For over a century, the most popular type of sound transducer has been dynamic loudspeakers. Traditional analog speakers are used everywhere. It is they who remain the last analog device in the sound reproduction path familiar to a modern person. But if analogue dynamic loudspeakers were discovered by archaeologists of some civilization in the distant future, they would probably puzzle over why their ancestors needed such illogical heating devices. The speaker converts most of the energy into heat and this is not its only problem.
At the same time, digital emitters of various types have been produced in limited quantities for a long time. The latter are little known to a wide range of consumers, are expensive and are used relatively rarely. Further, a brief history of digital sound emitters, the devices in which they were used and applied, as well as considerations about their prospects.
Prerequisites for the appearance
Since the mid-20s, undivided dominance in electroacoustics has remained with the electrodynamic loudspeaker, in its various variations. Neither electrostats, which at first burned great, disrupting the sessions of the first sound films in the 30s, and then became simply fabulously expensive, could not squeeze it out. Neither ionophones, which are not capable of adequate reproduction of low frequencies. Neither piezoelectric emitters, which could not withstand the competition due to their small frequency range.
burnt out subwoofer speaker coil
At the same time, the speakers can hardly be called a technically perfect solution. So, for tweeters, the coil temperature of 100 degrees Celsius is not the limit, the efficiency for this reason rarely exceeds 1%, and the temperature of the woofer coil of dynamic drivers can easily exceed 150 or even 200 degrees, when operating at rated power. Distortion, both frequency and non-linear, requires correction or technologies that can significantly reduce them. A similar story occurs with the transient response, which in expensive solutions makes you constantly chase a large frequency range, which, ideally, should go much further than the spectrum audible to the human ear.
But, despite all the shortcomings of the speaker, it was he who became the most demanded in terms of the combination of advantages. At the same time, tireless researchers did not stop looking for something more productive, energy efficient, and also more manageable. Engineers began looking for a way to convert digital signals to sound directly, without using a DAC.
acoustic experiments Bell Labs in the 1920s
In theory, digital speakers were first described by Bell Labs back in the 1920s. Their principle was simple enough. The least significant bit controls the speaker, in which a value of “1” drives it at its maximum amplitude, a value of “0” stops signaling completely. Further, the least significant bit doubled the initial radiation area, the next one doubled its area, etc. in accordance with the number of bits. In the 20s, there was no pressing need for this type of digital signal-to-sound conversion, and theoretically, the work lay on the table for many years.
Bell Lab Phone Loudspeaker
In early versions, the emission area of the next bit was located concentrically around the segment of the previous bit, but this rule is not required. The theory was first put into practice in 1980. Bell Lab also became the developer. It was a disc-shaped electrode on which a thin film membrane was fixed. The electrode was divided into isolated segments, with the area ratio described above for the number of discharges of 4.3, 2.1, 0. Segments were excited with a digital rectangular signal, in accordance with its value.
For telephone communication, the fidelity of reproduction was sufficient, but this emitter was unsuitable for reproducing music. The fact is that in order to obtain sufficient loudness, the area of the corresponding radiator in the form factor of the speaker system was unacceptably large. Also a problem was the distortion of the conversion, which in classical DACs can be eliminated with the help of filters. But in digital emitters, their use is impossible, since the conversion takes place directly and they are the final link in reproduction.
The next stage in the development of digital sound emitters was the creation of electret and piezoelectric digital loudspeakers by SONY. The principle of operation was not very different from that used at Bell Lab, but the design was different. The electrodes of such emitters were concentric sections with equal area. The sections were connected in groups, the number of groups depended on the capacity of the emitter.
A fundamentally different method of dividing sections of a digital loudspeaker was proposed by engineers at Matsushita Electric Corporation (today Panasonic Corporation). The patents, which are still owned by the company, propose to group the sound-emitting segments according to the weighting factor of the discharge.
None of the developments described in the section were developed due to the cost of production, high distortions, low manufacturability and other specific problems of newborn technology.
Attempts to create an electrodynamic digital emitter began almost immediately after the appearance of piezo and electret loudspeakers of this type. The problems of the latter consisted in a narrow frequency range and a kind of frequency response, which did not allow them to be effectively used anywhere, except for communication devices for reproducing voice and HF sections of the speaker.
drawing from Philips patent
Philips and Sony began experimenting with digital speakers back in 1982. The principle was that the number of coils in the emitter increases, while the number of sections corresponds to the capacity. The result was Philips patent # 4612420, shortly before that in Japan # 58-31699, showing a similar digital speaker design.
The multi-coil digital speaker can be considered to be one of the longest-lived digital driver options. The last mention of a similar development dates back to 2000, when a similar principle was applied by B&W, the flagship of the audiophile development.
University piezo emitters
In addition to electronics corporations, the digital emitter theme has been actively pursued at universities. A group of scientists from Shinzu University in Nagano focused their efforts on creating piezoelectric digital loudspeakers in the 1990s. They got the first result in 1993, and by 1999 they showed an emitter designed for a 16-bit signal with a sampling rate of 48 kHz.
We can say that this development was the first digital emitter, the characteristics of which were sufficient for limited multimedia use. The characteristics of the device were as follows:
- Frequency range: 40-10000 Hz;
- Uneven frequency response within 4dB.
- THD 3.5% at 50 Hz and 0.1% at 10000 Hz
- Sensitivity 84 dB
Quantization noise and other artifacts of this type of digital-to-analog conversion associated with low …