Handbook for Sound Engineers

Magnetic Recording and Playback 1081

of stereo audio, a very compact digital recorder, Fig.

28-47, has been made possible. New heads and metal

particle tapes have been utilized to produce a

long-playing cassette tape system with quality equal to

the compact digital disk.

The R-DAT format operates at two tape speeds,
8.15 mm/s (0.32 in/s) for recording and 12.23 mm/s
(0.48 in/s) for widetrack playback of prerecorded tapes.
In spite of the very slow tape speed, very high data rates
are made possible with a rotating head drum speed of
2000 rev/min and flying head velocity of 3 m/s. The
resulting slant tracks are 23.5 mm (0.93 in) long and
inclined at an angle of approximately 6.5( from
horizontal.
The data is digitized to 16 bit resolution and
recorded with double-encoded Reed-Solomon error
correcting coding with interleaving between not only
channels 1 and 2 but also adjacent scans of the flying
heads, Figs 28-48. A 60 m (65.6 ft) tape holds
2200 Mbytes of information capable of encoding
2 hours of stereo music. Search for a desired program
can be conducted at 60 times normal speed; rewind and
fast forward without search is up to 180 times normal
speed, allowing full rewinding in approximately 40 s.
The digital storage capacity of the audio R-DAT
format has been greatly enhanced by newer technology
to serve as a backup medium for computer hard disks.
The fourth generation of the Digital Data Storage
format (DDS4) jointly developed by Sony and
Hewlett-Packard from the original R-DAT format
boasts a capacity of 20 Gbytes per 150 meter tape. The
drum spins at 11,480 rev/min to achieve a data
throughput of 2.87 MB/s before data compression and
up to 7.62 MB/s after compression.

28.5.4 Packing Density Maximization with Rotary

Head Recorders

The professional analog multitrack formats are very

inefficient in the use of recording tape. Nearly half of

the tape width is devoted to guardbands between tracks.

These guardbands are required to minimize crosstalk

between channels due to fringing and crosstalk within

the heads. These problems are overcome in rotary head

recorders that do not require guardbands.

Several aspects of the rotary head system contribute

to the elimination of guardbands, including servo posi-

tioning of the tape, azimuth shifting on alternate scans,

and the elimination of low-frequency components in the

recorded signal.

The servo positioning of the helical scan tape during

playback is analogous to a conventional longitudinal

recorder with self-aligning guides to correct for any

guiding errors. Control signals recorded along the edge

of the tape are used to synchronize the motion of the

tape past the rotating drum to the spinning of the drum.

This synchronization adjusts the position of the

recorded tracks on the tape to exactly coincide with the

path of the flying head. The active servo control of the

tape motion duplicates any disturbances that may have

occurred during recording to maintain correlation

between the flying head path and the track, permitting

tracks to be recorded abutting each other.

This technique can be carried one step further if the

spinning head is augmented with a rapidly responding

positioning actuator. Fig. 28-49 shows a scanning head

mounted on a piezoelectric positioner called a bimorph.

If a voltage is applied to the bimorph, the head mount

deflects and moves the head.

Since the bimorph can respond much faster than the

servo system, tracking errors can be continuously

corrected throughout the helical scan of the tape. This

Figure 28-47. R-DAT tape transport mechanism.

Capstan

Intake

roller

guide

Incline guide Head drum

Tension

regulator

Roller guide

Fixed guide

Fixed guide

Roller guide

Fixed guide

Fixed guide

Incline guide

Figure 28-48. R-DAT tape format.

23.5 mm

60 o2' 52.5"

Optional Track 1

Guardband

Guardband

Optional Track 2

Subcode

Aft

Digital audio data

Aft

Subcode

Track width (2.61 mm)Tape width (8.81 mm)