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STV8227T

Digital Audio Decoder/Processor for A2 and NICAM Television/Video Recorders

STMicroelectronics 

STV82x7

3 Digital Demodulator

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.

Figure 10: 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)

AUTOSTD_STATUS (8Eh)

AGC

SIF

Amp

A/D

AGC

Control

AUTOSTD

DEMOD_STAT(0Dh)

ZWT_STAT (42h)

NICAM_STAT(3Fh)

AGC_CTRL (0Eh)

AGC_GAIN (0Fh)

AUTOSTD_TIMERS (8Dh)

AUTOSTD_CTRL (8Ah)

AUTOSTD_STANDARD_DETECT (8Bh)

AUTOSTD_STEREO_DETECT (8Ch)

Zweiton

Decoder

DCO2 +

Mixer

Channel

Filter

FIR2

CAROFFSET2 (3Ah)

FM

Demodulator

DQPSK

Demodulator

Channel 2 = Stereo/Mono Right

NICAM

Decoder

FM Stereo

(To Sound Preprocessing)

NICAM L

NICAM R

(To Sound Preprocessing)

3.1

Note:

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).

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

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