Loudspeakers

Details Loudspeakers Loudspeakers

1997-09-30

2002-04-16

Harvey, Minsun Oh (Department: 2644)

Electrical audio signal processing systems and devices

Having non-electrical feature

And loudspeaker

C381S332000

Reexamination Certificate

active

06373955

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to loudspeakers for providing sound from electrical signals which may be analogue or digital in nature.
BACKGROUND OF THE INVENTION
Conventional analogue loudspeakers rely for their operation on the motion of a diaphragm which is driven by some type of electromechanical motor, moving-coil being the most common variety, though electrostatic, piezoelectric and ionisation devices have all been tried and used. The analogue loudspeaker as a whole attempts to reproduce the desired sound by moving all or part of the diaphragm closely in synchronism with a smoothly varying analogue electrical signal which is usually interpreted as representing the instantaneous sound pressure that a listener to the loudspeaker device should hear. The inherent limitations of such analogue loudspeakers are in part related to stiffness of the diaphragm used, mass of the diaphragm, the linearity and efficiency of and power available from electromechanical motors with adequate bandwidth, and limitations on throw of the diaphragm. These and other factors combine to cause the analogue loudspeaker to operate with low efficiency and relatively high distortion levels.
With the current prevalence of high quality digital audio material available, frequently in 16-bit binary format with an inherent distortion level of close to 0.002%, it is clear that current analogue hi-fidelity loudspeaker systems operating close to the 1% distortion level (500 times worse) are now the limiting factor in audio quality when listening to reproduced sound. Recent trends in electronic equipment have also been to mininise power consumption, not only to reduce power wastage, but also to reduce equipment operating temperatures thus allowing mniniaturisation and high reliability, as well as portability, and allowing operation from small batteries. Again, the linear analogue power amplifier/loudspeaker combination operating at the 0.3% to 1% electro-acoustic efficiency level is out of step with these trends. Lastly, even though digital audio source material is now commonplace and becoming increasingly so with the advent of digital radio and television, all conventional hi-fidelity systems for the reproduction of digital source material need to contain a digital-to-analogue converter (DAC) at some point in the system, to produce analogue signals for application to the analogue loudspeaker. The DACs themselves produce further noise and distortion that adds to that already present in the system, and also add extra cost.
Attempts have been made previously to develop a digital loudspeaker design that overcomes some or all of the limitations of analogue loudspeakers mentioned above. These fall into several categories: Pseudo-digital loudspeakers comprising a digital signal processor driving a standard analogue speaker, Moving Coil Digital Loudspeakers with tapped “voice-coils”; Piezoelectric and Electrostatic drivers, where the area of the diaphragm is divided into separate regions with binary-related surface areas; and Pulse-width-modulation amplification which is really a digital amplifier technology. Most previous attempts at building a digital loudspeaker system have assumed that binary digital code was the digital signal medium, not only at the input of the device but also right through to the output transducers. This causes serious technical problems in practice.
In a signed n-bit system, the transducer used for the least significant bit (LSB) of the output operates at a power level 2
n−2
times less than the most significant (non-sign) bit (MSB). Because of the necessarily mechanical nature of sound-producing devices, this wide dynamic range imposes serious design constraints on the types of devices used for LSB and MSB transducers. and thus makes matching of the devices very difficult. In a binary-weighted transducer (or transducer-array) system, there are serious transient problems caused at points where the code changes from a value with many consecutive low order zeroes or ones to the next level (up or down) where there are many consecutive low order ones or zeroes. Multiple acoustic transitions occur at this code point change which will inevitably produce considerable sound energy, even though the code change represents only a least significant bit change in signal amplitude which would be preferably nearly inaudible.
In addition to the switching transient problem outlined there is also a level error associated with such zeroes-to-ones and ones-to-zeroes code changes. This is because in a real system the transducers cannot easily be matched precisely enough that the each transducer is precisely one least significant bit greater in effective power or amplitude than the sum of all the lesser-bit transducers acting in concert.
It is not unknown for loudspeakers to have arrays of many transducers which produce pressure pulses, individually and independently fed. But in the past this has been done with binary digital signals, e.g. Stinger U.S. Pat. No. 4,515,997, which fails practically for the reasons given above. No one has done it with unary digital signals because their advantages in these respects were not foreseen. Voltage-sampled signals, not discrete-time-sampled signals, have been used to determine what instantaneous number of identical transducers, arranged in a one or two dimensional array, should be switched on as a series of voltage thresholds has been reached, e.g. Nubert, DE Patent 4343807 A1, but this does not disclose unary encoding of binary (or other) digital codes, nor does it usefully convert the voltage levels that cause triggering of the various transducers into appropriate sound pressure levels, instead triggering near-instantaneous 3-dimensional-volume changes in the transducers, or constant strokes, thus creating a series of positive and negative pressure impulses at the trigger edges rather than continuous pulses of an appropriate polarity, the end result being the production of no useful sound power. Nubert, having described the operation of his invention entirely in terms of events occurring at certain voltage levels, does also suggest that analogue signals could be converted to digital form with an ADC; however, this in itself does not necessarily suggest regular time-sampled digital signals as, e.g., flash-ADCs are capable of digitizing without regular (or in some cases, any) clock signals. There is no clear statement at all in Neuberg's disclosure as to how to achieve control of individual transducers in the case w here a digital input signal is present and in particular, no clear indication of what and how to encode such digital input signals into any required form. Neuberg also discloses the idea of using constant stroke transducers, but does not disclose constant pressure pulse transducers.
Another problem not adequately addressed by existing digital loudspeaker designs is that of transducer dynamics and appropriate drive waveforms for producing the desired acoustic sound output waveform.
Binary to unary ends are known in the prior art U.S. Pat. No. 5,313,300 (Rabile) describes a technique of synthesising such encoders for video DACs, for wide binary words from unary sub-encoders each capable of encoding less-wide binary words. The technique described uses ‘tiers’ of small unary encoders interconnected in something like a tree-structure, with additional gating blocks to perform the final conversion to unary. A problem exists with extending this design to larger input binary bit-widths in that the interconnection system between all the unary sub-encoders becomes complex and is not amenable to a bus-structured approach (because of its tree-like tiered or cascade nature). In addition, U.S. Pat. No. 5,313,300's design presupposes the existence of the unary subecoder blocks but nowhere describes the operation (nor truth table) of such sub-encoders and their exact nature is therefore unclear, although a truth table for an entire encoder is disclosed.
Digital Pulse Width Modulation, PWM, generators are known e.g. Kirn U.S. Pat. No. 4,773,096, whic

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