Some technical information

Some datasheet, information, signal specification...

Last Update: 30/01/2007

Why this is working

I'll try to be short and effective :)

SCART is only a connector, IT IS NOT a video standard. Putting a s-video signal on a scart connetor is like cheating.. tha's why many (but not all) TVs cannot get colors: thay simply do not understand s-video signal.

S-video is a video standard to transmit video information by using 2 wires: one for b/w information, the other for color information.

Composite , the old video standard, is made by only one wire: b/w and color information travel together along the wire.

When we connect s-video to our Scart "adapter" we are in fact loosing entierly the Color signal because our Tv cannot understand where to get Color information. Croma signal is on PIN15, the PIN that out TV is missing to check.

Our Tv, on the SCART connector, is only listening to Pin20, which contains Luminance (luma) information, our B/W video.

Why ? because, referring to SCART connector standard, the TV on PIN20 should find a "composite" video signal, that like said before, is made to stay on one wire carrying Luma+Croma togheter. But this is not the case. Our cnnector is cherating, and is putting Luma on PIN20, and Croma on PIN15... but no-one told the TV to look on PIN15, yes... nobody said that!

That's why instead, if we connect the composite (yellow) cable on the scart adapter the Tv is working: it is forwarded to PIN20, TV read PIN20 and gather luma and croma information.

With this little "hack" we are recreating a pseudo Composite signal on PIN20, a signal that the TV is able to fully read.

The bottom line is: IF your TV cannot read directly s-ideo signal via the adapter, forget s-video and use directly Composite from your device to the TV if possible.

In case you are using a notebook (like me) this is the only method to have colors on TV.

Image quality: if we give 80% to true composite system and 100 to true s-video system, this methos yeld a quality not much than 80% but, somethimes depending on equipment used, as low as 40%... because of "physical" limitation of the entire setup.

Yes... some more recent TV are able to "discover" an s-video signal on SCART connetor, but just because they were built with all this in mind.

Some Tech Background

Firt, read these snippet of information (source: googling the web) to have a quick and dirty a-b-c in the sector:

S-VIDEO - Is made of 4 pins, 2 signal, 2 ground. The Chroma (C) signal gave you color, the Luminance (Y) signal which gave you black and white, Chroma is an extremely phase-sensitive signal. Additional cable length or additional adapter meant additional extra delay, additional extra phase shift, and too much of it be resultant of wrong color or no color at all, no color by default is Luminance only which got through and is black and white. The meaning is: color--> one cable with it's own ground, luma--> one cable with it's own ground.

Composite - The Chroma signal is encoded as a color burst (still Phase-sensitive) within the Luminance signal (in the front-porch segment). The end is: all the signal is on one cable.

Scart - is a standard for a cable (not video signal) which can carry multiple signal: audio, video, rgb, composite, s-video and many more. Some where standardized, others are standard de-facto, meaning i build it, so it will work this way [brand_X], and this is not working for Brand_y... when using converters you generally loose audio, so you need separate RCA audio cables (normally, red and white). Composite is LESS sensitive ("less sensitive" does not mean "impervious") to color loss than S-VIDEO from the fact that Chroma being embeded within Luminance partially compensated for the extra delay or extra phase shift (they shifted together better).

In composite video , the luminance signal is low-pass filtered to prevent crosstalk between high- frequency luminance information and the color subcarrier (essentially like FM receivers, multiple station filtered by your radio, so you can listen to one clear station at once). S-Video, however, separates the two (with two cables), so low-pass filtering is not necessary. This increases bandwidth for the luminance information, and also subdues the color crosstalk problem.

The luminance performance of S-Video is noticeably better than composite video; the chrominance performance with reduced crosstalk also shows noticeable improvement.

S-Video signals tend to degrade considerably when transmitted across more than 5 meters of cable with some cheaper S-Video cables.

Today, S-Video signals can be transferred through SCART connections as well. However the device that has the SCART connector must support S-Video as it is not part of the original SCART standard.

Some Additional Files

Some Files to download, mostly application notes made by electronics manufatrer of video chip for TV signals.

Maxim/Dallas video standards [.rar - 330Kb]
Intersil image separation tecniques [.rar - 290Kb]
Video Signal format explained [.rar - 220Kb]

Some video pattern to evalutate video detail losses [.rar - 160Kb]

Some links

Wikipedia s-video - info
Wikipedia composite - info

Question, info, suggetion, report ? did it worked, burns? fires? B/W again? totally black? ... maybe... COLORS !!!! mail me ! say something, to ask... or just because it is working :) let me know ! ;)

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