Compact Disc Player      

Found in the home, car, and even the palm of your hand, the compact disc player is a truly pervasive piece of consumer electronics equipment. The compact disc player allows anyone access to studio-quality sound nearly anywhere. Unfortunately, the elegance and complexity of the compact disc player’s operation and design are underappreciated by many users.

Today, analog audio is found in the cassette tape, the vinyl record, and in isolated cases, the eight-track tape. Similar to the first audio recording on a foil-covered cylinder made by Thomas Edison in 1877, these implementations reproduce an audio signal using mechanical or electrical contact with the recording media.

The cassette tape relies on the contact of a sensor on magnetic tape, and the record on the contact of a needle-like pickup in a vinyl groove to generate their audio signals. In either case, the playback mechanism must physically contact the media, causing wear to both. This is detrimental to the signal quality because in analog audio any signal is permissible. Consequently, the system has no means of differentiating noise, distortion, and damaged media from the original recorded signal. The appeal of the digital audio signal is that noise and distortion can be separated from the audio signal.

The compact disc player as a sound reproduction device fulfills the loop begun in the recording studio, returning the audio signal back to its original analog form. If all the theoretical guidelines have been followed in the equipment and processes between the musician and your audio system, the sound you hear is exactly the sound that was heard in the recording studio.

The specifications for the compact disc and compact disc players were jointly developed by Sony, Philips, and Polygram in 1982. This specification is contained in their standards document referred to as the Red Book. A summary of this standard is shown in Table 1.

Table 1. Red Book specifications for the compact disc system.

The information on the compact disc itself is stored as digital data, similar to data stored on a computer. When recorded, the audio signal is sampled at a rate of 44.1 kHz, or one sample every 22.7 microseconds. This means that for one second of audio, there are 44,100 16-bit samples. A sampling frequency of 44.1 kHz was chosen to adequately satisfy the Nyquist sampling theorem, which states that the sampling rate must be at least twice that of the highest frequency to be sampled. Because the upper limit of human hearing lies at 20 kHz, the sampling frequency chosen adequately satisfies the theorem. Each one of the 44,100 samples represents the audio signal level at an instant in time. These samples, along with other control and timing data, are what travel through the player’s subsystems to recreate an analog audio signal.

The compact disc player contains two main subsystems: the transport system and the audio data processing system. The transport system orchestrates the mechanical operation of the player and includes such items as the spindle motor, laser pickup, lens focus, servo mechanism, and the user interface. The audio data processing section covers all other player processes.

Unlike a record player, the spindle motor that drives the rotational motion of the disc revolves at a variable speed. To keep the rate of data constant, the spindle motor must rotate the disc faster as the laser moves towards the outside edge of disc. This variance in rotational speed is due to the increasing amount of data present in the track as the distance from the center of the disc increases.

The laser pickup represents the actual physical interface between the data on the compact disc media and the player. The pickup is typically mounted on a movable sled mounted under the disc. It is comprised of the laser, a system of lenses, a photodetector, and a motor which moves the sled. The actual laser is a semiconductor-type and emits light at a wavelength of 780 nanometers, just within the infrared spectrum.

Information on the compact disc is represented by a series of lands, or flat spots, and pits, or holes. However, when viewed from the bottom of the disc from where the data is read, the pits are actually bumps. The difference in height between the pits and lands is specified, so a unique property of light can be used to obtain an electrical signal.

The width of the data track on the compact disc is approximately 0.5 micrometers, or about 30 times narrower than the width of a human hair. Because of disc wobble and shocks or vibrations to the player, the data track may move significantly as it passes over the laser. To compensate, a complex auto-tracking and auto-focus system are integrated into the laser pickup. These functions are implemented by measuring the relative beam intensity at the photodetector, dividing the photodetector into separate regions, and comparing the intensities in each region.

The most common method of implementing the servo mechanism is through the use of a three-beam pickup design. Emitted from a single laser, the three beams are formed by passing the laser light through a beam splitter. The middle, most intense beam lands directly on the track and reads the data; the two other beams land to the sides of the track and are used for tracking control.

More simple and cheaper to implement, although not as accurate, is the one-beam design. It operates using a similar photodetector arrangement. Although the design offers equivalent auto-focus performance, it has compromised auto-tracking ability due to the lack of the two auxiliary control beams.

