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10
PRE-EMPHASIS SELECTION
The Model 708 Stereo Generator supports the 75-microsecond FM
broadcasting pre-emphasis standard common to the Western
Hemisphere and parts of the Orient, and the 50-microsecond standard
used in Europe and Asia. Pre-emphasis appropriate to the shipping
destination is jumpered at the factory, but this is easily changed if
necessary.
Beneath the top cover and about two inches behind the input barrier
strip are two jumper pin strips, each with a push-on jumper. These are
identified as
JMP1
and
JMP4
in the circuit board legend, and each is
designated with a
75µs
and a
50µs
marking to indicate proper jumper
placement for the two available pre-emphasis characteristics. Figure 3
illustrates the jumpering options
75-MICROSECOND
50-MICROSECOND
Figure 3 -
Pre-emphasis Jumpering
SUBCARRIER INPUTS
Internal combining is provided for two auxiliary subcarriers for SCA
and/or RDS services. The rear-panel
SUBCARRIER INPUT
connectors
are unbalanced inputs with a load impedance in the neighborhood of
10K-ohms.
An RDS subcarrier normally accounts for 5% or less of the total
composite multiplex output signal, making it 26dB or more below peak
program modulation. SCA subcarriers typically have a higher injection
level, about 10%, or 20dB below program peak deviation. Subcarrier
inputs at a level of 0dBu or greater may be fed to the
SUBCARRIER
INPUT
connectors, and can be regulated to the proper injection level by
the front-panel
SUB INJ.
level controls.
Front-panel metering of the RDS or SCA subcarrier levels allows
accurate adjustment of the injection level(s), even in the absence of a
Modulation Monitor with a specific display of subcarrier injection.