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The On-Screen
RDS Level
Display
If the peak-to-peak level of the composite/MPX for 100% carrier
deviation is known, the RDS level may be set with reasonable
accuracy using the built-in subcarrier measurement utility of
the 730. The composite/MPX is easily measured by bridging
the input of the exciter with an oscilloscope using a BNC “T”
adapter. Monitoring normal program material that has passed
through a typical audio processor, the 100%-modulation value
will be apparent on the ‘scope. Take this peak-to-peak figure
and multiply it by 3 to 4 percent, and then set the injection to
the calculated value. For example, if the composite/MPX level
is 4V p-p at 100% modulation, set the RDS level to 4 X .035, or
0.14V p-p for 3.5% injection.
RDS injection is not particularly critical, but should be main-
tained in the 3% to 5% range. Too little or too much injection
will make messaging unreliable.
In the United States, the FCC permits an extra 5% total carrier
modulation for each added FM subcarrier, with total modula-
tion not to exceed 110% with two or more subcarriers. Consult
current local regulations regarding this matter.
Subcarrier Phase
According to the RDS and RDBS Specifications, the 57kHz RDS
subcarrier may be either in phase or in quadrature with the
19kHz stereo pilot. These two phase relationships are shown
in Figures 5 and 6, respectively. Note that the quadrature rela-
tionship in Figure 6 yields a slightly lower peak level, which
suggests the possibility of marginally greater carrier modula-
tion by the program audio signal.
Figure 5 –
Subcarrier In Phase
Figure 6 –
Subcarrier In Quadrature
The 730 Encoder is skillfully and meticulously aligned at the
factory to ensure that the subcarrier has the proper phase rela-
tionship with the stereo pilot, whether the encoder is used in
the sidechain or in the loop-through operating mode. Although
some stereo generators are equipped to supply a pilot-sync ref-
erence for RDS (usually a TTL-level squarewave), this reference
may or may not be in exact phase with the actual stereo pilot
component in the composite/MPX signal. For this reason we
recommend using the composite/MPX signal for synchroniza-
tion. In the sidechain mode, the 730 Encoder simply bridges
the output of the stereo generator and does not load or other-
wise compromise the composite/MPX signal.
