WO2008126025A2

WO2008126025A2 - Method for processing a digital signal

Info

Publication number: WO2008126025A2
Application number: PCT/IB2008/051357
Authority: WO
Inventors: Alexander C. Geerlings; Robertus L. Tousain
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-04-17
Filing date: 2008-04-10
Publication date: 2008-10-23
Also published as: TW200908782A; WO2008126025A3

Abstract

A system receiving a digital signal (50) making a transition (50) from a first discrete signal value Il to a second discrete signal value 12 can show undesirable oscillations. The invention proposes a method for reducing such oscillations. The method comprises the step of generating a series of one or more precursor pulses (52,- 52, 53, 54, 55) prior to said signal transition, each precursor pulse having the second discrete signal- value. The i-th pulse has a duration Δi in a precursor time interval of duration Ti, and 'thus represents an average signal value Si = 11 + (12-11 ) &sdot,- Δi/Ti. This average signal value Si is continuously rising (if I2>I1) or falling (if I2<I1), preferably in a linear manner. A signal processing circuit capable of performing the method can be interposed between a. signal source and a receiving system, which may be a lighting system comprising at least one LED.

Description

Method for processing a digital signal

FIELD OF THE INVENTION

The present invention relates in general to a method for providing a truly digital signal to a resonant system or circuit. In a particular example, the present invention relates to a system for driving an LED, and the present invention will be specifically explained for this example, but it is to be noted that the invention is not limited to this example.

BACKGROUND OF THE INVENTION

In general, systems receiving an input signal exhibit a response characteristic, which characteristic is a property of the system. For instance, when the input signal makes a fast transition from one signal value to another, the system will exhibit a certain step response, which may involve an overshoot and/or a dampened oscillation, depending on eigenfrequencies of the system. This is illustrated in figure 1, which shows a graph of input signal (upper curve) and system response (lower curve) as a function of time (horizontal axis). Before time tl, the system is in an equilibrium state, where the input signal has a constant signal value II, and some system parameter has a constant value Pl. The system parameter may be a signal level, a light magnitude, a motor position, etc. At time tl, the input signal makes a stepwise change from constant signal value Il to constant signal value 12. The figure shows that the system parameter will eventually reach another equilibrium value P2, but the system parameter can not follow the rapid change. So, in this example, the system parameter will relatively slowly rise towards P2, will continue rising beyond P2 but at a reducing rise speed, and will end up oscillating around P2 with a reducing amplitude.

The problem can be solved by amending the design of the system such as to alter the eigenfrequencies. However, the present invention relates to situations where the system receiving the signal can not be altered, so that its eigenfrequencies are to be considered as a given.

In the case the input signal is an analogue signal, it is possible to amend the signal shape such that the system response to the amended signal better resembles the desired system response. For instance, it is possible that a signal step from value Il to value 12 is subdivided into two steps, the first step being from first value Il to an intermediate value 13, the second step being from the intermediate value 13 to the second value 12, the steps being shifted in time over a distance equal to half the period of the resonant oscillation of the system, as shown in the third curve of figure 1. It is also possible to apply a low-pass filter, which will effectively reduce the steepness of a step change, so that the receiving system can more easily follow the change.

However, in case the input signal is a digital signal, it is not possible to make such amendments to the signal shape, because a digital signal can only take discrete values (0 and 1) and can only make a stepwise transition from a 0 to a 1 or vice versa. As mentioned, one example where this problem plays a role is lighting, particular the dimming of light sources. In the case of an incandescent lamp, dimming can be achieved by applying a reduced voltage of constant magnitude to the lamp. However, LEDs are either ON or OFF, and dimming is achieved by applying duty cycle control or Pulse Width Modulation: the LED is powered by a sequence of power pulses, and the average light intensity is determined by the ratio of the pulse width to the pulse distance (or the ratio of the pulse width to the pulse period). Now, each pulse constitutes a transient which may excite an eigenfrequency of the system, i.e. the combination of LEDs and driving electronics. Once excited, such eigenfrequencies can cause fluctuating mechanical stresses in electrical components, and even audible noise. Noise is undesirable in view of being annoying to people, fluctuating mechanical stresses can affect the performance and reduce the lifetime of components.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a solution for the above problem.

