Abstract:
A method and system process a signal for PWM modulation. An amplitude control signal adjusts the amplitude of an input signal, and an offset is added to the amplitude-adjusted signal to produce an offset-adjusted signal. The offset is selected according to the amplitude adjustment applied to the input signal. The offset-adjusted signal is pulse-width modulated the to produce a pulse-width modulated signal, and the pulse-width modulated signal is filtered to reduce high frequency components thereof.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims priority under 35 U.S.C. § 119 from Korean Patent Application 10-2007-0045141, filed on 9 May 2007 in the name of Bong Joo Kim, the entirety of which is hereby incorporated by reference for all purposes as if fully set forth herein. 
       BACKGROUND AND SUMMARY 
       [0002]    1. Field 
         [0003]    This invention pertains to the field of pulse width modulation (PWM) and devices, such as audio amplifiers, that process data signals using PWM. 
         [0004]    2. Description 
         [0005]    A switching amplifier, or class-D amplifier, is an electronic amplifier where the active devices (especially in the output stage) are operated in on/off mode (i.e., as switches).  FIG. 1  shows a block diagram of one embodiment of a class-D amplifier  100  for processing an analog input signal. Amplifier  100  includes triangular wave generator  120 , comparator  140 , switching controller  160 , and low pass filter  180 . The output of amplifier  100  is provided to a relatively fixed load (e.g., a loudspeaker  10 , which typically might have an impedance of 8 ohms). 
         [0006]    Amplifier  100  employs pulse width modulation (PWM) to convey the information of the analog input signal (e.g., an audio signal). The input signal is converted to a sequence of pulses whose average value is directly proportional to the amplitude of the signal at that time. The frequency of the pulses is typically ten or more times the highest frequency of interest in the input signal. The output signal produced by switching controller  160  consists of a train of pulses whose width is a function of the amplitude &amp; frequency of the input signal being amplified, and hence amplifier  100  is also called a PWM amplifier. The output signal from switching controller  160  is filtered by low pass filter  180  to remove the aforementioned high frequency components of the pulses. PWM amplifier  100  feeds a varying audio signal voltage into loudspeaker  10 . 
         [0007]    The output signal contains, in addition to the required amplified input signal, unwanted spectral components (i.e. the pulse frequency and its harmonics) that must be removed by low pass filter  180 . Low pass filter  180  is typically fabricated using (theoretically) lossless components like inductors and capacitors in order to maintain efficiency. 
         [0008]      FIG. 2  is a functional block diagram of one embodiment of a PWM amplifier  200 . PWM amplifier  200  includes a volume control block  210 , an oversampler  220 , a Delta-Sigma modulator  230 , a PWM mapper  240 , and a filter  250 . 
         [0009]    In contrast to amplifier  100  in  FIG. 1 , PWM amplifier  200  operates with a digital audio input signal. It must be noted that all real world audio signals are continuous-time analog signals. Therefore, sampling and quantization must be applied to convert the continuous-time analog signal to a discrete-time digital representation for use with PWM amplifier  200 . 
         [0010]    PWM amplifier  200  receives at its input a digital audio signal as pulse-code modulated data PCM_DATA, and receives a volume control signal VOL_CON, and outputs an amplified output signal AUD_OUT. PCM is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a digital (usually binary) code. 
         [0011]    Volume control block  210  includes a volume table  211  and a multiplier  215 . Volume table  211  stores in a memory volume data VOL_DATA corresponding to each value of VOL_CON. VOL_DATA is a digital code (e.g. if VOL_CON is 4-bit data-&gt;Volume Table stores 16 values for VOL_DATA). In operation, volume table  211  receives the volume control signal VOL_CON and in response thereto generates a corresponding value for VOL_DATA which it outputs as the Volume. The value of Volume is then applied to multiplier  215  in order to adjust the level of PCM_DATA to output a volume-controlled audio signal VD. 
