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OPTIMOD-PC
OPTIMOD-PC's bandwidth can be adjusted from 10 kHz to 20 kHz to provide cor-
rectly anti-aliased audio for any of these systems. If any anti-aliasing filters following
OPTIMOD-PC's output are phase-linear and have integer-sample time delays, these
filters will pass the band-limited OPTIMOD-PC output without introducing overshoot
because they remove no further spectrum and do not cause their output samples to
become asynchronous with the peak-controlled samples at OPTIMOD-PC's output.
OPTIMOD-PC always operates at 48 kHz sample rate internally. Its output is
equipped with a sample rate converter that can output at 32 kHz, 44.1 kHz, 48, 88.2,
or 96 kHz. These rates can be synchronized to either AES3 digital input (see page 1-
22).
We expect that transmitters that transmit sample rates below 32 kHz will provide
internal sample rate conversion, and that most will probably accept audio at 48 kHz
sample rate regardless of the final sample rate of the transmission. Any sample rate
conversion may cause the transmitted sample to become asynchronous to the peak-
controlled samples emerging from OPTIMOD-PC and may therefore introduce over-
shoot. Fortunately, as the audio bandwidth becomes lower this becomes less of the
problem because the 48 kHz sample rate within OPTIMOD-PC oversamples the au-
dio. It is therefore less likely that peaks will "slip between the samples."
Subframe Delay
OPTIMOD-PC provides an adjustable time delay of up to 60 milliseconds. This allows
the installer to force the total delay through the processing to equal one frame
(which is useful in sound-for-picture applications). The definition of "frame" de-
pends on the system in which OPTIMOD-PC is installed.
The selections are
settings and whether 2-band or 5-band processing is active), 30 fps (NTSC
monochrome video), 29.97 fps (NTSC color video), 25 fps (most PAL
video), and 24 fps (film). You can also adjust the delay in one-millisecond
increments from 33 to 60 ms.
Setting Output/Modulation Levels
In a perfect world, one could set the peak level at OPTIMOD-PC's output to 0dBfs.
However, there are at several potential problems that may make it desirable to set
the modulation level slightly lower.
•
First is asynchronous re-sampling, which we have discussed at length earlier in
this chapter. (See page 1-24, for example.) If any digital processing that causes
its output samples to be asynchronous to its input samples is used after
OPTIMOD-PC's output, this can cause the peak levels of individual samples to
increase above the nominal threshold of limiting. This increase is typically less
than 0.5dB.
•
Second is additional processing, such as equalization. Equalization that applies
boosts at certain frequencies is very likely to add peak level and thus cause clip-
M
(typically 24 ms delay; depends on crossover
INIMUM
1-29
INTRODUCTION
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