Audio Engineering

Digital Audio Recording Basics 531

In the same way that the recorded wavelength of a magnetic recording is limited by the gap
in the replay head, the density of optical recording is limited by the size of light spot which
can be focused on the medium. This is controlled by the wavelength of the light used and
by the aperture of the lens. When the light spot is as small as these limits allow, it is said
to be diffraction limited. The recorded details on the disc are minute, and could easily be
obscured by dust particles. In practice the information layer needs to be protected by a
thick transparent coating. Light enters the coating well out of focus over a large area so that
it can pass around dust particles, and comes to a focus within the thickness of the coating.
Although the number of bits per unit area is high in optical recorders, the number of bits
per unit volume is not as high as that of tape because of the thickness of the coating.

Figure 17.6 shows the principle of readout of the compact disc which is a read-only disc
manufactured by pressing. The track consists of raised bumps separated by fl at areas. The
entire surface of the disc is metalized, and the bumps are one quarter of a wavelength
in height. The player spot is arranged so that half of its light falls on top of a bump,
and half on the surrounding surface. Light returning from the fl at surface has traveled
half a wavelength further than light returning from the top of the bump, and so there is

Bump

height

0.13μm

≈ λ/4

Bump

width

0.5μm

λ 

0.5

μm

Mirror

surface

Spot size

≈1.2μm

Track pitch

1.6μm

Figure 17.6 : CD readout principle and dimensions. The presence of a bump causes

destructive interference in the refl ected light.