Particularly, the present invention aims to adapt a truly digital signal which is used as an input to a resonant system or circuit, where the goal of the adaptation is to reduce or avoid ringing and overshoot in the system/circuit response.

Particularly, the present invention aims to provide a digital signal source capable of providing a digital signal in such a manner that a receiving system, receiving the digital signal, will show no or only little oscillation, or at least will show reduced oscillation.

To this end, a signal source generates at least one precursor pulse. Preferably, the signal source generates a series of two or more precursor pulses, in which case it is further preferred that later precursor pulses have larger pulse width. On average, the digital signal will have a relatively slowly changing signal level, which can be followed by the receiving system more easily.

Further advantageous elaborations are mentioned in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of the present invention will be further explained by the following description of one or more preferred embodiments with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: figure 1 is a graph showing signals as a function of time; figure 2 is a block diagram schematically illustrating a signal processing assembly; figure 3 is a graph illustrating the theoretical behavior of a LED; figure 4 is a graph illustrating precursor pulses according to the present invention; figure 5A is a graph illustrating a digital control signal with precisely one precursor pulse; figure 5B is a graph illustrating system response in the case of receiving a digital control signal with precisely one precursor pulse as compared to the system response in the case of receiving a digital control signal with no precursor pulses.

DETAILED DESCRIPTION OF THE INVENTION

Figure 2 schematically shows an assembly 1, comprising a signal source 10 for providing an original digital signal Sc, a signal receiving system 30, and a signal adaptation circuit 20 interposed therebetween. The signal adaptation circuit 20 receives the original digital signal Sc as an input signal, and produces an adapted digital signal Sa as output signal, which is received by the signal receiving system 30 as an input signal.

Without the signal adaptation circuit 20, the assembly may be a standard assembly. For instance, in the field of illumination, the assembly may be a standard lighting assembly, wherein the signal receiving system 30 is the combination of one or more LEDs plus driver, and wherein the signal source 10 is a controller generating a digital control signal Sc for the driver, dictating the driver when the LED(s) should be ON or OFF. In the following, it will be assumed that control signal value "1" corresponds to LED ON, and that control signal value "0" corresponds to LED OFF. Figure 3 is a graph illustrating the theoretical, intended behavior of the LED(s) in relation to the digital control signal Sc. When the digital control signal Sc (upper curve) has value "0", the LED driver does not generate LED current, and the LED does not generate light (light level 0). When the digital control signal has value "1", the LED driver generates nominal LED current, and the LED generates nominal light output (magnitude Lmax). Thus, the LED switches ON and OFF at a pace dictated by the digital control signal Sc. In practice, the frequency of the digital control signal Sc will be chosen such that the ON/OFF switching (flickering) of the LED is not visible to the human eye, for instance by having the frequency well above 100 Hz. To the human eye, the LED behaves as producing a constant average light level Lav.

However, as mentioned above, the steep transients of the digital control signal Sc may cause oscillation problems in the system 30.

Figure 4 is a graph, similar to figure 1 yet on a larger scale, illustrating the operation of the signal adaptation circuit 20 of the present invention. The figure shows an input signal Sc (upper graph, curve 40) as received by the signal processing circuit 20, and an output signal SA (lower graph, curve 50) as outputted by the signal adaptation circuit 20. For explaining the invention, the figure only shows a transient from "0" to "1" of the input signal Sc, but it should be clear to a person skilled in the art that a similar explanation would apply for a transient in the opposite direction.

The explanation starts with a steady state situation where the input signal Sc is "0" while the output signal SA is also "0". At time tl, the input signal Sc makes a transition 49 to "1". At time t2, the output signal SA makes the same transition 59. It is noted that the mutual timing of the input signal Sc and the output signal SA is not relevant for understanding and practicing the present invention. In the exemplary embodiment, with a

"normal" control circuit 10 and a separate adaptation circuit 20 adapting the output signal of the control circuit 10, there will undoubtedly be some delay between tl and t2, due to inevitable response times of circuit components. However, it is also possible that control circuit 10 itself is adapted to practice the present invention, so that, in stead of providing a "normal" control Sc, the adapted control circuit provides an adapted control signal SA; in that case, the software of the control circuit 10 may be arranged so that time t2 corresponds with time tl. It is noted that, at some later time not shown in the figure, the input signal Sc will drop down to "0" again, and the output signal SA will do the same. Thus, the input signal Sc has a pulse 41, defined by said two subsequent transitions from "0" to "1" and back, and the time duration between these two subsequent transitions will be indicated as "pulse duration" or "pulse length".

