Compact
Disc
lnteresting facts about
the compact disc
The
information
"groove"
of a
compact
disc
has a length of
approximately
5
km and contains a complex data signal:
alternatingly
arranged digital
values from two
audio channels carry the
music
information. Ancillary
information
for
displaying the selections
(track.
index),
playing
time, etc. are also
encoded in this information
"groove'l
On compact discs
this information
is
encoded
on an ultrathin
aluminium layer
The
digital
information
is
embossed
in this layer
in the
form of
"pits'1
The surface
is
scanned
by means of a laser beam.
the
reflected light of
which is converted by
photo
diodes
to electrical,
digi-
tal signals.
Structure of
the compact
disc
The
maximum
playing
time of
a compact disc is 74 minutes Special
information
has
been
encoded on the disc throuqh
which individual
selections or a
passage
can be
quickly
found.
O The individual selections
on the compact disc are separated
by a
brief
pause
and are designated
as TRACKS.
O
Individual
movements or sections
(mostly
classical
pieces)
are
identified by certain disc
producers
with indices
(INDEX)
O Each compact disc is also subdivided
into minutes and seconds.
Any
position
can
be
accessed
with
one second accuracy
and thus
remains reproducible at
any
line.
Handling
and care of
the compact discs
Handling of the compact disc
is much
easier and
less
problematic
in
comparison with conventional LP records. Nevertheless
certain
precautions
are to be observed.
For this
purpose
it is necessary to
explain the design of a disc
The ultrathin aluminium layer. that
holds
the information, is applied
to
a
transparent substrate.
The label is
subsequently
printed
on this
aluminium layer.
Although the compact
disc
is scanned through
the substrate,
the side
carrying
the
label is more
sensitive
to
scratches because the
printing
ink
and the varnish are the only
protection
for the extremely thin
aluminium coating.
On the
printed
side a
scratch can
interfere with the
reproduction
process
and
render it
even impossible, while on the
scanning side a scratch
goes
virtually unnoticed.
B
The developed
aggregate
signal
is
separated
into music and
informa
tion
data,
and
checked.
Erroneous or
missing
data
are corrected or
complemented
by a
powerful
error detection system.
Missing data
(resulting
from scratches)
are
replaced
by
interpolation
(mean
value
from
scanning
before and after the fault).
lf
the errors
are too frequent
for beinq
correctable, the output is muted.
We
distinguish
between two types of time:
DISC TIME:
playing
time in minutes and seconds since the start
of
the disc.
TRACK TIME:
playing
time in minutes and seconds since
the start of
the selection
(TRACK)
Allthese
informative data are
defined
and encoded on the
information
"groove"
by
the
CD
producel
which means
that they
are independent
of the digital
audio
player
and cannot be altered.
The spacing
between
the
information
"grooves"
on the compact disc
is
only 1.6
pm
and the mean
bit
length is approximately 2
pm
(two
thousandths
of a millimeter).
lf we consider that a
human hair has a
thickness of
40
to 60
pm,
it becomes evident that a scratch
in
the
aluminium layer can have
disasterous consequences.
Cleaning of
the compact
disc is not necessary as long as
it is
only
touched along
the rim
and always returned to the original
packing
sleeve.
Dirty compact
discs
can be cleaned
on
the
scanning side
(specular
surface) by wiping it ln
straight line from the centertoward the rim
with
a
piece
of
lint free cloth.
Solvents or abrasives must not be used under
any circumstances.
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