Theatre Design & Technology - Spring 1984 - 38

SOUND

Probably the hottest topic in the
sound fidelity field today is the Compact Disc (CD). Is it here to stay? Will
it replace regular records? Is it really
better than records? Should I be thinking of replacing my turntable with a
CD? Just about all the expert in the
field agree that the answers are: yes,
yes, yes, maybe.
Let's not dwell on how the CD
works; that has been well documented
in the stereo consumer press for the past
two years (see High Fideli!y and Audio
magazines). What is important to us
are the functional features that make
the CD superior to analog records for
use in the theatre,
First, what's wrong with the analog
record-our old friend the LP? Under
ideal, laboratory conditions there is
probably no discernable di fference between the playback sound from a CD
and an LP. We do not, however, live
and work under laboratory conditions.
In the real world the LP is subject to
five families of problems:
(I) contamination by dirt, which produces obvious and annoying noise;
(2) weardown of the groove walls, resulting in the loss of high frequencies and increasing background
nOIse;
(3) distortion caused by incorrect stylus tracking;
(4) groove jumping or locking caused
by stylus tracking failure;
(5) wow and flutter caused by irregularities in the motor drive system and
by disc slippage on the turntable.
Each of these problems has more
than one cause; some can be prevented
or minimized through careful handling
of the records and regular maintenance
of the arm, cartridge and stylus. There
are other problems (hUIll, rumble, etc.)
that come from using low-cost equipment. However, even with good equipment and meticulous record care the
risks of groove jumping and the buildup of noise with repeated plays are so
great that no one (I hope) ever plays a
record during a live performance. The
36

recording is always transferred 10 tape
for use in the show,
Because physical construction and
mel hod of playing the CD are different
from the LP, the playback problems of
the LP are almost lotally eliminated.
The CD is encased in a smooth, sealed
plaslic jacket thaI can be wiped clean if
it should acquire enough dirt to be a
problem (chocolate ice cream, perhaps),
There is no stylus 10 wear down groove
walls, so quality does not deteriorate
with use. While there are no grooves
for the stylus to jump or lock into, the
playback laser does have to track the
spirals of digital pulses laid down on the
disc and mistracking is a possibility-a
very unlikely possibility.
In addition to overcoming the mechanical problems of the LP, the CD offers several features not possible with
the analog record. Perhaps the most
important one is the CD player's ability
to recover from and hide the effects of
a misreading of the track. With an LP
playback, any misreading (caused by
dirt, groove jump, etc.) results in audible noise and a bit of missed program.
When a CD player misreads a track,
an internal "smart" circuit recognizes
the error and suppresses any garbage
sounds. It then tries to recover or reconstruct the missing program material.
In most cases this error-correcting procedure works so well that the average
rate of audible errors is less than one
per hour. This performance certainly
beats the several errors per minute of
most analog playbacks.
Another feature unique to the CD is
the ability to encode non-program information on the disc. International standards have established eight sub-code
channels that run along with the two- or
four-channel stereo program, The subcode channels are used for such things
as turning off the outpul between selections and during search operations, for
identifying the Iype of pre-emphasis
used, and for identifying two- and fourchannel stereo programs. The subchannels are also used for indexing (like

Theatre Design & Technology/USITT /Spring 1984

the tab on a typewriter) which helps iI
finding a particular passage in a lonl
cut. Several of t he channels do nOl ye
ha\'e standardized applications; lhe~
are being held for future de\'elopmenls
ot all disc manufacturers make use 0
the sub-code channels.
In addition to the sub-code channel
that co-exist with the program, there i
a table of contents recorded on thi
"lead-in groove" before the progran
begins. This TOC contains the numbe
of tracks and their starting times meas
ured from the start of the disc, This in
formation allows the player to be pro
grammed 10 play only selected "CU1S'
in any desired sequence.
The record on a turntable has onl
practical operating advantage over an:
closed system (tape or CD): the opel'
atar can visually and manually cue thl
record to a desired starting point vel':
quickly. The sub-code channels ane
TOC of the CD are a help in findin)
tracks, and the very expensive($6000 +
broadcast machines even have a largl
knob for tight cueing. The problem 0
fast and tight cues is more importan
for disc jockey work in broadcast in)
and disco clubs than in theatre. It i
likely that CDs will often be copied te
tape for use in live performances. Pos
sible direct use of the CD includes ap
plications where tight starting cues an
not important such as entr'acte musil
or, perhaps, music for dance. Excep
for the cueing problem, there is ne
reason for not using a CD in a livi
performance.
There are a few temporary problem
with the CD: cost, availability, and lim
ited program catalog.
At this writing (Thanksgiving 1983)
the cost of consumer-type CD player
ranges from $500 to $1200 with profes
sional (broadcast) machines in thi
$6000+ range. The industry prognos
ticators assure us that we will see anum
bel' of machines in the less-Ihan-$50(
category by mid-1984. The cost di ffer
ence among the machines is (as in videc
tape machines) largely a matter of use

Table of Contents for the Digital Edition of Theatre Design & Technology - Spring 1984