1. Field of Invention
The present application is related generally to the field of high-fidelity speaker systems where motional feedback signal is used to enhance the performance.
2. Prior Art
(Magnetic) Flux modulation is a modulation of magnetic flux passing through the air gap of the motor structure as a function of applied current. Such modulation is undesirable in that it can cause distortion in the sound reproduced from the speakers. The cause of flux modulation can be explained with reference to
Flux modulation is getting worse in recent years because high power amplifiers and speakers (>1 kW) are becoming popular. Higher power means higher current and therefore higher flux modulation. And this problem not only affect the drivers used to low frequency (as often referred to as subwoofer), it also affect the drivers used at the higher frequency. In the past, short circuit rings have been placed around the pole piece to reduce the effect of flux modulation, such as one described in paper “Moving-coil Loudspeaker Topology as an Indicator of Linear Excursion Capability” by Mark R. Gander in Journal of AES Vol 29 No 1/2 1981 January/February, pp 10-26. The issue with short circuit rings is the physical size of the ring may have to be huge in order to be effective, and that increases the cost. And smaller rings are not effective at reducing flux modulation at low frequency. Other similar techniques include U.S. Pat. Nos. 5,815,587 and 5,151,943.
All of the above-mentioned techniques address the flux modulation from a magnetic circuit's operating point of view. The present invention addresses this issue using a nonlinear feedback system with the feedback signal as a nonlinear function of two components: 1) current through the driver voice coil (the main voice coil that converts the electrical energy to mechanical energy), and 2) the motional feedback signal which is derived from a sensing coil wound on the same former as the main voice coil, which is also referred to as the driver coil. In other words, the present invention is completely orthogonal with the usage of short-circuit rings and can be applied at the same time.
Other patents of possible interest include U.S. Pat. No. 5,764,781 issued Jun. 9, 1998, U.S. Pat. No. 6,104,817 issued Aug. 15, 2000, and U.S. Pat. No. 7,215,590 issued May 8, 2007.
Nonlinear feedback systems have been proposed in the past to linearize the BL profile over a large travel distance of the voice coil. One technique is U.S. Pat. No. 5,542,001, which taught a method of linearizing the back-emf motional signal by introducing a multiplier, integrator, and correction generator in an inner feedback loop within the motional feedback path. That is, there is a local feedback loop inside the global feedback loop.
Yet, another technique was proposed when A. J. M. Kaizer examined the issue of correcting the nonlinearity of electrodynamic loudspeakers; and the result was published in the paper “Modeling of the Nonlinear Response of an Electrodynamic Loudspeaker by a Volterra Series Expansion”, appeared in Journal of Audio Engineer Society, vol. 35, no 6, 1987 June, page 421-433. The paper proposed a voltage drive and a current drive 2nd order distortion reduction circuits. However, both circuits operate on a feedforward principle instead of a feedback principle and A. J. M. Kaizer subsequently obtained U.S. Pat. No. 4,709,391. The main difference between a feedforward system and a feedback system is that the former needs a forward prediction of how the system will perform and generates a forward correction term based on the prediction to achieve a lower distortion. As a result, the unit-to-unit variation and change of operation condition can render the techniques less effective.
The objectives of the above-mentioned non-linear feedback techniques were to linearize the BL profile and therefore they are insufficient in addressing the issue of flux modulation.
It is desirable to provide nonlinear motional feedback to reduce the distortion by injecting a distortion compensation signal into the system.
Vs=Bslv [1]
In the presence of flux modulation, Bs is not a constant value. As a result, we can rewrite Bs as a sum of a constant value Bs(0) plus a component, f(I), which is a function of the current through driver coil:
Bs=Bs(0)+f(I) [2]
where Bs(0) is the Bs value at 0 current, and I is the current through the driver coil, and f(I) is a function of I. Note in Equation [2] the current in the sensing coil is omitted for the sake of simplicity. If the current in the sensing coil is significant enough, we can include such effect in f(I).
Equation [1] can be rewritten as:
Vs=B
s(0)lv+f(I)lv [3]
The second term on the right-hand side is the distortion term. From Equation [3], it can be seen that if one would like to maintain non-distorted velocity v in the presence of current, the motional signal needs to contain the distortion term. It is therefore desirable to generate the non-linear component in the motional signal, f(I)Lv, such that we get non-distorted velocity. However, the derivation of this term faces several issues in practice. The first issue is derivation of velocity signal v from the sensing signal due to the mutual inductance between driver coil and sensing coil. The second issue is that the function f(I) is frequency-dependent due to the eddy current in the pole piece and the Faraday current through the short-circuit ring when it is present. The third issue is the design of feedback loop gain such that distortion compensation is effective over a wide frequency range. Therefore it is also desirable if the non-linear feedback system can effectively reduce the effect of flux modulation on the motional sensing signal over a portion, or even the entirety of the frequency range in which the system is intended to operate.
