The wire-line system is illustrated in block diagram form in Figure 1,
and the elementary radio system is illustrated in Figure 2.
B. Wireless FM
If the telephone line interconnection is not used, the control signals
are sent to the transmitter site by using subcarrier techniques. As
shown in Figure 2, the control generator at the studio frequency-
modulates a subcarrier generator. The output of this subcarrier
generator is then applied to the multiplex or subcarrier input of an
STL (microwave) transmitter. In this manner, the control signals
“ride piggyback” on the STL going to the transmitter site. This sub-
carrier generator is a standard addition to the studio unit of the
TRC-15A.R for wireless operation.
At the transmitter site the subcarrier output from the STL receiver
is applied to a subcarrier demodulator located in the TRC-15AR trans-
mitter unit. The output from this demodulator is a replica of the
control signal which originated at the studio. This signal is then
internally applied to the control demodulator in the TRC-15AR as in
the basic system. The subcarrier demodulator at the transmitter
site is a standard addition to the transmitter unit of the TRC-15AR for
wireless operation.
In a similar manner, the metering signals may be returned from the
transmitter to the studio by using a subcarrier. As shown in Figure 3,
the metering generator frequency-modulates a subcarrier generator
at the transmitter site. This subcarrier generator is usually a part
of the main FM transmitter. As with the control signals, the meter-
ing signal now rides piggyback on the broadcast carrier in the form
of an SCA signal which is then received at the studio. Here the sub-
carrier is extracted and demodulated by an SCA receiver. The out-
put of this receiver or SCA demodulator is a replica of the metering
signal generated at the transmitter site.
When the metering signal is returned to the studio in this manner, it
is customary to use the frequency range of 20 Hz to 30 Hz for the
metering signal. In this way, program material such as background
music may also be broadcast on the SCA subcarrier. It has been
found in practice that there is little interaction between the two. This
low-frequency metering signal, which is referred to as the subaudible
metering signal, is usually adjusted to modulate the SCA subcarrier
about 14 dB below program. Because it is in the low audio spectrum
and because it modulates the subcarrier at a low level, it causes
little degradation of the music service. By the same token, low-pass
TRC-15A -4-
