Tech Talk

A compact disc, or CD, is an optical storage medium with digital data recorded on it. The digital data can be in the form of audio, video, or computer information. When the CD is played, the information is read or detected by a tightly focused light source called a laser (thus the name optical medium). This article will focus on audio compact discs, which are used to play back recorded music.

The history of the compact disc can be traced back to the development of electronic technology and particularly digital electronic technology in the 1960s. Although the first applications of this technology were not in the recording area, it found increasing use in audio components as the technology evolved.

During the same period, many companies started experimenting with optical information storage and laser technology. Among these companies, electronic giants Sony and Philips made notable progress in this area.

By the 1970s, digital and optical technologies had reached a level where they could be combined to develop a single audio system. These technologies provided solutions to the three main challenges faced by the developers of digital audio.

The first challenge was to find a suitable method for recording audio signals in digital format, a process known as audio encoding. A practical method of audio encoding was developed from the theories published by C. Shannon in 1948. This method, known as pulse code modulation (PCM), is a technique that samples a sound during a short time interval and converts the sample to a numerical value that is then modulated or stored for later retrieval.

The storing of audio signals in digital form requires a large amount of data. For instance, to store one second of music requires one million bits of data. The next challenge, therefore, was to find a suitable storage medium to accommodate any significant amount of sound. The solution to this problem came in the form of optical discs. An optical disc can store large amounts of data tightly compressed together. For example, one million bits of data on a CD can occupy an area smaller than a pinhead. This information is read by means of a laser beam that is capable of focusing on a very narrow area as small as 1/2500th of an inch.

The final challenge of digital audio was to process the densely packed information on compact discs quickly enough to produce continuous music. The solution was provided by the development of integrated circuit technology, which allow the processing of millions of computations in just micro-seconds.

By the late 1970s, a common set of standards for the optical storage discs had been developed by the joint efforts of Sony and Philips. A consortium of 35 hardware manufacturers agreed to adopt this standard in 1981 and the first compact discs and compact disc players were introduced in the market in 1982.

Direct-Stream Digital (DSD) is the trademark name used by Sony and Philips for their system of recreating audible frequencies which uses pulse-density modulation encoding, a technology to store audio signals on digital storage media which is used for the super audio compact disc (SACD).

The signal is stored as delta-sigma modulated digital audio, a sequence of single bit values at a frequency sampling rate of 64 times the CD Audio sampling rates of 44.1 kHz, for a rate of 2.8224 MHz (1 bit times 64 times 44.1 kHz). Noise shaping occurs by use of the 64x oversampled signal to reduce noise/distortion caused by the inaccuracy of quantization of the audio signal to a single bit. Therefore it is a topic of discussion whether it is possible to eliminate distortion in 1-bit Sigma-Delta conversion (see Audio Engineering Society Convention Paper 5395 in external links section below). Due to the way 1-bit Sigma-Delta converters work, DSD encoded audio for lower frequencies has better resolution than standard CDs, while for higher frequencies the resolution, and therefore the fidelity, drops down below that of standard CDs.

Direct Metal Mastering for vinyl records yields unsurpassed clarity in the high-end and warm bass. DMM records also sound louder, which is great for radio, or just for putting more distance between signal and noise. The groove is cut directly in copper metal. Transient response is greatly improved. Stampers are plated directly from the DMM Copper Master, eliminating two of the three plating steps required for lacquers. In short, DMM yields better detail resolution and a lower noise ratio. This is especially good for long play albums, or audiophile material.

DualDisc is a type of double-sided optical disc product developed by a group of record companies including EMI Music, Universal Music Group, Sony/BMG Music Entertainment, Warner Music Group, and 5.1 Entertainment Group and now under the aegis of the Recording Industry Association of America (RIAA). It features an audio layer similar to a CD (but not following the Red Book CD Specifications) on one side and a standard DVD layer on the other. In this respect it is similar to, but distinct from, the DVD Plus invented in Europe by Dieter Dierks and covered by European patents.

DualDiscs first appeared in the United States in March 2004 as part of a marketing test conducted by the same five record companies who developed the product. The test involved thirteen titles being released to a limited number of retailers in the Boston, Massachusetts, and Seattle, Washington, markets. The test marketing was seen as a success after 82% of respondents to a survey (which was included with the test titles) said that DualDiscs met or exceeded their expectations. In addition, 90% of respondents said that they would recommend DualDisc to a friend.

