STV6411 datasheet(18/20 Pages) STMICROELECTRONICS

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APPLICATION NOTE

1 - Audio Part

1.a - Inputs

The audio inputs are designed to follow sources up

to (at least) 2VRMS (that is around 6VPP) with an

expected DC level of VCC/2 (4V typ.). That’s why

the device is providing this DC polarization. That

means that in most of the cases the inputs are AC

coupled via chemical capacitors. The recom-

mended values are 1

µF, 2.2µF or 4.7µF (internal

polar. is made via a 50k

Ω resistor). I want to point

out that the internal polarization is filtered by an

external capacitor (on Pin called ‘VREF’). This ca-

pacitor contribute to good performance of the de-

vice. Its value should be 47

µF or more (coupled

with an 47nF HF cap. for internal video references).

220

4.7

m F

12k

Audio In

STV6411A

L/R Audio

Figure 14 : Audio Inputs

NB: In some particular cases (loopback from out-

puts to inputs) the AC coupling capacitor can be

removed... but some small offsets in the audio

chain can cause some noise while switching from

one input to another.

1.b - Outputs

Audio output buffers are able to provide more than

2.1VRMS (around 6VPP) on a typical load of 10k

Ω

(in fact a 2k

Ω load is acceptable). The DC level is

once more VCC/2 for best dynamic performance.

Usually some AC coupling capacitors are used at

the outputs. To drive some typical 10k

Ω loads, it is

normal to use capacitors with value 5 to 10 times

the value of the input capacitors. That gives a value

between 4.7

µF and 47µF. Moreover it can be a

good idea to insert resistors (220

Ω or 470Ω) in the

audio outputs. That will provide a protection for

output stages. No external drivers or buffers are

needed in typical use of the device.

2 - Video Part

2.a - Inputs

Video inputs need to be AC coupled. But only some

small capacitor values are requested thanks to the

internal clamps provided by these devices. Usually

some 100nF HF capacitors (47nF to 220nF) are

enough to provide good performances on Y,

CVBS,RGB and C inputs.

Chrominance inputs : - average clamp - that means

that the DC is measured as the average value of the

input signal and set to an internal reference (close

to 3V). The dynamic allowed is more than 1.5V.

RGB, Y, CVBS inputs : - bottom sync top clamp -

that means that the DC level is measured at the

lowest value of the input signal and set to an

internal reference (close to 2V). The dynamic al-

lowed is more than 1.5V.

470

4.7

m F

STV6411A

L/R Audio

Audio Out

Figure 15 : Audio Outputs

220

100nF

Video In

STV6411A

Y/C/CVBS/RGB

Figure 16 : Video Inputs

2.b - Outputs

On these devices the video outputs are NOT ABLE

to drive 150

Ω. That means that external buffers (one

simple NPN-Transistor per output) are needed. To

reduce the external components, the output DC

level have been chosen to allow a direct drive of the

base of the output follower (NPN). The emitters of

the NPNs will be polarized to ground via 1k

Ω resis-

tors (more or less) and will drive the outputs through

some 75

Ω resistors. Do not forget to bufferize your

favourite UHF modulator video input...

Chrominance outputs have a DC of 2.3V (it is an

average value) and Luminance type output have a

DC of 1.3V (it is a bottom value).

(4.7k

W )1kW

STV6411A

Video Out

Video

Figure 17 : Video (and Fast Blanking) Outputs

STV6411A

18/20

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