Problems of Digital Speaker,Size and Ultrasonic output,smart hardware China

Problems of Digital Speaker

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-bitaudio CDstandard, 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 aDACwhere 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 aliasingis unavoidable, so that the audio output is "reflected" at equal amplitude in the frequency domain, on the other side of thesampling 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 ultrasonics accompanying the desired output.

In electronic digital to analog conversion, this is addressed by the use oflow-pass filters 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 loud speaker. 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.

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