Compact discs store music in digital format (a series of 0s and 1s
referred
to as data). CD players have built-in D/A converters that turn the digital
data on a CD into an analog signal - which we hear as music. Ideally, all
the data stored on the disc should be converted to the analog format. In
reality, many factors cause some data to be lost and sound reproduction to
deteriorate. CD-CHECKTM tests the key components of a player that
are responsible for the
delivery of all data stored on the disc: the pickup
assembly and the error correction system.
CD players overcome numerous obstacles
The pickup assembly of a CD player performs a number of complex tasks. It
tracks, focuses on, and reads digital information that is aligned in rows
of pits and flats 0.0005 mm wide (700 times smaller than a pinprick) at a
speed of 1.2 to 1.4 m/sec. While performing this task, the pickup's laser
beam encounters many obstacles: disc manufacturing flaws and errors that
develop during the normal course of use. Mass-manufactured CDs always
contain imperfections and the manufacturing standard allows a certain
degree of error, expressed as bit error rate (BER). Such errors are
typically caused by deformed pits, micro air bubbles in the disc's plastic
substrate, or pinholes in its metal layer. Industry standards also allow
for a certain degree of disc warpage, or even small displacement of the
disc's centre. In addition to manufacturing defects, dust, dirt, and
scratches accumulate on the disc's surface. Any of these are overwhelming
in size when compared with the pits and flats that represent the signal.
All result in considerable data loss (dropout) during playback.
Another common source of data loss is the pickup assembly itself. Its
ability to correctly track and read the information off the disc depends
on
many factors and changes with time and usage. Tracking misalignment, a
dirty or scratched laser lens, internal or external vibrations, and many
other conditions will result in data loss in addition to loss caused by
the
errors on the disc medium.
How is data loss managed?
CD players manage to reproduce music truthfully despite these ever present
problems.
100% accurate sound.
As a first line of defense, CD players employ a sophisticated
error
correction system designed to accurately restore the missing data and
enable exact reproduction of the recorded signal. The system uses
redundant
information on the disc to reconstruct the lost content. If not for the
error correction circuitry, the player would never be able to reproduce
the
music without sound distortion or alteration -- even when playing new,
seemingly perfect discs.
Less than 100% sound.
When data loss is too large to be handled by the error correction system,
an error concealment circuit is employed to
interpolate or "mute over" the
lost signal. Interpolation can be compared to building sonic bridges over
the data dropout (by computing approximate values for missing data from
surrounding information). "Muting" means inserting a silent pause in place
of an error, either with or without fade-in/fade-out of the adjacent
signal. Whatever method is used, error concealment - as opposed to error
correction - will cause varying degrees of sound distortion, affecting
sound clarity, stereo imaging and dynamics. Hard to detect as not always distinctly
audible, concealment may prevent the listener from hearing the full music
spectrum of recordings. If the CD player conceals errors frequently, it is
also an indication of a poor error correction system and/or a
deteriorating
pickup.
Failure.
When both error correction and concealment fail, distinct clicks will
result and, in more severe cases, track skipping or stalling will take
place.
Function of CD-CHECKTM
CD-CHECKTM disc contains a series of tracks that uniquely test the performance
of the pickup and error correction systems. The tests quickly rate a player's
capacity to deliver accurate sound without reliance on error concealment to
mask data loss, while encountering common pickup flaws and disc defects. CD-
CHECKTM allows objective verification of:
-
Whether the player processes complete digital data necessary for high
fidelity sound reproduction, while playing the typical mass-manufactured
discs.
-
A player's capacity to manage data dropouts without affecting sound
clearity, stage imaging, transient response, and dynamics.
-
A player's reliance on error correction to completely reconstruct the lost
data as opposed to masking the loss through error concealment.
-
The capacity of error correction to provide backup for mechanical
deterioriation of the pickup and ensure a player's consistent performance over
time.
-
Whether laser lens cleaning, or performance improving pickup adjustments are
required/effective.
-
Effectiveness of CD performance enhacement devices and techniques.
-
A player's reliability in comparison with other CD playing equipment.
