BW Broadcast technical manual
Page 25
Installation and setup
2.54 Stereo with internal limiters disabled.
The limiter can be disabled internally if you do not require it. This effectively allows any audio input signals
straight through to the stereo encoder unaltered. You may wish to disable the limiter when you have an external
processor or limiter that may be of a higher performance than the internal limiter in this transmitter. You can still
have the limiter active even with external limiters in operation, as the limiter will not discolor the sound when fed
with pre-processed audio. We leave that up to you. For safety purposes you may wish to leave the clippers on the
limiter and the 15 kHz filters in to provide extra protection when you are unsure of the peak output characteristics
of the external piece of equipment. When you know that the piece of audio processing equipment is band-limited
to 15 kHz and peak limited you can leave out the clippers safe in the assumption that the external processor will
be able to handle the level control completely.
Advanced setup procedure.
1
. Firstly make sure that the exciter is set to your chosen operating frequency. If you have not already done so
then remove the top cover from the transmitter.
2
. Connect the transmitter to a dummy load.
3
. Turn on the transmitter and within 15 seconds it should lock to frequency.
4
. Set the internal limiter to OFF with jumper J8. Set the limiter clippers to ON with jumpers J6 and J7. Set the 15
kHz filter clipper jumpers J9 and J10 to ON. Remove the pre-emphasis jumpers from J2 and J completely for
now. Make sure the loopthrough jumper J1 is switched to loopthrough and the mono from left jumper J12 is set to
the OFF position. Disable the stereo pilot by setting jumper J11 to mono. Set the limiter mode jumper J4 and J5 to
CLARITY
These settings will allow any audio straight through the limiter and into the stereo encoder without any form of
level control apart from the peak level clippers, so we can set the system internal levels correctly.
5
. We should check that any offsets internal to the limiter are nulled out to keep distortion to a minimum. Connect
a multimeter set to millivolts to testpoint TP1 which is located next to the right clipper jumper. You will also need
an earth point for the meter. The bolt on REG2 is a good earth. The case of the transmitter is not a good earth
due to the allo-chrome finish on the case. Adjust the blue multiturn pot VR in the center of the limiter section for
the minimum voltage reading on the meter. Aim for below 10 millivolts. Repeat for the left channel with VR4 and
by taking the reading from test point TP2.
6
. Connect an audio source to both channels and apply a 400Hz tone with a level of +6dBu. Make sure the gain
controls are set to maximum (Fully clockwise). These are multi-turn so make sure you hear / feel the pots click
indicating end of travel.
7
. Work out the the pilot level you intend to use. It is usually between 8 and 10 percent of the modulation with 9
percent being standard in most countries. 9 percent corresponds to 6.75 kHz deviation so for a total peak devia-
tion of 75 kHz (the industry standard for 100 percent modulation) we need to adjust the multiplex level for a peak
deviation of 68.25 kHz (91 percent modulation). This corresponds to the total minus the intended pilot level that
we will re-introduce in a moment. We recommend further backing off the deviation by 4 % which equates to
kHz deviation. This 4 % acts as a guard-band for any small overshoots that may occur in the limiter and the ster-
eo encoder filters. You should at this point with your tones applied have a peak deviation of 75 kHz - 6.75kHz (or
intended pilot level) - kHz = 65.25kHz. If you have not already done so, adjust the multiplex input level on the
back panel to this level. If you are using the LCD display metering then 65 kHz would be an acceptable setting.
8
. Re-instate the pilot by setting jumper J11 to STEREO. Now adjust the pilot level control VR6 for a peak devia-
tion of 75kHz peak deviation (100 percent modulation)
The transmitter should now have the total multiplex and the pilot setup at the correct level. All that is left to do is
to set the input gain controls to suit your external equipment.
9
. Set the limiter clippers J6 and J7 to OFF and set the filter clippers J10 and J11 to OFF.
10
. Connect a 400 Hz tone at your desired level to the left channel and adjust the left channel gain control on the
