STV0056 datasheet(10/27 Pages) STMICROELECTRONICS

10 / 27 page

CIRCUIT DESCRIPTION

Video Section

The composite video is first set to a standard level

by means of a 64 step gain controlled amplifier. In

the case that themodulation is negative,an inverter

can be switched in.

One of two different external video de-emphasis

networks (for instance PAL and NTSC) is select-

able by an integrated bus controlled switch.

Then energy dispersal is removed by a sync tip

clamping circuit, which is used on all inputs to a

video switching matrix, thus making sure that no

DC steps occur when switching video sources.

The matrix can be used to feed video to and from

decoders, VCR’s and TV’s.

A bus controlled black level adjustment circuit is

provided on the decoder output allowing a direct

connection to an on-board Videocrypt decoder.

Additionaly all the video outputs are tristate type

(high impedance mode is supported), allowing a

simple parallel connections to the scarts (Twin

tuner applications).

Audio Section

The two audio channels are totally independent

except for the possibility given to output on both

channelsonly one of the selectedinput audio chan-

nels.

To allow a very cost effective application, each

channel uses PLL demodulation. Neither external

complex filter nor ceramic filters are needed.

The frequency of the demodulated subcarrier is

chosen by a frequency synthesizer which sets the

frequency of the internal local oscillator by com-

paring

its phase with the internally generated

reference. When the frequency is reached, the

microprocessor switches in the PLL and the de-

modulationstarts. Atany moment the microproces-

sor can read from the device (watchdog registers)

the actual frequency to which the PLL is locked. It

can also verify that a carrier is present at the wanted

frequency(by reading AMPLK status bit) thanks to

a synchronous amplitude detector, which is also

used for the audio input AGC.

In order to maintain constant amplitude of the

recovered audio regardless of variations between

satellites or subcarriers, the PLL loop gain may be

programmed from 56 values.

Any frequency deviation can be accomodated

(from

±30kHz till ±400kHz).

Two different networks can be permanently con-

nected for either 75

µs or J17 de-emphasis. If 50µs

de-emphasisis required, this can be inserted by an

internal switch, thus allowing a worldwide applica-

tion.

The STV0056AF is intended to be compatible with

Wegener Panda System.

Two types of audio outputs are provided : one is a

fixed 1VRMS and the other is a gain controlled

2VRMS max. The control range being from +12dB

to -26.75dBwith 1.25dB steps. This output can also

be muted.

A matrix is implemented to feed audio to and from

decoders VCR’s and TV’s.

Noise reduction system and de-emphasis can be

inserted or by-passed through bus control.

Also all the audio outputs are tristate-type (high

impedance mode is supported), allowing a simple

parallel connections to the scarts (Twin tuner appli-

cations).

Others

A 22kHz tone is generated for LNB control.

It is selectable by bus control and available on one

of the two pins connected to the external video

de-emphasis networks. One general purpose I/O

is also available on the STV0056AF.

By means of the I

2C bus there is the possibility to

drive the ICs into a low power consumption mode

with active audio and video matrixes. Inde-

pendantly from the main power mode, each indi-

vidual audio and video output can be driven to high

impedance mode.

STV0056AF

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