Orban OPTIMOD-FM 5700i, Operating Manual

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INTRODUCTION ORBAN MODEL 5700i
kHz, it can be passed without additional overshoot by equally well by any link with
44.1 kHz or higher sample frequency.
Currently available sample rate converters use phase-linear filters (which have con-
stant group delay at all frequencies). If they do not remove spectral energy from the
original signal, the sample rate conversion, whether upward or downward, will not
add overshoot to the signal. This is not true of systems that are not strictly band-
limited to 15 kHz, where downward sample rate conversion will remove spectral en-
ergy and will therefore introduce overshoot.
If the link does not have an AES3 input, you must drive its analog input from the
5700i’s analog output. This is less desirable because the link’s analog input circuitry
may not meet all requirements for passing processed audio without overshoot.
NICAM is a sort of hybrid between PCM and lossy data reduction systems. It uses a
block-companded floating-point representation of the signal with J.17 pre-
emphasis.
Older technology converters (including some older NICAM encoders) may exhibit
quantization distortion unless they have been correctly dithered. Additionally, they
can exhibit rapid changes in group delay around cut-off because their analog filters
are ordinarily not group-delay equalized. The installing engineer should be aware of
all of these potential problems when designing a transmission system.
Any problems can be minimized by always driving a digital STL with the 5700i’s AES3
digital output, which will provide the most accurate interface to the STL. The digital
input and output accommodate sample rates of 32 kHz, 44.1 kHz, 48 kHz, 88.2 kHz,
and 96 kHz.
Composite Baseband Microwave STLs
The composite baseband microwave STL carries the standard pilot-tone stereo base-
band, and therefore receives the output of a stereo encoder located at the studio
site. The receiver output of the composite STL is the stereo baseband signal, which is
applied directly to the wideband input of the FM broadcast transmitter’s exciter.
Thus, no stereo encoder is needed at the transmitter.
In general, a composite microwave STL provides good audio quality, as long as there
is a line-of-sight transmission path from studio to transmitter of less than 10 miles
(16 km). If not, RF signal-to-noise ratio, multipath distortion, and diffraction effects
can cause serious quality problems. Where a composite STL is used, use the 5700i’s
stereo encoder to drive the composite STL transmitter.
Uncompressed digital composite baseband microwave STLs, if properly designed,
have excellent performance and we recommend them highly. They are particularly
desirable in a 5700i installation because they allow you to use the 5700i’s composite
limiter to increase on-air loudness. However, the fact that they are digital does not
eliminate the requirement that they have low frequency response that is less than 3
dB down at 0.15 Hz. Any such STL should be qualified to ensure that it meets this
specification.