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OPTIMOD 6300 DIGITAL
Studio-Transmitter Link
The following information is mainly relevant to digital radio and TV broadcasters
who have their transmitters and studios at different locations. Most netcasters will
not find the information in this section relevant because if netcasters need to ship
audio beyond their LAN, they ordinarily ship it from one location 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 might randomly disturb the audio feed.
STLs are used in two fundamentally different ways. Either they can pass unprocessed
audio for application to OPTIMOD 6300's input or they can pass OPTIMOD 6300's
peak-controlled output. The two applications have fundamentally different per-
formance 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 lossy processing will al-
most invariably distort peak levels and such links must therefore be carefully quali-
fied before you use them to carry the peak-controlled output of OPTIMOD 6300 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
nevertheless 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 6300'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.
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INTRODUCTION
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, or
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Troubleshooting