The output signal SA will likewise have a pulse 51, which will be indicated as "main pulse". According to the invention, the signal adaptation circuit 20 also generates a series of precursor pulses 52, 53, 54, 55 just before the main pulse 51.

The number of precursor pulses is not essential. In the example of figure 4, four precursor pulses are shown, but there may also be one, two or three precursor pulses or five or more.

In case the number of precursor pulses is two or more, the different precursor pulses may have mutually the same width, i.e. precursor pulse duration. However, it is preferred that the precursor pulses have successively increasing precursor pulse width, as clearly shown in figure 4. The mutual distance between the successive precursor pulses may remain constant, but in the example shown these distances are successively decreasing, so that the precursor pulse period remains constant.

Alternatively, it is possible that the precursor pulses have constant precursor pulse duration but successively decreasing mutual distance. Several other variations will be clear to a person skilled in the art after reading the following explanation.

In the embodiment of figure 4, the first precursor pulse 52 has its rising edge at time ti l and its falling edge at time tl2, thus a first precursor pulse duration Δl=tl2-tl 1. The second precursor pulse 53 has its rising edge at time t21, so the distance Dl between the first precursor pulse 52 and the next precursor pulse 53 is equal to Dl=t21-tl2. A first precursor time segment, defined as the time segment from ti l to t21, has a first time segment duration T 1 =t21 -t 11 , which corresponds to the precursor pulse period.

The second precursor pulse 53 has its falling edge at time t22, thus a second precursor pulse duration Δ2=t22-t21. The third precursor pulse 54 starts at t31 and ends at t32, thus a third precursor pulse duration Δ3=t32-t31. The fourth precursor pulse 55 starts at t41 and ends at t42, thus a fourth precursor pulse duration Δ4=t42-t41. Further, second, third and fourth precursor time segments are defined, having durations T2=t31-t21, T3=t41-t31, T4=t2-t41, respectively.

In the preferred embodiment of figure 4, T1=T2=T3=T4. Further, in the preferred embodiment, Δ2=2-Δ1, Δ3=3-Δ1, Δ4=4-Δ1. More generally, it is preferred that the precursor pulse durations increase in a linear manner, so that for the i-th pulse the precursor pulse duration Δi is equal to Δl-i/(n+l), with n indicating the number of precursor pulses. Although the precise length of the precursor pulses, or the precise value of the time segment duration, is not essential, each of the precursor pulses has a duration much smaller than the duration of the main pulse 51. More particularly, if the receiving system 30 has a response time τ, each of the precursor pulses has a duration smaller than the response time τ. Here, the response time τ means the time needed for the measured parameter (see the lower curve in figure 1) to reach its final equilibrium value P2 for the first time. Briefly stated, this means that the receiving system 30 can not follow the short precursor pulses. Effectively, it might be said that the input of the receiving system 30 behaves as a low-pass filter for the precursor pulses. The response of the receiving system 30 will be equal to, or comparable to, its response for the case that it would receive a signal identical to the time-average of its input signal SA- This time-average is shown as the sloping line 61 in figure 4. Thus, effectively, the result of the invention is that the receiving system 30 responds to its input signal SA as if this had a reduced steepness, so that the overshoot and possible oscillations are greatly reduced, while actually the input signal for the receiving system 30 is still a truly digital signal.

Describing the invention at the level of individual precursor pulses, each precursor pulse represents an average signal value S defined as Si = Δi/Ti, wherein Si > Sj for i>j. It should be clear that this can be effected with constant Ti if Δi > Δj for i>j, or with constant Δi if Ti < Tj for i>j, for example.

The above applies for a rising transient. For a transient from a "1" to a "0", Si < Sj for i>j applies. In the above, the signal values are indicated as "0" and "1", respectively. More generally, if the signal values are indicated as Il and 12, respectively (see figure 1), Si = Il + Δi/Ti (I2-Il) applies.

