STV82X6 Datasheet, PDF (10/95 Pages) STMicroelectronics – Multistandard TV Audio Processor and Digital Sound Demodulator

Demodulator Block
2 Demodulator Block
STV82x6
Figure 6: Demodulator Block Diagram
Channel 1 = Mono Left
DCO1+
Mixer
Channel
Filter
FIR1
CAROFFSET1(22h)
AM
AM
Demodulator
FM
Demodulator _FML
AM/FM Mono
(To Sound Preprocessing)
AGC
SIF
Amp
A/D
AUTO_STAT(54h)
AUTOSTD
AGC
Control
AGCC(0Eh)
AGCS(0Fh)
AUTO_CTRL(50h)
AUTO_SCKM(51h)
AUTO_SCKST(52h)
DCO2 +
Mixer
Channel
Filter
FIR2
CAROFFSET2(3Ah)
DEMOD_STAT(0Dh)
ZWT_STAT(41h)
NICAM_STAT(3Fh)
Zweiton
Decoder
FM
Demodulator
DQPSK
Demodulator
NICAM
Decoder
FM Stereo
(To Sound Preprocessing)
NICAML
NICAMR
(To Sound Preprocessing)
Channel 2 = Stereo/Mono Right
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.
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