Electrical audio signal processing systems and devices – With mixer
Reexamination Certificate
2001-12-11
2004-11-02
Mei, Xu (Department: 2644)
Electrical audio signal processing systems and devices
With mixer
C369S004000
Reexamination Certificate
active
06813361
ABSTRACT:
TECHNICAL FIELD
The invention relates to apparatus and methods for determining the absolute position of a movable member with respect to one or more fixed members. More specifically, the invention relates to apparatus and methods for determining the absolute position of a manual control for audio systems.
BACKGROUND OF THE INVENTION
A wide variety of prior art devices and techniques have been developed for determining the absolute position of a movable member with respect to a nonmovable member, or vice versa. Such systems typically rely on a unique physical characteristic which exists with respect to each relative position of the movable and nonmovable members. Such characteristic may be due to resistive, capacitive, inductive, optical, or magnetic properties of the relatively movable members. Systems which are based on one of the above physical properties typically have widely varying characteristics with respect to: i) resolution (the size of the smallest incremental movement which can be detected by the system); ii) repeatability (the error within which a given position can be reproduced by the system); and iii) accuracy (the absolute deviation between the desired target position and the actual position of the relatively movable parts of the system). Furthermore, systems of the above type often vary dramatically with respect to cost, assembly complexity, and susceptibility to ambient conditions which affect resolution, repeatability, and/or accuracy.
Audio systems such as public address systems, audio control panels for mixing audio sources in various venues (e.g., radio studios, theaters, discotheques, etc.) and even home audio systems employ a variety of manual controls in which the output of the manual control is indicative of a particular position of the control with respect to the control panel. In most systems of the type described, the manual control does not itself directly conduct audio power, but rather acts as an indirect control for an amplifier (e.g., a voltage controlled amplifier) which then attenuates or amplifies the audio signal of interest. In this type of application while accuracy and repeatability are important, resolution is more so. Nevertheless, the most important feature for most consumers relates to the lack of electrical noise in the sensor which is otherwise also amplified by the voltage controlled amplifier. For discriminating consumers, the feel of the control (i.e., constant drag with change of position and over time) and durability are also important. Secondary considerations include cost and the absence of “bleed” in audio fader controls. “Bleed” represents the ability of the audio fader control to completely attenuate an audio signal which is controlled by the manual sensor.
Traditionally, potentiometers and variable linear resistors have been used in audio fader control systems because of the relatively low cost of components of this type and large travel distance (particularly with respect to linear resistive control elements) which is highly desirable to audio mixer artists. Conventional wire wound variable resistors having movable wiper arms have been supplanted by the variable resistor of the type having a carbonized resistive element imprinted on a printed circuit board. This mechanical arrangement advantageously facilitates the manufacture of a variable resistor having variable incremental resistance. That is, each incremental movement of the linear fader control produces a non-linear change in the resistor value. This can be achieved by either varying the width or thickness of the resistive trace on the printed circuit board. Such variability is highly desirable because where controls of this type are used for volume control, the human ear's impression of constant volume change is itself non-linear. Those of ordinary skill in the relevant art are well aware that human perception of increasing volume is not only non-linear, it is essentially logarithmic. This relationship has been quantified by a variety of specialists in the art and is commonly known as a “standard listener curve”. By matching the thickness and/or width of the resistive carbon material on the printed circuit board of a linear variable resistor with a standard listener curve, variability of resistance with respect to position can be produced so as to closely match the standard listener curve. Thus, ordinary amplification circuitry can be employed such that an arithmetic physical displacement of the manual control produces an apparent logarithmic equivalent attenuation in volume of an audio signal which is operatively coupled to that control.
Unfortunately, printed circuit board linear variable resistors of the type described above suffer from a number of defects. First among these defects relates to travel noise associated with movement of the fader control. That is, as the wiper arm traverses the resistive path, microscopic arcing occurs because the travel path itself is not perfectly smooth. Indeed, as the travel path begins to wear, arcing becomes more pronounced, is amplified through the voltage control amplifier, and is heard as a popping or crackling sound by the listener. This problem is only exacerbated by further use and wear. A professional audio mixing artist may cycle an audio fader control up to twenty cycles per second for hours on end. Yet, the life cycle of a typical linear resistive element of the type described may be as little as 10,000 cycles. As the carbonized surface of the printed circuit board begins to wear under the action of the wiper arm, not only is the noise problem exacerbated, but the linear resistor begins to bleed. That is, bringing the wiper to the fully attenuated position associated with its end of travel no longer fully attenuates the signal. A second problem associated with contact type controls such as linear resistors is that the “feel” of the control (i.e., the coefficient of friction of the carbonized surface) is non-constant either over time, or from one end of travel to the other. A good feel which is constant over time is an important characteristic for artists in this field. Finally, the most demanding artists require significant accuracy with respect to controls of this type. However, faders with resistive elements of the type described above typically have an accuracy of no better than ±20% total travel length. Thus, even if the repeatability and resolution of the control are good (which typically is not the case), scale markings on the fader control are of little use to the artist due to the low accuracy of the control itself.
Those of ordinary skill in the prior art have recognized the above described limitations of conventional resistive controls, and therefore have developed non-contact position sensors of the capacitive, inductive, optical, and magnetic type. With respect to optical control systems, an optical encoder/decoder system is described by Yochum in U.S. Pat. No. 4,412,812 for use as an audio fader control. In that system, a movable control handle is connected to a linear shutter which is disposed between corresponding pairs of light emitting diodes and light detectors. By appropriately positioning the emitter/detector pairs with respect to various apertures in the shutter, any unique, absolute position of the shutter can be instantaneously determined by digital logic circuitry connected to the light detectors. In addition, the digital information which corresponds to a unique position of the shutter (and thus control handle) can be used to access a digital look-up table in the form of a read-only memory (ROM) which may be provided with a non-linear transfer function, presumably to match the position of the control handle to a standard listener curve. Yochum further describes that more than one ROM can be provided, selected by an appropriate switch, to provide two or more listening curves.
The system described by Yochum avoids all of the disadvantages associated with resistive, contact-type fader control. Nevertheless, Yochum's system requires at least 13 optical emitter/detector pairs
Jeffs Philip R.
Mathews Paul
Black Lowe & Graham PLLC
Mei Xu
Rane Corproration
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