WO2010052506A1 - Procede et appareil de repartition de l'energie spectrale d'une forme d'onde de commutation - Google Patents

Procede et appareil de repartition de l'energie spectrale d'une forme d'onde de commutation Download PDF

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Publication number
WO2010052506A1
WO2010052506A1 PCT/GB2009/051499 GB2009051499W WO2010052506A1 WO 2010052506 A1 WO2010052506 A1 WO 2010052506A1 GB 2009051499 W GB2009051499 W GB 2009051499W WO 2010052506 A1 WO2010052506 A1 WO 2010052506A1
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WO
WIPO (PCT)
Prior art keywords
waveform
switching
modulated
signal
switching waveform
Prior art date
Application number
PCT/GB2009/051499
Other languages
English (en)
Inventor
Keith Noel Jenkins
Gary Bryan Wordsworth
Michael Stuart Evans
Original Assignee
Iti Scotland Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Iti Scotland Limited filed Critical Iti Scotland Limited
Publication of WO2010052506A1 publication Critical patent/WO2010052506A1/fr

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Classifications

    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/3406—Control of illumination source
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/34—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters by control of light from an independent source
    • G09G3/3406—Control of illumination source
    • G09G3/3413—Details of control of colour illumination sources
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B45/00—Circuit arrangements for operating light-emitting diodes [LED]
    • H05B45/10—Controlling the intensity of the light
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2310/00—Command of the display device
    • G09G2310/08—Details of timing specific for flat panels, other than clock recovery
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/02—Improving the quality of display appearance
    • G09G2320/0247—Flicker reduction other than flicker reduction circuits used for single beam cathode-ray tubes
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/06—Adjustment of display parameters
    • G09G2320/0626—Adjustment of display parameters for control of overall brightness
    • G09G2320/064—Adjustment of display parameters for control of overall brightness by time modulation of the brightness of the illumination source

