, the switching transistor T.sub.1 has its 
collector-emitter path connected in series with an inverse-current 
preventing diode D.sub.1 and a current limiting coil L.sub.2 in a circuit 
that is in parallel with the choke coil L.sub.1. Further, base voltage 
setting resistors R.sub.1 and R.sub.2 are suitable connected with the 
transistor T.sub.1. By way of example, the base voltage setting resistors 
R.sub.1 and R.sub.2 may have resistance values of 470.OMEGA. and 
47.OMEGA., respectively, while the current limiting coil L.sub.2 has an 
inductance value of 470 .mu.H. 
Horizontal distortion rates have actually been measured for a television 
receiver provided with a conventional horizontal output circuit having a 
saturable reactor for correction of linearity distortion, and for a 
television receiver provided with a linearity distortion correcting 
circuit according to this invention, for example, as in FIG. 7. The data 
obtained by such measurements of the horizontal distortion rates are 
graphically shown on FIG. 8 in which the horizontal image positions are 
indicated along the abscissa and the distortion rates are indicated along 
the ordinate. Further, on FIG. 8, the dotted line A represents the 
distortion rates obtained at various horizontal image positions in the 
case of the television receiver according to the prior art which uses a 
conventional saturable reactor for correction of linearity distortion. On 
the other hand, the solid line B indicates the distortion rates at 
different horizontal image positions for the television receiver provided 
with the linearity distortion correction circuit embodying the present 
invention. It will be seen that, in the case of the prior art (the line A) 
distortion rates as high as .+-.3.5% is obtained, where-as, in the case of 
the television receiver provided with a linearity distortion correcting 
circuit according to this invention, the maximum distortion rates were 
reduced to no more than .+-.1.5%. Such improvement in linearity is 
particularly conspicuous to a viewer of a large-screen television 
receiver. 
In the case of the linear distortion correcting circuit 10A shown on FIG. 
7, a transistor T.sub.1 is employed to act as a switching element. 
However, it will be understood that such switching function could be 
performed by the diode D.sub.1 alone. 
By way of summary, it will be appreciated that a horizontal deflection 
output circuit according to the present invention includes a linearity 
distortion correcting circuit by which a correcting current is fed to the 
S correcting capacitor to modulate or modify the amplitude of the terminal 
voltage of the latter, whereby the characteristic deterioration and 
variation resulting from temperature fluctuations can be reduced, as 
compared with the linearity correction effected in accordance with the 
prior art by the use of a saturable reactor. Further, through the use of 
the present invention, as described above, the correction characteristic 
can be accurately varied so as to achieve remarkable minimizing of the 
distortion rates. 
Although illustrative embodiments of this invention have been described in 
detail herein 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 affected therein by one 
skilled in the art without departing from the scope or spirit of the 
invention as defined in the appended claims.