STV6411A ST Microelectronics, STV6411A Datasheet - Page 18

• Manufacturer Part Number: STV6411A
• Description: AUDIO/VIDEO SWITCH MATRIX
• Manufacturer: ST Microelectronics

STV6411A
APPLICATION NOTE
1 - Audio Part
1.a - Inputs
The audio inputs are designed to follow sources up
to (at least) 2V
expected DC level of V
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 F, 2.2 F or 4.7 F (internal
polar. is made via a 50k resistor). I want to point
out that the internal polarization is filtered by an
external capacitor (on Pin called ‘V
pacitor contribute to good performance of the de-
vice. Its value should be 47 F or more (coupled
with an 47nF HF cap. for internal video references).
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
2.1V
(in fact a 2k load is acceptable). The DC level is
once more V
Usually some AC coupling capacitors are used at
the outputs. To drive some typical 10k 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 F and 47 F. Moreover it can be a
good idea to insert resistors (220 or 470 ) 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 15 : Audio Outputs
18/20
STV6411A
L/R Audio
Audio Out
RMS
(around 6V
12k
CC
RMS
/2 for best dynamic performance.
4.7 F
220
(that is around 6V
PP
) on a typical load of 10k
CC
/2 (4V typ.). That’s why
470
4.7 F
REF
STV6411A
L/R Audio
Audio In
PP
’). This ca-
) with an
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.
Figure 16 : Video Inputs
2.b - Outputs
On these devices the video outputs are NOT ABLE
to drive 150 . 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 resis-
tors (more or less) and will drive the outputs through
some 75 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).
Figure 17 : Video (and Fast Blanking) Outputs
Y/C/CVBS/RGB
STV6411A
Video Out
75
(4.7k )
220
100nF
1k
75
STV6411A
Video In
Video