GB2094114A - Scanning circuit for a television - Google Patents

Scanning circuit for a television Download PDF

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Publication number
GB2094114A
GB2094114A GB8106465A GB8106465A GB2094114A GB 2094114 A GB2094114 A GB 2094114A GB 8106465 A GB8106465 A GB 8106465A GB 8106465 A GB8106465 A GB 8106465A GB 2094114 A GB2094114 A GB 2094114A
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United Kingdom
Prior art keywords
signal
amplitude
scan signal
display device
field
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Granted
Application number
GB8106465A
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GB2094114B (en
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CROMA RESEARCH Ltd
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CROMA RESEARCH Ltd
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Filing date
Publication date
Application filed by CROMA RESEARCH Ltd filed Critical CROMA RESEARCH Ltd
Priority to GB8106465A priority Critical patent/GB2094114B/en
Publication of GB2094114A publication Critical patent/GB2094114A/en
Application granted granted Critical
Publication of GB2094114B publication Critical patent/GB2094114B/en
Expired legal-status Critical Current

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N3/00—Scanning details of television systems; Combination thereof with generation of supply voltages
    • H04N3/10—Scanning details of television systems; Combination thereof with generation of supply voltages by means not exclusively optical-mechanical
    • H04N3/16—Scanning details of television systems; Combination thereof with generation of supply voltages by means not exclusively optical-mechanical by deflecting electron beam in cathode-ray tube, e.g. scanning corrections
    • H04N3/22—Circuits for controlling dimensions, shape or centering of picture on screen
    • H04N3/223—Controlling dimensions

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  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Details Of Television Scanning (AREA)

Abstract

This invention relates to a system for maintaining a constant picture aspect ratio on a television display device. There is provided a television display device having a line scan circuit (10,12,14,20) and a field scan circuit (24,26,28,30,32) in which the amplitude of the line scan signal is selectively variable and the field scan circuit (24,26,28,30,32) is controlled in dependence upon the amplitude of the line scan signal such that the amplitude of the two scan signals remain in a predetermined ratio to one another. A respective detector (20,32) is included in each scan circuit in order to provide D.C. output signals proportional to the respective scan currents. The two D.C. signals are applied to a differential amplifier (22) which serves as a comparator and error signal amplifier. Output from the differential amplifier (22) is connected to a voltage controlled monostable (34) the duty ratio which is variable in dependence upon the error signal derived from the differential amplifier (22). The monostable multivibrator (34) controls a switchable attenuator (26) included in the field scan circuit (24,26,28,30,32) such that when the amplitude of the line scan signal is changed to vary the width of the picture, the field scan signal is automatically controlled so as to maintain the aspect ratio constant. A constant frequency signal having for example a frequency equal to eight times the line frequency is applied to the trigger input (36) of the monostable multivibrator (34). <IMAGE>

