Where Access To The Transmitter Is Not Possible; Optimod 6300 At The Transmitter: Gain Control Before The Stl - Orban OPTIMOD 6300 Operating Manual

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INTRODUCTION

Where access to the transmitter is not possible

OPTIMOD 6300 at the transmitter: gain control before the STL

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 up to 0.5 dB of overshoot caused by asyn-
chronous re-sampling (see page 1-16).
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.
Where maximum quality is desired, it is wise to request that all equipment in the
signal path after the studio be carefully measured, aligned, and qualified to meet
the appropriate standards for bandwidth, distortion group delay and gain stability.
Such equipment should be measured at reasonable intervals.
The audio received at OPTIMOD 6300's input should have the highest possible qual-
ity. To achieve the full audible benefit of OPTIMOD 6300 processing, use a studio-
transmitter link (STL) that is as flat as the bandwidth of OPTIMOD 6300 as used in
your plant (usually 20 kHz). Ideally, you should use a 20-bit (or better) uncompressed
digital link with at least 44.1 kHz sample frequency.
ORBAN MODEL 6300