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[If
you
are interested
in
an explanation
of
the
mechanism
causing the
distortion increase
in
PROOF
mode,
refer
to 5.b of
Appendix
B
(Circuit
Description).
The
relevant
paragraph
begins:
"PROOF
mode...".]
Stereo
Performance
Verification: These
measurements
are
usually
made
most
meaningfully
by defeating
compression and
limiting action
while leaving
ail
active
circuitry unbypassed.
The
existing
PROOF/OPERATE
and
LOUDNESS
CONTROLLER
OPERATE/DEFEAT
switches
provide
this
function.
(See
Mono
Proof immediately
above.)
Special attention should be paid
to
headroom.
The
clipping
point
of
the
VCA's
in
the
Dual-Band Compressor
is
approximately 5dB above
100%
modulation,
although
distortion
will
begin
to
rise
substantially
as
clipping
is
approached.
In
the
BTSC
Stereo
setup instructions
in
Part 4
of
this
Manual,
instructions
were
provided
for
adjusting
the
8182A's
OUTPUT ATTENUATORS
such
that
100% L+R
modulation
(i.e.,
+25kHz
deviation)
is
achieved
when
both
Left
and
Right
Channels
are
simultaneously
driven.
This
means
that only
50%
L
or
R
modulation
is
achievable
when
a
single
channel
is
driven
alone
unless
the
OUTPUT ATTENUATORS
are
temporarily turned
up
by
6dB
.
If
this
is
done,
100%
modulation
of
the
Left and
Right channels can be
achieved.
There
is
no
need
to
readjust
the
8182A's
OUTPUT ATTENUATORS
to
achieve
100% L+R
or
L-R:
L+R
at up to
100%
modulation can be generated by
driving
both
Left and
Right channels
in-phase
,
while
L-R
at
up
to
100%
modulation can be
generated
by
driving
both Left and Right channels out-of-phase
.
(To
make
the out-
of-phase
connection, temporarily reverse the
"+"
and
input
connections
of
one
channel
only.)
When
trying
to
create pure
L+R
or
L-R,
null
undesired
L-R
or
L+R
(respectively)
by
balancing the Left and
Right channel
gains as closely
as
possible.
There
will
always
be
some
residual linear crosstalk
which cannot be
nulled.
This
is
caused by
relative phase
shifts
between
the
Left and Right
channels.
Because
this
crosstalk
is
linear,
it
introduces
no
distortion
products
into
the
signal
and
has
no
audible
significance
unless
it
is
large
enough
(perhaps -35dB
or
greater)
to
cause
deterioration of stereo imaging.
Full
assessment
of
crosstalk
performance
requires a
baseband
spectrum
analyzer
to
separately
measure
the
linear
crosstalk
(which
is
usually
innocuous)
and
the non-
linear
crosstalk
(which can cause
audible distortion
if
large enough).
In
the
BTSC
system,
if
the
frequency
of
the
test
tone
is
F
and
f.,=l5.
734kHz,
then
linear
crosstalk
is
found
at
F
(subchannel-to-main
channel)
and
at
2f^+F
(main
channel-to-subchannel).
All
other
parts of the
spectrum
are non-linear crosstalk or
other
spurious
signals
except
for
the
stereo
pilot
and
the
desired
modulation.
6-2
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