Orban OPTIMOD 6200, Operating Manual

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OPTIMOD INTRODUCTION
1-7
Studio-Transmitter Link
Transmission from Studio to Transmitter
There are five types of studio-transmitter links (STLs) in common use in broadcast ser-
vice: 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 are used in two fundamentally different ways. They can either pass unprocessed
audio for application to the 6200’s input, or they can pass the 6200’s peak-controlled
output. The two applications have fundamentally different performance requirements. In
general, 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 doesn’t need as low a noise floor as a link passing unprocessed audio. However, its
transient response is critical. In DAB applications 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 second 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 the 6200 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 sys-
tem.
It is practical (though not ideal) to use lossy data reduction to pass unprocessed audio to
the 6200’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.
Because the 6200 uses multi-band 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 6200’s multi-band processing will not unmask 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 384kB/second or higher.
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