Audio Engineering

Digital Audio Recording Basics 561

The combination of azimuth recording, an active tracking system, and high coercivity
tape (1500 Oersteds compared to 200 Oersteds for analogue audio tape) allows the tracks
to be incredibly narrow. Heads of 20 μ m width write tracks 13 μ m wide. About 10 such
tracks will fi t in the groove of a vinyl disc. Although the head drum spins at 2000 rpm, the
tape speed needed is only 8.15 mm per second.

The subcode of RDAT functions in a variety of ways. In consumer devices, the subcode
works in the same way as in CD, having a table of contents and fl ags allowing rapid
access to the beginning of tracks and carrying signals to give a playing time readout.

In professional RDAT machines, the subcode is used to record time code. A time code
format based on hours, minutes, seconds, and DAT frames (where a DAT frame is one
drum revolution) is recorded on the tape, but suitable machines can convert the tape code

B

head

A

B

A

B

head

A

B

A

(a)

(b)

Figure 17.28 : (a) A correctly tracking head produces pilot-tone bursts of identical amplitude.

(b) The head is off-track, and the fi rst pilot burst becomes larger, whereas the second

becomes smaller. This produces the tracking error.