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INTRODUCTION ORBAN Model 8400
Some links may use straightforward PCM (pulse-code modulation) without lossy data
reduction. If you connect to these through an AES/EBU digital interface, these can be
very transparent provided they do not truncate the digital words produced by the devices
driving their inputs. Because the 8400’s output is tightly band-limited to 15 kHz, it can
be passed without additional overshoot by 32, 44.1 or 48 kHz links equally well.
Currently available sample rate converters use phase-linear filters (which have constant
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 energy and will therefore
introduce overshoot.
If the link does not have an AES/EBU input, you must drive its analog input from the
8400’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 quan-
tization 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 8400’s
AES/EBU 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, and 48 kHz.
Composite Baseband Microwave STLs
The composite baseband microwave STL carries the standard pilot-tone stereo baseband,
and is therefore fed from the output of a stereo encoder located at the studio site. The re-
ceiver output of the composite STL is the stereo baseband signal, which is applied di-
rectly 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 seri-
ous quality problems. Where a composite STL is used, use the 8400’s stereo encoder to
drive the composite STL transmitter.
Dual Microwave STLs
Dual microwave STLs use two separate transmitters and receivers to pass the left and
right channels in discrete form. Dual microwave STLs offer greater noise immunity than
composite microwave STLs. However, problems include gain- and phase-matching of the
left and right channels, overloads induced by pre-emphasis, and requirements that the
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