STV8206 datasheet(12/97 Pages) STMICROELECTRONICS

12 / 97 page

Demodulator Block

STV82x6

12/97

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

DCO2 +

Channel

Filter

FIR1

Channel

Filter

FIR2

Demodulator

DQPSK

Demodulator

AUTOSTD

AGC

Control

SIF

NICAM

Decoder

DCO1+

Zweiton

A/D

NICAML

NICAMR

AM/FM Mono

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)

Decoder

AGC

Amp

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