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This invention relates to lighting control.
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Known drive or control techniques for light-emitting devices typically use pulse
width modulation (PWM) to drive light-emitting diodes (LEDs) and LED arrays.
These techniques have significant latency, in that a required change of illumination or
display light level from the light-emitting device can be implemented only during a
next complete PWM cycle after a new demanded level information is received.
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Moreover, known PWM techniques typically provide only a linear scale of
modulation in that each incremental change in demanded input level gives rise to a
linear change in a width of a period for which a drive mechanism is switched on.
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Such drive technologies are known from, for example, US-A-6,016,038 and US-A-6,150,774.
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It is an object of the present invention at least to ameliorate the aforesaid
deficiencies in the prior art.
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According to a first aspect of the invention, there is provided a lighting control
apparatus for controlling a lighting system dependent on a received input signal
representing a required perceived illumination level, the apparatus comprising a
sigma-delta converter having an input filter for providing a filtered input from the
input signal to a precision comparator for comparing the filtered input signal with a
feedback input signal from a system model module simulating an effect of a
comparator output signal on the lighting system to compare a resultant perceived
illumination level with the required perceived illumination level, such that the
precision comparator provides the comparator output signal for controlling a power
supply to the lighting system to provide the required perceived illumination level.
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Preferably, the precision comparator outputs bit stream data to the system model
module and as the output signal.
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Conveniently, where the apparatus is for controlling a lighting system having a
plurality of lighting array channels, the sigma-delta converter comprises a plurality of
precision comparators for providing a plurality of output signals to the respective
lighting array channels.
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Advantageously, the system model module comprises a respective system model
for each of the lighting array channels.
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Conveniently, the lighting control apparatus further comprises means for
monitoring a supply drive level to the lighting system for input to the system model
module.
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Advantageously, the lighting control apparatus further comprises ambient
lighting sensing means for inputting an ambient lighting signal to the system model
module.
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Preferably, the ambient lighting sensing means provides signals corresponding
to a plurality of discrete wavelengths to provide information on the effective colour
characteristics of an ambient light level at a user's eyes.
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Advantageously, the lighting control apparatus further comprises feedback
illumination sensing means for sensing illumination received at a user location from
the lighting system and for inputting an illumination feedback signal to the system
model module.
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Preferably, the system model module monitors and models at least one of
lighting system supply drive level, time and linearity responses of lighting drive
circuitry, time and linearity responses of the lighting system, models of light
perception characteristics of a user's eyes, perception of ambient light levels and
perception of ambient light colours.
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Conveniently, the lighting control apparatus is arranged for controlling a light-emitting
diode display or illumination system.
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Advantageously, the output signal is arranged for controlling at least one power
switch for controlling power supply to the lighting system.
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Preferably, the lighting control apparatus is arranged for driving a three channel
RGB array of light-emitting devices precisely to control a colour perceived by a user,
dependent on ambient lighting colour and level.
-
Advantageously, the lighting control apparatus is arranged for controlling a
lighting system precisely to control a colour perceived by a user, dependent on at least
one of known characteristics and defects in the user's optical perception, and of
characteristics of optical transmission media through which light from the lighting
system reaches the user.
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Advantageously, the lighting control apparatus is arranged to provide nonlinear
modulation of an output power of the lighting system.
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According to a second aspect of the invention, there is provided a method of
controlling a lighting system comprising the steps of: receiving an input signal
representing a required perceived illumination level; passing the input signal through a
sigma-delta converter comprising filtering the input signal to produce a filtered input
signal, inputting the filtered input to a precision comparator, feeding back a
comparator output signal from the precision comparator through a system model
module, which simulates an effect of the comparator output signal on the lighting
system, to the precision comparator to compare a resultant perceived illumination level
with the required perceived illumination level; and outputting the comparator output
signal from the precision comparator for controlling a power supply to the lighting
system to provide the required perceived illumination level.
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Preferably, the step of outputting the comparator output signal comprises
outputting bit stream data.
-
Conveniently, where the method is for controlling a lighting system having a
plurality of lighting array channels, the step of passing the input signal through a
sigma-delta converter comprises passing a respective plurality of input signals and
inputting the filtered input to a plurality of precision comparators for providing a
plurality of comparator output signals to the respective lighting array channels.
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Preferably, the system model module comprises a respective system model for
each channel.
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Advantageously, the method comprises a further step of monitoring a supply
drive level to the lighting system for input to the system model module.
