Operating instructions

OPTIMOD-AM DIGITAL TROUBLESHOOTING
5-3
compromising the 9400’s noise level, we could eliminate a control that
was frequently misadjusted.
If you are using an external processor ahead of the 9400, be sure it is not clipping or
otherwise causing problems.
The 9400's highly processed output puts great demands on transmitter performance
Some transmitters cannot handle the very high average power in the 9400's output
Section 1 discusses this in detail
The distortion of tube-type transmitters will increase substantially as the tubes go
flat with use The first thing to go is asymmetrical positive peak capability, so, if it is
impractical to replace the modulator tubes at this time, reduce the setting of the
9400 P
OSITIVE PEAK control until the transmitter no longer compresses the peaks. In-
deed, some transmitters cannot handle asymmetrical positive peaks without com-
pression even with good tubes, Never try to run these transmitters with asymmetry.
The codecs used in the HD AM and DRM systems operate a very low bit rates. They
tend to produce more artifacts as program material becomes denser. To prevent the
9400’s processing from increasing density excessively, we recommend setting the MB
RELEASE control in the digital radio processing close to SLOW and to use minimal
amounts of look-ahead limiting.
Unfortunately, some CDs are now mastered with so much compression, limiting, and
clipping that they cause codecs to misbehave without further processing in the
broadcast chain. Until the broadcast industry successfully pressures the record indus-
try to supply broadcasters with lightly processed broadcast-mastered music, this
problem will continue.
Audible Noise on Air
(See also “RFI, Hums, Clicks, or Buzzes” on page 5-1.)
Excessive
compression will always exaggerate noise in the source material. The 9400
has two systems that fight this problem.
1. The compressor gate freezes the gain of the AGC and compressor systems when-
ever the input noise drops below a level set by the threshold control for the process-
ing section in question, preventing noise below this level from being further in-
creased. There are three independent compressor gate circuits in the 9400. The first
affects the AGC, while the second and third affect the Multiband Compressors in the
AM analog and digital radio chains respectively. Each has its own independent
threshold control. (See M
B GATE on page 3-49.)
2. The dynamic single-ended noise reduction (see D
WNEXP THR on page 3-50) can be
used to reduce the level of the noise below the level at which it appears at the in-
put.
If you are using the 9400's analog input, the overall noise performance of the sys-
tem is usually limited by the overload-to-noise ratio of the analog-to-digital con-
verter used by the 9400 to digitize the input. (This ratio is better than 108 dB.) It is