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OPTIMOD-PC
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.
Transmission from Studio to Transmitter
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
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 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.
It is practical (though not ideal) to use lossy data reduction to pass unprocessed au-
dio to OPTIMOD-PC's input. The data rate should be at least of "contribution qual-
ity"—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.
Because OPTIMOD-PC uses multiband limiting, it can dynamically change the fre-
quency response of the channel. This can violate the psychoacoustic masking as-
sumptions made in designing the lossy data reduction algorithm. Therefore, you
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INTRODUCTION
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