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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