It is noted that the adapted signal SA is used as a control signal for a receiving circuit 30, wherein the overall duty cycle of the control signal determines a property of the receiving circuit 30, for instance a light output level of a LED, as explained above. The invention is preferably practiced in such a way that the overall duty cycle of the adapted signal SA is equal to the overall duty cycle of the original control signal SQ. This may mean that the duration of main pulse 51, starting from transition 59, is reduced by an amount corresponding to the summation of the precursor pulse durations Δi, taking into account any precursor pulses at the end of the main pulse.

In a particular embodiment of the application, only one precursor pulse is used. Figure 5A is a graph comparable to figure 4, showing the waveform of the adapted digital signal SA for this case. In that case, the width Δl of the precursor pulse 52 and the time Dl between the precursor pulse 52 and the rising signal 59 of the main pulse 51 should be chosen compliant with the resonance frequency of the circuit or system 30. A possible embodiment is one in which the sequence times are updated continuously based upon the actual resonant frequency. The effect of the smartly shaped pulse sequence on the current/voltage in the circuitry is given in figure 5B, which is a graph comparable to the second and fourth graphs of figure 1. Line 61 indicates the current/voltage response to an unadapted control signal Sc; it can be seen that substantial ringing occurs: the peak current/voltage is almost twice the value of the desired step. Line 62 indicates the current/voltage response to the adapted control signal SA of figure 5 A; almost no overshoot occurs and ringing is largely suppressed.

Summarizing, the present invention provides a method for reducing the undesirable oscillations which may occur in a system 30 receiving a digital signal 50 making a transition 59 from a first discrete signal value Il to a second discrete signal value 12 can show undesirable oscillations. The method comprises the step of generating a series of one or more precursor pulses (52; 52, 53, 54, 55) prior to said signal transition, each precursor pulse having the second discrete signal value. The i-th pulse has a duration Δi in a precursor time interval of duration Ti, and thus represents an average signal value Si = Il + (I2-Il)-Δi/Ti. This average signal value Si is continuously rising (if I2>I1) or falling (if I2<I1), preferably in a linear manner.

A signal processing circuit 20 capable of performing the method can be interposed between a signal source 10 and a receiving system 30, which may be a lighting system comprising at least one LED. The intermediate processing circuit influences the behavior of the receiving system without having to modify the receiving system, while the receiving system still receives a digital signal. Alternatively, instead of a separate unit 20 connected in between a signal source 10 and a signal receiver 30, the signal processing circuit 20 of the present invention may also be integrated with the signal source 10 or the signal receiver 30.

In both of the above types of implementation, the signals are advantageously generated by software, which makes implementation of the present invention relatively simple, even in existing devices, where a relatively simple amendment of the software suffices.

While the invention has been illustrated and described in detail in the drawings and foregoing description, it should be clear to a person skilled in the art that such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments; rather, several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. For instance, instead of an illumination system, the invention can be applied to systems of quite different type, such as for instance a stepper motor.

Further, instead of a digital signal where the signal can take only two discrete values, the present invention is also applicable in cases where the signal can take three or more discrete values. In the following, therefore, such signal will generally be indicated as a discrete signal.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.

Claims

CLAIMS:

1. Method for reducing undesirable oscillations in a system (30) receiving a discrete signal (SA) making a transition (59) from a first discrete signal value Il to a second discrete signal value 12 differing from the first discrete signal value; the method comprising the step of, prior to said signal transition (59), applying to the system (30) a series of one or more digital precursor pulses (52; 52, 53, 54, 55) each precursor pulse having the second discrete signal value.

2. Method according to claim 1, wherein said series contains precisely one precursor pulse (52) with duration tuned to the dominant resonance frequency of the receiving system (30).

3. Method according to claim 1, wherein said series contains two or more pulses; wherein the i-th pulse has a duration Δi in a precursor time interval of duration Ti, and thus represents an average signal value Si = Il + (12-11 ) Δi/Ti; and wherein Si > Sj for i>j if I2>I 1 or Si < Sj for i>j if I2<I 1.

