Orban Optimod-AM 9400, Руководство по эксплуатации

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OPTIMOD-AM DIGITAL INTRODUCTION
1-11
Studio-Transmitter Link
Transmission from Studio to Transmitter
There are several types of studio-transmitter links (STLs) in common use in broadcast
service: uncompressed digital, digital with lossy compression (like MPEG, Dolby
®
, or
APT-x
®
), microwave, analog landline (telephone / post line), and audio subcarrier on
a video microwave STL.
STLs in AM service are used in two fundamentally different ways. They can either:

pass unprocessed audio for application to the 9400’s input, or

pass the 9400’s peak-controlled analog or digital left and right audio outputs for
application to the transmitter.
These applications have different performance requirements. In general, a link that
passes unprocessed audio should have very low noise and low nonlinear distortion,
but its transient response is not important. A link that passes processed audio does
not need as low a noise floor as a link passing unprocessed audio. However, its tran-
sient response is critical. At the current state of the art, an uncompressed digital link
using digital inputs and outputs to pass audio in left/right format achieves best re-
sults. We will elaborate below.
Digital Links
Digital links may pass audio as straightforward PCM encoding or they may apply
lossy data reduction processing to the signal to reduce the number of bits per sec-
ond required for transmission through the digital link. Such processing will almost
invariably distort peak levels; such links must therefore be carefully qualified before
you use them to carry the peak-controlled output of the 9400 to the transmitter. For
any lossy compression system the higher the data rate, the less the peak levels will
be corrupted by added noise, so use the highest data rate practical in your system.
As stated above, links using lossy data reduction cannot pass an OPTIMOD-AM–
processed signal. However, it is practical (though not ideal) to use lossy data reduc-
tion to pass
unprocessed audio to the 9400’s input. The data rate should be at least
of “contribution quality” — the higher, the better. If any part of the studio chain is
analog, we recommend using at least 20-bit A/D conversion before encoding. Be-
cause the 9400 uses multiband limiting, it can dynamically change the frequency re-
sponse of the channel. This can violate the psychoacoustic masking assumptions
made in designing the lossy data reduction algorithm. Therefore, you need to leave
“headroom” in the algorithm so that the 9400’s multiband processing will not un-
mask quantization noise. This is also true of any lossy data reduction applied in the
studio (such as hard disk digital delivery systems).
For MPEG Layer 2 encoding, we recommend 384 kB/second or higher.
Some links may use straightforward PCM (pulse-code modulation) without lossy
data reduction. If you connect to these through an AES3 digital interface, these can
be very transparent if they do not truncate the digital words produced by the de-