Orban OPTIMOD-AM 9300, Руководство по эксплуатации

Страница: 44 / 242

1-24
INTRODUCTION ORBAN MODEL 9300
for conventional receivers (which implies a relatively extreme preemphasis), the
newer receivers might sound strident or exceptionally bright. If the choice favored
the newer receivers (less preemphasis and probably less processing), the majority of
receivers would be deprived of much high-end energy and would sound both qui-
eter and duller.
NRSC Standard Preemphasis and Low-pass Filtering
In response to this dilemma, the National Radio Systems Committee (NRSC) under-
took the difficult task of defining a voluntary recommended preemphasis curve for
AM radio that would be acceptable to broadcasters (who want the highest quality
sound on the majority of their listeners' radios) and to receiver manufacturers (who
are primarily concerned with interference from first- and second-adjacent stations).
After a year of deliberation, a modified 75-microsecond preemphasis/deemphasis
standard was approved (See
Figure 1-2 ). This provides a moderate amount of im-
provement for existing narrowband radios while optimizing the sound of wideband
radios. Most importantly, it generates substantially less first-adjacent interference
than do steeper preemphasis curves. The second part of the NRSC standard calls for
a sharp upper limit of 10 kHz (at –15dB) for the audio presented to the transmitter.
(See
Figure 1-3 .)
OPTIMOD-AM's NRSC low-pass setting is essentially flat to 9.5 kHz and substantially
exceeds the NRSC standards above that frequency. This essentially eliminates inter-
ference to second and higher adjacencies. While some have protested that this is in-
adequate and that 15 kHz audio should be permitted, the unfortunate fact is that
interference-free 15 kHz audio could only be achieved by a complete re-allocation
of the AM band.
On April 27, 1989, The FCC (U.S.A.) released a Report and Order that amended sec-
tion 73.44 of the FCC Rules by requiring all U.S. AM stations to comply with the oc-
Figure 1-2: NRSC Modified 75 µs Deemphasis