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In low band systems, the four outputs are sent to four driver modules, which in turn
are used to drive four PA modules. These four outputs drive four 15 kW visual PA
cabinets (see 15 kW system, above).
In high band transmitters, the four phase and gain module outputs to four driver
modules. Each driver module output is split using a two-way divider, whose outputs
each drive a PA module. The PA module outputs are recombined using four two-way
combiners, before passing to the four 15 kW PA cabinets (see 15 kW system, above).
The visual cabinet outputs are recombined in pairs, using two 3 dB quadrature
hybrids. Finally, the pairs are recombined with a final 3 dB hybrid.
1.2.10 Aural Signal Path
Again, the basic topologies of the aural paths in the various models are similar. As in the
visual path, the exciter aural output passes to an AGC module which monitors a sample of
the aural System output. The AGC module output then drives the aural RF chain. (Refer to
the transmitter block diagram in the drawing package for this discussion.)
1.2.10.1 Exciter
Either monaural audio and SCA, or externally generated composite stereo, is fed to the
exciter and modulated onto an IF frequency, which is lower than that of the visual IF by an
amount equal to the difference between the desired aural and visual carriers. The modulator
is a voltage-controlled oscillator whose center frequency is held constant by a phase-locked
loop (PLL).
IF group delay correction (optional) can be used at this point to improve stereo separation
in systems where notch diplexers are used. The IF signal is converted up to channel using a
mixer and the same LO as in the visual chain. The resulting signal is bandpass filtered and
amplified, becoming the exciter’s aural output. As in the visual path, if dual exciters
(optional) are used, each exciter’s aural output is routed to the exciter switcher.
1.2.10.2 Dual-Carrier Systems
In systems where dual aural carriers are generated, the exciter path takes a different form.
The two signals are modulated onto two different IF carriers, and the modulated carriers are
added together. Linearity pre-correction is added to prevent intermodulation of the two
carriers. The resulting signal is mixed up to channel with the same LO as used in the visual
chain, and is bandpass filtered and amplified, becoming the exciter output.
1.2.10.3 AGC Module
As in the visual path, the exciter switcher output or single exciter feeds an AGC module,
which holds the aural transmitter gain constant by controlling aural RF drive based on
samples of exciter drive and transmitter aural output.
1.2.10.4 Phase and Gain Module
