S-video to composite video adapter
This simple adapter can be used to convert Y/C
video (S-video) to a composite video. This adapter is useful in cases where your
video output device has only S-video output but your signal source accepts only
composite video input. This circuit works with both PAL and NTSC video
standards.
Y-ground------------------+
+---------- RCA/composite ground
C-ground------------------+
Y-------------------------+
+--------- RCA/composite video
C------------||-----------+
470pF
This circuit can be quite easily build inside
a the S-video connector case if a physically small size 470 pF (ceramic)
capacitor is used. Larger capacitor values will also work, but cause picture to
become "softer". The voltage rating of capacitor can be 10V or more.
This circuit works in practice quite well even
though the circuit operation is not ideal. This means that impedances and signal
levels not matched exactly right, but near enough to work accetably. The picture
quality you get from this circuit is is good, but not as good as with best
possible composite video output circuitry.
Here is the pinout of the S-video connector
shown from the end with the FEMALE PINS (picture is a view on the equipment
back/front panel):
1 Y ground
2 C ground
3 Y (luminance+sync)
4 C (crominance)
What if my PC graphics card has a 7-pin S-video connector instead of 4-pin ?
The four pin S-video connector as shown above
is the standard connector for carrying S-video. Those seven pin connectors seen
on some PC graphics cards are non-standard connectors for carrying S-video. The
use of the pins on those seven pin connectors is not standardized and can vary
from manufacturer to manufacturer. (For some strange reasons some manufacturers
in PC industry just keep constanly breaking the industry standars and create
lots of confuzion to users when doing so).
Generally the four pins on those 7-pin
connectors on the same places as the standard four pin S-video connector have
practcly always the same fuctionality as those pins in S-video connector. The
other three pins can have then some extra signals which are not part of S-video
(usually some pins of those carry composite video and some control signals, but
the use of those three extra pins vary quite much). So if you encounter 7 pin
connector for S-video, then forget the three center pins... just use the four on
the standard positions. The circuit shoudl work with those connectors as well.
Can the circuit used in other way ?
If you try to connect the circuit in opposite
way so that you have a composite video signal which yoiu want to feed to S-video
input then you might wonder would this circuit work also in that way. The answer
is that the circuit sort of work also in this way. If you connect a comoisute
video source to a S-video input of your TV using this circuit, you will get a
full color picture on your TV screen. The downside of this is that the picture
quality will be worse than if you were using the real composite vidoe input of
your TV. The reason for this is that after the circuit the color information is
still in the brighness signal, you you see some interference on the screen
caused by color subcarrier which gets to the screen. This all means that you can
use this circuit as last change in situations where you don't have a composite
video input. The picture quality is degraded, but propably still usable for some
not so demanding applications. A real well working solution would be much more
complicated than this circuit.
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