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Multiplex
Output
Servo
The DAVID IV employs an open-loop servo system to maintain
the multiplex output at the precise level set by the user. As ei-
ther the 19kHz stereo pilot level or the RDS injection level is
adjusted (or even turned off), the composite/MPX output re-
mains constant so that no back-and-forth tweaking is required.
RDS COMBINING PROVISION
The Radio Data System, or RDS (called the Radio
Broadcast
Data
System, or RBDS in the US), enables the FM broadcaster to
transmit certain digital ‘metadata’ along with his program au-
dio. The transmitted information includes certain mandatory
housekeeping functions that identify the station format, keep
track of rebroadcast translators, send time information, etc. It
can also display advertising or music artist and title info on the
faceplate of RDS radios.
An external encoder is required to generate the RDS subcarrier.
When RDS is enabled, the DAVID IV supplies a synchronizing
signal to lock the encoder to the stereo pilot. The DAVID IV has
a self-metering combining network to add the RDS subcarrier to
the composite/MPX output signal, and a servo to maintain con-
stant MPX peak level.
19kHz Sync
Output
The rear-panel
19kHz SYNC
BNC connector gives a TTL-
compatible square-wave output at the 19kHz stereo pilot fre-
quency when the RDS input is enabled, both in normal stereo
operation and in any of the monaural modes. This signal is
used to synchronize an RDS encoder with the stereo pilot. It is
a 5V p-p square wave with a 50% duty cycle, in-phase with the
stereo pilot component of the FM multiplex signal.
RDS Input
The
RDS INPUT
BNC connector accepts the 57kHz RDS subcar-
rier for combining with the FM multiplex program signal. The
p-p level of the subcarrier can range from 0.5V to 5V, corre-
sponding to a typical injection level approximately 5% of total
carrier deviation.
Injection
Metering
The DAVID IV has an accurate and helpful internal metering
utility for adjusting the RDS subcarrier injection. The number
shown the RDS area of the
Setup
menus is the actual injection
level as a percentage of total carrier modulation. This is quite
useful during installation of the RDS encoder, especially when a
modulation monitor that displays RDS subcarrier injection is
not available.
Setting
RDS Injection
The RDS injection level is adjusted in the
Setup
menus under
RDS Input
. But before connecting your RDS encoder, be sure
first to set the
Level:
(Web interface) or
RDS Injection
(front-
panel) slider all the way down. Note that the numbers associ-
ated with the slider do not change during this operation.