Audio Engineering

534 Chapter 17

Random access memory (RAM) offers extremely short access time, but the volume of data
generated by digital audio precludes the use of RAM for anything more than a few seconds
because it would be too expensive. In addition loss of power causes the recording to be lost.

Tape has the advantage that it is thin and can be held compactly on reels. However,
this slows down the access time because the tape has to be wound, or shuttled, to the
appropriate place. Tape is, however, inexpensive, long lasting, and is appropriate for
archiving large quantities of data.

However, discs allow rapid access because their entire surface is permanently exposed
and the positioner can move the heads to any location in a matter of milliseconds. The
capacity is limited compared to tape because in the case of magnetic discs there is an
air gap between the medium and the head. Exchangeable discs have to have a certain
minimum head fl ying height below which the risk of contamination and a consequent
head crash is too great. In Winchester technology the heads and disc are sealed inside
a single assembly and contaminants can be excluded. In this case the fl ying height can
be reduced and the packing density increased as a consequence. However, the disc is no
longer exchangeable. In the case of optical discs the medium itself is extremely thick and
multiple platter drives are impracticable because of the size of the optical pickup.

Theory

Measurement with less than

perfect focus

Maximum operating

frequency

EquivalentFc

(^0) Fc
0.5
1.0
Output level
Figure 17.8 : Frequency response of laser pickup. Maximum operating frequency is about half
of cutoff frequency F (^) c.