Advertisement
APPENDIX D:
Field Audit-of-Performance
GENERAL
This
Appendix
provides
instructions
enabling
OPTIMOD-TV
users
to
check
the
performance
of
their
units
using
test
equipment
likely
to
be
found
in
a well-
equipped TV station. This procedure is a starting point for detecting and diagnosing
a problem that you believe is caused by OPTIMOD-TV. It is also useful in routine
maintenance,
and
can
be
used
at
Proof
time
to
check
routine
equipment
performance, thus providing more data than the Proof alone provides. By its nature,
it
is
limited in scope
to discovering static problems. A dynamic problem
in the
AGC circuitry ( caused by the failure of a timing module on Card # 5, for example)
would not tend to be discovered by performing these tests.
For
this
reason, measurements must always be complemented by listening. If you
are
well-acquainted with the " sound" of OPTIMOD-TV as adjusted to your tastes,
then faults that develop will ordinarily be readily detectable by ear.
If
audio
problems
develop,
many
engineers
immediately
blame
their
processing.
However,
as
is
the
case
with
any
processing,
faults
in
the
audio
equipment
preceding OPTIMOD-TV will be magnified by the action of the processing. Program
material
that
is marginally
distorted
at
the OPTIMOD-TV
input,
for example,
is
likely to be unlistenable by the time it emerges from the output when aggressive
processing is
used. In addition, be sensitive
to possible defects in
the
monitoring
equipment;
verify that a problem can be observed on at least two receivers before
pushing the panic button.
REQUIRED
EQUIPMENT
a)
Audio
Oscillator.
An
ultra- low- distortion
type
like
the
Sound
Technology
1710B
is preferred. However, a Heathkit or Eico-type oscillator ( such as Heath
10-72) can be used to obtain approximate results, provided that residual distortion
has been verified to be below 0.1%.
b)
Noise
and Distortion ( N&D) Test Set. Once again, a high-performance type
like the Sound Technology is preferred, but not required.
c)
General- Purpose
Oscilloscope.
DC- coupled,
dual- trace,
with
at
least
5mHz
vertical
bandwidth. This
is
used to monitor the output
of the N&D
Test
Set.
AUDIO
It
is often more convenient to make measurements on the bench away from high
PROCESSING
RF
fields
which could affect results. For example, in a high RF field it is very
difficult to accurately measure the very low THD produced by a properly- operating
OPTIMOD-TV at most frequencies. However, in an emergency situation ( is there any
other
kind?!),
it
is usually possible to do measurements under high- RF
conditions
which will reveal many of the grosser faults which could develop in the OPTIMOD-
TV circuitry.
The rear- panel TEST JACKS serve as a convenient source of preemphasized audio.
By
strapping
deemphasis IN
on
Card # 7 ( see Part
3: Installation, Fig.
3-2),
an
unbalanced deemphasized output
is simultaneously
available between the (+)
audio
output and circuit ground on the rear- panel barrier strip. To assure correct response
from
the output RFI suppression network, load the main output
with a 300 ohm
+5% resistor between the (+) input and circuit ground.
D-1
Advertisement
