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INTRODUCTION
Studio-Transmitter Link in Digital Broadcasting
Transmission from Studio to Transmitter
Digital links
mixer. This allows the sample frequency of the 1101's Wave outputs to be
locked to the chosen reference.
The following information is mainly relevant to digital radio broadcasters who have
their transmitters and studios at different locations. It assumes that you are using
OPTIMOD-PC as a replacement for a stand-alone audio processor. In this context,
you use OPTIMOD-PC's external analog or digital inputs and outputs and do not
route the audio through the host computer.
Most netcasters will not find the information in this section relevant because if net-
casters need to ship audio beyond their LAN, they ordinarily ship it from one loca-
tion to another in the form of encoded audio through low-capacity Telco-supplied
digital links like ISDN or E-1/T-1.
There are five types of studio-transmitter links (STLs) in common use in broadcast
service: uncompressed digital, digital with lossy compression (like MPEG, Dolby
®
APT-x
), microwave, analog landline (telephone/post line), and audio subcarrier on a
video microwave STL.
At this writing, we believe that the Internet is insufficiently reliable to
serve as a carrier for a real-time STL because of the risk that network in-
terruptions may randomly interrupt the audio feed.
STLs are used in two fundamentally different ways. Either they can pass unprocessed
audio for application to OPTIMOD-PC's input or they can pass OPTIMOD-PC's peak-
controlled output. The two applications have fundamentally different performance
requirements.
•
A link that passes unprocessed audio should have very low noise and low non-
linear 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 transient response is critical. In DAB ap-
plications, such a link must be uncompressed digital and must use digital inputs
and outputs to achieve best results. We will elaborate below.
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, and such links must therefore be carefully qualified
before you use them to carry the peak-controlled output of OPTIMOD-PC to the
transmitter. For example, the MPEG Layer 2 algorithm can increase peak levels up to
4dB at 160kb/sec by adding large amounts of quantization noise to the signal. While
the desired program material may psychoacoustically mask this noise, it is neverthe-
less large enough to affect peak levels severely. For any lossy compression system the
ORBAN MODEL 1101
®
, or
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