Daewoo DV-K882SY-AN, Service Manual

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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 controlling. 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.
4. DIGITAL AUDIO FLOW
The demodulator function includes integrated baseband filters for pulse shaping and unwanted signal rejection, automatic
gain control, a low jitter integrated VCO.
the decoder function performs the descrambling, de-interleaving and reformatting operations required to recover the
original data words.
The data words are processed through a stereo digital filter, digital de-emphasis network, second order noise shaper, and
256 times over sampling bitstream audio DAC.
The whole device supported by 8.192MHX crystal oscillator, and I
2 C serial data bus for communications with µ-com.
In the output stage out selector can automute the nicam signal to normal audio when the digital data have noise more
than error rate.
It also can be switched manually by user.
PRE-
AMP
SAW
FILTER
IF
AMP
AM
DE MOD
O db
LIMITER
AMP
L/L' control PAL SOUND
Standard S/W
AUDIO OUT
mute
BASE BAND
FILTER
&
AGC GAIN
STAGE
PHASE DETECT
&
DATA
SLICERS
NICAM728
DECODER &
DEVICE
CONTROLLER
DIGITAL
FILTER
&
NOISE
SHAPER
BITSTREAM
DAC
OUT S/W
&
BUFFER
OUT.L
OUT.R
DQPSK
I C DATA X-TAL NORMAL AUDIO
DECODER DEMODULATOR
2