Orban OPTIMOD-FM 5700i, Operating Manual

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INTRODUCTION ORBAN MODEL 5700i
Further, it is common for a microwave STL to bounce because of a large infrasonic
peak in its frequency response caused by an under-damped automatic frequency
control (AFC) phase-locked loop. This bounce can increase the STL’s peak carrier de-
viation by as much as 2dB, reducing average modulation. Many commercial STLs
have this problem.
Some consultants presently offer modifications to minimize or eliminate this prob-
lem. If your exciter or STL has this problem, you may contact Orban Customer Service
for the latest information on such services.
Analog Landline (PTT/Post Office Line)
Analog landline quality is extremely variable, ranging from excellent to poor.
Whether landlines should be used or not depends upon the quality of the lines lo-
cally available, and upon the availability of other alternatives. Due to line equalizer
characteristics and phase shifts, even the best landlines tend to veil audio quality
slightly. They will certainly be the weakest link in a FM broadcast chain.
Slight frequency response irregularities and non-constant group delay characteristics
will alter the peak-to-average ratio, and will thus reduce the effectiveness of any
peak limiting performed prior to their inputs.
STL and Exciter Overshoot
Earlier in this section, we discussed at length what is required to prevent STLs from
overshooting. There are similar requirements for FM exciters. Nevertheless, in some
installations some overshoot is inevitable. If this is a problem in your installation, the
5700i’s remote control feature offers the means to reduce the peak level of the
5700i’s audio output as necessary. This way, you can still use the 5700i’s line-up tone
to adjust the steady-state deviation to
±
75 kHz. Yet, the reduced peak level of the
audio emitted from the 5700i ensures that the carrier deviates no further than
±
75
kHz after overshoot. This overshoot reduction can be selected on the input/output
screen, and the remote operation can be selected in System Setup: Network > Re-
mote.
Using Lossy Data Reduction in the Studio
Many stations are now using lossy data reduction algorithms like MPEG-1 Layer 2 or
Dolby AC2 to increase the storage time of digital playback media. In addition, source
material is often supplied through a lossy data reduction algorithm, whether from
satellite or over landlines. Sometimes, several encode/decode cycles will be cascaded
before the material is finally presented to OPTIMOD-FM’s input.
All such algorithms operate by increasing the quantization noise in discrete fre-
quency bands. If not psychoacoustically masked by the program material, this noise
may be perceived as distortion, “gurgling,” or other interference. Psychoacoustic
calculations are used to ensure that the added noise is masked by the desired pro-
gram material and not heard. Cascading several stages of such processing can raise
the added quantization noise above the threshold of masking, such that it is heard.