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2. OPERATION
Each band is preceded by a 1 kHz start signal the end of which coincides with
the 10 Hz point of the sweep. This signal is intended for actuating the auto-
matic start system provided by the Response Test Unit Type 4409 in connection
with the Level Recorder Type 2305 (a paper speed of 3 mm/s should be used
on the Recorder). In cases where wow and rumble from the turn table would
disturb the measurements, their influence may he reduced by isolating the
frequency range of interest by means of a sharp high-pass filter. Another
solution is the use of a better turn table. With a good turn table, it is possible
to carry investigations further down in frequency by rotating the record at
16 2/3 rpm, thus dividing the frequencies by a factor of two.
In the case of electrodynamic
pick-ups,
which are velocity sensitive, the
6 dB/octave variation of velocity on the constant amplitude sweep may be
compensated for by means of
a
series resistance of 63 kQ followed by a
parallel
cap~citance
of 1 ,p,F connected in the measuring circuit.
The recorded level given on the label is checked on each sweep at 100 Hz to
within
,±
o.5 dB. In addition, the amplitude constancy from 10 Hz to 100 Hz
is controlled to within
±
o.5 dB.
Test of Tape Recorders etc.
Tape recorders or other sound recording/reproducing equipment are tested
by using the B & K Beat Frequency Oscillator Type 1022 as a constant
reference input (see Fig. 2.13). The frequency tuning of this oscillator is
logarithmic, continuously covring the range 20
Hz-
20 kHz, and may be
mechanically driven in synchronism with the frequency scale on the paper
of the Level RecordPr 2305. This is automatically realised in the Combined
Unit Type 3308, Fig. 2.14.
As the paper drive of the Level
Recorder,
when connected to a 4409, can only
be started by the cessation of a 1 kHz start signal this condition can be
simply provided for by use of the "1ooo Hz Ref." push button of the B.F.O.,
see also the instruction manual for this instrument.
The plotting of the frequency response of the tape recorder (including both
recording and reproducing devices) may be made simultaneously or conse-
cutively. See Fig. 2.15.
As the equipment usually includes an output amplifier with low output
impedance, it can be directly connected to the Level Recorder 2305 which
has an input impedance of 16-18 k.Q and a maximum sensitivity of 10 mV
(0 dB line).
A frequency response can then he automatically plotted by preparing and
replaying a reference tape as outlined above. The synchronization between the
start of the paper drive and the beginning of the frequency sweep on the tape
is obtained by means of the 4409 actuated by the 1 kHz starting signal
originally obtained from the "1ooo Hz Ref." signal of the B.F.O. The output
signal level at 1 kHz should, however, be high enough to actuate the synchro-
starter. See specifications, p. 34.
30
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