Digital Links - Orban OPTIMOD 6300 Operating Manual

OPTIMOD 6300 DIGITAL

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.
Because OPTIMOD 6300 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
need to leave "headroom" in the algorithm so that OPTIMOD 6300's multiband
processing will not unmask quantization noise. This is also true of any lossy data re-
duction 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 AES3 digital interface, these can
be very transparent if they do not truncate the digital words produced by the de-
vices driving their inputs and they do not require downward sample rate conversion.
Downward sample rate conversion can cause overshoot due to spectral
truncation and asynchronous re-sampling of the 48 kHz peak-controlled
samples.
If the link does not have an AES3 input, you must drive its analog input from
OPTIMOD 6300's analog output. This is not recommended because OPTIMOD 6300's
monitor output will overshoot in the analog domain because of the physics of the
system. The use of external digital-to-analog converters should also be avoided for
many of the same reasons.
Peak control in OPTIMOD 6300 occurs at a 48 kHz sample frequency. This
is sufficient to prevent any samples from exceeding the threshold of lim-
iting. However, after reconstruction, the analog output may overshoot
the nominal 100% level because these overshoots "fall between the
samples," so the processing cannot be aware of them. If you use this
output to feed the analog input of a digital STL, the new samples in the
STL will not be synchronous with the samples inside OPTIMOD 6300.
Therefore, they may well fall on the overshoots, causing loss of peak
modulation control. It is thus very important to use a link with an AES3
input to ensure correct peak control.
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INTRODUCTION