AUDIO SIGNAL PATH
Audio Signal Processing (Figure 19)
All Audio Processing is performed by 7651,
which is located on the SSB. The Sound IF is
fed to Pin 50 and to the Sound Demodulator
for decoding. The internal Switch selects
between AV1, AV2, and AV3. The Audio is
digitally processed inside the IC to separate
the Second Audio Program and process
volume levels to maintain a constant audio
level if the AVL feature is selected by the
user. The AUDIO-L and AUDIO-R are fed to
the audio amplifier on the Large Signal Board.
The Headphone output on Pins 18 and 17 is
fed to the Headphone amplifier on the LSB.
SC1_OUT-L and SC1_OUT_R on Pins 28 and
29 are fed to the monitor audio jacks on the
LSB. The AUDIO-SW, Subwoofer output, is
fed to the LSB and then to the Subwoofer
module. Audio frequencies above 100 Hz are
filtered out for this output. Volume levels are
controlled in the Audio Processor IC.
Audio Amplifier (Figure 20)
Audio from the SSB is fed to the Audio
Amplifier on the LSB. Left Channel Audio is
fed to Pin 2 of 7702 while Right Channel
Audio is fed to Pin 5. Audio is output to the
speakers on Pins 12 and 7. Left and Right
outputs are also fed to the Surround Sound
Speaker Jack. The output of the Surround
Jacks is the difference between the Left and
Right Channels. The Amplifier is powered
by a 27 volt supply which is present as
soon as power is applied to the set. When
the set is in the Stand By mode, a High on
the Stand By line turns transistors 7710
and 7704 On placing a Low on Pins 11 and
3 of 7702. This turns the output stage of
7702 Off during Stand By. When the
User selects the Mute function, the
SOUND-ENABLE line goes Low, turning
transistor 7701 Off. Pin 8 of 7702 will go
High, muting drive to the output stage.
When Sound is present, the SOUND-
ENABLE line is High. When Power is
removed from the set, the POWER-DOWN
line goes Low, turning transistor 7711 On,
causing the SOUND-ENABLE line to go
Low. This mutes the audio output during
Power Down to prevent a POP in the
speakers.
Headphone Amplifier (Figure 21)
Headphone audio from the SSB is fed to
the Headphone Amplifier, 7703-A, located
on the LSB. The Audio is then output to
the Side Jack panel. This circuit is
powered by the +5 volt supply.
Page 24
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