Inovonics 525N, Инструкция по установке и эксплуатации

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selected audio cutoff options have no effect on modulation
measurements.
Navigate to the
SET OPTIONS
screen, No. 3. SEL buttons will
navigate the flashing brackets to the current
AUDIO CUT:
set-
ting, and
MENU/EDIT
buttons may be used to set the audio cut-
off to
FLAT
, or in 1kHz increments from
10kHz
down to
2kHz
.
Figure 2 on Page 7 plots the family of cutoff curves. The fre-
quency indicated by the display is the –3dB point of a fourth-
order (24dB/octave) low-pass filter. This cutoff applies only to
the rear-panel
PROGRAM AUDIO OUT
connector and the front-
panel
PHONES
jack. Again, modulation measurement always
conforms to the
FLAT
plot shown in Figure 2.
NRSC
De-Emphasis
AM broadcasting in the US employs fixed transmission pre-
emphasis defined by the National Radio Systems Committee
(NRSC) in their July, 1985 Specification. To obtain flat overall
response, the receiver supplies a complementary de-emphasis
curve. Figure 3 on Page 7 shows this truncated 75-microsec-
ond NRSC characteristic.
NRSC de-emphasis for AM broadcasting was established in the
mid-1980s when 10kHz audio bandwidth was proposed as a
quality benchmark in the US. But the full advantage of com-
plementary pre- and de-emphasis has been negated as many
broadcasters have voluntarily opted to chop-off high frequen-
cies drastically so as not to ‘waste modulation.’ Their ra-
tionale is that silky highs can’t be recovered by AM radios that,
themselves, have implemented narrow bandwidths to lessen
the effects of manmade RF noise.
The 525N allows de-emphasis to be selected from the front
panel for any of the audio cutoff settings, except for the
FLAT
position that automatically defeats de-emphasis.
With the
MENU/EDIT
buttons, bring up Menu No. 3,
SET OP-
TIONS
. Then use the
SEL
buttons to bracket either
IN
or
OUT
after
NRSC DE-EMPHASIS:
. The selection may then be toggled
with the
MENU/EDIT
buttons.
De-emphasis is normally left
IN
for listening, but may be
switched
OUT
when making measurements through the rear-
panel
PROGRAM AUDIO OUT
connector. Again, de-emphasis is
automatically switched-out when
AUDIO CUT:
is set to
FLAT
.
Just as with the audio bandwidth selection, de-emphasis affects
only the audio outputs and has no effect on modulation meas-
urements.
RECEPTION AND MEASUREMENT CONSIDERATIONS
Consistent with signal strength and noise concerns already
covered, the carrier peak modulation readings will be trustwor-
thy under most all circumstances.