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Specifically, it is necessary to minimize skew so that the closely spaced characters
on tape can be correctly interpreted and distinguished from each other.
illustrates three of the causes of skew in an exaggérated manner.
are magnified when the skew written into a tape by one tape drive is opposite to the
skew associated with reading on another tape drive.
problem are:
1. Head skew.
This is inherent in the manufacture of the head and cannot be ad
justed in the field. It is derived from the fact that mechanically and magnetically the
individual tracks (pole tips) are not in perfect alignment nor are they completely iden
tical in shape.
2. Electrical skew.
with their associated circuitry, e. g. in reading identical tape signals, are not exactly
alike.
(See item 4 b elow .)
3. Angular alignment of the head and tape.
(section 15.2.6) and is of great importance. While the average alignment is subject
to adjustment, the rapid fluctuations in alignment called tape "flutter" are difficult to
remove completely.
The angular alignment of tape, as well as the lateral position of
tape (tape tracking), is determined by the accurate location of the vacuum columns,
split idlers and the read-write head (guide posts).
tion 15. 2. 5.
4.
Signal amplitude—tracking.
if one signal is weaker than the other even though their wave form s are sim ilar and
peak at the same time.
on the leading edge of each pulse before the weaker signal does, and this is the time
that the corresponding read register trigger is set by the final amplifier.
in a strict sense, is the lateral agreement of the individual tracks of the read write
head with the signal tracks on tape.
with respect to the read-write head and also the lateral position of each individual track
with respect to the reference edge.
tory. Inasmuch as signal amplitude is dependent on tracking, skew is also dependent
on tracking to some extent. For this reason, the pre-am ps are temporarily adjusted,
if necessary, to obtain 20v peak-to-peak output from the master tape when checking
the skew adjustment.
5.
Speed of tape.
tape, it is necessary to use a character gate in the control unit.
is turned on by the first bit in the character, and turned off after a fixed interval
such as 33. 6 usee.
Speed, therefore, is a factor in determining the effective skew
because, when reading, the slower the tape moves the greater is the time interval
between bits as a result of the geometric skew already present.
writing, the speed determines the distance between characters.
speed of the tape will probably not deviate far from the prescribed 75 inches/second
(unless the belt is slipping, or there is an appreciable bind), the fluctuations of speed
called "jitter" add to the skew problem. Jitter is aggravated by loose and worn parts
in the moving capstan drive system 0
Skew can even be observed when reading forward a tape that has just been written
on the same head.
This occurs with no apparent change in tape alignment and with
all signal amplitudes at 20v peak-to-peak.
and (2) electrical skew» A probable contributing factor is that the location of the bit
on tape as it is written forward is determined by the right-hand pole tip (effective),
while the leading edge of the signal pulse in reading forward is determined by the
The wave form s produced by different tracks in conjunction
Two tracks have an effective skew between them
This is true because the stronger signal reaches + 2 .5 volts
This depends on the lateral position of the tape
The latter is held to close tolerances at the fa c
To distinguish skewed successive characters coming from
This can be attributed to: (1) head skew,
44
Figure 16
The effects of skew
The factors involved in the skew
This can be adjusted in the field
This is explained further in sec
The character gate
Furthermore, in
While the average
Tracking,
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