         [0012]      FIG. 3  illustrates a block diagram of oversampler  300  which is one possible embodiment of oversampler  220 . Oversampler  300  includes a first sampler operating at a frequency Fs, a low pass interpolation filter, and a second sampler operating at a much higher frequency (e.g., 64 Fs) that the first sampler. In signal processing, oversampling is the process of sampling a signal with a sampling frequency significantly higher than twice the bandwidth or highest frequency of the signal being sampled. Oversampling reduces quantization noise and increases resolution. Oversampler  220  oversamples the volume-controlled audio signal VD which is the output by volume control block  210  and outputs an oversampled signal DSM_IN. 
         [0013]      FIG. 4  illustrates a block diagram of Delta Sigma Modulator  400  which is one possible embodiment of Delta Sigma Modulator  230 . Delta Sigma Modulator  400  includes summer  410 , loop filter  420 , and quantizer  430 . Loop filter  420  performs noise shaping by moving the quantization noise to higher frequencies which the ear can&#39;t hear. Quantizer  430  requantizes the signal output by loop filter  420 . The output of quantizer  430  is fed back to summer  410  quantizer  430  for quantization noise reduction. 
         [0014]    Delta Sigma Modulator  230  quantizes the oversampled signal DSM_IN to produce an output signal DSM_OUT having a fewer number of bits. With current technology (e.g., a system clock of 100˜200 MHz), one can not make a PWM pulse of high resolution (e.g. 16 bits), so it needs to be re-quantized to a smaller number of bits (e.g. 4˜5 bits) by Delta-Sigma Modulator  230 . 
         [0015]    PWM mapper  240  receives the PCM signal DSM_OUT and in response thereto produces and outputs a PWM signal. PWM mapper  140  modulates the width of the pulse in the PWM signal in proportion to the volume of the input signal DSM_OUT. PWM uses a square wave whose duty cycle is modulated resulting in the variation of the average value of the waveform.  FIG. 5  illustrates an operation of PWM mapper  140  in the case where a three-bit PCM signal is converted to a one-bit PWM signal. 
         [0016]    Low Pass Filter (LPF)  250  is a filter that passes low frequency signals (i.e., the required amplified signal) and removes unwanted spectral components (i.e., signals at the pulse frequency). Beneficially, LPF  250  is made with theoretically lossless components like inductors and capacitors. 
         [0017]    A properly designed class-D amplifier offers the following benefits: small size and weight; low power (heat) a dissipation and hence a small heatsink requirements (or no heatsink at all); low cost due to the small heat sink requirements and compact circuitry; and very high power conversion efficiency, usually ≧90%. 
         [0018]    Hereinafter, the current which is consumed by transferring the amplified signal to the speaker is called “dynamic current” and the current which is consumed by low pass filter filtering the unwanted spectral components is called “static current.” The total current that the PWM amplifier consumes is the sum of the dynamic current and the static current. 
         [0019]      FIG. 6  illustrates the relationship between the static current and the total current consumption in the conventional PWM amplifier. As can be seen in  FIG. 6 , when the amplitude of the signal (i.e., the volume of an audio signal) is at its maximum value, then the load current (i.e. the dynamic current) which is passed by the low pass filter and transferred to the load (i.e., the loudspeaker) is the greatest portion of the total current consumption of the amplifier. But as the amplitude of the signal decreases, then the total current consumption decreases while the static current consumed in the low pass filter increases so as eventually to be in excess of the load current and therefore become the greatest portion of the total current consumption of the PWM amplifier. 
         [0020]    In practice, an audio signal is rarely set at its maximum value, and is more typically at a much lower amplitude. As a result, most of the current consumption of the PWM amplifier is attributed to the static current consumed by the low pass filter. This static current is effectively wasted power and therefore diminishes the power efficiency of the PWM amplifier. 
         [0021]    The relationship illustrated in  FIG. 6  can be explained as follows. 
         [0022]    First, the duty ratio of the PWM signal is defined as the ratio between the period of time when the PWM signal is at the logic HIGH state and the period of time when the PWM signal is at the logic LOW state. The amount of static current in the PWM amplifier depends on the duty ratio of the PWM signal. As the duty ratio approaches 1:1, the static current increases, and as the duty ratio increases in magnitude (e.g., 1:2, 1:3 . . . ), then the static current decreases. 
