If you're designing the analog portions of audio systems, it helps to know two things: what your listeners expect, and how you're going to objectively demonstrate that you're delivering on those expectations. Yet it's difficult to relate engineering measurements to the subjective experience of hearing. The idea that hearing isn't a linear continuum started to crystallize for me in June.
National Semiconductor wanted to show me some products it had optimized for the absolutely purest consumers of audio systems-the hardcore, two-speaker audiophile. These products comprised a family of high-voltage op amps with extremely low distortion and an audio power-amplifier driver. Both are designed to replace large handfuls of discrete components in top-end audiophile pre-amps and amplifiers. Designers of ne plus ultra audio systems still insist on discretes, and National has set out to change their minds.
National set up an acoustically ideal sound room with a pair of Wilson Audio Specialties Watt-Puppy speakers and a reference-design analog audio chain built around the new chips. It also had its own CDs to demonstrate characteristics that true believers listen for, but I was invited to bring my own disc, too. So, I grabbed a CD of Beethoven piano sonatas from the carrier on my car's sun-visor. The grand piano is a subtle instrument, and Beethoven piano sonatas are unlikely to undergo much in the way of post-production "enhancements."
My expectations were low, because my hearing has been attenuated by age, automobiles, airplanes, and motorcycles. Knowing that, I walked into that sound room thinking its $26,000 worth of speakers connected to National's custom electronics was all overkill for my $50 ears. That is, I figured that $50 was all it was worth spending on headphones or speakers for my personal use. The upshot? I was shocked at what I heard.
On National's demo recordings, I could tell which cymbals rang longest. I also could hear the singer's lips make that sound when you separate your lips and there's some saliva on them. On my Beethoven CD, I could hear the pedal work-not the mechanical action, but the difference in tone as the artist's foot released-sustain exactly on the beat.
I was puzzled. I have the furthest thing from "golden ears" that one can have without wearing prostheses. How could I be getting so much new experience from a recording I had listened to hundreds of times? I put that question to Mark Brasfield, principal audio applications engineer at National and creator of its sound room (Fig. 1).
Interestingly, Brasfield confessed that he also has some hearing loss. Yet he said industry studies revealed that, up to a point, people with reduced hearing acuity actually get more out of high-end audio than people with perfect hearing.
Test experts To get an idea of what kinds of tests you could run to compare audio systems, I spoke to Bruce Hofer, founder and president of test-equipment maker Audio Precision (see "The Challenges Of Audio Testing" at www.electronicdesign. com, Drill Deeper 16803). Audio Precision is probably the most respected maker of purely audio test gear (Fig. 2). But if I was hoping for unambiguous metrics, Hofer discouraged me.
"After three decades now, one realizes that you don't understand what you don't understand about the ear and the brain system," Hofer said. "At Audio Precision, we're all about making objective measurements on audio devices. But the world is also about perception and measurements, which, by their very nature, are not going to track perfectly with that which you're really trying to determine."
Hofer said that Audio Precision doesn't have an audio distortion analyzer that says "Great/Good/Bad" yet. His test gear simply provides numbers, measures of performance, usually cut along different axes like frequency response, distortion factor, or noise. While these metrics all are valid, Hofer said, the ear's behavior is more subtle than that.
The conversation then veered into the broader topic of the way companies and researchers use human subjects, rather than precision test gear, in their evaluations. Hofer said that the best examples include Fraunhofer and Dolby, which use human subjects listening to real recordings to gauge how well their lossy compression algorithms capture real performances (see "Testing For Audio Transparency," Drill Deeper 16802).
The analytical measurements that Audio Precision analyzers make are meant to avoid the downfalls of subjective measurements by describing signals that are highly defined mathematically, explained Hofer. It's a straightforward process to describe the effect of distortion products, hum products, and noise products, with respect to an original mathematically defined signal.
"But even then, if you digitally recorded a broad multitone input, the measurement will show some tones, not attenuated, but entirely missing," said Hofer. "The lossy compression algorithm made the decision not to waste any bits on certain parts of the signal because those parts didn't meet the threshold for audibility."
And if the algorithms are truly optimized, one's ears and brain won't notice the missing information, even though an analog test instrument will.
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