TECHNICAL TALK, JULIAN D. HIRSCH (Feb. 1976)
AES CONVENTION: A number of the technical papers presented at the recent 52nd Convention of the Audio Engineering Society have implications likely to affect the high-fidelity components reaching the market in months and years to come. The AES, being an engineering-oriented organization, is not directly concerned with audiophile hardware, but the research that goes into its creation is the basis for much of the technical-pro gram content at the conventions.
Some of the papers concerned products which have already been announced to the public and will be available for purchase in the near future, perhaps by the time this appears in print. For exam ple, high-polymer (HP) plastic materials can be used to make piezoelectric transducers that resemble electrostatic drivers but do not require expensive, bulky, high-voltage power supplied. Last year, Pioneer introduced headphones based on HP elements, and they have now employed this technique in omnidirectional tweeters for their new HPM-200 speaker system. According to the data presented in their paper, the cylindrical HP tweeters are not only omnidirectional in the horizontal plane, but they have extremely low distortion (on the order of 0.3 percent) at the fairly loud listening level of 90 dB. A paper by Matsushita (parent company of Technics, JVC, and Panasonic) also described an electret tweeter (which somewhat resembles the Pioneer device in its operating characteristics) with horn loading to raise its efficiency.
Until recently, most applications of the phase-locked loop (PLL) in consumer equipment have been in the multiplex demodulators of stereo FM tuners and receivers. Sony has made use of the PLL in the servo-control system of the turntable in their new $800 direct-drive record player. By locking the phase (rather than the speed) of the rotating platter to the output of a crystal-controlled oscillator, they have protected the turntable control system from the vagaries of the commercial power lines. The data shown in their paper indicate a startling reduction (by a factor of perhaps 10 to 100 times) of turntable speed perturbations, as compared with the performance of the velocity-feedback systems used in other direct-drive turntables.
Many people might consider this a form of overkill, considering the superiority of many "conventional" direct-drive systems over less sophisticated and less expensive-methods of record rotation. But at least one need not have any lingering doubts about the record-playing speed with the new Sony turntable; its accuracy is that of the crystal-oscillator frequency, or better than 0.003 percent! The tone arm of the Sony record player is conventional in design and appearance, but not in its composition. Instead of a metal tube, its construction employs a carbon-filament material whose physical properties minimize the unwanted resonant modes occurring in the region of hundreds to thousands of hertz, which can add "glitches" to an otherwise smooth response curve. Most metal tone arms we have tested exhibit this behavior at some frequencies, but the aberrations are of such a narrow bandwidth that they are difficult to measure, much less hear. However, the purist can take heart from the reduction in their amplitude with Sony's tone-arm material.
The majority of papers presented at the AES convention were directed to the engineer rather than to the consumer., For example, the advanced measurement instruments and methods developed by some companies have made it possible, often for the first time, to detect and measure the transient behavior of electronic and electroacoustic audio components. Engineers at the Danish firm of Bruel & Kjaer, whose beautifully conceived line of test equipment is found in laboratories the world over, described a method of making virtually any type of free-field acoustic measurement (these must usually be performed in an anechoic room) in a normal room environment. The approach is simple, although the equipment required is not. By using tone-burst signals and "gating" the test instruments to respond only to the steady-state acoustic levels within the tone burst, echoes and other room effects are completely excluded. This measurement method can be extended to include frequency response, directional characteristics, harmonic distortion, and phase response.
In a second paper by B&K engineers, a system for making swept measurements of harmonic distortion, conventional intermodulation distortion, or difference-tone intermodulation distortion was described. To some degree, the distortion characteristics of most audio components are frequency-dependent.
In the case of transducers (loudspeakers and phono cartridges as well as tape recorders), the dependency is so complete that measurements made at one or two discrete frequencies give an inadequate and often misleading picture of the distortion characteristics of the device.
The B&K swept distortion measurement provides a new insight into the performance of any audio component. Similar systems have been built in the past based on modified audio-spectrum analyzers (I recall working on such a sys tem over fifteen years ago), but, in addition to their considerable bulk and cost (more in 1960 dollars than B&K's system in 1975 dollars), they were limited to the audio-frequency range and were unable to measure distortion levels much below 0.1 percent. In contrast, the B&K instrument, which is available commercially, spans a range from 2 Hz to 200 kHz and, judging from the data presented with the B&K paper, can make measurements down to 0.001 percent. It is a remarkable measurement system.
Probably the most impressive and advanced measurement described at the AES Convention was made in the laboratories of the Japan Victor Company (JVC). The wave front emerging from a loudspeaker, which is theoretically spherical in shape, is rarely ideal. Loud speaker transient response is acknowledged to be imperfect, but most of the measurement techniques devised for its analysis also leave much to be desired.
JVC engineers drove loudspeakers (tweeters of various types) with shaped signal pulses and measured the acoustic pressure in the emerging wavefront at 3,000 (!) points in a horizontal plane in front of the speaker. The data were converted to digital form, processed by a computer, and stored in a digital magnetic recorder. The data playback was further computer-processed and displayed on a cathode-ray-tube screen on an expanded time scale so that the 100-millisecond measurement period could be viewed over a period of several seconds.
The screen was photographed with a motion picture camera, and the films were shown to the audience at the presentation of the paper.
Speaking for myself (and I think the audience shared my reaction), it was rather eerie to see the pulse waveform emerge from the speaker and fan out into a circular arc. Overshoot and ringing, as well as the drop in amplitude off the central axis of the radiator, were clearly visible. Since the presentation began with a display of an ideal pulse response, we were able to judge how closely each of the drivers came to achieving theoretical perfection.
The test setup required for JVC's measurements was rather elaborate and involved several types of expensive instrumentation. Little was said about the possible correlation of the observed wave propagation characteristics and the subjective sound of the speakers, and in a case such as this it is risky to read more into a measurement than appears on the surface. Nevertheless, it seems probable that continuing investigations of this type, and some of the others described in the convention papers, will eventually result in consumer products with improved performance. Based on what I heard and saw, I am happy to report that audio progress has not stagnated, and that tomorrow's high-fidelity components will be even better than today's.
Dokoder 1140 Tape Deck Onkyo Model 8 Speaker System Tandberg TCD-310 Cassette Deck Rotel RA-1412 Amplifier
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Also see:
HARD ROCK, SOFT EARS--Can loud music cause loss of hearing?