System and method for compensating memoryless non-linear distortion of an audio transducer

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System and method for compensating memoryless non-linear distortion of an audio transducer DTS, Inc. C.192260 0 WO 2008/048413 PCT/US2007/020652

System and Method for Compensating Memory less Non-Linear Distortion of an Audio Transducer 5 BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to audio transducer compensation, and more particularly to a method of 10 compensating non-linear distortion of an audio transducer such as a speaker, earphone or microphone.

Description of the Related Art

Audio transducers preferably exhibit a uniform and 15 predictable input/output (I/O) response characteristic. In a speaker, the analog audio signal coupled to the input of a speaker is what is ideally provided at the ear of the listener. In reality, the audio signal that reaches the listener's ear is the original audio signal plus some 20 distortion caused by the speaker itself (e.g., its construction and the interaction of the components within it) and by the listening environment (e.g., the location of the listener, the acoustic characteristics of the room, etc) in which the audio signal must travel to reach the 25 listener's ear. There are many techniques performed during the manufacture of the speaker to minimize the distortion caused by the speaker itself so as to provide the desired speaker .response. In addition, there are techniques for mechanically hand-tuning the speaker to further reduce 30 dis tort ion.

Distortion includes both linear and non-linear components. Non-linear distortion such as "clipping" is a function of the amplitude of the input audio signal whereas linear distortion is not. Klippel et al, 'Loudspeaker 35 Nonlineaxities - Causes, Parameters, Symptoms' AES Oct 7-10 1 WO 2008/048413 PCT/US2007/020652 2005 des c r ibes the r elat ionship between non-1inear distortion measurement and nonlinearities which are the physical causes for signal distortion in speakers and other transducers - 5 There are many approaches to solve the linear part of the problem. The simplest method is an equalizer that provides a bank of bandpass filters with independent gain control. Techniques for compensating non-linear distortion are less developed. 10 Bard et al "Compensation of nonlinearities of horn loudspeakers", AES Oct 7-10 2005 uses an inverse transform based on frequency-domain Volterra kernels to estimate the nonlinearity of the speaker. The inversion is obtained by analytically calculating the inverted Volterra kernels from 15 forward frequency domain kernels. This approach is good for stationary signals (e.g. a set of sinusoids) but significant nonlinearity may occur in transient non-stationary regions of the audio signal. 20

SUMMARY OF THE INVENTION

The present invention provides a low-cost, real-time solution for compensating memoryless non-linear distortion in an audio transducer. 25 This is accomplished with an audio system that estimates signal amplitude and velocity of an audio signal, looks up a scale factor from a look-up table (LUT) for the defined pair (amplitude, velocity), and applies the scale factor to the signal amplitude. The scale factor is an 30 estimate of the transducer's nonlinear distortion at a point in its phase plane given by (amplitude, velocity). The transducer's nonlinear distortion over the phase plane is found by applying a test signal having a known signal amplitude and velocity to the transducer, measuring a 35 recorded signal amplitude and setting the scale factor 2 WO 2008/048413 PCT/US2007/020652 equal to the ratio of the test signal amplitude to the recorded signal amplitude. The test signal(s) should have amplitudes and velocities that span the phase plane. This approach assumes that the sources of nonlinear distortion 5 are 'memoryless', which for most transducers is a reasonably accurate assumption. Scaling can be used to either pre- or post-compensate the audio' signal depending on the audio transducer. The compensated audio signal will exhibit lower harmonic distortion (HD) and intermodulation 10 distortion (XMD) , which are the typical specifications for nonlinear distortion of a speaker.

These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred 15 embodiments, taken together with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of an audio transducer; 20 FIGs. 2a and 2b are block and flow diagrams for computing a phase plane LUT for pre-compensating an audio signal for playback on an audio transducer; FIGs. 3a, 3b, 3c and 3d are plots of an exemplary test signal and its phase plane; 25 FIG. 4 is a plot of a recorded signal including HD and IMD of the speaker;· FIG. 5 is a diagram of the phase plane that is mapped to the LOT; FIGs. 6a and 6b are block diagrams of an audio system 30 configured to use the phase plane LUT to compensate non linear distortion of the speaker; and FIG. 7 is a diagram of the compensated recorded signal. 3

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WO 2008/048413 PCT/US2007/020652

