Table of Contents
Advertisement
1-26
INTRODUCTION
Analog Telephone/Landline (PTT/Post Office Line)
Location of OPTIMOD-PC in Digital Radio Service
At the Transmitter is Best
Where Access to the Transmitter is not Possible
configured for composite operation (even if a composite FM stereo signal is not ac-
tually being applied to the link).
Further, it is not unusual for a microwave STL to bounce because of a large infra-
sonic peak in its frequency response caused by an under-damped automatic fre-
quency control (AFC) phase-locked loop. This bounce can increase the STL's peak car-
rier deviation 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 quality is extremely variable, ranging from excellent to poor. (For-
tunately, they are largely obsolete, having been replaced by digital links.) Whether
landlines should be used or not depends upon the quality of the lines locally avail-
able, and upon the availability of other alternatives. Due to line equalizer character-
istics and phase shifts, even the best landlines tend to veil audio quality slightly,.
They will certainly be the weakest link in a DAB 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.
The best location for OPTIMOD-PC is as close as possible to the transmitter so that
OPTIMOD-PC's AES3 output can be connected to the transmitter through a circuit
path that introduces no change in OPTIMOD-PC's output bitstream. A high-quality
AES3 cable is ideal.
Sometimes it is not possible to locate OPTIMOD-PC at the transmitter. Instead, it
must be located on the studio side of the link connecting the audio facility to the
transmitter. If the transmitter is not accessible, all audio processing must be done at
the studio, and you must tolerate any damage that occurs later.
If an uncompressed digital link is available, this is an ideal situation because such a
link will pass OPTIMOD-PC's output with little or no degradation. However, such a
link is not always available.
If only a 32 kHz sample rate link is available, the sample rate conversion
necessary to downsample the audio will cause overshoots when
OPTIMOD-PC is operated at 20 kHz bandwidth because the sample rate
converter removes spectral energy. In this case, you can minimize over-
shoot by operating OPTIMOD-PC at 15 kHz bandwidth. (Set it from the
Configuration tab in the I/O Mixer application.)
ORBAN MODEL 1100
Advertisement
Table of Contents
