Orban OPTIMOD 6300, Operating Manual

Page: 31 / 255

OPTIMOD 6300 DIGITAL INTRODUCTION
1-9
Location of OPTIMOD 6300
This section assumes that OPTIMOD 6300 will be used as a transmission processor.
See the
Using OPTIMOD 6300 as a Studio Level Controller on page 1-10 if OPTIMOD
6300 is to be used as a studio AGC.
At the transmitter is best
The best location for OPTIMOD 6300 is as close as possible to the transmitter or en-
coder so that OPTIMOD 6300’s AES3 output can be connected to the transmitter or
encoder through a circuit path that introduces no change in OPTIMOD 6300’s output
bitstream. A high-quality AES3 cable is ideal.
Where access to the transmitter is not possible
Sometimes it is not possible to locate OPTIMOD 6300 at the transmitter. Instead, it
must be located on the studio side of the link connecting the audio plant to the
transmitter. If the transmitter plant is not accessible, all audio processing must be
done at the studio and you must tolerate any damage that occurs later.
If an uncompressed digital link is available, this is an ideal situation because such a
link will pass OPTIMOD 6300’s output with little or no degradation. However, such a
link is not always available.
If only a 32 kHz sample rate link is available, the sample rate conversion
necessary to downsample the audio will cause overshoots when
OPTIMOD 6300 is operated at 20 kHz bandwidth because the sample rate
converter removes spectral energy. In this case, you can minimize over-
shoot by operating OPTIMOD 6300 at 15 kHz bandwidth because the
bandwidth limiting occurs before OPTIMOD 6300’s peak limiter, which
then accurately controls the peak level of the 15 kHz band-limited signal.
Unless the path is a digital path using no lossy compression, this situation will yield
lower performance than if OPTIMOD 6300 is connected directly to the transmitter
because artifacts that cannot be controlled by OPTIMOD 6300 will be introduced by
the link to the transmitter. These artifacts can decrease average modulation by 2-
4dB and can also add noise and audible non-linear distortion. With lossy digital
compression, this deterioration will be directly related to the bit rate. For an analog
path, the deterioration will depend on the amount of linear and non-linear distor-
tion in the path. In addition, there will be an unavoidable amount of overshoot
caused by asynchronous re-sampling (see page 1-13).
One strategy is to apply to OPTIMOD 6300’s output signal the same lossy compres-
sion that the DAB transmitter would apply. If a digital link is available with sufficient
bit rate to pass this compressed signal, it can then be passed directly to the DAB
transmitter without further processing if synchronization issues can be resolved.
Consult with the manufacturer of your DAB transmitter to see if this can be done.
Where only an analog or lossy digital link is available, feed the audio output of
OPTIMOD 6300 directly into the link. If present, the transmitter’s protection limiter
should be adjusted so that audio is normally just below the threshold of limiting.
The transmitter protection limiter should respond only to signals caused by faults or
by spurious peaks introduced by imperfections in the link.