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1-36
INTRODUCTION
Subframe Delay
Setting Output/Modulation Levels
oscillator. In addition, OPTIMOD-PC's output is equipped with a sample rate con-
verter 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 or to the sync input (see page 1-29).
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.
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 M
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 vid-
eo), and 24 fps (film). You can also adjust the delay in one-millisecond in-
crements from 33 to 60 ms.
In a perfect world, one could set the peak level at OPTIMOD-PC's output to 0 dBfs.
However, there are at several potential problems that may make it desirable to set
the modulation level slightly lower.
•
First is the fact that the peak limiter operates at 192 kHz sample rate and not at
an infinite rate. This ensures that overshoot after phase-linear sample rate con-
version or D/A conversion will not exceed 0.5 dB and will typically be much less.
•
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-
ping. However, equalization that attenuates certain frequencies can also cause
overshoots because of added phase shifts. So be wary of any equalization and al-
low headroom to accommodate it.
•
Third is headroom in lossy data compression systems. A well-designed perceptual
encoder will accept samples up to 0 dBfs and will have internal headroom suffi-
cient to avoid clipping. However, there is no guarantee that receiver manufac-
turers or decoder providers will implement perceptual decoders with sufficient
headroom to avoid clipping overshoots. Such overshoots are the inevitable side
effect of increasing the quantization noise in the channel, and can be as large as
3-4dB. Most perceptual encoder algorithms are designed to have unity gain from
input to output. So if peak levels at the input frequently come up to 0dBfs, peak
levels at the output will frequently exceed 0dBfs (and will be clipped) unless the
decoder algorithm is adjusted to be less than unity gain.
Canny engineers will therefore familiarize themselves with the performance of
(typically 24 ms delay; depends on crossover
INIMUM
ORBAN MODEL 1101
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