Operating instructions

OPTIMOD-AM DIGITAL INTRODUCTION
1-27
receivers would be deprived of much high-end energy and would sound both qui-
eter and duller.
In response to this dilemma, the National Radio Systems Committee (NRSC) under-
took the difficult task of defining a voluntary recommended pre-emphasis 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 pre-emphasis/de-emphasis
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 pre-emphasis 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.)
NRSC Standard Pre-emphasis and Low-pass Filtering
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-
cupied bandwidth specifications of the NRSC-2 standard by June 30, 1990. The NRSC-
2 standard is an RF mask that was derived from the NRSC-1 audio standard. The
purpose of the NRSC-2 RF mask is to provide a transmitted RF occupied bandwidth
standard that any station with a properly operating transmitter will meet if NRSC-1
audio processing is used prior to the transmitter and if the station is not overmodu-
lating.
OPTIMOD-AM complies fully with the NRSC-1 standard when the 9.5 kHz NRSC low-
pass filter is in use, the HF
SHAPE control is set to NRSC, and the HF GAIN control is
set to +10.
Unfortunately, at this writing, the trend towards wider band receivers has reversed
and most receivers are no wider than they were in the 1970s. For this reason, many
engineers feel that using a third-order equalizer with 10 dB of ultimate boost pro-
vides a more intelligible sound on the average radio than does the NRSC curve. The
9400’s HF shelving equalizer can provide such a boost.
When a station is transmitting with 5 kHz audio bandwidth, the 9400’s 5 kHz low-
pass filter can produce audible ringing that some find objectionable. Using the
9400’s bell-shaped HF parametric EQ tuned to 3 kHz can reduce the effects of this
ringing by reducing the boost at 5 kHz by comparison to the 9400’s HF shelving EQ,
which maintains full boost all the way to 5 kHz. Additionally, you can use the LPF
Shape control to trade off brightness against ringing.