Once the data samples have been read off the disc by the transport system, they are fed into the audio data processing system. This system consists of the demodulation and error detection circuits, error concealment and demultiplexing circuits, digital-to-analog converters, and the output filter.

During the recording process, each data sample to be put on the disc is encoded in two steps. The latter of the two steps is a system known as eight-to-fourteen modulation. In this system data is encoded when it is recorded, and decoded when it is played. During encoding, a series of 8 data bits is translated into an arbitrary set of 14 data bits. The purpose of this modulation is two-fold; disc storage efficiency is increased by reducing the number of pits, and a rudimentary type of error correction is implemented. During decoding, the 14 data bits are returned to the original 8-bit format.

During the disc mastering process, each data sample to be put on the disc is encoded in two steps. The latter of the two steps is a system known as eight-to-fourteen modulation. In this system, data is encoded when it is recorded, and decoded when it is played. During encoding, a series of 8 data bits is translated into an arbitrary set of 14 data bits. The purpose of this modulation is two-fold; disc storage efficiency is increased by reducing the number of 0-to-1 transitions in each data sample, and a rudimentary type of error correction is implemented. During decoding, the 14 data bits are returned to the original 8-bit format.

The other step implements a type of error correction capable of recovering lost data due to both manufacturing errors and the occasional hair, scratch, or fingerprint that may get on the surface of the disc. Any errors that are not corrected by the error correction circuitry are passed through an error concealment circuit. This circuit will simply silence the error. For a small number of samples, this concealment is inaudible to the ear. However, as a greater number of data samples are silenced, the error manifests itself as a skip, or audible silence.

Once all demodulation, error correction and concealment have taken place, the data is in one continuous stream. The final step is to break this stream apart into the left and right audio channels within the demultiplexing circuitry. From this point, the left and right data streams are passed along to the digital-to-analog converters to be reconstructed into an analog waveform.

The role of the digital-to-analog converter is to take each 16-bit audio sample and recreate an analog waveform. However, what results is only a rough approximation which has a very stepped, or jagged appearance.

Within the marketplace, the digital-to-analog conversion step in the decoding process reflects a great deal of variety in implementation. Players may be found featuring 16, 18-, 20-, low-, and 1-bit digital-to-analog converters.

Players introduced in and around the time of the first CD players in 1982 primarily used 16-bit DACs. In the ensuing years, players with 18- and 20-bit converters began to appear alongside and gradually displace their 16-bit counterparts. The use of a 18- or 20-bit DAC does not give true 18- or 20-bit audio performance. The extra bits used by these converters may be either thrown away, be left unused, or be put to other intelligent uses which enhance the overall performance of the player.

Although these 18- and 20-bit converters can enhance the performance of a compact disc player they, like 16-bit converters, are still plagued a variety of errors; all of which introduce harmonic distortion and degrade signal stability, imaging, and staging.

Addressing the problems with these multi-bit converters, a variety of competing low-bit conversion technologies have been developed. Rather than converting whole data words in parallel at the sampling frequency, far shorter word lengths are converted serially at significantly higher rates. This serial data conversion has been made possible in part by the powerful digital signal processors available today.

Regardless of the implementation of the DAC, a 16-bit sample is converted into an analog voltage. The value of this voltage is then held for the duration of one sample period, 1/44,100 Hz or 22.7 microseconds. This operation, known as sample-and-hold, gives the waveform its step-like appearance. At this point, the original analog audio signal is almost completely reconstructed; the final step is output filtering.

The role of the output filters is to smooth out the waveform from the digital-to-analog converters. Waveforms with very sharp, square signal transitions are rich in very high-frequency components, which lie outside the audio band. Since it is known that only frequencies within the audio band were recorded in the studio, it is safe to remove these from the signal. What results is exactly the signal that was recorded originally, without any of the jaggedness seen at the output of the DAC’s.