In the accompanying drawings:
Essentially, the power amplifier receives two linear feedbacks and one non-linear feedback. In contrast, the prior-art implementation will only have two linear feedbacks as shown in
The purpose of the motional signal correction circuit 18 is to filter out the mutual inductance component of the signal from the sensing coil used in
The frequency characteristic of the motional signal correction circuit depends on the driver's parameters and can be approximated via the following steps. First, plot the motional signal versus the current that flows through the speaker. That is Vs1/I, where I is the current through 11 (or Re). This plot is referred to as the motional impedance plot.
The equivalent circuit of motional impedance is well known. For example, the equivalent circuit of the motional impedance in
The function is essential a 2nd-order low pass filter with very high Q value, as shown in
The reason for the current filter is that the flux modulation is frequency dependent. At higher frequency, the effect is less significant. This is primarily because the eddy current in the pole piece can reduce flux modulation. A short-circuit ring around the pole piece also has similar effect, except the reduction of flux modulation is more significant. A second-order lowpass filter with Q<1 is found to be effective for this current filter. In the case of a 12″ driver with a shortcircuit ring, it is found that the corner frequency is around 60 Hz. However, this should not be interpreted as a limitation in the scope of the present invention. For those who are skilled in the art that use a different driver, one can use a different corner frequency and Q value to achieve similar results. And the order of the attenuation can range from a 1st-order characteristic to higher-order characteristic.
The multiplier performs the function of multiplying a current signal with a true motional signal because through an actual implementation of the current invention, it was found the first-order approximation of f(I)=kI, where k is a constant, is good at lower frequencies. At higher frequencies, a function of the current filter is used to provide a better approximation. In addition, the polarity of the feedback from an actual present-day implementation also indicates the nonlinear feedback is a positive feedback. That is the reason the polarity of the motional signal going into input B of the multiplier is reversed. This indicates the sign of the constant k in the first-order approximation of f(I) is positive. In the case when k is negative, we need to change the positive feedback to negative feedback on the non-linear feedback. Also the nature of positive feedback prompts us to prefer under-correction to over-correction. Note that in
The first purpose of the post-multiplier filter is to provide a relatively flat (linear feedback) loop gain, such as one shown in
For sealed-box speakers the above arrangement is very effective. The main reason is the true motional signal is by and large high enough so the approximation in Equation [1]-[3] is sound. For speaker configurations such as vented box, the cone movement can suddenly drop to almost zero at the box tuning frequency. At the frequency where the cone movement is very small, the accuracy of using motional signal from sensing coil to approximate real world cone velocity can be gross enough that the nonlinear feedback causes additional unwanted distortion, instead of canceling the distortion. In this case, it is necessary to modify the motional signal correction circuit to attenuate the motional signal around F1 such as one shown in
The present invention can also be used to partially compensate for other types of distortion such as spider nonlinearity, in addition to addressing the distortion caused by flux modulation. This is mainly because the final evaluation of distortion reduction requires real-world distortion measurements. It is possible to separate the distortion components attributable to flux modulation from those attributable to other factors because the contribution of flux modulation is minimal when the impedance is high. By varying the location of the impedance peak using either additional mass attached to speaker cone, or enclosures of various sizes, one can easily measure the distortion attributable to flux modulation and from other sources. While it is possible to do so, what is desired is to achieve an overall lower distortion. In other words, in the context of a more complicated distortion compensation feedback system that addresses multiple distortion sources, the present invention can be implemented as a component of such a system. One such an example has the multiplier block replaced with a multiplier network that generates multiple higher order nonlinear terms in addition to the simple nonlinear term A times B shown in block 20 of
Finally, those person skilled in the art would appreciate that the embodiments disclosed herein are merely to illustrate several of many forms which the invention may take in practice and numerous modifications can be made without departing from the present invention, to achieve various levels of distortion reduction.
This application claims the benefit of U.S. application No. 60/970,465 filed Sep. 6, 2007.
Number | Date | Country | |
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60970465 | Sep 2007 | US |