Description

SPECIFICATION Scanning circuit for a television This invention relates to a system for maintaining a constant picuture aspect ratio on a television display device.
With any television display, the aspect ratio is determined at the transmitting end and is normally 4 : 3. Thus, a display device must have precisely the same aspect ration to reproduce a picture faithfully.
The usual way of achieving this is to have separate line and field scan amplitude controls on the television display device so that the aspect ration may be adjusted correctly.
However, if a change of picture size is required, as is frequently the case on a broadcast monitor, both controls need to be adjusted in order to maintain the aspect ratio constant. Alternatively, two sets of controls may be provided with a switch to select one or other of the two sets thus allowing two sizes to be present.
The present invention seeks to provide a system wherein the picture size may be altered by means of a single control without affecting the aspect ratio.
In accordance with the present invention, there is provided a television display device with a line scan circuit and a field scan circuit in which the amplitude of the line scan signal is selectively variable and the field scan circuit is controlled in dependence upon the amplitude of the line scan signal such that the amplitude of the two scanning signals remain in a predetermined ratio to one another.
Conveniently, the two scanning signals, namely the line scan signal and the field scan signal are detected to produce D.C. output signals proportional to the respective scan currents and the two D.C.
signals resulting from detection are compared to produce an error signal serving to control the amplitude of the field scan signal in a ciosed feedback loop.
Thus, when the amplitude of the line scan signal is changed to vary the width of the picture, the field scan signal is automatically controlled to maintain the aspect ratio constant by the follow-up servo loop without the need for a separate control.
Conveniently, the amplitude of the field scan circuit is controlled by means of a variable attenuatorwhich, in order to provide good linearity may be constituted by a switched attenuator with the switching taking place at a multiple of line frequency.
The invention will now be described further, by way of example, with reference to the accompanying drawing which is a block circuit diagram of a system in accordance with the invention.
In the accompanying drawing, a line scan circuit 10 applies a sawtooth current to the line scan coils 12 by way of a resistor 14. The amplitude of the detection signal is variable by means of a potentiometer 18 connected to the line scan circuit.
A detector 20 connected across the resistor 14 rectifies and smooths the line scan signal to produce a D.C. signal which is applied to one input of a differential amplifier 22 serving as comparator and error signal amplifier.
The field scan signal is derived from a field sawtooth generator 24 the output of which is applied by way of a switchable attenuator 26 and a filter and output stage 28 to field scan coils 30. The field scan signal is similarly detected by mean of a detector 32 and applied to the second input of the differential amplifier 22. The output of the differential amplifier 22 is connected to a voltage controlled monostable 34 the duty ratio of which is variable in dependence upon the error signal derived from the differential amplifier 22. A constant frequency signal having for example a frequency equal to eight times the line frequency is applied to the trigger input 36 of the monostable multivibrator 34 the output of which controls the switchable attenuator 26 which has two branches one of which includes an attenuator in the form of a resistor.Switching between the two branches is effected under the control of the voltage controlled monstable 34.
In operation, the potentiometer 18 is adjusted in dependence upon the desired width of the picture.
No separate control is provided for the field scan signal and instead in response to the change in the line scan signal amplitude an error signal is produced by the differential amplifier 22 which now varies the duty ratio of the monostable 34 to vary the mean level of attenuation of the signal derived from the field sawtooth generator 34 by way of a switchable attenuator 26. The field scan signal amplitude thus varies in the sense determined by the error signal until the two signals compared by the differential amplifier 22 once again match one another, thereby retaining the desired aspect ratio.
CLAIMS (Filed on 2 March 1982) 1. A television display device having a line scan circuit and a field scan circuit in which the amplitude of the light scan signal is selectively variable and the field scan circuit is controlled in dependence upon the amplitude of the line scan signal such that the amplitude of the two scanning signals remain in a predetermined ratio to one another.
2. Ateievision display device as claimed in claim 1, wherein the line scan signal and the field scan signal are detected to produce D.C. output signals proportional to the respective scan currents, the two D.C. signals resulting from detection being compared to produce an error signal serving to control the amplitude of the field scan signal in a closed feedback loop.
3. A television display device as claimed in claim 1 or 2, wherein the amplitude of the field scan circuit is controlled by means of a variable attenuator constituted by a switched attenuator in which switching occurs at a multiple of the line frequency.
4. A television display device as claimed in claim 2 or 3, comprising a first detector which rectifies and smooths the line scan signal to produce a D.C. signal which is applied to one input of a differential amplifier which serves as a comparator and error signal amplifier and a second detector which rectifies and smooths the field scan signal to produce a D.C. signal which is applied to another input of the
**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Scanning circuit for a television This invention relates to a system for maintaining a constant picuture aspect ratio on a television display device. With any television display, the aspect ratio is determined at the transmitting end and is normally 4 : 3. Thus, a display device must have precisely the same aspect ration to reproduce a picture faithfully. The usual way of achieving this is to have separate line and field scan amplitude controls on the television display device so that the aspect ration may be adjusted correctly. However, if a change of picture size is required, as is frequently the case on a broadcast monitor, both controls need to be adjusted in order to maintain the aspect ratio constant. Alternatively, two sets of controls may be provided with a switch to select one or other of the two sets thus allowing two sizes to be present. The present invention seeks to provide a system wherein the picture size may be altered by means of a single control without affecting the aspect ratio. In accordance with the present invention, there is provided a television display device with a line scan circuit and a field scan circuit in which the amplitude of the line scan signal is selectively variable and the field scan circuit is controlled in dependence upon the amplitude of the line scan signal such that the amplitude of the two scanning signals remain in a predetermined ratio to one another. Conveniently, the two scanning signals, namely the line scan signal and the field scan signal are detected to produce D.C. output signals proportional to the respective scan currents and the two D.C. signals resulting from detection are compared to produce an error signal serving to control the amplitude of the field scan signal in a ciosed feedback loop. Thus, when the amplitude of the line scan signal is changed to vary the width of the picture, the field scan signal is automatically controlled to maintain the aspect ratio constant by the follow-up servo loop without the need for a separate control. Conveniently, the amplitude of the field scan circuit is controlled by means of a variable attenuatorwhich, in order to provide good linearity may be constituted by a switched attenuator with the switching taking place at a multiple of line frequency. The invention will now be described further, by way of example, with reference to the accompanying drawing which is a block circuit diagram of a system in accordance with the invention. In the accompanying drawing, a line scan circuit 10 applies a sawtooth current to the line scan coils 12 by way of a resistor 14. The amplitude of the detection signal is variable by means of a potentiometer 18 connected to the line scan circuit. A detector 20 connected across the resistor 14 rectifies and smooths the line scan signal to produce a D.C. signal which is applied to one input of a differential amplifier 22 serving as comparator and error signal amplifier. The field scan signal is derived from a field sawtooth generator 24 the output of which is applied by way of a switchable attenuator 26 and a filter and output stage 28 to field scan coils 30. The field scan signal is similarly detected by mean of a detector 32 and applied to the second input of the differential amplifier 22. The output of the differential amplifier 22 is connected to a voltage controlled monostable 34 the duty ratio of which is variable in dependence upon the error signal derived from the differential amplifier 22. A constant frequency signal having for example a frequency equal to eight times the line frequency is applied to the trigger input 36 of the monostable multivibrator 34 the output of which controls the switchable attenuator 26 which has two branches one of which includes an attenuator in the form of a resistor.Switching between the two branches is effected under the control of the voltage controlled monstable 34. In operation, the potentiometer 18 is adjusted in dependence upon the desired width of the picture. No separate control is provided for the field scan signal and instead in response to the change in the line scan signal amplitude an error signal is produced by the differential amplifier 22 which now varies the duty ratio of the monostable 34 to vary the mean level of attenuation of the signal derived from the field sawtooth generator 34 by way of a switchable attenuator 26. The field scan signal amplitude thus varies in the sense determined by the error signal until the two signals compared by the differential amplifier 22 once again match one another, thereby retaining the desired aspect ratio. CLAIMS (Filed on 2 March 1982)
1. A television display device having a line scan circuit and a field scan circuit in which the amplitude of the light scan signal is selectively variable and the field scan circuit is controlled in dependence upon the amplitude of the line scan signal such that the amplitude of the two scanning signals remain in a predetermined ratio to one another.
2. Ateievision display device as claimed in claim 1, wherein the line scan signal and the field scan signal are detected to produce D.C. output signals proportional to the respective scan currents, the two D.C. signals resulting from detection being compared to produce an error signal serving to control the amplitude of the field scan signal in a closed feedback loop.
3. A television display device as claimed in claim 1 or 2, wherein the amplitude of the field scan circuit is controlled by means of a variable attenuator constituted by a switched attenuator in which switching occurs at a multiple of the line frequency.
4. A television display device as claimed in claim 2 or 3, comprising a first detector which rectifies and smooths the line scan signal to produce a D.C. signal which is applied to one input of a differential amplifier which serves as a comparator and error signal amplifier and a second detector which rectifies and smooths the field scan signal to produce a D.C. signal which is applied to another input of the differential amplifier.
5. A television display device as claimed in claims 3 and 4, wherein the output of the differential amplifier is connected to a voltage controlled monostable the duty ratio of which is variable in dependence upon the error signal derived from the differential amplifier and the output of which controls the switchable attenuator.
6. Atelevision display device as claimed in claim 5, wherein a frequency equal to eight times the line frequency is applied to a trigger input of the monstable multivibrator.
7. A television display device as claimed in any preceding claim dependent upon claim 3, wherein the switchable attenuator has two branches one of which includes an attenuator in the form of a resistor.
8. A television display device substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.
GB8106465A 1981-03-02 1981-03-02 Scanning circuit for a television Expired GB2094114B (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
GB8106465A GB2094114B (en) 1981-03-02 1981-03-02 Scanning circuit for a television

