Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Reexamination Certificate
1999-02-02
2001-12-11
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
C315S371000
Reexamination Certificate
active
06329768
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention relates to a process for correction of the deformation of a picture formed by a cathode-ray tube and a device implementing this process. It relates more particularly to the adjusting of the S correction in a television set.
2. Description of the Related Art:
Cathode-ray tubes are presently the most commonly used display elements in television sets. Represented diagrammatically in
FIG. 1
a
is a television
6
comprising a conventional cathode-ray tube. The latter comprises an electron gun
1
which produces an electron beam
2
which is accelerated and focused onto a luminescent screen
3
to form a point of the picture. Deflection coils
4
, outside the tube, deflect the electron beam electromagnetically so as to scan the screen and form a complete picture.
Let us recall that a television picture is generally formed of two fields, even and odd, and that the electron beam scans the screen twice from top to bottom and from left to right in order to display a complete picture.
In what follows we shall be interested solely in the vertical deflection of the electron beam.
In
FIG. 1
c
we have represented the current flowing through the coils
4
of the vertical deflector. The general shape of this signal is that of a sawtooth whose rising segments
8
correspond to the scanning of the screen by the electron beam from top to bottom (the active part of the picture), and whose segments
9
correspond to the return of the electron beam to the top of the screen (field blanking interval), the video signal sending a black level to the screen throughout the duration of the segment
9
(the inactive part of the picture). The duration of time between the start of a segment
8
and the end of a consecutive segment
9
is equal to the duration of a field.
Returning to
FIG. 1
a
, it may be noted that the distance between the electron gun
1
and the surface of the screen
3
is not constant: it increases as we travel from the centre of the screen towards the top or the bottom. Indeed, in recent cathode-ray tubes termed “flat screens” the radius of curvature of the screen
3
onto which the electron beam
2
is projected is much greater than the radius of curvature R between the centre of deflection C of the electron beam and its theoretical surface of projection
5
.
The consequences of this are that, when no correction is made, the picture which appears on the screen
3
is deformed, especially at the top and bottom of the picture. Thus, if a test pattern having equidistant horizontal lines is projected onto the screen, a picture such as that represented in
FIG. 1
b
is obtained in which the gap between the lines is greater at the top and bottom of the picture than in the middle.
To correct this phenomenon, it is known practice to apply a current to the coils of the vertical deflector, not a perfect sawtooth current, as represented in
FIG. 1
c
, but one in which the sawtooth exhibits “S”-shaped portions
18
, as represented in
FIG. 1
e
. This correction is dubbed “S correction”. The image obtained when the correction is optimal is represented in
FIG. 1
d.
This S correction must be set at the end of the television production line by altering the amplitude of the “S” shape of the portions
18
relative to the uncorrected shape of the sawtooth
8
′. During this adjustment, three points
10
,
11
,
12
of the portion
18
of the curve are not modified by the changes of amplitude of the S correction, these points will be called the fixed points of the S correction hereinafter.
In order for the correction of the picture to be optimal, it is also necessary for the S correction to be centred relative to the screen. That is to say, at the instants at which the sawtooth current passes respectively through the points
10
,
11
and
12
, the electron beam must scan the first line, the middle line and the last line of the screen respectively.
Moreover, in certain television receivers it is known practice to generate a few overscan lines at the top and bottom of the screen, these lines being used to perform tests. For example, these lines can be used to transmit colour information and to carry out auto-regulation of the colour temperature.
These overscan lines must not of course be displayed on the screen since they carry no information relating to the “useful” television picture transmitted. Hence, an adjustment must be made at the end of the television manufacturing line such that the electron beam does not scan the surface of the screen during these overscan lines.
Hence, at the end of a television receiver production line an operator must adjust a number of parameters: the position of the overscan lines, the position and amplitude of the S correction, etc.
In practice, he proceeds as follows:
firstly he dispatches a test pattern to the television, termed the convergence pattern and composed of a grid of equidistant lines, and he marks the zones of the picture which correspond to the fixed points of the S correction, that is to say he modifies the amplitude of the S correction so as to view on the screen those lines which remain still;
once the position of these lines has been marked, he must secondly position them at the top, middle and bottom of the screen respectively by adjusting the amplitude and the offset of the sawtooth current flowing through the coils of the deflector; the purpose of this is to centre the fixed points of the S correction with respect to the picture formed on the screen and this makes it possible furthermore to avoid displaying the overscan lines on the screen;
thirdly, he must readjust the amplitude of the S correction until the lines of the grid pattern appear equidistant on the screen.
SUMMARY OF THE INVENTION
This series of adjustments turns out to be fairly lengthy and an object of the invention is to simplify these operations in such a way as to reduce the time required to adjust each television set.
To this end, the subject of the invention is a process for S correction of the deformation of a picture formed by a cathode-ray tube, the amplitude of the S correction applied to the sawtooth current flowing through the deflection coils of the cathode-ray tube being variable. According to the invention, the process comprises the step of displaying visual marks on the screen at the location of the picture points which remain stationary as the amplitude of the S correction is varied.
By virtue of the invention, the first step of the above-described adjustment performed at the end of production is eliminated since the operator directly views the position of the zones of the picture corresponding to the fixed points of the S correction. Moreover, this marking is more accurate than in the prior art since it does not depend on the operators subjective perception of the position of the lines of the convergence pattern.
According to one aspect of the invention, the visual marks displayed are black lines.
It is thus sufficient for the operator to dispatch for example a white test pattern to the screen so as easily to mark the black lines corresponding to the fixed points of the S correction.
According to a particular embodiment, the step of displaying visual marks comprises:
the generation of pulses whose duration is at least equal to a video line at the instants at which the curve of the current flowing through the deflection coils passes through points, termed the “fixed points” of the S correction, which are not affected by the variation in amplitude of said S correction; and
the insertion of said pulses into the “Super Sand Castle” signal generated by the circuit controlling the scanning of the cathode-ray tube, said pulses indicating the position of those lines for which a black level is to be displayed on the screen.
Thus, the invention is implemented using a signal which already exists in the scan control circuit, the “Super Sand Castle” signal, which is generated, inter alia, so as to compel the video processor to deliver a black level to the electron guns of the cathode-ray tube duri
Chen Shih-Chao
Laks Joseph J.
Sragow Daniel E.
Thomson Licensing S.A.
Tripoli Joseph S.
LandOfFree
Process for correcting picture deformation and device... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for correcting picture deformation and device..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for correcting picture deformation and device... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2579451