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Conveniently, the method comprises a further step of sensing ambient light for
inputting an ambient lighting signal to the system model module.
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Preferably, the step of sensing ambient light comprises sensing ambient light
corresponding to a plurality of discrete wavelengths to provide information on the
effective colour characteristics of an ambient light level at a user's eyes.
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Advantageously, the method comprises a further step of sensing illumination
received from the lighting system at a user location and inputting a resultant
illumination feedback signal to the system model module.
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Preferably, the step of feeding back an output from the precision comparator
through a system model module, simulating an effect of the comparator output signal
on the lighting system, comprises monitoring with the system control module at least
one of lighting system supply drive level, time and linearity responses of lighting drive
circuitry and/or the lighting system, and/or modelling at least one of light perception
characteristics of a user's eyes and a user's perception of ambient light levels and
colours.
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Advantageously, the method is arranged for controlling a light-emitting diode
display or illumination system.
-
Conveniently, the step of outputting the output signal comprises controlling at
least one power switch for controlling power supply to the lighting system.
-
Preferably, the method is arranged for driving a three channel RGB array of
light-emitting devices precisely to control a colour perceived by a user, dependent on
ambient lighting colour and level.
-
Advantageously, the method is arranged for controlling a lighting system
precisely to control a colour perceived by a user, dependent on at least one of known
characteristics or defects in the user's optical perception and on optical transmission
media through which light from the lighting system reaches the user.
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Conveniently, the method is arranged for providing nonlinear modulation of the
lighting system.
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According to a third aspect of the invention, there is provided a computer
program comprising code means for performing all the steps of the method described
above when the program is run on one or more computers.
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The invention will now be described, by way of example, with reference to the
accompanying drawings in which:
- Figure 1 is a schematic diagram of a known digital audio system using sigma-delta
modulation;
- Figure 2 is a schematic drawing of a known lighting controller using pulse width
modulation;
- Figure 3 is a schematic drawing of a known multi-channel light-emitting array
controller using the light control system of Figure 2;
- Figure 4 is a schematic diagram of a single channel LED controller according to
the present invention; and
- Figure 5 is a schematic diagram of a multi-channel light-emitting array
controller according to the present invention.
-
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In the Figures, like reference numerals denote like parts.
-
Referring to Figure 1, use of sigma-delta modulation techniques is known in, for
example, a digital audio storage system 10. An input filter 17 receives an input signal
11 and outputs a filtered signal 111 to a first input of a precision comparator 14. A bit
stream output 12 from the precision comparator 14 is input to a system model
reconstruction filter 15. An output 151 of the system model reconstruction filter 15 is
fed back to a second input of the precision comparator 14. The bit stream output 12 of
the precision comparator is also output to a first data processing module 18. There is
an output from the first data processing module 18 to a data storage device 13 and an
output from the data storage device 13 to a second data processing module 19. Output
from the second data processing module 19 is to an input of an audio system 16 having
an output 161. The first data processing module 18 may, for example, be a component
of a compact disc recording apparatus and the second data processing module 19 may
be a component of a compact disc player. In this example, the sigma delta converter
models an audio response of the compact disc player.
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In use, the input signal 11 (which may be analogue or digital) is filtered by the
input filter 17 and a filtered output 111 is translated by the precision comparator 14
into the single bit data stream 12 which is processed and stored on the suitable data
storage device 13. A central feature of the conversion process or modulation technique
is a precision switch mechanism of the precision comparator 14, and modelling of
effects of the precision comparator on forward path system components by a suitable
simulation of the audio system or by a suitable reconstruction filter 15, the output 151
of that filter being used as an estimate of the eventual system output which it is
intended should accurately match the input data. This estimate is used instead of a
feedback signal from the system output, which is not available, and is compared with
the pre-processed demand input 111 filtered by the input filter 17 from the input signal
11. The input filter 17, precision comparator 14 and system model reconstruction
filter 15 together form a sigma-delta converter. A comparison by the precision
comparator 14 is used to select a most appropriate control switch action and may
include elements of either hysteresis or dither in a decision process. Multiple
weighted switched outputs may also be used, but are usually less advantageous in a
practical situation where the relative weightings have to be precisely defined, so that
sigma-delta modulation techniques have been developed essentially for use with single
switch mechanisms.
