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STV6411

AUDIO/VIDEO SWITCH MATRIX

STMicroelectronics 

STV6411A

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 recommend-

ed values are 1µF, 2.2µF or 4.7µF (internal polar.

is made via a 50kW resistor). I want to point out

that the internal polarization is filtered by an exter-

nal capacitor (on Pin called `VREF'). This capacitor

contribute to good performance of the device. Its

value should be 47µF or more (coupled with an

47nF HF cap. for internal video references).

Figure 17. Audio Inputs

L/R Audio

220W 4.7m F

Audio In

12kW

STV6411A

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 10kW

(in fact a 2kW 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 10kW loads, it is

normal to use capacitors with value 5 to 10 times

the value of the input capacitors. That gives a val-

ue between 4.7µF and 47µF. Moreover it can be a

good idea to insert resistors (220W or 470W) 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.

Figure 18. Audio Outputs

Audio Out

4.7m F 470W

L/R Audio

STV6411A

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 ref-

erence (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 inter-

nal reference (close to 2V). The dynamic allowed

is more than 1.5V.

Figure 19. Video Inputs

220W

Y/C/CVBS/RGB

100nF

Video In

75W

STV6411A

2.b - Outputs

On these devices the video outputs are NOT

ABLE to drive 150W. 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 1kW resistors (more or less) and will

drive the outputs through some 75W resistors. Do

not forget to bufferize your favourite UHF modula-

tor 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).

Figure 20. Video (and Fast Blanking) Outputs

Video Out

(4.7kW )

STV6411A

75W

1kW

Video

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