DETAILED DESCRIPTION OF THE INVENTION

The present invention describes a low-cost, real-time solution for compensating non-linear distortion in an audio transducer such as a speaker, earphone or microphone. As 5 used herein, the term waudio transducer" refers to any device that is actuated by power from one system and supplies power in another form to another system in which one form of the power is electrical and the other is acoustic or electrical, and which reproduces an audio 10 signal. The transducer may be an output transducer such as a speaker or earphone or an input transducer such as' a microphone. An exemplary embodiment of the invention will be now be described for a loudspeaker that converts an electrical input audio signal into an audible acoustic 15 signal. A reading of Klippel's paper led us to the observation that the primary non-linear distortion that contributes to HD and IMD is 'memoryless * . The physical causes of this distortion can be described entirely by a let order 20 approximation of the potential and kinetic energy of the audio transducer. To a good approximation, the potential and kinetic energy, hence the memoryless non-linear distortion can be uniquely described by the signal amplitude and signal velocity, respectively. • 25 As shown in Figure 1, an audio speaker 100 includes a diaphragm 102 that pushes the air to create sound waves. The diaphragm is suspended on a spider 104 and a surround 106, which are connected to a speaker frame (not shown) . Voice coil 108 is connected to the diaphragm and receives 30 electrical current (input signal). The diaphragm movement .- happens through interaction 112 of the magnetic field of a permanent magnet 110 with magnetic field of the coil 108. Permanent magnet is typically connected to the metallic construction 114 in the speaker to provide proper 4 configuration of the magnetic field and geometry of the gap 116 where voice coil is moving.

The total energy of the speaker is given by: E — Ep + Ek

Where: WO 2008/048413 PCT/US2007/020652

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potential energy 10 15

Ek=—~- - kinetic energy k - stiffness of the suspension (surround-4- sp i der) x - displacement of the diaphragm L - inductance of the coil J - current through coil, proportional to the signal amplitude m - mass of the diaphragm v - velocity of the diaphragm

These simplified formulas, which do not take into account that speaker is constructed from many parts or the interdependence of the parameters (£,/,£,_.) that would 20 require higher order nonlinear terms to fully describe the system, provide a good approximation of the system and the causes of the memoryless non-linear distortion.

The observation that the non-linear distortion is to a large extent 'memoryless' and that the audio transducer 25 energy can be represented to a good approximation by the signal amplitude and velocity, allows for a low-cost, realtime solution for compensating non-linear distortion in an audio transducer. An audio playback system estimates signal amplitude and velocity, looks up the closest scale 30 factor(s) from a look-up table (LUT) for the measured pair (amplitude, velocity), preferably interpolates to a scale 5

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WO 2008/048413 PCT/US2007/020652 factor for the measured pair, and applies the scale factor to the signal amplitude. The scale factor is an estimate of the transducer's nonlinear distortion at a point in its phase plane given by amplitude, velocity. The transducer's 5 nonlinear distortion over the phase plane is found by applying a test signal having a known signal amplitude and velocity to the transducer, measuring a recorded signal amplitude and setting the scale factor equal to the ratio of the test signal amplitude to the recorded signal 10 amplitude. The compensated audio signal will exhibit lower harmonic distortion (HD) and intermodulation distortion (IMD), which are the typical specifications for nonlinear distortion of a speaker. 15 Phase Plane Characterization

The test set-up for characterizing the memoryless non linear distortion properties of the speaker and the method of generating the LUT are illustrated in Figures 2 through 5. The test set-up suitably includes a computer 10, a sound 20 card 12, the speaker under test 14 and a microphone 16. The computer generates and passes a digital audio test signal 18 to sound card 12, which in turn drives the speaker. Microphone 16 picks up the audible signal and converts it back to an electrical signal. The sound card passes the 25 recorded digital audio signal 20 back to the computer for analysis. A full duplex sound card is suitably used so that playback and recording of the test signal is performed with reference to a shared clock signal so that the digital signals are time-aligned to within a single sample period, 30 and thus fully synchronized.

The techniques of the present invention will characterize and compensate for any memoryless source of non-linear distortion in the signal path from playback to recording. Accordingly, a high quality microphone is used 6 197915/2 WO 2008/048413 PCT/US2007/020652 such that any distortion induced by the microphone is negligible. Note, if the transducer under test were a microphone, a /high quality speaker would be used to negate unwanted sources of distortion. To characterize only the 5 speaker, the "listening environment" should be configured to minimize any reflections or other sources of distortion. Alternately, the same techniques can be used to characterize the speaker in the consumer's home theater, for example. In the latter case, the consumer's receiver or 10 speaker system would have to be configured to perform the test, analyze the data and configure the speaker for playback.