DualDisc titles received a mass rollout to retailers throughout the United States in February, 2005, though some titles were available as early as November, 2004. The recording industry had nearly 200 DualDisc titles available by the end of 2005 and over 2,000,000 units had been sold by the middle of that year.

A high-resolution audio format approved by the DVD Forum in early 1999. Like CDs, DVD-Audio uses PCM encoding. It provides for 16, 20 and 24-bit samples at a variety of sampling rates: 44.1, 48, 88.2, 96, 176.4 and 192 kHz, compared to 16 bit samples at 44.1 kHz for CDs. DVD-Audio (DVD-A) offers two channels at the highest sampling rates and six channels at lower rates. Channel rates can also vary; for example, the channels for the front speakers can be sampled higher than for the rear.

DVD-Audio supports other coding systems such as Dolby Digital, DTS and DSD and records them on alternate tracks.

The Video Zone: DVD-Audio discs may also contain video, graphics and text in the DVD-Video zone. The Video zone is located at the outer edge of the disc following the DVD-Audio zone that starts at the center of the platter. DVD-Audio players also play regular DVD movies.

The Flip Disc (DVD-A and CD): A Flip Disc, which became available in 2004, is a DVD-Audio disc that contains the same titles in the CD-DA format on the other side for compatibility with regular CD players. See SACD, MLP and DVD.

HDCD is a digital processing technique that increases fidelity on audio CDs, developed by Keith Johnson and Michael "Pflash" Pflaumer. HDCD uses a 20-bit master instead of 16 bits and stores the additional data in a subchannel that is processed on HDCD-equipped players. The format remains compatible with the traditional audio CD, and non-HDCD players can play HDCD discs as if they were regular CDs. Because HDCD players have a higher quality digital filter, regular CDs may sound better.

(Super Audio CD) A high-resolution CD audio format from Sony and Philips. SACD and DVD-Audio (DVD-A) are the two next-generation digital audio formats for enhanced sound quality. SACD uses Direct Stream Digital (DSD) technology which provides 1-bit pulse density modulation (PDM) encoding at 2,822,400 samples per second. Each sample is a 0 or 1 (up or down), depending on its relationship to the previous sample. Compared to CD, SACD boosts frequency response from 20kHz to 100kHz and dynamic range from 96 to 120 db. See PDM.

Multi-Channel Support: Introduced with only two channels for stereo, 5.1 multi-channel support was later added. In 1999, Sony's SCD-1 was the first SACD player on the market. Its $5,000 price placed it squarely in the audiophile league, but subsequent players moved SACD well into the consumer realm, with combo units that also play DVD-Audio, DVDs and CDs for less than $200.

Hybrid SACD: Introduced in 2003, hybrid SACD discs include a separate CD layer with the same titles so they can play in regular CD players.

XRCD (short for eXtended Resolution Compact Disc) is a special mastering and manufacture process patented by JVC that is used for producing high fidelity redbook Compact Discs. It was first introduced in 1995.

Overview: XRCDs are in every aspect regular CDs that just have been manufactured to stringent quality standards. This means that they can be played on any conventional compact disc player. It is interesting to note that an XRCD is supposedly priced about twice as high as a regular full-priced CD. JVC attributes this to the high cost of the quality mastering and manufacturing process.

Mastering process: If digital, the source material is first converted to analogue with a K2 20-bit or 24-bit digital-to-analog converter. The signal is then digitized with JVC's K2 analog-to-digital converters. This musical information is next encoded on a magneto-optical disk for transportation.

The disk is sent to JVC's Yokohama manufacturing plant, where after reducing jitter and distortion the musical signal is down-converted to 16-bit through a K2 "super coding" process. This 16-bit signal is then EFM-encoded before going through a high-precision DVD K2 laser using a proprietary "Extended Pit Cut" technology. This optimizes the linear velocity of the glass master (giving precise pit lengths), as well as eliminating any time jitters, all the while controlled by an extremely accurate Rubidium clock. All CDs are then stamped directly from the glass master.

XRCD2 and XRCD24 are improved versions of the original XRCD process. XRCD2 is the first to record to a magneto-optical disk via the digital K2 regenerator. XRCD24 upgraded the transported music signal's bit width from 20 to 24 bits.