Broadcast Warehouse TX 25/50, Техническое руководство

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TX25/50 FM Transmitter technical manual
page 19
Installation and setup
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 VR3 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 +6 dbu. Make sure the gain
controls are set to maximum (Fully clockwise). These are multiturn 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 3
KHz deviation. This 4 % acts as a guard-band for any small overshoots that may occur in the limiter and the
stereo 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 set-
ting.
8 . Reinstate the pilot by setting jumper J11 to stereo. Now adjust the pilot level control VR6 for a peak deviation of
The previous setting plus the intended pilot level. In our example this would mean setting the deviation at 72KHz.
This equates to our modulation and pilot leaving 3 KHz for our guard-band.
9 . Set the limiter to ON with jumper J8. Decide on Clarity or loud mode for the limiter and set jumpers J4 and J5
accordingly.
The transmitter should now have the multiplex and pilot setup at the correct level. All that is left to do is to set the
input gain controls to your desired settings. For most applications where you require a compressed loud competi-
tive sound we recommend setting the gain controls at maximum. But if you are supplying a very high level feed
you may have to turn the gain controls down to stop internal clipping occurring prior to the limiter. (Please see
note on pre-emphasis at the end of the section). You may also wish the limiter to start to limit for a given level of
input. To set the limiters input controls up for a given input level consult step 10. Otherwise leave the input gain
controls at maximum and skip to step 12.
10 . Apply your audio feed to the left channel input socket on the rear panel at the desired level you want limiting
to commence. This will need to be a minimum of 0dbu. The limiter inputs will need at least a 0dbu input to
achieve limiting even with the rear panel gain controls set at maximum. Read the pre-emphasis note at the end of
the section for more info on input level restrictions.
Set the LCD display to limiter gain reduction and with audio applied to the left channel reduce the left channel
input gain control until the display flickers from 0dB to 2dB gain reduction.This is the onset of limiting.
11 . Remove the left channel audio and repeat the procedure for the right channel. This procedure also ensures
that both inputs to the internal limiter are driven equally.
12 . 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. 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-empha-
sis and it is enabled then you should remove the pre-emphasis jumpers J2 and J3 to ensure that only one set of
pre-emphasis has been applied throughout the broadcast chain.
13 . Connect your studio feed to the transmitter. Turn the power off to the transmitter, connect your external
antenna, reconnect the power and you’re 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.