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB8106465A GB2094114B (en) 1981-03-02 1981-03-02 Scanning circuit for a television

Publications (2)

Publication Number Publication Date
GB2094114A true GB2094114A (en) 1982-09-08
GB2094114B GB2094114B (en) 1984-10-24

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3406224A1 (en) * 1983-02-21 1984-08-23 Rca Corp., New York, N.Y. Circuit for changing the image size of a television receiver
DE3607064A1 (en) * 1985-03-05 1986-09-11 Sgs Microelettronica S.P.A., Catania CONTROL CIRCUIT WITH COMPENSATION OF THE ANODE VOLTAGE VARIATIONS FOR THE VERTICAL DEFLECTION LEVEL OF A TELEVISION
GB2179828A (en) * 1985-08-14 1987-03-11 Rca Corp Selectable raster size for video display
US4763194A (en) * 1985-08-14 1988-08-09 Rca Licensing Corporation Selectable raster size for video display
US4795949A (en) * 1988-04-08 1989-01-03 Rca Licensing Corporation Deflection current correction circuit

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3406224A1 (en) * 1983-02-21 1984-08-23 Rca Corp., New York, N.Y. Circuit for changing the image size of a television receiver
FR2541543A1 (en) * 1983-02-21 1984-08-24 Rca Corp TV receiver image size adjustment circuit
DE3607064A1 (en) * 1985-03-05 1986-09-11 Sgs Microelettronica S.P.A., Catania CONTROL CIRCUIT WITH COMPENSATION OF THE ANODE VOLTAGE VARIATIONS FOR THE VERTICAL DEFLECTION LEVEL OF A TELEVISION
GB2179828A (en) * 1985-08-14 1987-03-11 Rca Corp Selectable raster size for video display
US4763194A (en) * 1985-08-14 1988-08-09 Rca Licensing Corporation Selectable raster size for video display
GB2179828B (en) * 1985-08-14 1989-08-02 Rca Corp Selectable raster size for video display
US4795949A (en) * 1988-04-08 1989-01-03 Rca Licensing Corporation Deflection current correction circuit

Also Published As

Publication number Publication date
GB2094114B (en) 1984-10-24

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PCNP Patent ceased through non-payment of renewal fee