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2. OPERATION
~)
While playing the frequency sweep, set METER RANGE to the position
giving normal pointer deflection on the meter throughout the frequency
sweep. Then give the Level Recorder writing stylus a convenient de-
flection by means of the INPUT ATTENUATOR and INPUT POTENTIO-
METER knobs.
d) Finally, when the paper drive is stopped, shift the paper by means of
the Finger Wheel Z (see the 2305 Manual) until the writing stylus rests
on the 20 Hz line. Approach the required position in the reverse
direction.
3) Before starting the automatic frequency response plot, check that the
rotation speed of the record player is exactly 45 r.p.m. when loaded by the
pick-up to be tested, e.g. when tracking a groove.
4) Engage the pick-up on the lead-in groove of the band which has been
selected for the test. During the first few seconds (playing of the start signal)
it will be possible to fine adjust the
1
kHz deflection if necessary. The plotting
of the frequency response takes place automatically when the 1 kHz start
signal ceases, and when the paper drive stops on the 20 Hz line of the next
chart, the set-up is ready for another plotting without any further adjustment.
Note: Use of 25 or 10 dB Potentiometer for detailed frequency response in-
vestigations. The only modification to be made in this case is that the
POTENTIOMETER RANGE knob of the Level Recorder has to be shifted from
"50" to "25" or "10" respectively.
Play-back Losses.
At high frequencies, the acceleration which is communicated to the reproducing
stylus attains a few hundred g (gravity)
.
Consequently, even if the stylus mass
has been reduced to a minimum, the reaction of the stylus on the record may
cause non-negligible elastic deformations of the
groove
walls. This phenomenon
which, in most cases corresponds to a reduction of amplitude and velocity of
the stylus movement, is commonly called "Play back loss".
The play back losses are greatly dependent on the construction of the repro-
ducing pick-up. They may be negative as well as positive and cannot be
determined without accurate knowledge of the mechanical constants involved.
A theoretical investigation of the phenomenon *) shows that play back losses
(or gains) increase rapidly with frequency in all
cases.
Practically, play back
losses attaining a few dB at 20 kHz are observed when tracing the inner bands
of the Gliding Frequency Records with commercial pick-ups of good quality.
Automatic Selective Cross-talk Measurements.
General.
Gliding frequency cross-talk measurements are carried out by means of the
record QR 2009 and the set-up of Fig. 2.2.
When testing good quality pick-ups, it will be noticed that the inherent surface
*)
See B
&
K Technical Review No 3-1962, Appendix
A.
19
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