THE MINUTE SCALE OF RECORDINGS
What makes the table system’s job so devilishly demanding is the
truly minute scale of the music signal that must be released from
the grooves. This signal must be magnified some 30,000 times from
the groove to the speakers in order to be heard.
Visualize the fineness of a record groove, and then consider that
it combines two distinct channels of information, each with completely
different modulations. Some of the signal modulations in the groove
are on the same order of size as a wavelength of light, which means
the stylus has to “read” a signal as small as a millionth of an
inch. Add in all the variation and complexities in the scale of
the music itself, from crescendos to pianissimos, from piccolos
to contrabassoons, and you can begin to see the stylus has quite
a job.
The recorded audio bandwidth is the range of frequencies (i.e.,
rates of vibration) the human ear can hear, which extends from
20 Hz to 20,000 Hz. Hertz (Hz) is the same as cycles per second.
You can hear a piccolo note whose fundamental vibrates as fast
as 4,698.6 Hz and whose harmonics extend well up to 18,000 Hz and
beyond, and a contrabassoon with a fundamental plunging as deep
as 29 Hz. Your ear can also detect ranges in loudness of 60 dB,
a ratio of 100,000 to 1. Not only is this tremendous range captured
in the record groove, but then the stylus has to release it.
For the half mile or so of record groove per LP side, the stylus
must precisely trace abrupt changes in the direction of the undulating
groove, sometimes traveling at speeds several times the acceleration
of gravity, without ever losing contact with either wall or blurring
together the modulations.
Groove friction heats the stylus up to 350 degrees Fahrenheit
and the groove vinyl momentarily liquefies each time the stylus
passes over it. (This is why one should let a record rest for at
least 30 minutes before replaying it, and preferably for 24 hours.)
Even though the cartridge tracking weight is commonly set at only
about 1.5 grams, the entire weight is supported on the minute side
edges of the stylus. As a result, the downforce applied to the
groove on a per-square-inch basis is several TONS.
Combine these extreme conditions of weight, heat, speed, and need
for exquisite maneuverability, then add in the scale of environmental
vibrations that interfere with the stylus as it retrieves the music
from the groove, and it’s extraordinary that ANY music (as opposed
to noise) is heard through an audio system. The feat of retrieving
all the music from the groove is analogous to an elephant trying
to thread a needle.
To help one better grasp the magnitude of the difficulties in
retrieving all the music from the record, the Boston Inch Scale
(developed by E. B. Meyer and published in the Boston Audio Society’s
magazine, The Speaker) converts signal and table measurements from
their real-life micron scale into inches. A micron is a millionth
of a meter, or one thousandth of a millimeter, which is equivalent
to 0.0039 inch.
Using the inch scale, a stylus is 30 feet high, affixed to a cantilever
50 feet thick and 275 feet long, which extends from a cartridge
body 2,000 feet long, sitting 80 feet above the record. The tonearm,
450 feet in diameter, crosses 1,300 feet above the record from
its pivot point 4 miles away. On a typical line-contact stylus,
the stylus down- force temporarily deforms the vinyl by as much
as an inch (20 times the size of a violin harmonic), leaving a
stylus footprint on the groove wall measuring 10 inches long and
4 inches wide.
A typical midrange signal demands that the stylus move 16 inches
from peak to peak of the wave form. A deep bass note 10 dB louder
requires the stylus to move 10 feet 6 inches whereas for a high-frequency
harmonic at a very low sound level, the stylus must move only 0.68
inch. Even the simplest piece of music is likely to contain, at
any one time, enormous numbers of frequencies at different levels.
(Incidentally, the same microsonic scale applies to compact discs.
Though it is technologically feasible to make the pits smaller
than they are now, and thus fit more information onto a single
disc, the laser fine enough to read those smaller pits has yet
to become commercially practical.) |