STV8217 datasheet(21/149 Pages) STMICROELECTRONICS

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STV82x7

Digital Demodulator

The Digital Demodulator (see Figure 10) is composed of two channels. The first channel

demodulates an FM or an AM signal. The second channel demodulates FM 2-carrier or NICAM

signals (stereo demodulation).

All channel parameters are programmed automatically by the built-in Automatic Standard

Recognition System (Autostandard) in order to find the correct sound standard. Channels can also

be programmed manually via the I²C interface for very specific standards not included among the

known standards.

3.1

Sound IF Signal

The Analog Sound Carrier IF is connected to the STV82x7 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 gain value allowing for 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 the transmitted

audio signal. A preset I²C parameter is provided to define the gain of the AGC used in Manual mode

(Registers AGC_CTRL and AGC_GAIN).

Note:

For optimum AM demodulation performance, it is recommended to use the MONO Input.

Figure 10: Demodulator Block Diagram

Mixer

DCO2 +

Channel

Filter

FIR1

Channel

Filter

FIR2

Demodulator

DQPSK

Demodulator

AUTOSTD

AGC

Control

SIF

NICAM

Decoder

DCO1+

Zweiton

A/D

NICAM L

NICAM R

AM/FM Mono

FML

FM Stereo

AUTOSTD_CTRL (8Ah)

AUTOSTD_STANDARD_DETECT (8Bh)

AUTOSTD_STEREO_DETECT (8Ch)

AUTOSTD_STATUS (8Eh)

DEMOD_STAT(0Dh)

ZWT_STAT (42h)

NICAM_STAT(3Fh)

CAROFFSET1 (22h)

CAROFFSET2 (3Ah)

Channel 1 = Mono Left

Channel 2 = Stereo/Mono Right

AGC_CTRL (0Eh)

AGC_GAIN (0Fh)

(To Sound Preprocessing)

Decoder

AGC

Amp

AUTOSTD_TIMERS (8Dh)

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