Handbook for Sound Engineers

Optical Disc Formats for Audio Reproduction and Recording 1133

30.1 Introduction

The digital storage of audio signals presents a technical
challenge. A 60 minute stereo musical selection, with a
sampling rate of 44.1 kHz and 16 bit pulse code modu-
lation, generates over 5 billion bits. To store this data
successfully, error correction, synchronization, and
modulation may push the total required capacity to over
15 billion bits. Recordings with a higher sampling fre-
quency, longer word-length, and additional channels
require correspondingly greater storage capacity. In
addition, commercial music storage media must provide
random access, small size, convenience, durability, low
cost, and ease of replication. Still other applications call
for write-once or recordable/erasable storage. Clearly,
digital audio’s storage requirements are formidable.
The CD was the first format able to meet these
demands. A CD can hold over an hour of high-fidelity
music on a robust and economically manufactured disc.
CD player hardware specifications have evolved to
surpass most listeners’ ability to audibly distinguish
between players. In short, the format is well suited to
the storage demands of stereo music. In addition, the
CD format is suitable for many extended applications.
As a result, a number of alternative CD formats were
developed. A CD-ROM disc may hold several hours of
music, along with video and text information.
Write-once and recordable/erasable formats (CD-R and
CD-RW) are widely used in both professional and
consumer applications.

The desire for higher performance specifications and
multichannel sound stimulated development of the
super audio CD (SACD) format; it uses direct stream
digital coding in place of PCM coding to store either
stereo or multichannel audio signals on a multilayer
disc. The need for increased storage capacity, particu-
larly for the storage of high-quality digital video,
encouraged development of the DVD format. A DVD
disc may store from 4.7 to 17 Gbytes of data, using one
or multiple data layers. As with the CD, DVD comes in
many guises. The DVD-Video format is used to store
motion pictures, DVD-Audio is used for high-quality
stereo and multichannel music, DVD-ROM is used for
computer applications, and a variety of DVD formats
have been devised for recording applications. The HD
DVD and Blu-ray disc formats use shorter wavelength
lasers and higher resolution optics to dramatically
increase storage density, allowing storage of high-defi-
nition video and audio. Disc formats such as these will
further extend the opportunities of optical disc storage
for professional and consumer applications.

30.2 CD Specifications

The compact disc digital audio (CD-DA) format is

sometimes known as the Red Book standard and is cod-

ified in the ISO/IEC 908 standard. The diameter of a

CD is 120 millimeters (mm) (4.7 in), its center hole

diameter is 15 mm (0.59 in), and its thickness is 1.2 mm

(0.047 in). The innermost diameter does not hold data;

it provides a clamping area for the player to secure the

disc to the spindle motor shaft. Data is recorded on a

35.5 mm (1.4 in) wide area. A lead-in area occupies the

innermost data radius, and a lead-out area occupies the

outermost radius; they contain nonaudio data used to

control the player’s operation.

A transparent polycarbonate plastic substrate forms

most of a disc’s 1.2 mm thickness, as shown in Fig.

30-1. Data is physically contained in pits that are

impressed on the top surface of the substrate. The pit

surface is covered with a very thin 50 nm to 100 nm

(nanometer) metal (e.g., aluminum or gold) layer and

another thin 10μm to 30μm (micrometer) protective

plastic layer, with the 5μm identifying label printed on

top. A laser beam is used to read the data. It is applied

from below, passes through the transparent substrate,

reflects from the metallized pit surface, and passes back

through the substrate. The laser beam is focused on the

metallized surface embedded inside the disc. Since data

on a disc is read by a light beam, playing a CD does not

cause wear to the data surface, or pickup.

30.2.1 Pit Track

Data is arranged as a pit track in a continuous spiral run-

ning from the inner circumference to the outer. A pit is

Figure 30-1. CD construction showing substrate, metallized

surface, protective layer, and label.

Printing ink

Protective laquer coating

Disc substrate

10,000 × magnification

120 mm

15 mm

1.2 mm

5 Mm

10–30 Mm

Metalized

pit surface

0.05–0.1 Mm