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With such single switch mechanisms, output of a switch forms a single bit
stream 12 of high speed data representing the input data. This bit stream 12 may
undergo data decimation, compression and further processing in a first data processing
unit 18 before storage on, for example, a computer or compact disc 13. When data is
retrieved from the computer or compact disc the data is reconstructed by a second data
processing unit 19 for output through an audio system 16 with characteristics similar
to those modelled by the system model reconstruction filter 15 in the sigma-delta
conversion process. This total process forms the basis of many known digital audio
data storage techniques.
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Referring to Figure 2, a known light control system 20 uses pulse width
modulation to control a light-emitting device 26. A power supply 23 is connected by
means of an electrically controlled switch 29 to the light-emitting device 26. A ramp
generator 22 is connected to a first input of a comparator 24 and a signal input to
control the light-emitting device 26 is connected to a second input of the comparator
24. An output of the comparator 24 is connected to a control input of the switch 29.
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In use a ramp waveform 221 is output from the ramp generator 22 to the first
input of the comparator 24. A control signal is input to the second input of the
comparator 24. When the control signal is above the ramp waveform input the
comparator 24 produces a high output, when the control signal is below the ramp
waveform input the comparator produces a low output. By varying the control signal a
variable width pulse output 211, 212 is produced by the comparator 24. The variable
pulse width signal is input to the switch 29 to control the brightness of the light-emitting
device 26.
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Referring to Figure 3, three such circuits as described above in relation to Figure
2 may be used in a known multi-channel lighting controller 30 to control red, green
and blue channels controlling arrays of red, green and blue light-emitting arrays
respectively. Similarly to the control system of Figure 2, a power supply 37 is
connected by a first switch 391 to an array of red light-emitting devices 361, 362. A
first comparator 341 has a first ramp waveform 321 applied to a first input and a red
control signal 31 input to a second input. A resultant pulse width modulated output
311 controls the switch 391 to control brightness of the red light-emitting array 361,
362.
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Similarly, the power supply 37 is connected by a second switch 392 to a green
light-emitting array 363, 364. A second comparator 342 has a second ramp waveform
322 applied to a first input and a green control signal 32 input to a second input. A
resultant pulse width modulated output 321 controls the switch 392 to control
brightness of the green light-emitting array 363, 364.
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Similarly, the power supply 37 is connected by a third switch 393 to a blue
light-emitting array 365, 366. A third comparator 343 has a third ramp waveform 323
applied to a first input and a blue control signal 33 input to a second input. A resultant
pulse width modulated output 331 controls the switch 393 to control brightness of the
blue light-emitting array 365, 366.
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The first ramp waveform 321, the second ramp waveform 322 and the third
ramp waveform 323 may be a same waveform or two or three different waveforms.
-
In accordance with the present invention, a related technique to that shown in an
audio control system in Figure 1 may be applied to control of a light-emitting device
or array similar to those shown in Figures 2 and 3 respectively.
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The characteristics of this technique are as follows. A sigma-delta modulation
technique is used to control light output from single channel or from multiple channels
of light-emitting devices or arrays used for either display or illumination purposes.
Modelling inherent in this technique may be used to compensate for linearity and time
response characteristics of all system components including those in the drive
circuitry, light-emitting devices and optical path, and of an observer's eye and colour
perception. The modelling inherent in this technique may further be used to combine
effects of several channels in a composite model which predicts an effect at an
observer's position of a combination of channel outputs, e.g. of red, green and blue
output wavelengths on a colour perceived by the observer.
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Referring to Figure 4, a single channel LED control system 40 according to the
invention has a LED lighting device or array 46 driven by a current source 43 through
a power switch 49. An input filter 47 for receiving an input signal 41 for controlling
the LED lighting device or array 46 has a filtered output 411 to a first input of a
precision comparator 44. The input filter 47 is used to adjust an overall dynamic
system response, typically being a low pass filter which both sets an overall system
time constant and significantly in this application smoothes noise or sharp
discontinuities in input demand data.
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A bit stream data output 42 of the precision comparator 44 is output to a first
input of a system model module 45. An estimated output signal 451 of the system
model module 45 is fed back to a second input of the precision comparator 44. The
input filter 47, precision comparator 44 and system model module 45 together form a
sigma-delta converter. The bit stream data output 42 of the precision comparator is
also output to a control input of the power switch 49 to control current from the
current source 43 to the LED device or array 46. A drive level signal 431 from the
current source 43 is input to a second input of the system model module 45. An
ambient light sensor 481 provides an ambient light signal 482 to a third input of the
system model module 45. Light from the LED light device or array may be observed
at an observer's eye 485. An optional output feedback sensor 483 may read the
illumination level received at the observer's eye 485 and provide an output feedback
signal 484 to an output feedback input of the system model module 45.
