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2. OPERATION
noise on the records hinders the measurements of cross-talk in the important
part of the frequency range, where
it
is only approximately 20c--26 dB below
the modulation level. When testing a velocity sensitive pick-up above 1 kHz,
a better signal-to-noise ratio (up to 32 dB) may be obtained by eliminating
the equalizing filter (FILTER SWITCH of 4409 in "direct" position). However,
the signaVcross-talk ratio of the records ( 40 dB from 500 Hz to 2 kHz, see
Fig. 1.13) is not completely utilized in these conditions. A means of reducing
surface noise disturbances, is the synchronous filtering of the cross-talk.
Stereophonic
reproducing head
under test
Preamplifier
Spectrometer
Level Recorder
I
'+--11---'1--'
2112
I
m•
L------------~~~~~~~~~;~~~~=~J
.f623!JO
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 Brii.el
&
Kjrer
up to 700 MQ paralleled by 3 pF. If the output voltage of the tested instru-
mentation is greater than o.1 V at 1 kHz with a 10 kQ load, the preamplifier
may be omitted. The spectrometer Type 2112 is equivalent to the combination
2603
+
1612. The combined unit Type 3313 (2305 integral with 2112) is not
recommended on account of a ground loop which has been avoided here by
using the single lead
AQ
0100.
The signal-to-noise ratio is for example 65 dB in a 1/3 octave filter at 1 kHz,
see Fig. 2.3. This filtering is provided by the Spectrometer Type 2112 (or
2603
+
1612) which features automatic switching of a standard pure logarith-
mic series of 1/3 octave filters (Fig. 2.4). In order to obtain the best possible
synchronization
.
b etween the filter switching frequencies and the instantaneous
gliding frequency on the record, a special adjustment of the recorder's auto-
matic stop position has to be made beforehand as described below in the
paragraph "Synchronous Filtering Adjustment".
Operating Procedure.
The operating procedure for synchronously filtered cross-talk measurement
is the same as for a frequency response plotting with the exception, that the
2112 (1612) has to be set to the Automatic Switching and 1/3 Octave condition,
(see also the respective manuals). The Channel Selector in the 4409 is set to the
20
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