
Demodulator Block
STV82x6
12/97
2
Demodulator Block
Note:
Zweiton is the Dual (Two Tone) FM stereo or A2 system.
2.1
Digital Demodulator
2.1.1
Sound IF Signal
The Analog Sound Carrier IF is connected to STV82x6 via the SIF pin. Before Analog-to-Digital
Conversion (ADC), an Automatic Gain Control (AGC) is performed to adjust the incoming IF signal
to the full scale of the ADC. A preliminary video rejection is recommended to optimize conversion
and demodulation performances. The AGC system provides a wide range of SIF input levels and is
activated for all standards, except L/L. In this particular case, the sound carrier is AM-modulated
and an automatic level adjustment would only damage transmitted audio signal. A preset I2C
parameter is required to define the gain of the AGC used in Manual mode (Registers
AGCC
and
AGCS
).
2.1.2
Demodulation
The demodulation system operates by default in Automatic mode. In this mode, the STV82x6 is able
to
identify and
demodulate any TV sound standard including NICAM and A2 systems
(see
Table 2
) without any external control via the I2C interface. It consists of the two demodulation
channels (Channel 1 = Mono Left and Channel 2 = Mono Right/Stereo) to simultaneously process
two sound carriers in order to handle all transmission modes (stereo and up to three mono
languages). The
built-in Automatic Standard Recognition System
(AUTOSTD) automatically
programs the appropriate bits in the I2C registers which are forced to Read-only mode for users (see
Section 9.1
). The programming is optimized for each standard to be identified and demodulated.
Figure 7: Demodulator Block Diagram
Mixer
Mixer
DCO2 +
Channel
Filter
FIR1
Channel
Filter
FIR2
AM
Demodulator
FM
Demodulator
FM
Demodulator
DQPSK
Demodulator
AUTOSTD
AGC
Control
SIF
NICAM
Decoder
DCO1+
Zweiton
Decoder
A/D
NICAML
NICAMR
AM/FM Mono
(To Sound Preprocessing)
AM
FML
FM Stereo
AUTO_CTRL(50h)
AUTO_SCKM(51h)
AUTO_SCKST(52h)
AUTO_STAT(54h)
DEMOD_STAT(0Dh)
ZWT_STAT(41h)
NICAM_STAT(3Fh)
CAROFFSET1(22h)
CAROFFSET2(3Ah)
Channel 1 = Mono Left
Channel 2 = Stereo/Mono Right
AGCC(0Eh)
AGCS(0Fh)
(To Sound Preprocessing)
(To Sound Preprocessing)
AGC
Amp