OPTIMOD-FM INTRODUCTION
1-13
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 three fundamentally different ways. They can either (1) pass unproc-
essed audio for application to the 8400’s input, (2) they can pass the 8400’s peak-
controlled analog or digital left and right audio outputs, or (3) they can pass the 8400’s
peak-controlled composite stereo baseband output. The three applications have 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 un-
processed audio. However, its transient 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 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 8400 to the transmitter. For example, the MPEG
Layer 2 algorithm can increase peak levels up to 4 dB at 160kB/sec by adding large
amounts of quantization noise to the signal. While the desired program material may psy-
choacoustically mask this noise, it is nevertheless large enough to affect peak levels se-
verely. 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 audio to
the 8400’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 8400 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 8400’s multiband 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.
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