General Instrument C6M-II, Installation Manual

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4-2 System Block Diagrams
C6M-II Installation Manual
Audio to IF Modulation System
Figure 4-2
Audio to IF modulation system
A Left and right audio signals are encoded per BTSC standards when the stereo encoder option is
installed. If the modulator option board is installed, audio switching is provided. The audio input
will follow the switched video input.
B The audio input range is selected from the Audio range (AUDR) entry on the Audio menu. Either
the low range (AUDR LOW) which is –10 to +10 dBm, or the high range (AUDR HI) which is +5 to
+25 dBm, may be selected.
C The audio input impedance is selected from the Audio impedance (AUDZ) entry on the Audio
menu. Either low impedance (AUDZ LOW) which is 600 Ohms balanced, or high impedance
(AUDZ HI) which is 15 kohms, may be selected.
D A variable gain stage sets the FM deviation to the required
±
25 kHz. This can be adjusted through
the Audio deviation entry on the Main menu.
E An optional AGC capability is provided for situations involving an unstable audio source or
switching between audio sources of different levels. This capability is enabled through the Audio
AGC entry on the Audio menu.
F The standard broadcast television pre-emphasis can be disabled.
G Frequency modulation is accomplished by applying the processed audio signal to a 9.0 MHz
voltage controlled oscillator (VCO). The center frequency of the sound carrier is precisely
maintained by a phase-locked loop with an internal 2.0 MHz reference.
H The audio VCO’s frequency is divided by two to produce the standard 4.5 MHz aural subcarrier