1080 Chapter 28
to account for fixed and variable timing errors. A 4/6
Modulator adds bits to the data to create easily recog-
nized data patterns that have an optimized bandwidth for
recording on the tape. The patterns were then fed
through the Write Amplifier to the write head.
Playback from the tape began at either of two read
heads, one located before the write head for
sync/overdub operations and one located after the write
head for confidence monitoring. The selected data was
fed to the Read Amplifier where the analog-looking
pulses from the tape were converted into digital pulses
by a differentiator and level detector. The patterns of
digital pulses passed through the 4/6 Demodulator to
strip off the extra bits added by the modulator during the
write process. The Timebase Corrector removed any
timing variations due to flutter in the tape transport,
restoring the exact sampling rate. The RSC Decoder and
Deinterleave used the Reed-Solomon data to correct any
correctable errors and put the data back into the proper
order. Any uncorrectable errors were concealed by the
Interpolater, which made a best guess attempt to hide
errors. If the errors are too large to hide, the output
mutes rather than passes faulty data.
Other functions include master timing circuits, tape
motion servos, extensive logic, and display functions
for metering and control.
28.5.2 Helical Scan Digital Tape Transports
The primary limitations of longitudinal magnetic tape
with a large number of narrow tracks were tape guiding
and crosstalk and of course expense. Helical scan tech-
niques greatly reduce these problems by using heads
with alternating azimuth angles on the record/play
heads to reduce crosstalk. Track-to-track spacing can be
virtually overlapping, with dynamic tracking of the
flying heads to eliminate any errors, Fig. 28-46.
The limiting factor for helical scan tape is the
throughput on the single digital track that is being
recorded. For example, a recorder capable of 8 channels
of 16 bit or 24 bit audio requires bandwidths of approxi-
mately 8 MHz or 12 MHz, respectively. The most
economical approach is to adapt a consumer format to
fit this requirement. For example, the popular ADAT
series manufactured by Alesis and others were based on
the S-VHS format that used ½ inch tape. Similarly, the
Tascam DTRS (Digital Tape Recording System) series
utilizes the technology developed for 8 mm handheld
video recorders. Both of these products offer 8 chan-
nels in an inexpensive package. Multiple machines can
be locked together to provide up to 128 tracks of audio
at about one-tenth the price of the equivalent DASH
recorder.
The basic helical scan transport consists of a rapidly
rotating head drum and a capstan to control the forward
speed of the tape. A spooling mechanism engages the
reel hubs in the cassette to provide proper winding of
the tape in all modes. Auxiliary functions include auto
loading mechanisms to load and eject the cassette and
auto threading mechanisms to extract the tape from the
cassette.
For most applications, the tape is wrapped around
the head drum to cover slightly more than half the
circumference of the drum. Heads are mounted in pairs
180 q apart on the drum, protruding slightly from the
face of the drum. The specific tape format determines
the diameter of the head drum. The drum spins many
revolutions per second to provide the high linear scan-
ning speed required for the digital data stream. For
example, in 16 bit mode the ADAT drum is 2.44 inches
in diameter and spins at 50 rev/s to yield a linear scan-
ning speed of 192 in/s. In comparison, the forward
speed of the tape is only 3.9 in/s, about one-fifieth of
the scanning speed.
The scanning drum is tilted slightly with respect to
the path of the tape, causing the spinning head to scan
the tape in diagonal stripes. For the ADAT example, the
angle is 7.5q, yielding a diagonal track length of slightly
less than 4 inches. In comparison, the R-DAT system
uses a 1.18 inch (30 mm) diameter drum inclined 6.5q
spinning at 33.3 rev/s to give a scanning speed of about
120 in/s.28.5.3 Rotary Digital Audio TapeThe R-DAT format shares technology with the 8 mm
camcorder VCRs. By optimizing a miniaturized helical
Figure 28-46. Helical scan transport layout. scan system using 4 mm tape for direct digital recording
Rotating head drumRead - write headCassette