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1-10
INTRODUCTION
Location of OPTIMOD-DAB
At the Transmitter is Best
The best location for OPTIMOD-DAB is as close as possible to the transmitter so that
OPTIMOD-DAB's AES/EBU output can be connected to the transmitter through a cir-
cuit path that introduces no change in OPTIMOD-DAB's output bitstream. A high-
quality AES/EBU cable is ideal.
Where Access to the Transmitter Plant is Not Possible
Sometimes it is not possible to locate OPTIMOD-DAB at the transmitter. Instead, it
must be located on the studio side of the link connecting the audio plant to the transmit-
ter. 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-DAB's output with little or no degradation. However, such a link is
not always available.
If only a 32kHz sample rate link is available, the sample rate conversion
necessary to downsample the audio will cause overshoots when the 6200 is
operated at 20kHz bandwidth because the sample rate converter removes
spectral energy. In this case you can minimize overshoot by operating the
6200 at 15kHz bandwidth.
Unless the path is a digital path using no lossy compression, this situation will yield
lower performance than if OPTIMOD-DAB is connected directly to the transmitter, be-
cause artifacts that cannot be controlled by OPTIMOD-DAB will be introduced by the
link to the transmitter. These artifacts can result in 2-4dB lower average modulation
level, and can also add noise and audible non-linear distortion. In the case of lossy digi-
tal compression this deterioration will be directly related to the bit rate. In the case of 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 1-8).
One strategy is to apply the same lossy compression to OPTIMOD-DAB's output signal
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-DAB directly into the link. If available, 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 spu-
rious peaks introduced by imperfections in the link.
ORBAN Model 6200
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