FIELD OF THE INVENTION

The present invention relates to method and apparatus for generating a switching waveform. In particular, it relates to generating switching waveforms for illuminating a LED backlight for a LCD in controlling the brightness of the backlight.

BACKGROUND OF THE INVENTION

Many techniques have been developed to control the brightness of LEDs in LCD backlights. Some 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 (PWM) 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.

In operation, 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.

Examples of known drive circuit for LCD backlights using a PWM waveform are disclosed by US Patent Applications Nos. 2007/236445 and 2004/0145560.

Prior art techniques switch the LEDs on and off altering the ratio of on and off time to achieve the required average brightness (as perceived by a viewer). However, in such known systems, interaction (intermodulation) of the video signal being displayed and the PWM signal occurs. This may result in motion artefacts, flicker and moving or static patterning providing undesirable effects to the display,

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.

This results in a signal with a spectrum containing strong harmonics at the repetition frequency and multiples of that frequency. When PWM is used to control the brightness of a LED backlight for a LCD the PWM pulse rate may beat with the video signal being displayed producing interference patterns or flicker.

One known solution to reduce the visible artefacts is to synchronise the PWM pulse rate to the video raster (the video line or frame rate). This makes any intermodulation artefacts static when viewed on the display and therefore much less detrimental to the perceived image quality.

However, synchronising the PWM waveform to the video raster, as in the prior art does not eliminate beating (interference patterns or flicker) between the PWM waveform and static or moving picture detail completely. This is because static and moving picture detail are effectively unpredictable and unconstrained (within the spatial temporal constraints of the video system). This is partly overcome by pseudo random modulation of a PWM waveform as disclosed by US2008/01 11503 to reduce spectral content and hence reduce electromagnetic interference. This technique spreads the spectrum into a series of spectral spurs of uniform amplitude, reducing amplitude of each spectral spur. This means that any spur causing interference can be reduced, thus reducing its interference. However, the modulated switching waveform intermodulates with harmonic and sub harmonic components at the frequency of a video raster or image spectrum, producing unwanted visible effects.

Another form of switching waveform commonly used in a variety of applications is Pulse Density Modulation (PDM).

One known technique of generating a PDM waveform uses a series of pulses, typically of constant period. The value of the signal being reconstructed is created by adjusting the ratio of on and off pulses ('T's and "0"s in a binary sequence). One such known method of creating a PDM waveform is referred to as error feedback in the BBC report BBC RD 1987/12 "accommodating the residue of processed or computed digital video signals within the 8 bit CCIR recommendation 601". This report describes the use of error feedback to round to 8 bits from a higher precision. The resultant waveform is referred to as Pulse Density Modulation (PDM) waveform.

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. However, when Error Feedback or Sigma Delta modulation is used to construct a signal, spurious spectral spurs (harmonics and aliases of the signal and its harmonics) are created. 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.

In a DAC the spurious frequency components produced by PDM are often suppressed by a low pass reconstruction filter. PDM has been used in audio amplifiers where the pulse rate is chosen to be well above the response of the speakers or the human ear. In the case of a LCD backlight the frequency components in the drive waveform cannot be removed by a low pass reconstruction filter as the resultant LED current would no longer be "ON" or "OFF". This would reduce power efficiency and introduce variations in colour when the brightness was varied.

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 use of PDM to modulate an LED backlight is disclosed in US 2008/01 11503. This discloses the use of random, or pseudo random sequences to reduce intermodulation artefacts and electromagnetic interference. This exploits the spread spectrum characteristics of random or pseudo random signals. The present invention discloses how other signals with a spread spectrum can give improved performance. SUMMARY OF THE INVENTION

The present invention seeks to provide a waveform having reduced spurious frequency components, reduced motion artefacts and/or flicker for driving a LED backlight.

This is achieved according to a first aspect by a method for generating a switching waveform for driving a LED backlight for a LCD, the method comprising the steps of: synthesisiπg 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.

This is also achieved by a second aspect by apparatus for generating a switching waveform for driving a LED backlight for a LCD, the apparatus 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.

As a result 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. When used to drive a LED backlight the intermodulation products are also lower resulting in fewer or less visible picture artefacts. Although the total RF energy in the waveform can be higher than a conventional switching waveform, 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. Alternatively, 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.

In this way 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. When 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. Furthermore the reduced artefacts are achieved without the need of timing signals from the video path which allows independent development of subsystems and interoperability of both backlight and display technologies. Further driving the LEDs with a PDM waveform maintains the operating point of the LEDs at all brightness levels and hence gives stable chromaticity.

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.

BRIEF DESCRIPTION OF DRAWINGS

For a more complete understanding of the present invention, reference is made to the following description in conjunction with the accompanying drawings, in which:

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%;

Figure 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;

Figure 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; and

Figure 7 is a simplified schematic of apparatus according another embodiment of the present invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

A first embodiment of the present invention will now be described with reference to Figures 3 to 5.

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.

In operation, the synthesiser 301synthesises 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.

As shown in Figure 5, 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 syπthesising 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.

In synthesisiπg the first signal by summing a set of sinusoids, 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.

The summation of the sinusoids or the Fourier Transform results in a complex analogue waveform. To produce the desired binary modulated waveform this must be quantised to one bit (2 levels - LED on, and LED off) either before or after modulating the LED drive waveform. This will modify the original spectrum as the various spectral lines will intermodulate, typically producing more lines of lower levels, spread over a wider frequency range. This may be very desirable, or further modification, (e.g. by time domain filtering and re-quantising) may be used to derive a modulation sequence with the desired frequency characteristics.

Figure 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.

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. In an alternative embodiment, 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.

Alternatively, the first signal is synthesised and stored in memory as shown in the embodiment of Figure 7. 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.

In operation, 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. Importantly, 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. Although the total intermodulatioπ 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. Hence 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.

Although embodiments of the present invention have been illustrated in the accompanying drawings and described in the foregoing detailed description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous modifications without departing from the scope of the invention as set out in the following claims.