Audio Engineering

(Barry) #1
Digital Audio Recording Basics 559

the deinterleave process and corrected or concealed, and the time compression and any
fl uctuations from the transport are removed by time base correction. The corrected, time-
stable samples are then fed to the DAC.


One of the reasons for the phenomenal recording density at which RDAT operates is
the use of azimuth recording. In this technique, alternate tracks on the tape are laid
down with heads having different azimuth angles. In a two-headed machine this is
easily accommodated by having one head set at each angle. If the correct azimuth head
subsequently reads the track there is no diffi culty, but as Figure 17.26 shows, the wrong
head suffers a gross azimuth error.


Azimuth error causes phase shifts to occur across the width of the track and, at some
wavelengths, this will result in cancellation except at very long wavelengths where the
process is no longer effective. The use of 8110 channel coding in RDAT ensures that no low
frequencies are present in the recorded signal and so this characteristic of azimuth recording
is not a problem. As a result the pickup of signals from the adjacent track is effectively
prevented, and the tracks can be physically touching with no guard bands being necessary.


As the azimuth system effectively isolates the tracks from one another, the replay head
can usually be made wider than the track. A typical fi gure is 50% wider. A tracking error
of up to /  25% of the track width then causes no loss of signal quality.


Azimuth
record head

Azimuth
recording

Reverse Azimuth
playback head

Azimuth
angle α

Playback response
with reverse
Azimuth

(a)

(b)
Figure 17.26 : In azimuth recording (a), the head gap is tilted. If the track is played with the
same head, playback is normal, but the response of the reverse azimuth head is attenuated (b).
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