         [0023]      FIG. 7  is a flowchart illustrating operation of the conventional PWM amplifier  200 . As can be seen in  FIG. 7 , the conventional PWM amplifier  200  maintains the duty ratio of the PWM signal close to 1:1 regardless of the volume or magnitude of the audio signal, because the audio signal is alternating between (+) and (−) values. However, as shown in  FIG. 7 , there are some differences in the operation of conventional PWM amplifier  200  between when the volume of the audio signal is at a maximum value and when it is not at its maximum value. When the volume of audio signal is at a maximum value, then the PWM region is fully used by the audio signal and the amount of static current is negligible as compared with dynamic current. In contrast, when the volume of the audio signal is not at a maximum value, then a portion of the PWM region is unused by the audio signal, and the amount of static current is substantial as compared with dynamic current. 
         [0024]      FIG. 8  illustrates signals in PWM amplifier  200  in the case where the volume of the audio signal is at a maximum value. In this case, it is seen that the total range of the PWM pulse width is used by the signal. 
         [0025]      FIG. 9  illustrates signals in PWM amplifier  200  in the case where the volume of the audio signal is not at a maximum value. In this case, it is seen that the total range of the PWM pulse width is not used by the signal. 
         [0026]    Although the relationship between static current and dynamic current in a PWM modulator has been explained in the context of an amplifier, and particularly an audio amplifier, in general the same relationship may apply in other devices employing a PWM modulator to modulate a signal, for example, a motor control system. 
         [0027]    Accordingly, it would be advantageous to provide a method of PWM data processing which has a reduced static current. It would also be advantageous to provide an device or system that employs pulse width modulation which exhibits a reduced static current. Other and further objects and advantages will appear hereinafter. 
         [0028]    In one aspect of the invention, a method of processing a signal comprises: adjusting an amplitude of an input signal according to an amplitude control signal; adding an offset to the amplitude-adjusted signal to produce an offset-adjusted signal, wherein the offset is selected according to the amplitude adjustment applied to the input signal; pulse-width modulating the offset-adjusted signal to produce a pulse-width modulated signal; and filtering the pulse-width modulated signal to reduce high frequency components thereof. 
         [0029]    In another aspect of the invention, a method of processing an input signal comprises pulse-width modulating the input signal with a pulse-width modulator (PWM) to produce a PWM signal, and then filtering the PWM signal to reduce high frequency components of the pulse-width modulated signal, further comprising adjusting a duty ratio of the PWM signal in response to an amplitude control signal. 
         [0030]    In yet another aspect of the invention, an audio processing system comprises: a volume control adapted to adjust a volume of an input signal in response to a volume control signal; an offset generator adapted to generate an offset to be applied to the volume-adjusted input signal, wherein the offset is selected in response to the volume control signal; a combiner adapted to apply the offset to the volume-adjusted input signal to produce an offset-adjusted signal; a pulse width modulator adapted to pulse-width modulate the offset-adjusted signal; and a filter adapted to reduce high frequency components of the pulse-width modulated signal. 
         [0031]    In still another aspect of the invention, a motor control system comprises: an amplitude control adapted to adjust an amplitude of an input signal in response to an amplitude control signal; an offset generator adapted to generate an offset to be applied to the amplitude-adjusted input signal, wherein the offset is selected in response to the amplitude control signal; a combiner adapted to apply the offset to the amplitude-adjusted input signal to produce an offset-adjusted signal; a pulse width modulator adapted to pulse-width modulate the offset-adjusted signal; and a filter adapted to reduce high frequency components of the pulse-width modulated signal. 
         [0032]    In a further aspect of the invention, a system adapted to process an input signal with a pulse-width modulator (PWM) to produce a PWM signal, and further adapted to filter the PWM signal to reduce high frequency components of the PWM signal, further comprises a duty-cycle adjustment element adapted to adjust a duty cycle of the PWM signal in response to an amplitude control signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  shows a block diagram of one embodiment of a class-D amplifier. 
           [0034]      FIG. 2  is a functional block diagram of another embodiment of a pulse width modulation (PWM) amplifier. 
