STV0042 View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

STV0042

STV0042/STV0056 APPLICATION NOTE

STMicroelectronics 

STV0042/STV0056 APPLICATION NOTE

3 - APPLICATION NOTES (continued)

Figure 6 : Video Input Clamp Function

Filtered

Unclamped

Video

I2

A

C11

I1

2.7V

Video

∆

Ts

T

I CH = I2 - I 1 during Ts

I CH = - I1 during T - Ts

3.1.2.2 - Switching Matrix

This switching matrix is simply an array of CMOS

switches which is driven by registers. All configura-

tions are possible. Any output can be connected to

any input. An input can be connected to several

outputs.

3.1.2.3 - Output Stages

Each video output has a 6dB output gain, and the

internal final stage is a emitter follower structure

pulled down with a 1.3mA current source.

The output voltage amplitude is 2VPP, and the

synchro bottom level is clamped at 1.3V typically.

In the application, an external current amplifier

(75Ω driver) is required.

Remarks :

- The S3VIDOUT of the STV0056 features a black

level adjust (corresponding to a DC level adjust).

This function may be used to more simply inter-

face with built-in Videocrypt decoder.

- When the high 2 low power mode is selected, the

output impedance becomes high (≈ 23kΩ typ.),

and the DC output levels drops to low values

(< 0.2V) consequently the external 75Ω driver is

also turned (power saving).

3.2 - FM Demodulation

3.2.1 - Hardware Description

This chapter is relevant to :

- the input filter,

- a study of the FM demodulatorwhich is integrated

in the STV42/56 circuits (including the external

components needed at Pins DET L/R, AM-

PLOCK L/R, CPUMP L/R, AGC L/R).

3.2.1.1 - Input Filter (C25, R18, R17, L4, C24, C23)

The FM demodulator integrated in the STV42/56

circuit havea goodrejection of the video signal (due

to the selective AGC stage see paragraph 3.2.1.2).

However, to get a good audio signal to noise ratio,

it is preferable to attenuate the video signal spec-

trum with an input filter.

The filter proposed in the typical application (see

Figure 7) contains a high pass cell (C25, R18,

fC ≈ 1.5MHz) and a chroma trap (R17, L4, C24).

C23 is required for capacitive coupling with the

FM IN Pin (C23 value is not critical).

The center frequency of the chroma trap is about

4.4MHz for PAL/SECAM application and 3.58MHz

in case of NTSC application (Remark : In case of

PAL/NTSC application, a double trap may be re-

quired.)

It is recommanded to directly connect the input filter

to the tuner baseband output,because at this stage

the video signal is still emphazed (giving a natural

14dB attenuation of the lower part of the video

spectrum where most of the energy is concen-

trated.

3.2.1.2 - FM Demodulators

The block diagram of the FM demodulation part is

given in Figure 8. This block diagram contains 2

independants FM demodulators able to process all

types of mono and stereo sound, including :

- mono signals with deviation from ± 30kHz up to

± 400kHz (remark the spec mentions : ± 19.5kHz

to ± 592kHz, in order to cover all possible disper-

sions).

- Stereo pair featuring a frequency spacing differ-

ent from 180kHz (between left and right sub-car-

riers).

Each FM demodulator section contains 2 parts :

- an automatically gain controlled input stage

- a PLL demodulator

The only common point to both FM demodulators

is the frequency synthesis circuit : SYNTHE-

SIZER.

The structure of the FM demodulators which is

different from conventional solutions have been

selected for the two following reasons :

- no need for costly selective filters(LC or ceramic),

- to have a variable bandwidth (to accomodate all

type of deviations).

7/37

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