E 2h Measurement
of Erase Efficlency
To
measure
the
erase efficiency
a 1000 c,/s band-pass
fllter or
a
wave-analyser
ls
required.
A
tone ls
recorded
and afterwards
erased
by
running
the
tape through the machine
in
"record"
a
second
time,
but with
the
input short-circuited. Subsequently,
the residual
voltage can be mea-
sured
in the
rrreplayrr
posltion
via the
above-mentioned
filter. The erase
efflclency depends to
some extent on
the
positlon
of
the erase head
gap
relative
to the tape.
E
3
Wow
and
Flutter
Measurements and Tape Sllppage
Wow and flutter in tape recorders
are
instantaneous or
periodic
fluctuations
in tape
speed
which
are
caused by slight imperfections in
the
transport
mechanism.
The irregularities in
the
tape transport are
malnly
due to shafts
or
idlers
which
are out of true
(
eccentric
)
or
to
bearing
faults
(
dirt.
rust,
etc.
).
Eccentrlcities
glve
more
or less slnusoidal
wow
components,
bearing faults tend to
give
sharp
peaks,
It
is, therefore,
a
good
practlce
which
ls belng
adopted
more and
more wldely,
not only to measure the
wow
and
flutter
but to
record
it
on
a
pen-recorder.
By
means
of a
pen-recordlng,
1t 1s
posslble
to
teII at every servicing whether
a
wow-component has increased
substantially
compared to a
previous
measurement
and
whether it
has
reached
objectlonable
value. Replacing or re-corditioning the faulty
part
in time, will
avoid
costly
repalrs
ard equlpment
breakdown. By means of
a
pen-recording
the faulty component
can
be
identified
very
quickly
as the
frequency
of the disturbance can be calculated
from
the wave-length
of the disturbance and
the
paper
speed.
The frequency of the disturbance is equal to the revolu-
tlons of
the troublesome
part
and
the wave
form of
the disturbance ls an indication
of
the
type
of
fauIt.
Instead of taking a
pen-recording,
the frequency distribution of the wow and
flutter
may be
analy-
sed by
means of
a
wave
analyser.
However, it is not
so
easy to arrive at a clear conclusion by
this method and therefore the
pen-recordlng
ls
generally preferred,
Values
for wow
and
flutter
should always
be
quoted
as
weighted
readings.
On the other hand,
pen-
recordings
should always
be made unweighted, because lt
makes
it easier to
pick
out the compo-
nent
frequencies
and consequently
to identify the
source
of any
fault.
In the
past,
wow and
flutter
figures have
mostly
been
quoted
as
RMS
values
but lt
is
now becoming more
and
more usual to
use the
peak
to
peak
readings
instead i.e.
to
give
the highest instantaneous
fluctuations
of the
tape speed
referred to
its
mean value. Due to the very complex nature of the
wave form,
the
ratio
between
peak
and
RMS readings is not
a
constant value. The
ratlo
tends to be about 2.5 to 5.
As these discrepancies could
Iead to a misjudgement
of
the
quallty
of a
partlcular
recorder,
it is
lmportant always
to
state clearly
which method of measurement has been used, Readings are best
taken with the wow and
flutter meter EMT 420 which has been designed to meet the latest
stand-
ard
recommendations on wow
and
flutter measurements. It may be used ln conjunctlon with the
pen-
recorder HELCOSCRIPTER EMT or the filter unit EMT
421. For
practlcal purposes
sufflciently
conslstant
measurements
of
welghted wow
and
flutter may be obtained with the older type
wow
and
flutter meter type EMT 4I4.
IdeaIIy, the
wow
ard
flutter
of a tape-recorder should
be measured with the
aid
of
a
test-tape
which has
a
low wow
and
flutter content compared with that of the
machlne
urder
test.
However,
such
tapes cannot
be
produced
for
a hlgh
quallty professlonal
recorder as lt is technlcally lmpos-
sible
to make
a
"master
recorder" that has
substantlally
less wow and
flutter.
It is therefore
To Next Page
Loading...
