Daewoo DV-F24S, Service Manual

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2. PAL AUDIO FLOW
The FM sound intercarrier signal passing through the 5.5MHz sound BPF is fed to a limiter amplifier before it is
demodulated.
This gives high sensitivity and AM suppression. The limiter amplifier consists of seven internal AC-coupled stages,
minimizing the DC offset.
The FM-PLL demodulator consists of an RC-oscillator, loop filter and phase detector. The oscillator frequency is locked
on to the FM intercarrier signal from the limiter amplifier. As a result of this locking, the RC-oscillator is frequency
modulated. The modulating signal voltage is used to control the oscillator frequency using this technique, the FM-PLL
works as a FM demodulator. The AF signal coming out of the FM-PLL demodulator is amplified and buffered in an output
stage.
3. SECAM AUDIO FLOW
Because the SECAM TV sound system uses amplitude modulation, we need an AM-sound demodulation process. The
sound IF signal passing through the SAW filter is fed to the sound IF amplifier which consists of three AC-coupled
differential amplifier stages each with about 20dB gain.
At the output of each stage is a multiplier for gain control. The automatic gain control voltage, used to maintain the AM
demodulator output signal at a constant level, is generated by a mean level detector. This AGC detector charges and
discharges the AGC capacitor controlled by the output signal of the AM demodulator which is compared to an internal
reference voltage.
The IF amplifier output signal is fed to a limiting amplifier.
The limiter output signal (which is no longer AM modulated) is also fed to the multiplier. In this way we get AM
demodulation (in phase demodulation).
After lowpass filtering (400KHz) for carrier rejection and buffering, the demodulator output signal is fed to an operational
amplifier with three input stages and 0dB gain. One input is SECAM sound, the other is PAL sound. A sound mute control
signal can also be used to mute the OP amplifier output.
VIDEO DET
& DEMOD
SOUND
BPF
intercarrier
VIDEO BLOCK
LIMITER
AMP
FM PLL-
DEMOD
AF
AMP
AUDIO OUT
PRE-
AMP
SAW
FILTER
IF
AMP
AM
DE MOD
O dB
LIMITER
AMP
L/L' control PAL SOUND
Standard S/W
AUDIO OUT
mute