           [0035]      FIG. 3  illustrates a block diagram of one embodiment of an oversampler. 
           [0036]      FIG. 4  illustrates a block diagram of one embodiment of a Delta Sigma Modulator. 
           [0037]      FIG. 5  illustrates an operation of PWM mapper in the case where a three-bit PCM signal is converted to a one-bit PWM signal. 
           [0038]      FIG. 6  illustrates the relationship between the static current and the total current consumption in the conventional PWM amplifier. 
           [0039]      FIG. 7  is a flowchart illustrating operation of a conventional PWM amplifier. 
           [0040]      FIG. 8  illustrates signals in a conventional PWM amplifier in the case where the volume of an audio signal is at a maximum value. 
           [0041]      FIG. 9  illustrates signals in a conventional PWM amplifier in the case where the volume of an audio signal is not at a maximum value. 
           [0042]      FIG. 10  is a functional block diagram of a first embodiment of a PWM amplifier. 
           [0043]      FIG. 11  is a flowchart illustrating operation of the PWM amplifier of  FIG. 10 . 
           [0044]      FIG. 12  illustrates exemplary signals in the PWM amplifier of  FIG. 10 . 
           [0045]      FIG. 13  illustrates how various signals in the PWM amplifier of  FIG. 10  vary as a function of volume. 
           [0046]      FIG. 14  illustrates exemplary signals in the PWM amplifier of  FIG. 10  in the case where the input signal is not at a maximum value. 
           [0047]      FIG. 15  illustrates some operating principles of the PWM amplifier of  FIG. 10 . 
           [0048]      FIG. 16  illustrates one variation in operation of the PWM amplifier of  FIG. 10 . 
           [0049]      FIG. 17  is a functional block diagram of a second embodiment of a PWM amplifier. 
           [0050]      FIG. 18  is a functional block diagram of a third embodiment of a PWM amplifier. 
           [0051]      FIG. 19  illustrates the relationship between the static current and the total current consumption in the PWM amplifiers of  FIGS. 10 ,  17  and  18 . 
       
    
    
     DETAILED DESCRIPTION 
       [0052]      FIG. 10  is a functional block diagram of a first embodiment of a PWM amplifier  1000 . PWM Amplifier  1000  includes a volume control block  1010 , an oversampler  1020 , a Delta-Sigma modulator  1030 , a PWM mapper  1040 , a filter  1050 , and an offset addition block  1060 . 
         [0053]    PWM amplifier  1000  receives at its input a digital audio signal as pulse-code modulated data PCM_DATA, and receives a volume control signal VOL_CON, and outputs an amplified output signal AUD_OUT. 
         [0054]    Volume control block  1010  includes a volume table  1011  and a multiplier  1015 . Volume table  1011  stores in a memory volume data VOL_DATA corresponding to each value of VOL_CON. VOL_DATA is a digital code (e.g. if VOL_CON is 4-bit data-&gt;Volume Table stores 16 values for VOL_DATA). In operation, volume table  1011  receives the volume control signal VOL_CON and in response thereto generates a corresponding value for VOL_DATA which it outputs as the VOLUME. The value of the VOLUME is then applied to multiplier  1015  in order to adjust the level of PCM_DATA to output a volume-controlled audio signal VD. 
         [0055]    Offset addition block  1060  includes an offset generator  1061  and an offset combiner  1062 . In one embodiment, offset generator  1061  stores in a memory (e.g., in a table) offset data OFFSET_DATA corresponding to each value of the VOLUME output by Volume Table  1011 . OFFSET_DATA is a digital code (e.g. if Volume is 4-bit data-&gt;offset generator  1061  stores 16 values for OFFSET_DATA). In operation, offset generator  1061  receives the VOLUME and in response thereto generates a corresponding value for OFFSET_DATA which it outputs as the Offset. The value of OFFSET is then applied to combiner  1062  in order to adjust the level of PCM_DATA to output an offset-adjusted volume-controlled audio signal OD. 
         [0056]    As will be explained in greater detail below, the value of OFFSET is chosen so that, when the volume of the audio signal is not at a maximum value, then the duty cycle of the audio signal is increased so as to increase the operating efficiency of PWM amplifier  1000 . 
