BW Broadcast technical manual
Page 28
Installation and setup
input gain controls must be set equally and correctly for the this mode to work correctly and create a true mono
signal. Any gain difference would create L-R subcarrier information which is not neccessary and wastes band-
width. Following the ‘mono from two independent channels’ advanced setup procedure should have you set up
the two channels input gain controls at the same level.
2. Set the MONO FROM LEFT bridge jumper J12 to ON. this disconnects the right channel socket internally and
feeds the left information to the internal right channel circuitry.
3. Apply your audio feed to the left channel input.
4. Connect your studio feed to the transmitter. Turn the power off to the transmitter, connect your external anten-
na, reconnect your power and your on the air!
5. Monitor your total peak deviation with your real world audio material and check with the deviation display that
your peak deviation does not exceed 75 kHz. If you have set up the transmitter as per the instructions in this sec-
tion then you should be within 1 kHz or so of this figure. If your deviation does exceed 75 kHz then adjust the
back panel multiplex control to keep your maximum deviation at 75 kHz.
2.6 OTHER SETUP CONSIDERATIONS
PRE EMPHASIS
Pre-emphasis note: with pre-emphasis active the input level required to achieve limiting is frequency dependent
due to the characteristics of the pre-emphasis curve. There is 17dB more gain into the limiter with a 15 kHz input
signal compared to a frequency which is below the start point of the pre-emphasis curve. The pre-emphasis curve
provides a 6db per octave boost from the 50 or 75 µs breakpoint. For this reason we have used 400Hz for our
tests because it is below the pre-emphasis filter curve. With pre-emphasis active the maximum usable input level
to the limiter is 10dBu when the gain controls are at maximum. Anything above this may cause high frequency
content to hit the internal supply rails, clipping the audio and introducing distortion. To ensure you can reduce
your input level or reduce the rear panel gain controls.
15 kHz FILTER CLIPPERS
PLEASE NOTE: In some situations clipping after 15 kHz filters can cause degradation of the pilot and stereo sub-
carrier. Our tests have concluded that when the limiter and limiter clippers are active any overshoots caused by
ringing in the filters when clipped will not contribute any significant harmonics to make the transmitter non com-
pliant. The 15 kHz clippers are set to clip at the same amplitude as the limiter clippers. In effect the 15 kHz filter
clippers are out of circuit except for instances where the 15 kHz filters overshoot caused by ringing in the filters.
The overshoots from the filters will increase the amplitude of the audio material past that of the limiter clippers clip
point and the 15 kHz clippers will clip them back down to almost the same amplitude. This setup only allows a
small amount of clipping to be applied to post 15 kHz filtered audio keeping the spectral output to an acceptable
level.
You can safely use the clippers and still conform to the FM broadcast specifications set out by the FCC and CCIT.
This is not an issue if you are using an external processor that has band-limited (15 kHz) processed audio as the
filters will not overshoot.
We don’t advise ever to use the 15 kHz clippers without the limiter clippers.
LOOPTHROUGH
The rear panel BNC sockets provide an easy interface to the multiplex path. You can insert an external piece of
equipment between the multiplex output of the internal stereo encoder and the modulator section.
To connect an external piece of equipment consult the external equipments documentation. It should be as easy
as configuring the transmitter as per normal and then simply setting the internal Loopthrough jumper to OFF and
taking the multiplex output to the input of the external equipment and the output of the external equipment back
into the multiplex input socket. If the external equipment is not unity gain you may have to readjust the multiplex
input gain slightly to take account for any gain differences. Most loopthrough designed equipment is unity gain to
make setup and use as simple as possible.
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