4. Method according to claim 3, wherein the receiving system (30) has a response time τ, and wherein Δi < τ for all precursor pulses.

5. Method according to claim 3, wherein the average signal value Si of the i-th pulse fulfils the relationship Si = Il + (12-11 )-i/(n+l), with n indicating the number of precursor pulses.

6. Method according to claim 3, wherein the duration Ti of the precursor time interval is equal for all precursor pulses.

7. Method for generating an adapted digital control signal (SA) for a system (30), the method comprising the steps of: receiving a discrete control signal (Sc) making a transition (49) from a first discrete signal value Il to a second discrete signal value 12 differing from the first discrete signal value; generating a discrete adapted control signal (SA) making a transition (59) from the first discrete signal value Il to the second discrete signal value 12 differing from the first discrete signal value; prior to said signal transition (59), generating a series of one or more digital precursor pulses (52; 52, 53, 54, 55) each precursor pulse having the second discrete signal value; and applying to the system (30) the series of precursor pulses followed by said transition (59).

8. Method for generating a digital control signal (SA) for a system (30), the method comprising the steps of: generating the digital control signal (SA) at a first discrete signal value II; - generating a series of one or more digital precursor pulses (52; 52, 53, 54, 55) each precursor pulse having a second discrete signal value differing from the first discrete signal value; causing the control signal (SA) to make a transition (59) from the first discrete signal value Il to the second discrete signal value 12 to generate a main pulse (51).

9. Signal source for generating for a receiving system (30) a digital control signal making a transition (59) from a first discrete signal value Il to a second discrete signal value 12 differing from the first discrete signal value; the signal source being designed for performing the method of any of claims 1-6.

10. Signal adaptation circuit (20) for adapting a discrete input signal (Sc) that can take only discrete signal values (II, 12), the processing circuit (20) being designed for receiving the input signal ((Sc) and generating an output signal (SA) having the same signal value as the input signal when the signal value of the input signal is constant; the adaptation circuit (20) being designed, in response to the input signal making a transition (49) from a first discrete signal value Il to a second discrete signal value 12 differing from the first discrete signal value: to generate a series of one or more precursor pulses (52; 52, 53, 54, 55), each precursor pulse having the second discrete signal value; and to subsequently let the output signal make a transition (59) from the first discrete signal value Il to the second discrete signal value 12.

11. Circuit according to claim 10, wherein said series contains precisely one precursor pulse (52) with duration tuned to the dominant resonance frequency of the receiving system (30).

12. Circuit according to claim 10, wherein said series contains two or more pulses; wherein the i-th pulse has a duration Δi in a precursor time interval of duration Ti, and thus represents an average signal value Si = Il + (12-11 ) Δi/Ti; and wherein Si > Sj for i>j if I2>I1 or Si < Sj for i>j if I2<I1;

13. Circuit according to claim 12, wherein the average signal value Si of the i-th pulse fulfils the relationship Si = Il + (12-11 )-i/(n+l), with n indicating the number of precursor pulses.

14. Circuit according to claim 12, wherein the duration Ti of the precursor time interval is equal for all precursor pulses.

15. Assembly ( 1 ), comprising : a signal source according to claim 9; and a signal receiving system (30) coupled to receive the output signal from the signal source.

16. Assembly (1), comprising: a signal source (10) for generating a discrete signal (Sc) that can take only discrete signal values (II, 12); - a signal adaptation circuit (20) according to any of claims 9-13, coupled to receive as input signal the discrete signal produced by the signal source (10); and a signal receiving system (30) coupled to receive the output signal (SA) from the signal adaptation circuit (20).

17. Assembly according to claim 15 or 16, wherein the receiving system (30) has a response time τ, and wherein Δi < τ for all precursor pulses.

18. Assembly according to claim 15 or 16, wherein the signal receiving system

(30) is a lighting system comprising at least one LED.

19. Digital signal (SA) for controlling a receiving system (30), the signal comprising: - a portion having a first discrete signal value II; a series of one or more digital precursor pulses (52; 52, 53, 54, 55) each precursor pulse having a second discrete signal value differing from the first discrete signal value; a transition (59) from the first discrete signal value Il to the second discrete signal value 12; a main pulse (51) having the second discrete signal value 12.