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Stereo Decoder and Blend (Cont.)
3.
blend and high frequency rolloff circuit controlled
by the IF AGC voltage at pins 7 and 5.
feature improves the S/N of
station by gradually reducing the separation and
high frequency response.
set by R43.
D.
AUDIO CIRCUIT
Filter/Amplifier
1.
contains an active high pass-low pass filter which
is utilized in the AM mode using input pins 15 and
17.
but the other mode is blocked by reverse biased
diodes.
AM and
Dynamic Noise Reduction - The noise reduction is
2.
accomplished in U106.
which
reduction by varying the audio bandwidth.
there is no program material present, the audio
bandwidth is electronically narrowed to cut out the
mid and high frequency component of the noise.
When music or speech with high frequency material
is present, the audio bandwidth is widened so that
the audio fidelity is not noticeably affected.
opening of the bandwidth takes only 500 usec.
the bandwidth is open, the noise is masked by the
program material.
60 msec after the high frequency material in the
program is
distinguished from the noise by a dynamic level
threshold detector.
the
separated from noise.
turned off, pin 2 of U102 pulls pin 9 of U106 to
ground, causing a constantly wide audio bandwidth,
and no noise reduction.
reduction
containing quiet spots and low modulation.
3.
Electronic - Volume, Tone, Balance and Fader - The
audio inputs to the Audio Control Circuit,
come
selector selects the proper source and rejects the
others.
(Infinity only) is applied through Cl63 to pin 8
and through Cl62 to pin 21.
the tape player is applied direct to pin 12 and to
pin 17.
- The filter/amplifier IC, U7,
Internal diodes allow the chosen audio to pass
The dual audio amplifier is used for both
FM.
gives up to
removed.
threshold at
is
greatest
from three sources.
The
input
The input
a
The blend threshold is
The IC contains circuitry
10 db of effective
The bandwidth closes back down
The program material is
Resistors R132 and R133 set
which
the
desired
When noise reduction is
The effect of the noise
on
program
The internal
source from the CD player
The input source from
for the radio is applied
source
very weak stereo
noise
audio is
material
U105,
source
This
When
The
When
l-21
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