As described in Figure 2b, to generate the LUT, the computer generates a test signal whose spectral content 15 should cover phase plane i.e., the full range of signal amplitudes and velocities for the speaker (step 30) . An exemplary text signal 41 consisting of two simultaneous sine waves 42 (0 to 6kHz with amplitude of -6db) and 44 (0 to 5kHz with amplitude of -3db) and the corresponding phase 20 46 are shown in Figures 3a and 3b, respectively. As shown, two sine waves with changing frequency and amplitude provide good coverage of the phase plane. Figure 3c is the phase plane 47 for a single sine wave with increasing frequency, which provicies no coverage at the center. 25 Figure 3d is the phase plane 48 for a single sine wave with changing amplitude and frequency, which provides better coverage but still not complete.

The computer then executes a synchronized playback and recording of the test signal (step 32) . For each sample n, 30 the computer calculates a scale factor as the ratio of the amplitude of test signal s (n) to the amplitude of the recorded signal r(n), e.g.·, SF = s(n)/r(n) (step 34).

Alternately, SF (n) = log (s(n)/r(n)) in which case the LUT is logarithmic. A 'bias' constant may be added to the 7

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WO 2008/048413 PCT/US2007/020652 denominator r(n) to prevent division by 0 when r(n)=0 or to reduce the influence of noise. In either case, the only independent variables in the scale factor computation are computed are s (n) and r(n). The computer then calculates 5 the velocity v(n) of test signal s(n) (step 36). This may be done analytically from equations used to generate the test signal or empirically from the test signals samples. The empirical calculation can be as simple as the change in amplitude from the previous to the current sample divided 10 by the sampling interval, the change in amplitude from the previous to the succeeding sampled divided by twice the sampling interval or by calculating gradient through a 5-or 7-point FIR filter. For each sample, the scale factor is stored in a table with an index of (s(n},v(n)) (step 38). 15 The scale factor represents the amount of memoryless nonlinear distortion associated with the speaker when driven at a given signal amplitude and velocity.

The computer performs steps 34, 36 and 38 for each sample in the test signal and uses the data to construct a 20 lookup table (LUT) of scale factors indexed by (s(n),v(n)) (step 39) . If multiple scale factors are calculated for a given index (s (n) , v(n)) , the scale factors are averaged or filtered to assign a single value to the index. The scale factors may be interpolated and resampled to produce a 25 table having a desired indexing e.g., uniform spacing along the amplitude and velocity axis, and values for every index. If the test signal does not quite span the range of amplitudes and velocities, the data can be extrapolated to assign those values. Alternately, these points may be 30 assigned a value of one. The larger the amplitude and velocity ranges and/or the finer the resolution of the indexing, the larger the size of the LUT. The selection of these parameters will depend on the particular application.

In certain implementations, it may be desirable to 8 WO 2008/048413 PCT/US2007/020652 10 15 20 25 approximate the LUT with a polynomial equation in which the only independent variables are the amplitude and velocity, e.g. SP = f (amplitude, velocity) (step 40) . During playback, a polynomial evaluation may be preferred in systems with very strict requirements on memory footprint, e.g. the polynomial is much smaller than the LOT. Evaluation of the polynomial at playback may be slower or faster than the LUT depending on such factors as the number of terms in the polynomial and the interpolation algorithm used in conjunction with the LUT. Bilinear interpolation is quite fast while bicubic interpolation is somewhat slower, A standard 2D polynomial fitting algorithm can be used to find the proper order and coefficients of the polynomial.

For an exemplary speaker, the spectral content 50 of the recorded signal for the test signal shown in Fig. 3a includes both IMD 52 and HD 54 in addition to the replicated test signal 41 as illustrated in Figure 4. IMD and HD are the primary distortion values that are specified for a speaker or other audio transducer. Therefore, reducing IMD and HD are of primary importance.

For the exemplary speaker and test signal, a phase-plane 60, i.e. the data for constructing the LUT, is illustrated in Fig. 5. The data can be interpolated and/or extrapolated and resampled to generate the LUT having a specified indexing and resolution. For this particular speaker, the distortion peaks near the mid-range of the amplitude and velocity and rolls off in all directions. Other speakers or audio transducers will have different properties and will exhibit different distortion.

