Audio Engineering

550 Chapter 17

Thus the frequency generated is less than the bit rate and a result is that more data can be
recorded on the disc than would be possible with a simple code.

17.4 Hard Disc Recorders .............................................................................................

The hard disc recorder stores data on concentric tracks, which it accesses by moving the
head radially. Rapid access drives move the heads with a moving coil actuator, whereas
lower cost units will use stepping motors, which work more slowly. The radial position
of the head is called the cylinder address, and as the disc rotates, data blocks, often called
sectors, pass under the head. To increase storage capacity, many discs can be mounted on
a common spindle, each with its own head. All the heads move on a common positioner.
The operating surface can be selected by switching on only one of the heads. When one
track is full, the drive must select another head. When every track at that cylinder is full,
the drive must move to another cylinder. The drive is not forced to operate in this way; it
is equally capable of obtaining data blocks in any physical sequence from the disc.

Clearly while the head is moving it cannot transfer data. Using time compression to
smooth out the irregular data transfer, a hard disc drive can be made into an audio
recorder with the addition of a certain amount of memory.

Data

bit period

00

0000

001

11 1

0.8 of

data bit

period

11 1

111

Figure 17.18 : In RDAT an 8/10 code is used for recording. Each 8 data bits are represented

by a unique waveform generated by 10 channel bits. A channel bit one causes a transition

to be recorded. The transitions cannot be closer than 0.8 of a data bit, and this is the jitter

resistance. This is rather better than FM, which has a jitter window of only 0.5 bits.