How Do Half Size Heads Work ?
The standard head size for a DAT machine is 30mm, many portables use 15mm heads for
compact
size and low weight. Both systems use a head cylinder rotation speed of 2000 RPM in the record
and
playback modes. For Fast Forward the speed increases to as much as 3000 RPM. In rewind it
slows
down to as low as 1000 RPM. Both systems use a helically scanned tape method. Tape winding
around the inclined head assembly results in an angle of approximately 6 degrees, 22 minutes, 59.5
seconds for 30 mm heads and 6 degrees, 23 minutes, 59.5 seconds for 15 mm heads. Tape speed
for
both systems is 8.15 mm/sec. Resulting in a read speed of 200 tracks every three seconds.
The 15mm head systems differ from the 30mm ones in that they use a 180 degree wrap around
the
head assembly. 30 mm systems use a 90 degree wrap. This causes a two problems due to the
relative
speed, which becomes one half that of 30 mm designs. The relative velocity in a 30mm system is
3.133m/sec. and 1.566m/sec. in a 15 mm design. The second change in 15 mm designs is that the
track angle increases. To compensate for theses differences several things must be done. The first
is to
change the head to tape angle by about 1 minute. The second is to increase the track length from
7.5ms to 15ms. This presents no overlapping problems because the head switching timing is 15ms,
with 30ms available to read data from both heads. However it does decrease the RF frequency for
the
data area of the waveform from 4.7MHz (30mm) to 2.35MHz 15mm). We feel this is why the
one
half sized heads will often playback a tape 30mm designs will not. To correct for the timing
differences this causes, Time Base Correction is applied to the signal. This TBC system
compresses
the time axis for playback data (from 15ms to 7.5ms) and expands it (from 7.5ms to 15.0ms) for
record data. This yields a continuous RF signal that is compatible with 30mm designs.
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