Digital speakers
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This article needs drastic modification, since digitally driven speakers without any form of an analogue signal path are now, indeed, to become reality. It is no longer true, as it previously stated in this article, that all modern speakers are necesserily analogue.
for more information on this, refer to http://www.patentstorm.us/patents/6492761/description.html
Digital speakers are a form of loudspeaker technology. Not to be confused with modern digital formats and processing, they are a mature technology, having been experimented with extensively by Bell Labs as far back as the 1920s.
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[edit] Principle of Operation
The concept is simple; the least significant bit drives a tiny speaker driver, of whatever physical design is chosen; a value of "1" causes this driver to be driven full amplitude, a value of "0" causes it to be off. This allows for high efficiency in the amplifier, which at any time is either passing zero current, or required to drop the output voltage by zero volts, therefore in a theoretical ideal amplifier dissipating no power as heat at any time. The next least significant bit drives a speaker of twice the area (most often, but not necessarily, a ring around the previous driver), again to either full amplitude, or off. The next least significant bit drives a speaker of twice this area, and so on.
[edit] Problems
Although digital speakers can function, there are various problems with this design which make it impractical for any normal uses at present.
[edit] Size
For the number of bits required for high quality sound reproduction, the size of the system becomes impractically large. For example, for a 16-bit system with the same bit depth as the 16-bit audio CD standard, starting with a 0.5 cm² driver for the least significant bit would require a total area for the driver array of 32,000 cm², or over 450 square feet (41 m²).
[edit] Ultrasonic output
The mechanical speaker drivers have a limit to their speed of movement, so produce substantial amounts of ultrasonics as they move.
Since this system is converting digital signal to analog, the effect of aliasing is unavoidable, so that the audio output is "reflected" at equal amplitude in the frequency domain, on the other side of the sampling frequency.
Even accounting for the vastly lower efficiency of speaker drivers at such high frequencies, the result was to generate an unacceptably high level of ultrasonics accompanying the desired output.
In electronic digital to analog conversion, this is addressed by the use of low-pass filters to eliminate the spurious upper frequencies produced. Since these frequencies are eliminated in the electricl signal, they are not passed to the speaker and thus ultrasonic airwaves are not generated.
However, electronic filtering is inherently unable to solve this problem with the digital loudspeaker. The speaker elements must operate ultrasonically to avoid introducing (high levels of) audible artifacts, and this means ultrasonic airwaves are inevitable. Electronics can filter electrical signals, but can not remove ultrasonic frequencies already in the air.
[edit] Efficiency
Although amplifier efficiency is good with this system, moving coil speakers operate at relatively low efficiency in the ultrasonic frequency region. Thus the original aim of the method is defeated.
[edit] Cost
The large number of speaker drivers in the array, and the equally large number of amplifier channels to drive them makes for a pricey system.
[edit] Improvements
There are ways to tackle the above issues, but none lead to a competitive or even issue free system.
[edit] Size
System size can be easily made practical by using less than 16 bits. With a 0.5 cm² LSB, system sizes are:
- 8 bit: 128 cm² total array area, or 11.3cm x 11.3cm (apx 4.5 inches x 4.5 inches)
- 10 bit: 22.6cm x 22.6cm array size.
A larger number of bits can be accommodated in a given space by varying the throw of the different elements as well as their area. This can achieve a magnitude or more of area improvement for a given bit depth. Thus one could fit a 13 bit array into a square foot, or a 16 bit array into 4 square feet.
[edit] Ultrasonics
A passive air coupled diaphragm fitted over the array of digital drivers can act as a mechanical low pass filter. However, a sharp frequency cutoff is impossible, so significant ultrasonics would still be present. Multiple passive diaphragms could improve this, but would never remove all ultrasonics, and only add further to the system's already high cost and complication.
[edit] Intractable problems
The complexity and thus cost are both high compared to standard moving coil speakers.
The efficiency of speakers operated at ultrasonic frequencies is low, wiping out any efficiency gain in the amplifier.
Practical speakers demand production of fairly high volumes from fairly small cabinets, a combination that is difficult to achieve using ultrasonic speaker drivers.
Full removal of ultrasonic output is impractical.
The large number of required speaker and amplifier elements reduces system reliability significantly
Other more modern approaches to high efficiency, particularly class D amplification, work much better and at much lower cost than digital speakers.
[edit] Speakers marked as digital
Modern speakers marked as 'digital' are always analogue speakers, in most cases driven by an analogue amplifier. The widespread use of the term 'digital' with speakers is a marketing ploy intended to lead some buyers to associate the product with the advantages of digital sound systems. If pressed, manufacturers may claim the term means the product is ready for input from digital players (a claim true of all speaker systems).
There are also a minority of class D digital amplifier driven analogue speakers, though these are not normally found in separate computer speakers or home stereo systems. These are common in laptops, where their higher cost is justified by battery power savings. The speakers in such equipment are still analogue.
