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2.
OPERATION
noise on
the
r ecords hinders th
e
measurements of cross-talk
in the important
part of th
e
frequen
cy
range, where
it
is only
approximately 20-26
dB
below
th
e
modul
a
tion level.
When
testing a
velocity sensitive
pick-up
above
1
kc/s,
a
better
signal-to-noise
r
a
tio (up to
32
dB
)
may
be
obtained
by
eliminating
the equalizing
filt er (FILTER
SWITCH of
4409 in
"direct"
position
).
However,
the
signal/cross-talk
r
a
tio of
the
records
(40
dB
from
500
C/S
to 2
kc/s,
see
Fig. 1.13) is not completely
utilized
in
these
conditions.
A m eans of reducin
g
surface
noise disturbances,
is
the
synchronous
filterin
g
of
the cross-talk.
Stereophon ic
reproducing heod
under test
Preamplifier
Spectrometer
Level Recorder
Fig.
2.2.
Set-up
for automatic
cross-talk
measurements with
synchronous
filtering.
N.B.
The input
impedance
of
the
2410
used as preamplifier
is
1.5
M.Q paralleled
by
20
pF. Higher input
impedance
amplifiers are
available from Briiel
&:
Kjrer
up to 700
MQ
paralleled by
3
pF.
If
the
output
voltage
of tll
e
tested instru-
mentation
is
greater than
0.1 V at
1
kc/s with
a
10
kQ
load,
the
preamplifier
may be
omitted.
The spectrometer
Typ
e
2112
is equivalent to th
e
combination
2603
+
1612.
Th
e combined
unit Type
3313 (2305
integral
with 2112)
is not
recommende'
d
on
account of
a
ground
loop which has been avoided
here
by
using
the
single lead
AQ
0100.
The
signal-to-noise
r
a
tio is for example
65 dB
in
a
1/3
octave
filter
at
1
kc/s,
see
Fig.
2.3.
This
filtering
is provided
by
the
Spectrometer
T yp
e
2112 (or
2603
+
1612)
which
features
a
utomatic
switching of
a
standard pure
logarith-
mic
series of
1/
3
octave
filters
(Fig.
2.4). In
order
to
obtain
th
e
b est possible
synchronization b etween the
fi lter switching frequencies
and the
instantan
eo
us
gliding fr equency
on
the
record, a special adjustment of
the r
ecorder's
auto-
m
a
tic
stop
position has to b
e
made beforehand
as
described b elow in the
paragraph
"Synchronous
Filtering Adjustment".
Operating
Procedure.
The
operating procedure for
synchronously
filtered cross-talk m easurement
is the
same as
for
a
fr equency r
esponse plotting
with the
exception,
that the
2112 (1612)
has to b
e
set
to
the
Automatic Switching
and
1/3 Octave
condition,
(see
also
the
respective
m anuals). The
Channel Selector
in the 4409 is set to the
20
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