Broadcast Warehouse TX 150/300, Technical Manual, Page: 21 / 41

TX FM Transmitter technical manual
page 21
Installation and setup
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
rear panel for a peak deviation of 75 KHz. Remove the left channel audio lead.
11 . Connect a 400 Hz tone at your desired level to the right channel and adjust the right channel gain control on
the rear panel for a peak deviation of 75 KHz.
It is a good idea to check that both of the audio input gain controls are set to provide equal gain to both channels.
To do this connect an oscilloscope to the multiplex output socket. Connect an identical 400Hz tone to both audio
inputs and set the pilot to off by setting J11 to MONO. Adjust one of the audio input gain controls slightly to mini-
mize any distortion of the 400Hz tone observed on the scope. The distortion is 38 KHz switching information
superimposed onto the sine wave. When you have adjusted the input gain control to provide the same gain as the
other channel you will observe a pure sine wave with no switching information present. This is because if R=L
then L-R=0 and the subcarrier is L-R, so no gain difference between R and L equates to no 38 KHz subcarrier.
Reinstate the Pilot by setting jumper J11 to STEREO.
You can set the limiter clippers and the 15KHz filter clippers back to ON if you require. These will prevent over-
modulation no matter what input level you apply.
12 . If you wish to employ pre-emphasis within the transmitter then you will need to setup the transmitter for a
peak deviation of 75KHz with 15KHz tones instead of 400Hz. This is to prevent overmodulation of the transmitter
from high frequencies, which would be the case if we set the transmitter up for 75KHz peak deviation at 400Hz.
The pre-emphasis filter can put the 15KHz audio 17db higher than 400Hz. We strongly advise you to ensure pre-
emphasis is built into any external equipment if you decide not to use the internal limiter. If you do need to use the
internal pre-emphasis then you can set the pre-emphasis jumpers J2 and J3 to the correct setting for your region:
75uS for the Americas and Japan and 50uS for the rest of the world.
13 . Connect your studio feed to the transmitter. Turn the power off to the transmitter, connect your external
antenna, reconnect the power and your on the air!
14 . 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.
2.55 Mono from two independent channels.
This is essentially the same as the stereo with limiter mode except the stereo pilot is disabled which will enable
receivers to receive you in mono. A stereo feed can be supplied to the transmitter’s left and right inputs. These
will be mixed in the stereo encoder as normal to produce a multiplex signal. The absence of the pilot will force
tuners to decode the broadcast in mono. If both the right and left channels have the same content then no stereo
subcarrier will be created and the multiplex signal will resemble true mono. However we recommend mono from
one channel input mode if both channels are identical to ensure that the subcarrier content is kept to a minimum.
This mode is for when you want to broadcast in mono but have a stereo sound source that can’t be supplied as a
single mono feed, such as a stereo mixer with no mono button.
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.