         [0057]      FIG. 3  illustrates a block diagram of oversampler  300  which is one possible embodiment of oversampler  1020 . Oversampler  1020  oversamples the offset-adjusted volume-controlled audio signal OD which is the output by offset addition block  1060  and outputs an oversampled signal DSM_IN. 
         [0058]      FIG. 4  illustrates a block diagram of Delta Sigma Modulator  400  which is one possible embodiment of Delta Sigma Modulator  1030 . Delta Sigma Modulator  1030  quantizes the oversampled signal DSM_IN to produce an output signal DSM_OUT having a fewer number of bits. 
         [0059]    PWM mapper  1040  converts a received PCM signal to an output PWM signal. PWM mapper  1040  modulates the width of the pulse in the PWM signal in proportion to the amplitude of the input signal.  FIG. 5  illustrates an operation of PWM mapper  1040  in the case where a three-bit PCM signal is converted to a one-bit PWM signal. 
         [0060]    Low Pass Filter (LPF)  1050  is a filter that passes low frequency signals (i.e., the required amplified signal) and removes unwanted spectral components (i.e., signals at the pulse frequency). Beneficially, LPF  1050  is made with theoretically lossless components like inductors and capacitors. 
         [0061]    In  FIG. 10 , PCM_DATA, VD, OD, DSM_IN, DSM_OUT, PWM_OUT are all digital signals. PCM_DATA, VD, OD, DSM_IN, and DSM_OUT are all PCM signals, and PWM_OUT is a PWM signal. AUD_OUT is an analog signal. 
         [0062]      FIG. 11  is a flowchart illustrating operation of the PWM amplifier  1000  of  FIG. 10 . As seen in  FIG. 11 , there are some differences in the operation of PWM amplifier  1000  between when the volume of the audio signal is at a maximum value and when it is not at its maximum value. When the volume of audio signal is at a maximum value, then the PWM region is fully used by the audio signal and the amount of static current is negligible as compared with dynamic current. In contrast, when the volume of the audio signal is not at a maximum value, then the audio signal is shifted by an OFFSET value so as to remove a portion of the PWM region comprising smaller PWM values (e.g., values 1-7) that would otherwise be unused. Accordingly, the duty cycle of the PWM audio signal is increased and the static current is decreased. 
         [0063]      FIG. 12  is a diagram illustrating exemplary signals in the PWM amplifier  1000  of  FIG. 10 . In particular,  FIG. 12  shows an exemplary 16-bit OD signal at 48 kHz, an exemplary oversampled 16-bit DSM_IN signal at 64*48 kHz, and an exemplary delta-sigma modulated oversampled 4-bit DSM_IN signal at 64*48 kHz. 
         [0064]      FIG. 13  illustrates how various signals in the PWM amplifier  1000  of  FIG. 10  are varied as the VOLUME is changed for an exemplary PCM_DATA input signal. As can be seen in  FIG. 13 , as the VOLUME decreases from it maximum value (e.g., 0 dB) to lower values (e.g., −20 dB), then the amplitude of the volume-controlled signal VD is reduced, but the duty cycle is maintained at 1:1. In order to increase the duty ratio of the audio signal to decrease the static current in PWM amplifier  100 , as the VOLUME decreases from it maximum value (e.g., 0 dB) to lower values (e.g., −20 dB) offset addition block  1060  adjusts the OFFSET value from 0 toward a minimum OFFSET value (b−a), where b is one half of the dynamic range of the volume-controlled signal VD, and a is a modulation margin that insures that the audio signal does not fold back upon itself and become distorted. The OFFSET is added to the volume-controlled signal VD to produce the offset-adjusted signal OD shown in  FIG. 13 . After oversampling and delta-sigma modulation, the input signal to the PWM mapper  1040  is DSM_OUT as shown in  FIG. 13   
         [0065]      FIG. 14  illustrates exemplary signals in the PWM amplifier of  FIG. 10  in the case where the input signal is not at a maximum value.  FIG. 14  shows how an unused PWM region in the range 1-7 is removed as a result of the OFFSET being added to the volume-controlled audio signal. In the example illustrated in  FIG. 14 , the volume is adjusted so that the audio signal ranges from (−max/2) to (+max/2), in which case the duty ratio is adjusted to be 1:3. 