Definitions

  • the present invention relates to method and apparatus for generating a switching waveform.
  • it relates to generating switching waveforms for illuminating a LED backlight for a LCD in controlling the brightness of the backlight.
  • LED backlights use white LEDs while others use a combination of coloured LEDs to produce the required white light. In the latter case, the resultant colour is very dependant on the relative brightness of the LEDs and the relationship between the forward current of a LED and its brightness which is not precisely linear. The precise colour of a white LED can also change with its forward current. For this reason the brightness of LED backlights is normally controlled by pulsing the LED on and off at a single current and adjusting the proportion of the time that they are switched on. The pulse rate is chosen so that the response of the human eye averages the perceived brightness and the backlight does not flicker.
  • Pulse Width Modulation is commonly used to control the brightness of LED backlights for LCDs.
  • Figure 1 illustrates a simple schematic of a simple PWM waveform generator employed for brightness control.
  • the PWM waveform generator 101 comprises a brightness controller 103 connected to a first input terminal 105 of a comparator 107.
  • the PWM waveform generator 101 further comprises a sawtooth generator 109 connected to a second input terminal 1 11 of the comparator 107.
  • the PWM waveform generator 101 further comprises an output terminal 113 which is connected to the output terminal 1 15 of the comparator 107.
  • the output terminal 1 13 of the PWM waveform generator 101 is connected to the input of a LED driver 117.
  • the output of the LED driver 117 is connected to at least one LED chain comprising a plurality of LEDs 1 19.
  • the brightness controller 103 provided by user input or other display controls that output the brightness level.
  • the output brightness level is compared with a sawtooth waveform, output by the sawtooth generator 109, by the comparator 107.
  • the resulting output of the comparator 107 is a pulse waveform in which the width of the pulse is varied in accordance with any variation in the determined brightness level.
  • the pulse waveform is then used by the LED driver 117 to turn the LEDs 119 on or off by a period determined by the pulse width and hence dependent on the brightness level.
  • the Pulse Width Modulation (PWM) signal typically consists of a series of pulses with a constant repetition rate, the resolution being conveyed by adjusting the on/off ratio or duty cycle of the signal.
  • PDM Pulse Density Modulation
  • PDM Pulse Density Modulation
  • Figures 2a to 2c illustrate the generation of a PWM and a PDM waveforms representing a DC value of 75%.
  • Figure 2a illustrates a dc value of brightness control of a LED backlight for a LCD, for example, at 75%.
  • Figure 2b illustrates a PWM waveform representing the DC value of 75% of Figure 2a.
  • Figure 2c illustrates a PDM waveform representing the DC value of 75% of Figure 2a.
  • the PWM waveform of Figure 2b comprises a plurality of identical pulses having a period ti the width of w of each pulse is such that the duty cycle is 75%.
  • the equivalent PDM waveform of Figure 2c comprises a plurality of identical pulses having a repeat pattern, period ti and a pulse density in each period ti of 75%.
  • US Patent No. 2927962 is an early reference of a technique for creating a PDM waveform. It uses error feedback to round to a single bit to be low pass filtered to construct an analogue waveform is often referred to as a Sigma Delta modulation or a Delta Sigma DAC and is widely used in audio converters such as CD players or audio amplifiers.
  • a Sigma Delta modulation or a Delta Sigma DAC is widely used in audio converters such as CD players or audio amplifiers.
  • spurious spectral spurs harmonics and aliases of the signal and its harmonics
  • This has been overcome previously by spreading the energy across a wide range of spurs by techniques such as adding low level noise to "dither" the process making the artefacts more noise like such as that disclosed by US Patent No. 5404427.
  • This discloses a form of Sigma Delta Modulator which spreads the spurious frequency components by introducing low level pseudo random noise to the error feedback path.
  • a Spread Spectrum clock Another technique in the prior art used in equipment such as computers is a Spread Spectrum clock. Digital systems operate on a clock to synchronise the transfer of data from one register to another. If that clock is not a pure frequency with low jitter but has designed modulation to spread the spectrum, each frequency component in the emissions from the equipment can be spread over a frequency range lowering the peak amplitude. This technique can only modulate the clock transitions with an amplitude of up to one clock cycle (in practice significantly less to ensure there are no clocking errors).
  • the present invention seeks to provide a waveform having reduced spurious frequency components, reduced motion artefacts and/or flicker for driving a LED backlight.
  • a method for generating a switching waveform for driving a LED backlight for a LCD comprising the steps of: synthesisi ⁇ g a first signal having a spectrum comprising many closely spaced low level harmonics without any very low frequency harmonics; modulating a temporal characteristic of a switching waveform with the synthesised first signal to shape the spectral energy of the switching waveform; and outputting the modulated switching waveform to drive at least one LED chain, the at least one LED chain comprising a plurality of LEDs.
  • apparatus for generating a switching waveform for driving a LED backlight for a LCD comprising: a synthesiser for synthesising a first signal having a spectrum comprising many closeiy spaced low level harmonics without any very low frequency harmonics; modulating means for modulating a temporal characteristic of a switching waveform with the synthesised first signal to shape the spectral energy of the switching waveform; and an output terminal for outputting the modulated switching waveform to drive at least one LED chain, the at least one LED chain comprising a plurality of LEDs.
  • the first signal with evenly distributed spectral energy can be synthesised in a number of ways such that it has a spectrum comprising many closely spaced low level harmonics but without any very low frequency harmonics that might themselves cause visible flicker. This can be done by, for example, summing a plurality of sinusoidal waveforms or by synthesising the frequency domain signal and Fourier transforming to obtain the time domain. It may be synthesised in real time or stored in a memory and read out.
  • a modulated switching waveform is generated having a required spectral energy which is also more evenly distributed across a greater number of lower amplitude harmonics than a conventional switching waveform reducing spurious frequency components.
  • the intermodulation products are also lower resulting in fewer or less visible picture artefacts.
  • the peak spectral spurs are lower. Reduced peak spectral spurs mean reduced interference with narrow band systems and is the significant parameter for emissions approvals.
  • the method disclosed can be used in conjunction with other techniques such as a spread spectrum clock.
  • the spectrum is shaped by modulation of the synthesised signal so that the harmonics in frequency ranges that are particularly likely to interact with the video raster are further reduced.
  • the switching waveform may comprise a pulse width modulated waveform and the temporal characteristic may comprise switching transitions of the pulse width modulated waveform.
  • the switching waveform may comprise a pulse density modulated waveform and the temporal characteristic may comprise a pulse pattern of the pulse density modulated waveform.
  • the PWM waveform or the PDM waveform with the desired DC or low frequency content (the required brightness in the application of a LED backlight) with a much greater number of lower level spectral components is generated, i.e. a spread spectrum signal is generated.
  • these intermodulate with the video raster or picture detail of a LCD for example there are also a much greater number of intermodulation products. This results in any beating patterns being at a much lower level, less strongly patterned and as a result less visible. As a result, the picture quality is improved with reduced motion artefacts, patterning and flicker. Further, reducing beating with other ambient frequencies such as ambient lighting, other displays etc.
  • the modified switching waveforms also result in reduced electromagnetic interference with other parts of the display for example the liquid crystal drive signals as well as reduced electromagnetic interference with other equipment and consequently reduced electromagnetic screening is required. Since the modified switching waveforms have attenuated emissions, this allows higher drive voltages and hence longer chains of LEDs to be used.
  • Figure 1 is a simplified schematic of a known technique of controlling brightness of LEDs of a LCD backlight
  • Figures 2a to 2c illustrate an example of conventional PWM and PDM waveforms representing a DC value of 75%
  • FIG. 3 is a simplified schematic of apparatus according an embodiment of the present invention.
  • Figure 4 is a simplified diagram of the spectrum of the switching waveform output by a conventional PWM generator
  • FIG. 5 is a simplified diagram of the spectrum of the output by the apparatus of Figure 3;
  • Figure 6 is a simplified schematic of the synthesiser of Figure 3 according an embodiment of the present invention.
  • FIG. 7 is a simplified schematic of apparatus according another embodiment of the present invention.
  • the apparatus 300 comprises a synthesiser 301.
  • the output of the synthesiser is connected to an input of a modulator 305.
  • the apparatus 300 further comprises a switching waveform generator 303 connected to an input terminal 302.
  • the output of the switching waveform generator 303 is connected to another input of the modulator 305.
  • the output of the modulator 305 is connected to an output terminal 307 of the apparatus 300.
  • the output terminal 307 of the apparatus 300 is connected to a LED driver 309.
  • the output of the LED driver 309 is connected to a LED chain of a plurality of serially connected LEDs 311.
  • the synthesiser 301 synthes a first signal having a spectrum comprising many closely spaced low level harmonics without any very low frequency harmonics.
  • a brightness level is input into the switching waveform generator 303 via the input terminal 302.
  • the switching waveform generator 303 generates a switching PWM or PDM waveform for the desired brightness level.
  • the switching waveform output by the generator 303 is modulated with the synthesised first signal by the modulator 305.
  • the modified switching waveform is output on the output terminal 307 for the LED driver 309 to switch the LED chain 311 on and off for the required brightness level.
  • the PWM waveform output by the apparatus 300 may switch a plurality of LED chains as required for the backlight via corresponding plurality of drivers (not shown here).
  • the apparatus 300 of the first embodiment of the present invention modifies the timing of the transitions of the PWM waveform. This spreads the spectral energy.
  • Figure 4 illustrates the spectrum output of a PWM waveform generated by a conventional means as described for example with reference to Figure 1 representing a static value (brightness). The harmonics typically fall with increasing frequency.
  • the spectrum of the PWM waveform output by the apparatus 300 of the first embodiment as shown in Figure 3 is generally shaped such that low frequency harmonics are reduced and harmonics are reduced at the raster frequency. This process makes the synchronisation of the PWM waveform to the video raster redundant. Importantly, the PWM waveform modified by the apparatus 300 of the first embodiment of Figure 3 reduces image artefacts due to beating between the PWM frequency and static or moving picture detail.
  • the synthesiser 301 of the apparatus 300 of Figure 3 may synthesise the first signal using numerous different techniques, for example summing a plurality of sinusoidal waveforms or by sy ⁇ thesising the frequency domain signal and Fourier transforming to obtain the time domain. It may be synthesised in real time or stored in memory for later retrieval as required.
  • the number and frequency ranges of the spectral lines can be selected to optimise the spectral characteristic. For example low frequencies that might introduce flicker can be avoided. Harmonics or sub-harmonics of frequencies in the video raster that might intermodulate to produce visible artefacts can be avoided. Harmonics or sub harmonics of frequencies sensitive to EMC such as radio carriers can also be avoided. The more sinusoids that are combined and the lower the level of each, the performance can be further improved. Alternatively the spectrum can be converted to the time domain by a Fourier transform.
  • FIG. 6 illustrates an embodiment of real time synthesis of the first signal.
  • the synthesiser 600 comprises a linear feedback shift register (LFSR) 601.
  • the output of the LFSR 601 is connected to a filter 603.
  • the output of the filter 603 is connected to a quantiser 605.
  • the output of the quantiser 605 is connected to an output terminal 607 of the synthesiser 600.
  • the output terminal 607 of the synthesiser 600 is connected to the modulator 305 of the apparatus 300 of Figure 3.
  • LFSR linear feedback shift register
  • the first signal is synthesised in real time.
  • the LFSR 701 generates a spread spectrum signal such as a Maximal Length Sequence. This is filtered by the digital filter 703 (for example a Finite Impulse Filter or an Infinite Impulse Response filter) to optimise the spectral characteristics of the first signal. For example, remove very low frequency harmonics and those at the frequencies of the video raster as shown, for example, in Figure 5.
  • the first signal is synthesised using a software program, either one of the commercially available programs or a custom written program, carrying out the steps described above.
  • the apparatus 700 comprises an input terminal 701 connected to the input of a processor 703,
  • the processor 703 is connected to a memory device 705 such as a ROM via memory interface logic 707.
  • the memory interface logic 707 is connected to an output terminal 709.
  • the output terminal 709 of the apparatus 700 is connected to a LED driver 711 and LED string 713.
  • a brightness control signal is provided on the input terminal 701. This processed by the processor 703 to output a required address for a required modulated switching waveform for that brightness level which is stored in the storage device 705. This is addressed by the memory interface logic 707 to retrieve the required modulated switching waveform which is provided on the output terminal 709 for the LED driver 71 1.
  • the apparatus of the embodiments above may be utilised with or without synchronisation of the PWIWPDM pulse rate to the video raster.
  • the apparatus of the embodiments above dramatically reduces image artefacts due to beating between the PDM frequency components and static or moving picture detail. It does this because the amplitudes of the spectral components are individually of much lower amplitude.
  • the total intermodulatio ⁇ energy may be much higher than some previous solutions, it is less coherent, less patterned more noise like and lower level particularly around sensitive frequencies.
  • the embodiments above spread the spectrum over a wider frequency range than achieved by using a spread spectrum clock, for example, and the spacing of the spectral components can be wider than the spread created by such a clock.
  • the apparatus of the embodiments described above may be combined with a spread spectrum clock to useful effect.
  • All the LEDs in the backlight may be driven by the same signal or individual LEDs or groups of LEDs may be driven by individual generators with uncorrelated random signals, or the same random signal with different phases.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Computer Hardware Design (AREA)
  • General Physics & Mathematics (AREA)
  • Theoretical Computer Science (AREA)
  • Liquid Crystal (AREA)
  • Liquid Crystal Display Device Control (AREA)
  • Control Of Indicators Other Than Cathode Ray Tubes (AREA)

