Digital Links - Orban OPTIMOD-FM 8500S Operating Manual

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OPTIMOD-FM DIGITAL INTRODUCTION
STLs are used in three fundamentally different ways. They can either (1) pass un-
processed audio for application to the 8500S's input, (2) they can pass the 8500S's
peak-controlled analog or digital left and right audio outputs, or (3) they can pass
the 8500S's peak-controlled composite stereo baseband output. The three applica-
tions have different performance requirements. In general, a link that passes un-
processed 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. 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 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 the 8500S to the trans-
mitter. 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 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 the 8500S'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 8500S uses multiband limiting, it can dynamically change the frequency
response 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 8500S's multiband processing will not un-
mask 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 384 kB/second or higher.
If you use a lossy codec between the Optimod's output and the transmitter's input,
the codec must see a "flat" signal to ensure that its psychoacoustic model works cor-
rectly. Hence, the Optimod output driving the codec must be set to F , which ap-
LAT
plies deemphasis to the output of the Optimod's preemphasis processing. 50μs or
75μs preemphasis must then be applied to the codec's output signal. This usually oc-
curs in the FM transmitter's stereo encoder or in a stand-alone stereo encoder like
an Orban 5518.
Some links may use straightforward PCM (pulse-code modulation) without lossy
data reduction. If you connect to these through an AES3 digital interface, these can
be very transparent provided they do not truncate the digital words produced by
the devices driving their inputs. Because the 8500S's output is tightly band-limited to