Audio Engineering

Digital Audio Recording Basics 533

All optical discs need mechanisms to keep the pickup following the track and sharply
focused on it.

The frequency response of an optical disc is shown in Figure 17.8. The response is best
at DC and falls steadily to the optical cut-off frequency. Although the optics work down
to DC, this cannot be used for the data recording. DC and low frequencies in data would
interfere with the focus and tracking servos. In practice the signal from the pickup is
split by a fi lter. Low frequencies go to the servos, and higher frequencies go to the data
circuitry. As a result, the data channel has the same inability to handle DC as does a
magnetic recorder, and the same techniques are needed to overcome it.

17.2 Recording Media Compared .................................................................................

Of the various media discussed so far, it might be thought that one would be the best
and would displace all the others. This has not happened because there is no one best
medium; it depends on the application.

Recording

Heat from laser

raises medium

above Curie point.

Coil can reverse

flux

Fromlaser

Initial magnetic

flux direction

Disk

motion

Flux

due to

coil

Record

current

Figure 17.7 : The thermomagneto-optical disk uses the heat from a laser to allow a magnetic

fi eld to record on the disk.