Table of Contents
Advertisement
1. DESCRIPTION
provided by the B
&
K measuring equipment. See Fig. 1.10. The determination
of the correcting filters results immediately from the mathematical definition
of the standard characteristics:
where w is the angular frequency and
h
=
50
p,s
(curve 2) or 75
ftS
(curve 3), 12
=
318 p,s, t3
=
3180
,flS.
The recording characteristics of the B
&
K test records corresponds to
h
=
0, t2
=
318
flS,
t3
=
3180 ,p,s.
As seen on the figures, each gliding frequency band is preceded by a 1 kHz
start and reference signal lasting a few seconds. The starting point of the
logarithmic sweep coincides with the end of the 1 kHz modulation. The
purpose of this 1 kHz signal is, in addition to the determination of the fre-
quencies in the results as described below (synchronization of test signal and
recording paper) to allow a quick adjustment of the measuring chain sen-
sitivity to be made before actual measurements are taken. On all records the
1 kHz start signal is modulated laterally (A+ B) or vertically (A
-
B) when
preceding a vertical (A
-
B) band.
Recorded Level Tolerances.
The accuracy in amplitude of the reference recordings is determined to within
o.5 dB in a limited frequency range by the optical method of the "Filtered
B-line light
pattern"
referred to in the American Standards. Over the whole
frequency range, the accuracy is checked by means of a professional electro-
dynamic pick-up especially selected for this purpose. The check is carried out
at two different rotation speeds in order to distinguish the non-linearities of
the pick-up from those of the record itself: a change in rotation speed
accompanied by a corresponding change of the time constant of the equalizing
filter, produces a frequency translation of the record defects, while the fre-
quency response of the pick-up is unchanged. The investigation is carried out
both on the nickel "master" and on the vinylite records.
The flatness in frequency characteristic obtained in serial production is better
than
:±
o.5 dB from 100 Hz to 10 kHz and
±
1 dB from 20 Hz to 16 kHz.
Between 16
k~z
and 20 kHz a dip in the frequency response which is due to a
spurious resonance in the disc recorder might be present and the tolerances
are therefore enlarged as shown on Fig. 1.12.
Another important characteristic of the stereophonic test records is the upper
limit of the recorded cross-talk on the unmodulated groove walls of bands
A and B. This corresponds to the separation of the disc recorder since it is
practically unaffected by disc copying operations. The determination of the
spurious cross-talk in the pick-up employed for checking the recorded cross-
11
Advertisement
Table of Contents
