Inovonics 531N, Инструкция по установке и эксплуатации, Страница: 33 / 60

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Forced-Mono
Reception
Press the
MONO
button
(20)
, which will light the adjacent red
LED indicator. With ‘forced mono’ engaged, the stereo decoder
is disabled and the station is received monaurally with left and
right program channels equally combined.
You should notice a couple of things in the
MONO
mode: 1) a
big difference in the sound, especially when monitored with
headphones. The stereo image will collapse and much of the
detail may be missing from the music. It may even sound as if
some of the top-end has disappeared. Also: 2) the L+R meter
will jump up a bit in mono, but the L–R meter will drop almost
off-scale. Actually, a perfect monaural signal should have no
L–R component, but circuit and component tolerances within
the Model 531N make something in the –50dB range about the
best L–R cancellation. Keep this in mind because this repre-
sents the best sub/main, main/sub crosstalk reading obtainable
from the 531N, even though actual system crosstalk may be
lower. See Figure 4 on Page # for typical crosstalk measure-
ment performance limits.
Program
De-emphasis
While listening to off-air program audio, press the
DE-EMP
but-
ton
(21)
. The green LED indicator next to the button will turn
off and program audio will sound considerably brighter, even
to the point of shrillness. Pressing the button again will turn
de-emphasis back on. As you cycle program de-emphasis on
and off, you may notice that, although the audio sounds quite
different, the left- and right-channel program audio displays
8
do not change much, if at all. Despite a very audible difference,
there is very little musical energy at the high frequencies. It’s
the subtle overtones that are boosted out of proportion without
proper de-emphasis.
Unless you are doing system crosstalk or other proof-of-
performance tests that require ‘flat’ program audio, the
DE-
EMP
button
(21)
should be left in the default ‘on’ position with
the green LED lighted. After listening for a short time with de-
emphasis turned off, a return to the proper tonal balance will
sound somewhat dull. This notion will pass after listening
properly again for a short while.
AM Noise
Measurement
While monitoring the local signal source, press
SELECT
button
(11)
until the
AM NOISE
indicator
(10)
lights. What you now
hear in the headphones should bear only a vague similarity to
the program audio signal monitored in the
LEFT / RIGHT
or
L+R / L–R
measurements modes. What you are listening to is
the amplitude-modulation component of the FM carrier.
A perfect FM carrier would have no ‘incidental’ or ‘synchronous’
AM component. What’s more, this parameter of the FM carrier
is difficult to measure off-air, as the receiver must have a
broad, flat bandwidth characteristic and a linear IF amplifier.
Without these qualities the receiver can introduce its own AM
component to the incoming signal.