17
INJECTION
LEVEL ADJUST
RDS injection level (subcarrier modulation) is set with
this multiturn trim control. A detailed procedure for
setting injection level follows.
SUBCARRIER INJECTION LEVEL
The front-panel
INJECTION LEVEL ADJUST
potentiometer sets the
level of the RDS subcarrier. This is a 15-turn potentiometer to allow
the required adjustment resolution.
In the
sidechain
mode of operation, the RDS subcarrier is the only
signal that appears at the rear-panel
RDS OR MPX OUTPUT
connector. Because some exciters require a high signal level at
auxiliary wideband (SCA) inputs, the RDS signal has been made
adjustable to any value between zero and 2 volts, peak-to-peak.
In
loop-through
operation of the encoder, the composite/MPX
program signal that is fed to the
PILOT OR MPX INPUT
appears with
unity gain at the
RDS OR MPX OUTPUT
. A typical peak-to-peak
level of the Composite/MPX program signal is 3 volts. The same
0-2V RDS subcarrier level range is available in this mode, so
obviously the
INJECTION LEVEL ADJUST
control will be closer to
the counterclockwise end of its travel to give a level of 0.1 volts p-p,
which would correspond to the typical 3% to 4% RDS injection.
Setting
Subcarrier
Amplitude
Injection level is best set with a modulation monitor that includes a
subcarrier measurement utility. Alternatively, a basic mod-monitor
can be used alone, but program modulation and the stereo pilot
must be turned off to resolve the low RDS injection level.
NOTE:
If using a mod-monitor that does
not
have separate
subcarrier measurement ability, keep in mind that mod-monitors
are
peak-responding
for total modulation readings near 100%
modulation, but may be
average-responding
for measurements at
low levels of modulation. Because the RDS subcarrier is a complex,
suppressed-carrier double-sideband signal, an average reading of 3%
RDS injection can represent a level that is substantially higher when
measured peak-to-peak. It is the
peak
level we are interested in with
broadcasting.
Perhaps a better means of setting injection when a subcarrier
monitor is not available is simply to bridge the input of the exciter
with an oscilloscope using a BNC
adapter. With a normal
program input, the peak-to-peak value of the composite/MPX signal
can be noted. With this figure in mind, turn off program and pilot
modulation (but do not
disconnect
the stereo generator) and set the
peak-to-peak value of the RDS subcarrier to about 3% of the
measured composite/MPX level. When the program signal is
returned to normal, the mod-monitor should indicate a 3% increase
in total modulation, though this may be difficult to resolve near the
100% mark. Readjust the output level of the stereo generator to
fine-tune the original total-mod figure.
