Digital Formats: CD, DVD, SACD

The Compact Disc Digital Audio (CD-DA) standard was developed by Philips and Sony and introduced into the market in Japan in October 1982 and in USA and Europe in early 1983.

Features of the Compact Disc

Compact Discs are superior to vinyl discs and cassettes in a number of ways:

• Superior sound quality without clicks, hiss or other defects
• Fast random access to any track
• Long-life; compact discs do not wear out
• Compact size: only 12cm in diameter so they take up little storage space

The superior quality of CDs and their compact size is made possible by the use of digital technology.

CD digital audio should provide the quality needed for all audio applications, but for the purist this is not always enough. For this reason an enhanced format (HDCD) has been introduced and the new DVD-Audio format incorporates new features including higher sampling rate, more bits per sample and multi-channel surround sound.

Technologies needed to make CD and DVD possible:

• Pulse Code Modulation (PCM), invented in 1937 by Alec Reeves who was working in for STL in London at the time.  He received a CBE in 1969 for his work on PCM.
• Reed-Solomon error correction codes, which were invented by Reed and Solomon in 1960, but were based on earlier work by Hamming in 1950.
• The Laser, which was invented at the Bell Labs in 1958.

Other new technologies have been developed alongside the CD. One of these, the MPEG-1 video standard, allows 74 minutes of medium quality video to be stored on a CD, such as on a Video CD.

Laser Technology

CDs rely on laser technology to read (and write) the data on discs. The word LASER stands for Light Amplification by Stimulated Emission of Radiation. Lasers generate coherent light, which allows the light beam to be focused to a very small spot size, the spot diameter being equal to the wavelength divided by the numerical aperture. 

The advent of lasers and in particular low cost laser emitting diodes has allowed the compact disc technology to become one of the most successful consumer electronic technologies of all time.

LaserDisc

In the late 1960s, Philips developed the laser video disc, the first such application of the laser for a consumer electronics product. The 30cm disc was capable of storing up to 60 mins of analog video per side.

A low power laser was used to read the video information stored in pits in the disc surface. The video and audio signals are represented in analog form by these pits which were arranged in a spiral pattern, like vinyl records.

Compact Disc

CD players use light emitting diode lasers, which are compact and low cost, to read the data contained in pits in the surface of the disc.    

The laser diode is mounted on a swivel arm which can be moved in a radial direction across the disc surface while the disc is rotated.  This allows the laser beam to follow the pits accurately.

A semi-reflective mirror allows the reflected light to pass back to a photo detector. When the laser beam falls on a pit, very little is reflected. The changing light pattern detected is then converted into a series of zeros and ones which are then decoded into the original audio or computer data signal.

Unlike laserdiscs, CDs use a digital technique where the pits indicate whether a data bit is '0' or '1'. Also laserdiscs can be either CAV (Constant Angular Velocity) or CLV (Constant Linear Velocity), but all CDs use CLV.  This means that the pit sizes do not vary from inside to outside of the disc but the angular velocity does vary.

The compact disc format is limited in musical performance by its sampling rate of 44.1kHz and 16-bit quantization. When an audio signal is converted to digital for release on CD, the analog-to-digital converter "samples" (looks at) the audio waveform 44,100 times each second, in a process called sampling. The converter assigns a number to each sample that represents the waveform's amplitude at the sample point, a process called quantization. The CD format uses 16-bit quantization, meaning that each sample's amplitude is represented by a 16-bit binary (ones and zeros) number.

Sampling and quantization are the cornerstones of digital audio; sampling preserves the time information in an audio signal, quantization preserves the amplitude information. The faster the sampling rate, the wider the range of audio frequencies that can be preserved. The more bits in the quantization "word," the greater the dynamic range, the lower the noise, and the greater the resolution.

When the compact disc was developed in the late 1970s, 44.1kHz sampling and 16-bit quantization were chosen to achieve 74 minutes of playing time on a 120mm (12cm) disc. Moreover, the D/A converters of the day were limited to 14-bit resolution. Some argued at the time that 14-bit quantization was sufficient for CD, but fortunately, the standard was established at 16 bits. The compact disc has been a massive commercial success, with more than 500 million players and 12 billion discs sold.

Technological advances have greatly expanded the storage capacity of the 120mm optical disc. In 1995, Sony and Philips, inventors of the CD, announced Multi-Media CD, which could store about four times the data of a conventional CD. Simultaneously, Toshiba and Time-Warner proposed Super Density (SD) disc, based on similar technology. These new discs could hold a full-length movie with digital surround sound, or a huge amount of computer data. Unfortunately, the two formats were incompatible with each other.

