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1.
DESCRIPTION
maximum limit to the amplitude of modulation at high frequencies. The
conditions obtained on the B
&
K Gliding Frequency Records, where the
recorded velocity is limited at 20 kHz to 3.5 cm/s peak in each channel, are
illustrated in Fig. 1.11.
Band 1
Band 8
't62J62
Fig. 1.11. Views in the plane of modulation showing the relative dimensions
of the maximum allowed tip radius and the minimum radius of curvature
e
on the record QR 2009's outer and inner bands presenting a diameter of
28.2 and 12.6 em respectively at 20 kHz. Note that
e
is proportional to the
square of the diameter D (or of the groove speed). The speed of rotation being
45 r.p.m., i.e. o.75 r.p.s., the expression for
e
is
_
1r
(o.75 D)2 _
88
___!!!__
e-
2
tv
- o.
f
v
where f is the frequency and v the peak channel velocity (3.5 cm/s).
The advantage of the standardized recording characteristic is to give the
optimum signal-to-noise ratio by increasing the recorded amplitude at low fre-
quencies, where the rumbles and vibrations of the turntable are important,
and the recorded velocity at high frequencies where the surface noise increases
rapidly. Using any other characteristic will result in a decrease of the overall
signal-to-noise ratio after correction of the characteristic by an equalizing
filter. In the case of the B
&
K Gliding Frequency Records the signal-to-noise
ratio is decreased by about 6 dB after equalization, but this does not affect the
measuring possibilities.
In addition, the correction of the recording characteristics can easily be made
by means of a simple RC filter. Using different filters enables the selection of
different characteristics. This is of interest since there are differences between
the various national standards. Using a suitable filter, a constant recorded
velocity reference can also be obtained for testing electrodynamic pick-ups
without their associate equalizers. Three different characteristics are thus
10
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