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PART
6:
In-System Performance Verification
NOTE
The
FCC ( U.S.A.)
has
eliminated
requirements
for
periodic
Proof- Of-
Performance
measurements.
However,
any
performance
standards
specified
in
the
FCC
Rules
must
still
be
met.
Many
stations
will
doubtless
wish
to
make
periodic
performance
measurements.
The
text
below
provides
the
general
information
which
is
needed
to
perform
measurements
verifying
the
performance
of
a transmission
system
including
the
8182A.
Instructions
for
bench- top
verification
of
8182A
performance
outside of the
transmission
system are
found in Appendix
D: Field Audit-Of-Performance.
Mono
Performance
Verification:
This
is
very
straightforward.
Switch
both
PROOF/OPERATE switches to PROOF, switch the loudness controller to DEFEAT
(by
means of
its local OPERATE/DEFEAT switch),
and drive
the
input
with
test
signal. Sufficient headroom exists to modulate well beyond 100% at all frequencies,
50-15,000 Hz.
The PROOF
mode
turns
the Gate ON,
forcing
the unit
into
10dB
gain
reduction
("0"
on
the
G/R meter). To avoid apparent frequency response errors due to the
gain ' s changing between readings, wait at least two minutes after entering PROOF
mode before making measurements.
With single- chassis
units,
the only potential difficulty
involves STL
signal-to-noise
ratio. Because PROOF mode forces the unit into 10dB gain reduction, the measured
signal-to-noise ratio should accurately reflect the noise performance of the system
under normal operating conditions, provided that the system is normally adjusted to
run with 10dB gain reduction. Only the poorest- quality STL links will cause a noise
problem such that standards are not met under these conditions, and the OPTIMOD-
TV
Studio Accessory Chassis or a compandor-type noise reduction system may be
required
to achieve satisfactory results. (See Studio/Transmitter Links on p.2-1 for
a complete discussion.)
NOTES
1. OPTIMOD-TV
frequency
response
falls
off
extremely
rapidly
above
15.0kHz. If the test oscillator is miscalibrated, OPTIMOD-TV may appear
not
to meet proof at 15kHz. Before blaming OPTIMOD-TV, measure the
output frequency of the oscillator with an accurate counter to make sure
that
it
is
actually
producing
15.0kHz,
and
not
some
slightly
higher
frequency.
2. An
artifact
of
the
operation
of
the
Hilbert- Transform
Clipper
in
PROOF
mode
can
cause
falsely- high
distortion
readings
under
certain
conditions. THD between 4 and 5kHz may measure as high as 0.25%. If
SMPTE
IM distortion is measured in PROOF mode, it may read as high
as 1%.
While
these
readings
are
far
below
the
level
which
could
cause
a
problem
in a Proof situation, they are nevertheless not representative of
the
distortion performance capabilities of the OPTIMOD-TV circuitry in
OPERATE
mode. In OPERATE mode,
distortion is
highly
dependent on
level
and
frequency
such
that
audible
distortion
of
the
test
signal
is
avoided. This illustrates how the OPTIMOD-TV system is " tuned" to the
ear's psychoacoustic perception of distortion.
6-1
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