Abstract

Selon l'invention, une forme d'onde de commutation est générée pour entraîner des chaînes de DEL pour un rétroéclairage d'écran à cristaux liquides. L'énergie spectrale de la forme d'onde est répartie par modulation (305) d'une onde de forme de commutation présentant un niveau de luminosité souhaité avec la sortie d'un synthétiseur (301) pour émettre (307) une forme d'onde de commutation dans laquelle une caractéristique temporelle, telle qu'une synchronisation de transition d'une forme d'onde à modulation d'impulsions en largeur ou du motif d'une forme d'onde à modulation d'impulsions en durée, est modifiée pour étaler son spectre de façon plus régulière avec des amplitudes harmoniques inférieures pour entraîner (309) un réseau de DEL (311) servant à rétroéclairer un écran à cristaux liquides.
PCT/GB2009/051499 2008-11-10 2009-11-10 Procede et appareil de repartition de l'energie spectrale d'une forme d'onde de commutation WO2010052506A1 (fr)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
GB0820539.5 2008-11-10
GB0820539A GB2465194A (en) 2008-11-10 2008-11-10 Randomly or pseudo-randomly modulated switching waveform for LED backlight
GB0919551.2 2009-11-09
GB0919551A GB2465078A (en) 2008-11-10 2009-11-09 Generating a switching waveform for driving a LED backlight for a LCD

Publications (1)

Publication Number Publication Date
WO2010052506A1 true WO2010052506A1 (fr) 2010-05-14

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PCT/GB2009/051499 WO2010052506A1 (fr) 2008-11-10 2009-11-10 Procede et appareil de repartition de l'energie spectrale d'une forme d'onde de commutation

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GB (2) GB2465194A (fr)
TW (1) TW201032201A (fr)
WO (1) WO2010052506A1 (fr)

Cited By (3)

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WO2015044442A3 (fr) * 2013-09-30 2015-08-13 Elmos Semiconductor Ag Procédé de génération d'une séquence de mots de code binaires d'un code multi-bits destiné à un signal de commande destiné à un consommateur
JP2016006454A (ja) * 2014-06-20 2016-01-14 株式会社コンフォートビジョン研究所 バックライト発光制御方法ならびにプロジェクタ表示方法および自発光映像表示方法
US9433049B2 (en) 2014-12-01 2016-08-30 Nxp, B.V. LED controllers, drivers and lighting circuits

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US8259058B2 (en) * 2007-07-12 2012-09-04 Semtech International Ag Method and device for controlling the backlighting of a flat screen

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US20080111503A1 (en) * 2006-11-13 2008-05-15 Cypress Semiconductor Corporation Stochastic signal density modulation for optical transducer control
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WO2015044442A3 (fr) * 2013-09-30 2015-08-13 Elmos Semiconductor Ag Procédé de génération d'une séquence de mots de code binaires d'un code multi-bits destiné à un signal de commande destiné à un consommateur
WO2015044447A3 (fr) * 2013-09-30 2015-08-13 Elmos Semiconductor Ag Dispositif d'éclairage
JP2016006454A (ja) * 2014-06-20 2016-01-14 株式会社コンフォートビジョン研究所 バックライト発光制御方法ならびにプロジェクタ表示方法および自発光映像表示方法
US9433049B2 (en) 2014-12-01 2016-08-30 Nxp, B.V. LED controllers, drivers and lighting circuits

Also Published As

Publication number Publication date
GB2465194A (en) 2010-05-12
GB2465078A (en) 2010-05-12
GB0820539D0 (en) 2008-12-17
TW201032201A (en) 2010-09-01
GB0919551D0 (en) 2009-12-23

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