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A core of a drive mechanism of the control system 40 is the current source 43,
being a transistor or similar device or a combined drive component, current from
which can be switched either on or off by the power switch 49 to control the LED or
light-emitting array 46. This type of drive mechanism has well-known advantages in
relation to low dissipation levels within component parts of the system. In some
systems this drive mechanism is a combination of such switches providing suitably
weighted output drive levels. A drive mechanism with variable analogue drive level is
equally within the scope of this light-emitting device driving mechanism although it is
not normally associated with sigma-delta modulation.
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In use, a decision about whether power to the light-emitting device or array 46
should be switched on or off is made by comparing a required output light level, from
a pre-processed command present at the system input 41, with a current estimate of a
perceived light level 451. This estimate is generated in the system model module 45
which may include some or all of the response characteristics of the system
components including device supply drive level 431, time and linearity responses, and
which may include models of light perception characteristics of the observer's eye
485, and may further include modelling to incorporate effects on perception of
ambient light levels and colour, for example as received by the ambient light sensor
481. A comparison by the system model module 45 of an estimated output light level
and the demand input 41 informs a decision of an optimum condition for the drive
component 49, and the component 46 may then be switched on or off as most
appropriate. Effects of this choice can then be estimated to great precision in the
system model module 45 to inform a next decision.
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In this application there is normally little advantage in storing the resultant high
speed bit stream data 42, and the bit stream information is applied directly to the
power switch 49 to control the light-emitting device or array 46 and hence system
output.
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The input filter 47, decision method and system model 45 can be realised either
as physical components as illustrated or partially or wholly within software running on
a suitable processor.
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Resultant dynamics of the control system 40 may be manipulated by changing
characteristics of the input filter 47, or characteristics of the system model module 45
as is most appropriate.
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At its simplest, the control system 40 will work when the system model module
45 is given an impulse response of a form c(t) = k.exp(-t/tor), where tor defines an
appropriate system time constant, and where the input filter 47 has unity gain. More
generally models of, for example, the drive mechanism, light-emitting device,
transmission media and eye response can be cascaded and combined into a total
system model to give a required estimate of a system output to great precision.
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The ambient light sensor 481 may typically measure an ambient light level over
a broad spectrum, but may more usefully provide measurements at a number of
discrete wavelengths, providing information as to effective colour characteristics of an
ambient light level at the observer's eye 485.
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A feedback signal 484 on an outer feedback loop, shown as a broken line in
Figure 4, directly sensing the system output, or some relevant components of the
system output, using the feedback sensor 483, can be used for periodic adjustment,
calibration or correction of the system model or of its state or of its outputs. In zero
ambient light conditions the ambient light sensor 481 might be used for this purpose.
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Multiple light-emitting device arrays of two or more devices can be controlled
by using multiples of this single channel system, using different system models to
reflect different device, drive or eye characteristics appropriate to the components in
use on each channel.
-
An embodiment of the invention applied to control of two arrays of light-emitting
devices is illustrated in Figure 5.
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Referring to Figure 5, a multi-channel lighting control system 50 according to
the present invention has first and second light-emitting arrays 561, 562 driven
independently by a current source 53 through respective first and second power
switches 591, 592. An input filter 57 for receiving respective input signals 511, 512
for controlling the first and second light-emitting arrays 561, 562 has respective
filtered first and second outputs 5111, 5121 to respective first inputs of respective first
and second precision comparators 541, 542. Respective first and second bit stream
data outputs 521, 522 of the respective first and second precision comparators are
output to respective first and second inputs of a system model module 55. Respective
estimated output signals 551, 552 of the system model module 55 are fed back to
respective second inputs of the first and second precision comparators 541, 542. The
input filter 57, first and second precision comparators 541, 542 and system model
module 55 together form a sigma-delta converter. The respective bit stream data
outputs 521, 522 of the first and second precision comparators 541, 542 are also output
to respective control inputs of the first and second power switches 591, 592 to control
currents from the power supply 53 to the first and second light-emitting arrays 561,
562 respectively. A drive level signal 531 from the power supply 53 is input to a third
input of the system model module 55. An ambient light sensor 581 provides an
ambient light signal 582 to a fourth input of the system model module 55. The first
and second light-emitting arrays 561, 562 may be observed by an observer's eye 585,
either directly or through an optional transmission medium 565, shown by broken
lines in Figure 5. An optional output feedback sensor 583 may read an illumination
level received at the observer's eye 585 and provide an output feedback signal 584 to
an output feedback input of the system model module 55.