Like the digital-to-analog converter, the output filter stage has witnessed a great deal of improvement since 1982. Early 16-bit players used brickwall reconstruction filters. These filters are so named because of their very sharp frequency cutoff to attenuate noise lying immediately adjacent to the audio band. This sharp cutoff however, comes with the penalty of added distortion. Early and current 18-, 20-, low-, and 1-bit players have taken advantage of digital oversampling filters placed upstream of the DAC along with a gentle analog reconstruction filter. These filters have a gentler cutoff characteristic than the brickwall filters, because the oversampling filter shifts noise outside the audio band.

Sibling technologies

Although the hardware to effectively implement them did not exist in 1982, the designers of the compact disc standard exercised a great deal of foresight by allowing for variations on the type of data stored on the disc. Among these alternative uses are acronyms such as CD-ROM, CD-R, CD-i, Photo CD, CD-V, CD+G/M, and game discs (which are merely specialized CD-ROMs). Rather than storing audio data, all of these formats have the ability to store any type of digital data (CD-ROM and CD-R), still or motion video images (Photo CD and CD-V), and a mixed stream of audio, video, and other data (CD-i and CD+G/M). Regardless of what is stored on these discs, the fundamentals behind their operation are nearly identical to that of the audio compact disc.

Two other technologies in the marketplace that share some similarities with the compact disc include the video laser disc and Sony’s MiniDisc. The laser disc stores its information in an analog format. While this yields the benefits of an optical reading mechanism, none of the benefits of a digital signal are realized. Digital Video Disc, recently standardized by Sony and Toshiba, promises to remedy this. It will offer a digital storage format several times that of today’s compact disc, while maintaining the same small size of, as well as backward compatibility with, the compact disc. The MiniDisc shares many similarities with the compact audio disc in terms of data representation. However, the MiniDisc has a smaller form factor, is stored on a self-enclosed readable/writable magneto-optical disc, and utilizes data reduction that slightly lessens its sonic reproduction quality.

Compact discs have moved from an expensive audio alternative to an attractive storage and playback medium that has permeated all aspects of our cultural, social, and work lives. Recent developments in storage and usage have affirmed the success of the compact disc, and promise to push the technology even further.

References

Eargle, John. "Bitter Jitter and Sweeter Dither," Audio. Vol. 76, No. 1 (Jan. 1992). 24+

Erickson, Grant. "A Fundamental Introduction to the Compact Disc Player." University of Minnesota: Minneapolis, MN. World wide web document, http://www.tc.umn.edu/nlhome/g496/eric0139/Papers/paper.html. November 29, 1994.

Hicks, Christopher. "The Application of Dithering and Noise-shaping to Digital Audio." Cambridge University: Cambridge, England. World wide web document. http://www2.eng.cam.ac.uk/~cmh/audio_papers/n_shape.txt. June 1994.

Hicks, Christopher. "Aspects of Sampling, Oversampling, Quantisation, Dither and Noise-shaping, as Applied to Digital Audio." Cambridge University: Cambridge, England. World wide web document. http://www2.eng.cam.ac.uk/~cmh/ audio_papers/oversamp.txt. November 1994.

Kuhn, Kelin; et al. "The Technology of Compact Discs." University of Washington. Seattle, WA. World wide web document, http://www.ee.washington.edu/ class/ConsElec/cd.html.

Magnavox Corp. "The Compact Disc," Electronics Reference: Audio Technology Handbook. World wide web document, http://www.philipsmagnavox.com/index.html .

Oppenheim, Alan V. Signals and Systems. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1983.

Pohlmann, Ken C. The Compact Disc Handbook–The Computer music and digital audio series. 2nd Ed. Madison, WI: A-R Editions, Inc. 1992.

Shah, Prasanna. "Music of the Bitstream," Audio. Vol. 75, No. 1 (Jan. 1991). 56 - 60+.

Strawn, John. Digital Audio Engineering—The Computer music and digital audio series. Los Altos, CA: William Kaufmann, Inc., 1985.

Strawn, John. Digital Audio Signal Processing—The Computer music and digital audio series. Los Altos, CA: William Kaufmann, Inc., 1985.

Watkinson, John R. The Art of Digital Audio. 2nd Ed.. Boston, MA: Focal Press, 1994.

Watkinson, John R. Coding for Digital Recording. Boston, MA: Focal Press, 1990.

Whyte, Bert. "Every Little Bit Helps," Audio. Vol. 76, No. 8 (Aug. 1992). 19-21.

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