Digital speakers
Encyclopedia
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.
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.
Other approaches are possible. For example instead of doubling the area of the next most significant diaphragm segment, it could simply be driven so it stroked twice as far. The digital principle of operation and attendant amplifier efficiency benefits would remain.
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 34 square feet (3.2 m²).
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. One solution would be to overclock the conversion elements, introduce a digital filter and follow them with an acoustical low pass filter.
Even accounting for the vastly lower efficiency
of speaker drivers at such high frequencies, the result was to generate an unacceptably high level of ultrasonic
s accompanying the desired output.
In electronic digital to analog conversion, this is addressed by the use of low-pass filter
s to eliminate the spurious upper frequencies produced. Since these frequencies are eliminated in the electrical 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.
operate at relatively low efficiency in the ultrasonic frequency region. Thus the original aim of the method is defeated.
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
. one could fit a 13 bit array into a square foot, or a 16 bit array into 4 square foot (0.37161216 m²).
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.
recordings), or impute 'higher technology' than some other speaker, and perhaps higher price. If pressed, manufacturers may claim the term means the product is 'ready' for input from digital players; this is true of essentially all speaker systems.
There are also a minority of Class D
and Class T
digital amplifier driven analog 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 analog.
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
technology. Not to be confused with modern digital formats and processing, they are a mature technology
Mature technology
A mature technology is a technology that has been in use for long enough that most of its initial faults and inherent problems have been removed or reduced by further development...
, having been experimented with extensively by Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
as far back as the 1920s.
Principle of operation
The concept is simple; the least significant bitLeast significant bit
In computing, the least significant bit is the bit position in a binary integer giving the units value, that is, determining whether the number is even or odd. The lsb is sometimes referred to as the right-most bit, due to the convention in positional notation of writing less significant digits...
drives a tiny speaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
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.
Other approaches are possible. For example instead of doubling the area of the next most significant diaphragm segment, it could simply be driven so it stroked twice as far. The digital principle of operation and attendant amplifier efficiency benefits would remain.
Problems
Although digital speakers can function, there are various problems with this design which make it impractical for any normal uses at present.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 CDRed Book (audio CD standard)
Red Book is the standard for audio CDs . It is named after one of the Rainbow Books, a series of books that contain the technical specifications for all CD and CD-ROM formats.The first edition of the Red Book was released in 1980 by Philips and Sony; it was adopted by the Digital Audio Disc...
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 34 square feet (3.2 m²).
Ultrasonic output
To work properly, all of the individual diaphragm elements would have to operate cleanly at the clock frequency. The natural frequency response of the various elements will vary with their size. This creates a DAC where the various bits have different bandpass characteristics. Large short-term errors can be expected.Since this system is converting digital signal to analog, the effect of aliasing
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
is unavoidable, so that the audio output is "reflected" at equal amplitude in the frequency domain, on the other side of the sampling frequency. One solution would be to overclock the conversion elements, introduce a digital filter and follow them with an acoustical low pass filter.
Even accounting for the vastly lower efficiency
Efficient energy use
Efficient energy use, sometimes simply called energy efficiency, is the goal of efforts to reduce the amount of energy required to provide products and services. For example, insulating a home allows a building to use less heating and cooling energy to achieve and maintain a comfortable temperature...
of speaker drivers at such high frequencies, the result was to generate an unacceptably high level of ultrasonic
Ultrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...
s accompanying the desired output.
In electronic digital to analog conversion, this is addressed by the use of low-pass filter
Low-pass filter
A low-pass filter is an electronic filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter...
s to eliminate the spurious upper frequencies produced. Since these frequencies are eliminated in the electrical 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.
Efficiency
Although amplifier efficiency is good with this system, moving coil speakersLoudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
operate at relatively low efficiency in the ultrasonic frequency region. Thus the original aim of the method is defeated.
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.Improvements
There are ways to tackle the above issues, but none lead to a competitive or even issue free system.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 11.3 cm x 11.3 cm (apx 4.5 inches x 4.5 inches)
- 10 bit: 22.6 cm x 22.6 cm 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
Bit depth
Bit depth may refer to:* For bit depth in graphics, see color depth* For bit depth in audio, see audio bit depth* For bit depth in drilling for petroleum, see drilling rig...
. one could fit a 13 bit array into a square foot, or a 16 bit array into 4 square foot (0.37161216 m²).
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 complexity.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
Class D
The term Class D may refer to:* Class D, a planet classification from the fictional Star Trek universe* Class D, a modern stellar classification* Class D , a defunct class in minor league baseball in North America...
amplification, work much better and at much lower cost than digital speakers.
Speakers marked as digital
Modern speakers marketed as 'digital' are always analog speakers, in most cases driven by an analog amplifier. The widespread use of the term 'digital' with speakers is a marketing ploy intended to claim better suitability with 'digital' source material (e.g., MP3MP3
MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression...
recordings), or impute 'higher technology' than some other speaker, and perhaps higher price. If pressed, manufacturers may claim the term means the product is 'ready' for input from digital players; this is true of essentially all speaker systems.
There are also a minority of Class D
Switching amplifier
A class-D amplifier or switching amplifier is an electronic amplifier where all power devices are operated as binary switches. They are either fully on or fully off. Ideally, zero time is spent transitioning between those two states....
and Class T
Class T amplifier
A Class T amplifier is an audio amplifier IC design. Rather than being a separate "class" of amplifier, Class T is a registered trademark for Tripath's amplifier technologies....
digital amplifier driven analog 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 analog.