BW Broadcast technical manual
Page 20
Installation and setup
This option affects the character of the sound passing through the limiters inside the board. The sound can
be either (a) processed to be competitively loud, with some sacrifice in fidelity (hard limiting, the Loud position)
or (b) treated more gently, with high fidelity, but some loss in volume (soft limiting, the Pure position).
It’s instructive to listen to and compare the options while passing a representative selection of typical pro-
gramme material through the board. This will help you establish which sound you prefer.
As before, both jumpers 4 and 5 should always be in the same position. These jumpers, like all the following
ones, do not affect the operation of the board when external MPX is used with Jumper 1 off.
Jumpers 6 and 7: Limiter clippers on/off
The default position for these jumpers is ON.
If they are to be switched off when external audio processing is used, careful monitoring of deviation is recom-
mended. The 15 kHz post-filter clippers must be switched off at the same time (J9/10).
These jumpers control the clippers applied to the signal after the limiter but before the 15kHz low-pass fil-
ters on the way into the stereo coder. Again, these jumpers work as a pair and do not affect external MPX
inputs.
Jumper 8: Limiter on/off
The default position for this jumper is ON, i.e. limiter active.
If you are using a pre-processed and limited signal and do not want the additional protection of the internal
limiters, their action can be disabled by putting Jumper 8 in the OFF position. This one jumper controls both
channels, because the gain-reduction control voltage (which this jumper disables) is common to left and
right.
Even with the limiter disabled you may still wish to use the pre-filter clippers (J6/7) and post-filter clippers
(see below, J9/10) for protection. Always observe the deviation produced very carefully for excesses above
75 kHz if you ever remove the action of the limiter or clippers.
Jumpers 9 and 10: 15kHz filter clippers on/off
The default position for these clippers, placed after the 15kHz filters on the input to the coder, is ON.
These clippers protect against over-deviation caused by signals which ‘ring’ in the 15kHz filter, even after
having been caught by the clipper before the filter. With certain pre-processed and filtered inputs, however,
they may be switched out of circuit. But keep a close eye on the deviation.
Don’t have these clippers switched on unless you also have the limiter clippers active. Otherwise, your signal
could go out of specification.
These jumpers work as a pair and don’t affect external MPX drives.
Jumper 11: Mono / Stereo
The default position is STEREO, pilot tone on.
This option allows you to remove or restore the stereo pilot tone, at a frequency of 19 kHz, normally sitting at
a level between 8 and 10% of total deviation.
It is this tone which alerts stereo FM receivers to the need to switch on their stereo decoders. The presence
of a pilot tone is all that is required for the ‘stereo’ beacon to light on a receiver. If no 19kHz tone is received,
the receiver will operate in mono. It will not decode any L-R information modulated on the 38 kHz subcarrier,
even when it is still present.
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