DVD

The stage was set for a format war between the two competing proposals. Fortunately, the two sides reached an agreement on a single format that came to be known as DVD, and which embodied aspects of each camp's proposal. Though a DVD is the same size as a CD, it has in its maximum configuration seventeen times the storage capacity.

If we don't care about video, we can exploit DVD's huge storage capacity to encode digital audio with much higher resolution than was possible on CD. The recording industry's three major trade associations formed a group called the International Steering Committee to explore this possibility, and to standardize a new high-quality digital audio carrier. The three trade associations are the Recording Industry Association of America (RIAA), the Recording Industry Association of Japan (RIAJ), and Europe's International Federation of Phonographic Industry (IFPI).

The International Steering Committee issued 13 requirements for the new format, including improved sound quality, multichannel sound, and some form of copy protection. Another ISC requirement was that the new disc be backward-compatible with existing CD players (i.e., the disc must play in CD players as well as in high-resolution players). This last provision makes possible a single inventory of music titles for record labels and retailers-retailers won't need to stock two versions of the same title. The technical details for the new format were to be decided by a group of hardware representatives, called Working Group 4 (WG4).

Unfortunately, the WG4 immediately became politicized, with each member representing their companies' economic interests, not the interests of high-quality music or even consumers. For example, the consortium that holds many of the patents on DVD wanted the new disc to be based on DVD technology. That way, they would receive royalties on every DVD sold. Sony and Philips argued that the new carrier should be an extension of the compact disc-technology they developed and on which they would continue to receive royalties. (Although Sony and Philips also hold several key DVD patents.)

To give you an idea of how economic considerations take precedence over what should be engineering decisions, consider why 96kHz sampling is proposed by the DVD proponents. It would be much easier for everyone-and better-sounding for the music lover-if the sampling rate were 88.2kHz, exactly double the CD's sampling rate. Professional recorders would operate at 88.2kHz, with these master recordings converted to 44.1kHz for most consumers who don't have high-resolution players. Converting 88.2kHz to 44.1kHz is very simple, and the filtering involved only moderately degrades sound quality. A 44.1kHz version is necessary because most consumers will listen to conventional CD with 44.1kHz sampling for years, perhaps a decade. But 96kHz sampling involves enormous (and expensive) mathematical processing that audibly degrades the sound. In fact, the math is so difficult that 96kHz must first be converted to 48kHz in one step, then from 48kHz to 44.1kHz in a second step.

So why would anyone propose 96kHz sampling? Because the DVD-video proponents wanted to distinguish DVD from CD, and chose sampling frequencies of 48kHz and 96kHz for the new video format. It's shameful that millions of music listeners may listen to needlessly degraded sound quality because corporate suits, not engineers, ultimately determine a format's specifications.

Through Working Group 4, Sony and Philips proposed a new disc, called the Super Audio CD (SACD), that met the ISC's requirements, but their offer was rejected by other committee members who had business interests in seeing DVD as the next audio carrier. Super Audio CD provides 74 minutes of high-quality digital audio in two-channel and six-channel modes, along with conventional 16-bit/44.1kHz audio on the same disc for backward compatibility with CD. After more months of wrangling with no resolution in sight, Sony and Philips announced that they would introduce the Super Audio CD format on their own, without the consensus of WG4.

About the same time, audiophile record labels began releasing high-resolution music recordings by taking advantage of a provision of the DVD-Video specification that allows the disc to hold two channels of audio sampled at 96kHz with 24-bit quantization. Meanwhile, the WG4 continues to work toward a new format (DVD-Audio) that contains multichannel, high-resolution digital audio. We have the makings of a format war that will make the VHS/Beta battle look like a skirmish.

To recap, a music disc conforming to the DVD-Video standard will play two channels of 24-bit/96kHz digital audio on most DVD players designed for movie playback. To date, only audiophile record companies are releasing discs in this format.

DVD-Audio will contain high-resolution multichannel sound.

Super Audio CD is the Sony/Philips disc that is backward-compatible with CD and offers both two-channel and multichannel versions of the music. SACD players were introduced in the U.S. in summer 1999.

None of these formats is compatible with the others.

From a marketing perspective, a new audio format will be a much tougher sell to the general public than was CD. The average consumer instantly saw the advantages of a wear-free 5" optical disc over his aging 12" record collection. CD was an easy sale because it represented a wholesale improvement in convenience, and promised superior sound quality. A high-resolution audio format is a different matter. To the average consumer, who thinks CDs are the ultimate in fidelity, a new audio format may be considered superfluous. That's one of the reasons both SACD and DVD-Audio include multichannel audio. Surround sound is easier to demonstrate and sell than higher-quality two-channel quality.

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