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2. OPERATION
channel separation measurements (Fig. 2.9). In the laboratory, this is generally
made by first plotting the four charts corresponding to one channel on the
record bands 1 to 4, or 5 to 8, and then shifting, the paper back to the starting
point for superposing the other channel. See Fig. 2.10. Writing pens with
different ink colours may be selected for each channel.
However, the two-channel selector built-in in the Test Unit Type 4409 allows
the recording of both channels signals to be made simultaneously, when the
Channel Selector knob is in LEFT-RIGHT position. The selector is then
actuated from a multivibrator at a frequency of about 1.2 Hz. This means that
the measurements are taken alternately, with about o.4 second measuring time,
on each channel. The Level Recorder writing system is able to record level
difference of up to 40 dB in this time interval at the specified writing speed.
See Fig. 2.11.
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........
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Fig.
2.11.
Automatic separation measurements obtained with the two-channel
selector of tl1e Unit Type 4409. The tested pick-up and the set-up are the
same
as in the case of Fig. 2.5. After this first chart, the set-up continues the
plotting of the three other charts without any intervention.
N.B. An attenuation of a few dB appears when a measuring point coincides
with the switching from one filter to another (see Fig. 2.4). The black dashes
on the upper scale indicate the filter center frequencies where no attenuation
is present.
By this method, the four chart plot is completely automatic and can be
obtained in approximately five minutes. The inherent drawback is that the
curves are obtained like "dotted lines" for
each
channel, giving less details
than the continuous monochannel plotting. On the other hand when the
possible irregularities are known beforehand, as for series checks, the method
will provide a great gain of time.
In addition, the only manual work to be done is placing the pick-up on the
record. The paper drive of the Level Recorder automatically stops when a
complete plotting is carried out. This offers the opportunity for the operator
to readjust previously checked pick-ups during that time, or to control
several measuring set-ups at one time.
27
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