Optimod-Surround 8685 V1.0 Operating Manual - Orban

1-14
INTRODUCTION ORBAN MODEL 8685
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, overshoot decreases as bit rate increases, so use the highest bit rate practical
in your system. In addition, there will be an unavoidable amount of overshoot
caused by asynchronous re-sampling (see page 1-14).
When using lossy digital compression, it is particularly important to minimize the
amount of peak limiting in the 8685. Heavy peak limiting may introduce audible artifacts
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.
The Dolby Digital transmission encoder is lossy and introduces overshoot. Hence, it is
usually unwise to use substantial amounts of peak limiting when the 8685 is used as
a final transmission processor to drive a Dolby Digital encoder. If a reasonable value
of Dialnorm is used, little or no peak limiting should be required in the 8685 because
the Dolby Digital signal path will have a generous amount of headroom.
Peak control in OPTIMOD 8685 occurs at a 48 kHz sample frequency. This is sufficient
to prevent any samples from exceeding the threshold of limiting. 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 pass the 8685’s output through a digital>analog>digital conversion, the
new samples will not be synchronous with the samples inside OPTIMOD 8685. Therefore,
they may well fall on the overshoots, causing loss of peak modulation control.
Typically, this introduces less than 1 dB of overshoot, so allowing 1 dB of headroom
in the transmission chain should be adequate to prevent audible problems.
The 8685’s outputs are equipped with sample rate converters that can output at 32
kHz, 44.1 kHz, 48, 88.2, or 96 kHz. Setting the output sample rate below 48 kHz can
cause overshoot due to spectral truncation and asynchronous re-sampling of the 48
kHz peak-controlled samples, just as in the D>A>D case described above. Spectral
truncation can cause overshoots substantially larger than 1 dB. To prevent this, set
the 8685’s bandwidth control (in the active Setup) to so it is lower than the bandwidth
of the downstream link. For example, a 32 kHz sample rate link requires setting
the 8685’s bandwidth to 15 kHz or lower.
Sample Frequency Synchronization
Virtually all digital broadcasting systems require audio to be synchronized to a master
clock. The 8685 can synchronize the audio sample rate appearing at its AES3id
outputs and optional SDI output to one of several sources. Although the these outputs
can emit different sample rates, they must all be synchronized to the same
source.
If a given output is set to a different sample rate than the sync source, the output
sample rate will equal the sync source’s sample rate multiplied by the ratio of two
integers. For example, if the sync source is 48 kHz and the output sample rate is 32
kHz, the output sample rate will be exactly 2/3 of the sync sample rate.