Working in the Front Line Page 5

It took a full three years of commercial development before CD sound broke through the "10" level, which in 1990 represented the average for the whole industry. Players scoring 5 on the current 24-maximum scale are usually found in cheap music centers, while a player that wins respect in the more critical areas of the industry will score 14 or more. Genuine audiophile players are rare and score above 16, generally costing in excess of $1000. Recent high-end players and processors using both Bitstream and multi-bit DAC technology have shown that a score of 24 is attainable; no doubt this reference level will be bettered by further design refinements in the future.

Those who do not care either consider such differences inaudible, or deliberately deem them irrelevant or inconsequential. However, these differences are crucial factors in determining purchase for those who do care and do listen.

Comparisons may be drawn between some of these subjective differences and those found in the more familiar area of loudspeakers. For example, the treble reproduction of a good pure ribbon or electrostatic drive-unit can reach a high standard of naturalness and purity. The contrast with a budget dome or paper-cone tweeter is an obvious one, the latter often characterized by grainy, sibilant, and fizzy effects combined with a masking of fine detail and harmonic subtlety. Moreover, these differences would seem to be confirmed by delayed resonance and frequency response measurements.

When a CD player is evaluated with a ribbon or equivalent high-quality transducer in the chain, treble differences are observed which resemble those that would result from substituting an inferior tweeter. Yet in this case, there is no obvious measured parameter that correlates with this aspect of CD performance.

Moving down in frequency, varying the total "Q" factor of a loudspeaker system's low-frequency alignment leads to measured changes in bass response that relate well to the subjective changes. A similar variability in subjective bass quality, akin to Q variations in a loudspeaker system, can be heard between CD players of identically and perfectly flat frequency response, generally extended (-3dB) to below 3Hz.

Well-behaved loudspeakers of low stored energy characteristic and uniform axial and off-axis frequency responses tend to sound good. They can also present good stereo images, developed with a pleasing impression of image depth where the recorded material so allows. We also know that a more resonant class of speaker tends to mask low-level detail, ambience clues, and the like, and produces "ping-pong" stereo with little depth or ambience. A comparable effect may be heard with CD players; some give a rather flat, sterile image while others deliver rewarding levels of depth and clearly reproduced ambience, corresponding well to the original recorded acoustic. Again, no measurement can pinpoint such variations.

A further aspect concerns timbre, or tonal balance. Anyone who has recently heard orchestral string sound will testify that most reproduced string tone is a travesty, even with high-quality equipment. All the processes involved in recording and reproducing seem to impart a cumulative hardening, embrittlement, and congestion to orchestral strings (see Sidebar). CD players are no exception, though significant differences can be observed between them. As with the other parameters discussed, this is not amenable to laboratory analysis. Weighing the timbral differences in the context of a loudspeaker's sound, one might suspect significant variations of about 1.5dB magnitude in the lower presence-range octave; eg, from 1kHz to 2kHz.

Now for an even more contentious area.

The sound quality of passive electronic components: capacitors, resistors, inductors, cables
Very small differences in subjective sound quality can be identified. For example, listening tests have revealed audible differences between groups of metal-film and other types of resistor used in audio equipment (footnote 3). In these tests, the listeners had no interest or foreknowledge of the resistor types, and would not have known how to identify them even had they felt like trying. These auditioning results have been given strong practical confirmation by real amplifier designs.

Similar subjective tests involving capacitors (footnote 4) have resulted in a number of improved-sounding products employed in loudspeakers and amplifiers. In one double-blind listening sequence, a group of electrolytic power-supply capacitors was assessed for their contribution to the sound of a complete high-grade stereo amplifier. All of the capacitors tested were used well within their ratings. Their internal design, foils, and electrolyte chemistry were different, however. The capacitors were properly formed, then uniformly disguised and soldered directly into circuit by an independent operator in a location remote from the listeners. There were no other variables in the experiment. The listeners were asked both to assign merit scores to each presentation and describe the sound quality.

The results showed good consistency for the limited number of repeats employed; the engineers involved were astonished to find that the capacitor differences were highly significant, determining between 20% and 30% of the overall performance of the amplifier. Each type showed complex differences in virtually all of the normal subjective audio characterizations, including bass damping, stereo focus and depth, timbre and treble distortion, and/or treble brightness. No measurable differences were observed for the complete amplifier using any of these capacitors.

Footnote 3: Martin Colloms, "Pièce de Résistance," HFN/RR, June 1987. See also Hephaistos, "Enquête sur des résistances au-dessus de tout soupçon" (An investigation into resistors above all suspicion), L'Audiophile, Paris.

Footnote 4: Martin Colloms, "A Capacity to Change," HFN/RR, October & December 1985.