Orban OPTIMOD 6300 Operating Manual page 40

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INTRODUCTION
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.
If the link does not have an AES3 input, you must drive its analog input from
OPTIMOD 6300's analog output. Starting with V2.2 software, peak control in
OPTIMOD 6300 occurs at a 192 kHz sample frequency, which limits potential over-
shoot to 0.5 dB or less after D/A conversion or sample rate conversion.
Downward sample rate conversion can cause overshoot due to spectral
truncation. To prevent this, always set the 6300's lowpass filter cutoff
frequency to complement the lowest sample frequency you are using in
your transmission system. Use 20 kHz for sample frequencies of 44.1 kHz
and above, and use 15 kHz for a sample rate of 32 kHz.
When using lossy digital compression, it is particularly important to minimize the
amount of peak limiting in the 6300. Heavy peak limiting may introduce audible ar-
tifacts as a side effect of precisely controlling peak levels. It is pointless to introduce
such artifacts if the lossy compression compromises the benefits of the limiting by
adding overshoots. Instead, allow a generous amount of headroom when setting
the drive level into the STL. Most lossy digital STLs have a noise floor that is low
enough to make this practical. Using a reference level (Dialnorm setting) recom-
mended in ATSC A/85 (–24 LKFS) or EBU R 128 (–23 LUFS) ensures that little, if any,
peak limiting will occur.
The Dolby Digital transmission encoder is lossy and introduces overshoot. Hence, it is
usually unwise to use substantial amounts of peak limiting when the 6300 is used as
a final transmission processor to drive a Dolby Digital encoder. If a reasonable value
of Dialnorm is used (–23 dB or lower), little or no peak limiting should be required in
the 6300 because the Dolby Digital signal path will have a generous amount of
headroom.
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
are ordinarily not group-delay equalized. The installing engineer should be aware of
all of these potential problems when designing a transmission system.
You can minimize any problems by always driving a digital STL with OPTIMOD 6300's
AES3 digital output, which will provide the most accurate interface to the STL. The
ORBAN MODEL 6300