The described approach is particularly applicable to earphones, where the full size of the earphone is smaller then (or comparable to) the wavelength (and therefore the system can be better approximated by momentary values) . Assume an average earphone size is 1cm and the highest 30 9

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WO 2008/048413 PCT/US2007/020652 audio frequency is 16kHz. The wavelength of the 16kHz sound wave in air is 3 3 0m/sec / 16kHz = 2cm. Inside the earphone the sound waves will propagate faster than in air, but the wavelength of the highest frequency remains comparable to 5 the earphone size. The time of wave propagation from one end of the system to the other can be approximated to be zero. Consequently the memory effects will be negligible.

Distortion Compensation and Reproduction 10 In order to compensate for the speaker's memoryless non-linear distortion characteristics, the audio data samples d(n) having amplitude a(n) must scaled prior to its playback through the speaker. This can be accomplished in a number of different hardware configurations, two of which 15 are illustrated in Figures 6a-6b.

As shown in Figure 6a, a speaker 150 having three amplifier 152 and transducer 154 assemblies for bass, midrange and high frequencies is also provided with the processing capability 156 and memory 158 to precompensate 20 the input audio signal to cancel out or at least reduce memoryless non-linear speaker distortion. In a standard speaker, the audio signal is applied to a cross-over network that maps the audio signal to the bass, mid-range and high-frequency output transducers. In this exemplary 25 embodiment, each of the bass, mid-range and high-frequency components of the speaker were individually characterized for their memoryless non-linear distortion properties. The LOT 160 is stored in memory 158 for each speaker component. The LUT can be stored in memory at the time of manufacture, 30 as a service performed to characterize the particular speaker, or by the end-user by downloading them from a website and porting them into the memory. Processor (s) 156 executes a filter 164 that measures the signal amplitude a(n) , computes the velocity v(n) and extracts the scale 10 WO 2008/048413 PCT/US2007/020652 factor(s) closest to the index a(n), v(n> . Filter 164 suitably interpolates the extracted scale factor(s) using* for example, a bilinear or bicubic algorithm to obtain the scale factor. Bilinear interpolation requires the four 5 nearest scale factors whereas bicubic interpolation requires the sixteen nearest. The filter multiples the data sample d(n) by the scale factor. The scaled data samples d(n) are forwarded to the processor's D/A and than on to the amplifier 152. 10 As shown in Figure 6b, an audio -receiver 180 can be configured to perform the precompensation for a conventional speaker 182 having a cross-over network 184 and amp/transducer components 1B6 for bass, mid-range and high frequencies. Although the memory 188 for storing the 15 LUT 190 and the processor 194 for implementing the filter 196 are shown as separate or additional components for the audio decoder 200 it is quite feasible that this functionality would be designed into the audio decoder. The audio decoder receives the encoded audio signal from a 20 TV broadcast or DVD, decodes it and separates into stereo (L,R) or multi-channel (L,R,C,Ls,Rs, LFE) channels which are directed to respective speakers. As shown, for each channel the processor applies the filter to the audio signal and directs the precompensated signal to the 25 respective speaker 182. The filter performs in same manner as described above.

In an alternative embodiment, the speaker or application only requires that a low-frequency band be compensated. In this case, the audio samples d(n) can be 30 downsampled to that low-frequency band, the filter applied to each sample and than upsampled to the full frequency band. This achieves the required compensation at a lower CPU load per sample.

Precompensation using the LUT will work for any output 11 WO 2008/048413 PCT/US2007/020652 audio transducer such as the described speaker or headphones. However, in the case of any input transducer such as a microphone any compensation must be performed "post" transducing from an audible signal into an 5 electrical signal, for example. The analysis for constructing the LUT changes slightly. The scale factors are indexed against the (amplitude, velocity) of the recorded signal instead of the test signal. The synthesis for reproduction or playback is very similar except that it 10 occurs, post-transduction.

Testing &amp;amp; Results

The general approach set-forth of characterizing and compensating for the memoryless non-linear distortion 15 components is validated by the spectral response 210 of the output audio signal measured for a typical speaker as shown in Figure 7. As shown, the input signal including the high and low frequency sine waves 42 and 44, respectfully are faithfully reproduced and the IMD 52 and HD 54 are heavily 20 attenuated. The distortion compensation is not perfect because the energy equations for the system are only approximations, interpolation error in the scale factors and the presence of non-linear distortion having memory. However, the described solution for compensating memoryless 25 non-linear distortion in an audio transducer is fast, cost-effective and highly effective.

While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those 30 skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims. 12 cra&amp;amp;’an , crnxan rwzn ατα inia^n pnow pnszn irn πτ “|»oa ,ρνιη ηχΰπ naoana ma™ na^maa np’ioa .zruwan rwaa mpnan pmi1? oxnm οιηπη Pi?

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