SOME
SPECIFIC
APPLICATIONS
Adjusting
the
424A
To
Provide Peak Overload Protection
The
typical
"peak operating level"
of
an
analog
tape
recorder
is that
level
producing 3% total
harmonic distortion;
of a
power amplifier,
that
level
which
drives
the
amplifier to its rated output
voltage (power will
vary
according
to load
impedance); and of
an
STL
transmitter,
that
level
which drives
the transmitter to
100% modulation.
The following procedure matches the
424A
to the driven device.
It assumes
that
the input attenuator of
the driven device (if it
exists) is fixed, and adjusted to
produce approximately correct operating levels when
driven by
a
+4dBm or +8dBm
line.
(If
the
driven
device
is
significantly more
sensitive,
you
may find that the
424A OUTPUT ATTENuator must be
operated very
close to its counterclockwise
end stop, and
may be
touchy to adjust. In this case, it may
be desirable to
insert
a
loss
pad between the 424A
and the
input
of
the driven device.)
First,
you
must
decide if
you
will leave the
control
loop
in
its
normal "bass
rolloff" mode,
or
defeat the
bass rolloff
so that
the
control
loop
can sense
the
true peak
level
of
the output.
We recommend leaving the control
loop
in "bass
rolloff" mode to
achieve
most
natural sound, and to control any
transient peak
overshoots which may
occur
by
permitting
the VCA to clip them. However,
if
you
wish
to use
the
424A
as a
fast-attack true-peak
limiter, then you should restrap
the
control loop
rolloff
according
to
instructions
provided in the Control Loop
Bass
Rolloff
paragraph
in
Part B
above.
After
you have restrapped the control loop as necessary,
apply a 400Hz
sinewave
oscillator to the
input of
the 424A. Switch both
424A OPerate/DEFEAT switches
to
DEFEAT, and adjust
the
424A
INPUT
ATTENuator
and/or the oscillator
output
attenuator as necessary
to
produce
a
reading
of
"+2dB"
on the VCA
LEVEL meter.
This level is
within
IdB
of
the clipping level
of
the VCA
output.
Adjust the
424A
OUTPUT
ATTENuator
until the driven device is
driven
to the following
level:
Analog
Tape Recorder: 5dB
above
"Operating
Level" (i.e., 5dB above
the
nominal 1% total harmonic
distortion level).
Audio Power Amplifier: IdB
below
clipping.
Studio-Transmitter Link Transmitter:
90%
modulation if flat; 40% modulation if
pre-emphasi zed at
50
or 75us.
(NOTE: The
40%
modulation
figure
may be
increased
if
the de-esser
is
used to control occasional high frequency peaks.
However, beware
of
possible audible
side-effects if
this is attempted!)
U.S.A. Standard Telephone Line:
+
l6dBm into the line
(through
a
Telco-
supplied repeat coil).
(NOTE:
Place a 240 ohm +5%
1/4
carbon film
resistor
in
series
with each
leg
of
the
424A balanced output
to assure
that the
phone line
is driven by a true
600 ohm
source impedance. The
+l6dBm
must be
measured
on the phone line side
of the
resistors.)
Other
Devices: IdB
below the nominal overload point if flat; if
pre-
emphasized, on the basis of experience.
19
To Next Page
Loading...