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INTRODUCTION
should leave "headroom" in the algorithm so that OPTIMOD-PC's multiband proc-
essing will not unmask quantization noise. This is also true of any lossy data reduc-
tion applied in the studio (such as hard disk digital delivery systems).
For MPEG Layer 2 encoding, we recommend 384 kb/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, provided they do not truncate the digital words produced by
the devices driving their inputs and they do not require downward sample rate con-
version.
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-PC's monitor output. This is not recommended because OPTIMOD-PC's
monitor output will overshoot in the analog domain because of the physics of the
system. For many of the same reasons, avoid using external digital-to-analog con-
verters.
Peak control in OPTIMOD-PC occurs at a 48 kHz sample frequency. This is
sufficient to prevent any samples from exceeding the threshold of limit-
ing. However, after reconstruction, the analog output may overshoot the
nominal 100% level because these overshoots "fall between the sam-
ples," 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-PC. Therefore,
they may well fall on the overshoots, causing loss of peak modulation
control. It is thus important to use a link with an AES3 input to ensure
correct peak control.
The same sort of thing can happen if you use the output sample rate
converter, because the output samples are no longer synchronous with
the peak-controlled samples in the processing. Always use 48 kHz output
sample rate to achieve best peak control.
If you must use an analog input, you may bypass any anti-aliasing filters
in digital links driven by OPTIMOD-PC because OPTIMOD-PC's output
spectrum is tightly controlled. This ensures the most accurate possible
transient response, given the limitations of asynchronous sampling de-
scribed above.
Using analog outputs or non-48kHz sample rates will seldom introduce
more than 0.5 dB of overshoot, so it is usually sufficient to allow 0.5 dB
of headroom in these non-ideal setups. For example, one would set
OPTIMOD-PC's digital output level to –0.5 dB to prevent clipping.
NICAM is a sort of hybrid between PCM and lossy data reduction systems. It uses a
block-companded floating-point representation of the signal with J.17 preemphasis.
Older technology converters (including some older NICAM encoders) may exhibit
quantization distortion unless they have been correctly dithered. Additionally, they
can exhibit rapid changes in group delay around cut-off because their analog filters
ORBAN MODEL 1101
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