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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
lOdB
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
lOtB
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
lOdB
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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