What are Digital Speakers?


Digital speakers or Digital Sound Reconstruction (DSR) systems are a form of loudspeaker technology. You should not confuse them with modern digital formats and processing. This is because engineers still have not developed them as a mature technology. Bell Labs have been experimenting extensively with them as far back as the 1920s. However, they were not realized as commercial products.


Two main types


There are two main types of digital loudspeakers that bring closer the signal in the digital form to the transducing process and convert the electrical signals into air-pressure variations or sound energy. Theoretical work is proceeding on a loudspeaker whose sound generating section is a matrix of small digital or on/off elements. An array of electromagnetic solenoids or alternatively by piezo actuators energize these elements.

Modeling indicates that significantly greater excursion may be generated by spiral forms of the piezo exciter. By suitably driving the array for digitally processed signal, preferably to the full 16-bit 44/48 kHz PCM resolution or better, the matrix of elements should reconstruct the original acoustic wavefronts in time, frequency and amplitude. The problems of ultrasonic and beat-frequency noise generation will be considerable. In this basic form, the system may never be commercially released.


Digital conversion


Digital conversation from digital signals to sound pressure is the ultimate objective and may be possible with micro-machine pumps. In an alternative earlier proposal, the voice-coil of a direct radiator driver subdivides and each element drives digitally. Data integration occurs over the complete coil. Half the bit range- the upper, easier half- is handled by a split-coil winding while the lower half. In this case, the remaining 8 bits, operates in the analog domain, with the simple DAC and the requirement for only a low-power, linear amplifier for this part.


Active design


An intermediate for the digital speaker which is successful in production begins with an analog or conventional speaker of active design. Here the designer can choose to place a D/A converter in the loudspeaker electronics. A digital audio processor may also be present. Digitally coded signals can be fed directly to the system. This avoids loses in intermediate analog control electronics, as well as inline-level linking cables. Another digital line achieves the control of the signal replay level. In this case, a managing microcomputer is in the speaker.

In a domestic realization, the latter would be ideal to have an interface via a hand-held infrared remote control. The concept of remote D/A and A/D converters is likely to find the increasing application. The more that engineers handle the audio signals in digitally coded form, there is less danger of degradations in the transmission chain in comparison to analog electronic interfaces and cables. The trend is apparent in digital studio practice. For example, remote A/D converters were introduced at the microphone position.


System 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 34 square feet (3.2 m²).


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