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A particular advantage of this technique is an ability to combine modelling of
two or more channels to model a total effect at the observer's eye 585 of light output
or of differently coloured outputs from each channel. This enables sophisticated
modelling and control of colour or illumination perception to be achieved, whilst often
yielding an advantage in reducing a total execution period of the system models if
realised in software. With sophisticated modelling of this type, input demand
parameters may not be the same as those being controlled by the data bit streams, the
conversions being executed in a multiple input, multiple output filter such as the input
filter 57 or system model module 55. Similarly the components of the output which
are sensed and fed back to the system model may not be directly related to the
parameters controlled by the bit stream data or to the input data 511, 512.
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A typical use of this technique is driving a three channel RGB array of light-emitting
devices precisely to control a colour that an observer perceives, with
compensation for ambient lighting colour and level, and modelling (if required) of
known characteristics or defects in an observer's optical perception or in an optical
transmission medium 365 through which light from the controlled lighting or display
device is observed.
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This three-channel, three-colour system is a main application for this drive or
control technique. However, a four colour system, for example, may also be used.
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The sigma-delta modulation technique described herein allows the
implementation of lighting systems with latency periods typically hundreds of times
shorter than in the prior art to give much improved speed of control over perceived
light levels, with higher bandwidth compensation if required for 100 Hz supply
variations. With much shorter cycle times and non-linear modelling available through
sigma-delta modulation techniques, non-linear modulation schemes become available,
and provide a technique of compensation for both system component and eye response
non-linearities.
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Some of the advantages of the present invention over existing technology are as
follows. Existing drive techniques for light-emitting devices typically use PWM to
drive LEDs (light-emitting diodes) and LED arrays. This technique has significant
latency, in that a required change of illumination or display light level from the light-emitting
device can only be implemented during a next complete PWM cycle after the
new demanded level information is received. The sigma-delta modulation technique
described herein is able to implement systems with latency periods typically hundreds
of times shorter to give much improved speed of control over perceived light levels,
with higher bandwidth compensation if required for 100 Hz supply variations.
-
Existing PWM techniques typically provide only a linear scale of modulation in
that each incremental change in demanded input level gives rise to a linear change in a
width of a period for which the drive mechanism is switched on. With the much
shorter cycle times and non-linear modelling available through sigma-delta
modulation techniques, non-linear modulation schemes become available, and provide
a technique of compensation for both system component and eye response non-linearities.
'
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Advantages of this technique may therefore be summarised as one or more of the
following:
- ability to use a relatively simple, switched, low-dissipation drive
technique;
- superior time response over existing modulation techniques in use (for
example, no period latency as with PWM);
- ability to use precision system modelling techniques in order to
incorporate correction for non-linear system characteristics;
- use of a higher effective modulated carrier frequency so that drive circuit
filter components are usually smaller and of lower dissipation than those
necessary with other techniques;
- use of a higher effective modulated carrier frequency so that periodic
supply current fluctuations associated, for example, with synchronous
PWM are eliminated;
- ability to synchronise modulation process with disturbances on a power
supply to minimise effects of the said fluctuations;
- precision control of single light-emitting device output with
compensation for both non-linearities and time responses due to all
system components such as power drive circuit, power drive circuit
filtering, light-emitting device characteristics, light transmission media
and receiver's eye response;
- precision control of multiple light-emitting device array light output with
compensation for both non-linearities and time responses due to all
system components such as power drive circuits, power drive circuit
filtering, light-emitting device characteristics, light transmission media
and receiver's eye response;
- precision control of the light output of multi-coloured arrays of light-emitting
devices with compensation for both non-linearities and time
responses due to all system components such as power drive circuit,
power drive circuit filtering, light-emitting device characteristics, light
transmission media and receiver's eye response;
- a method of controlling the illumination/display colour perceived by an
observer to provide correction for problems of colour perception
including partial colour blindness; and
- a mechanism within the technique for compensation against changes in
ambient light level or of ambient light colouration which will maintain a
constant colour perception in the observer despite such changes.