BW Broadcast TX1000, Technical Manual

Page: 24 / 56

BW Broadcast technical manual
Page 24
Installation and setup
6 . 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 3
kHz deviation. This 4 % acts as a guard-band for any small overshoots that may occur in the limiter and the ste-
reo encoder filters. You should at this point with your tones applied have a peak deviation of 75 kHz - 6.75kHz (or
intended pilot level) - 3 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 J6 to STEREO. Now adjust the pilot level control VR4 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.
9 . Connect your studio feed to the transmitter. Turn the power off to the transmitter, connect your external anten-
na, reconnect the power and your on the air!
10 . 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.
2.54 Mono
Mono from one channel input: You can provide one audio feed to the transmitter via the left balanced input. This
has the advantage of supplying two exact audio signals to the stereo encoder section that will in turn cancel the
stereo subcarrier leaving only a pure mono signal with content falling to zero above 15 kHz.
Mono from two independent channels: A stereo feed can be supplied to the transmitter’s left and right inputs. If
DIP switch 3 is set to mono these will be summer to mono, ensuring the subcarrier contect is kept to a minimum.
The absence of the pilot will force tuners to decode the broadcast in mono.
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 10 seconds it should lock to frequency.
4
. Turn the pre-emphasis off for now by setting DIP switch 1 to the right. Make sure the loopthrough jumper J5 is
switched to loopthrough. Set DIP switch 3 to mono and disable the stereo pilot by setting jumper J6 to mono.
5
. Connect an audio source to both channels and apply a 400Hz tone with a level of +6 dbu
6
. Adjust the multiplex gain control on the back panel for a peak deviation of 72 kHz. This allows a 3 kHz guard-
band for slight overshoots that may occur in limiter and filters inside the system.
7
. If your audio feed has gone through an external processor prior to this transmitter then check to see if that unit
has pre-emphasis capability and if it is switched on. If it has pre-emphasis and it is enabled then you should leave
the pre-emphasis DIP switch, set to off to ensure that only one set of pre-emphasis has been applied throughout
the broadcast chain. Otherwise, switch it back to on and set DIP switch 2 to the correct setting for your region,
75µs for the Americas and Japan and 50µs for the rest of the world.