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More about CD players and CD-CHECKTM

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.

Use CD-CHECK to test error correction and tracking capability of a CD / DVD player

Use DED for continuous monitoring of the digital output signal and the digital link from CD / DVD players, DAT recorders and other digital sources

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