3. Stereo Decoder and Blend (Cont.)
blend and high frequency rolloff circuit controlled
by the IF AGC voltage at pins 7 and 5. This
feature improves the S/N of a very weak stereo
station by gradually reducing the separation and
high frequency response. The blend threshold is
set by R43.
D. AUDIO CIRCUIT
1.Filter/Amplifier - The filter/amplifier IC, U7,
contains an active high pass-low pass filter which
is utilized in the AM mode using input pins 15 and
17. Internal diodes allow the chosen audio to pass
but the other mode is blocked by reverse biased
diodes. The dual audio amplifier is used for both
AM and FM.
2. Dynamic Noise Reduction - The noise reduction is
accomplished in U106. The IC contains circuitry
which gives up to 10 db of effective noise
reduction by varying the audio bandwidth. When
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. The
opening of the bandwidth takes only 500 usec. When
the bandwidth is open, the noise is masked by the
program material. The bandwidth closes back down
60 msec after the high frequency material in the
program is removed. The program material is
distinguished from the noise by a dynamic level
threshold detector. Resistors R132 and R133 set
the threshold at which the desired audio is
separated from noise. When noise reduction is
turned off, pin 2 of U102 pulls pin 9 of U106 to
ground, causing a constantly wide audio bandwidth,
and no noise reduction. The effect of the noise
reduction is greatest on program material
containing quiet spots and low modulation.
3. Electronic - Volume, Tone, Balance and Fader - The
audio inputs to the Audio Control Circuit, U105,
come from three sources. The internal source
selector selects the proper source and rejects the
others. The input source from the CD player
(Infinity only) is applied through Cl63 to pin 8
and through Cl62 to pin 21. The input source from
the tape player is applied direct to pin 12 and to
pin 17. The input source for the radio is applied
l-21 |
ACEC