         [0066]      FIG. 15  illustrates some operating principles of a PWM amplifier of  FIG. 10 . In  FIG. 15 , the volume of the audio signal is set at a value below its maximum. The signals on line (a) in  FIG. 15  correspond to an example where an offset has not been applied, as in the conventional art PWM amplifier  200  of  FIG. 2 , and the signals on line (b) correspond to an example where an offset has been applied to the audio signal, as in PWM amplifier  1000  in  FIG. 10 . In  FIG. 15 : V 1  represents the range of PWM pulse width fluctuations when the volume of the audio signal is set at a value below its maximum; VM represents the range of PWM pulse width fluctuations when the volume of the audio signal is at its maximum value; C 1 /C 1 ′ and C 2 /C 2 ′ indicate the centers of the peak-to-peak swing for the audio signals on line (a) (conventional art with no offset) and line (b) (PWM amplifier with offset), respectively; and P 1 , P 2  indicate unused PWM regions for the audio signals on line (a) and line (b), respectively. 
         [0067]      FIG. 16  illustrates one variation in operation of the PWM amplifier of  FIG. 10 . When the OFFSET value changes dramatically according to a change in the VOLUME, the change in the PWM pulse width results in a “tic-noise.” To reduce this tic-noise, beneficially the minimum step (ST 2 ) in the value of the OFFSET is made smaller than the minimum step (ST 1 ) in the value of the VOLUME. Accordingly, as shown in  FIG. 16 , if the VOLUME is changed by one step, the OFFSET is controlled to change in multiple steps. Beneficially, in one embodiment his feature of controlling the minimum step of the OFFSET can be provided to offset addition block  1060 . 
         [0068]      FIG. 17  is a functional block diagram of a second embodiment of a PWM amplifier  1700 . PWM amplifier  1700  is similar to PWM amplifier  1000  of  FIG. 10 , and so for the sake of brevity, only the differences will be explained here. Whereas PWM amplifier  1000  includes oversampler  1020  following offset addition block  1060 , PWM amplifier  1700  includes instead oversampler and audio effects block  1770  preceding volume control block  1010 . 
         [0069]      FIG. 18  is a functional block diagram of a third embodiment of a PWM amplifier  1080 . PWM amplifier  1800  is similar to PWM amplifier  1000  of  FIG. 10 , and so for the sake of brevity, only the differences will be explained here. Whereas PWM amplifier  1000  includes offset addition block  1060  following volume control block  1010 , PWM amplifier  1800  includes instead offset addition block  1060  following oversampler  1020 . 
         [0070]      FIG. 19  illustrates the relationship between the static current and the total current consumption in the PWM amplifiers of  FIGS. 10 ,  17  and  18 . As can be seen in  FIG. 19 , when the amplitude of the signal (i.e., the volume of an audio signal) is at its maximum value, then the load current (i.e. the dynamic current) which is passed by the low pass filter and transferred to the load (i.e., the loudspeaker) is the greatest portion of the total current consumption of the amplifier. As the amplitude (volume) of the audio signal decreases, then the dynamic (load) current decreases. However, in contrast to the conventional PWM amplifier performance illustrated in  FIG. 6 , in the PWM amplifiers  1000 ,  1700  and  1800 , the static current consumed in the low pass filter also decreases when the amplitude (volume) of the audio signal decreases, dues to the OFFSET value added to the audio signal. As a result, at volume levels that are less than the maximum volume, the total current consumption of the PWM amplifiers  1000 ,  1700  and  1800  is reduced compared to the total current consumption of PWM amplifier  200 . 
         [0071]    Although the principles of adding an OFFSET to a signal in a PWM modulator have been explained in the context of an amplifier, and particularly an audio amplifier, in general the same principles may apply in other devices employing a PWM modulator to modulate a signal, for example, a motor control system. 
         [0072]    While preferred embodiments are disclosed herein, many variations are possible which remain within the concept and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims.