Film-Tech Forum ARCHIVE: SCART Switch

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Posted by Joe Redifer (Member # 3) on 05-21-2006, 01:48 PM:

I am looking for a SCART switcher that has at least 5 SCART inputs and 1 output. I would prefer it to be active and it MUST handle RGB and audio on all connections. But a lot of SCART switches I have seen don't specify if they handle RGB or if they are composite only. I don't even know why composite exists, everyone on the planet without exception hates it. It serves no useful purpose anymore. I hope whoever invented it scratches off a lottery ticket thinking that they have won the jackpot but instead wins nothing. That'll teach 'em.

Anyway, you this think one handles RGB? It doesn't specify:
Clickity clicky click

Of course I'll have to buy it from the UK, and it is quite expensive at 65 lbs plus shipping.

Posted by Paul Mayer (Member # 355) on 05-21-2006, 02:48 PM:

I don't see anything that looks like RGB ins or outs on that back panel. But since SCART supports RGB it looks like you could make up or buy some adapters to bring the RGB pins on the SCART connectors out to some RCA connectors to connect to your system.

Posted by Joe Redifer (Member # 3) on 05-21-2006, 05:47 PM:

Huh? No, the RGB never needs to be anything other than SCART, so I don't need any fancy connectors. But I have read that since it is European, some SCART systems only support composite because Europeans are weird.

Posted by Paul Mayer (Member # 355) on 05-21-2006, 09:04 PM:

Oh. Never mind. Not to worry, SCART supports both RGB and composite on separate sets of pins. You should be good to go.

Posted by Stephen Furley (Member # 1277) on 05-22-2006, 04:19 AM:

The SCART standard includes connections for both composite and RGB connections (S-video was added later) but a device isn't required to implement all of the signals. A VHS recorder for example would only have the composite pins wired. A switch box may, or may not, handle RGB signals, the only way to tell is to look at the spec, or open it up and look, or try it and see.

Posted by Joe Redifer (Member # 3) on 05-22-2006, 12:29 PM:

I don't want to spend $100 USD just to "try it and see".

Posted by Stephen Furley (Member # 1277) on 05-22-2006, 05:46 PM:

That's one of the problems with SCART; it's not obvious what signals it provides, or accepts. Could be composite only, composite+RGB, composite+s-video; I've even seen one which handled only audio signals, no video at all!

Then there's the lack of support for Y,Pr,Pb, and the fact that the RGB connections are input or output, not both at the same time, though neither of these would have been a problem in the original intended use. They were intended mainly for home use, where most people at that time were connecting video equipment to televisions via a RF cable to the ariel socket, even where video/audio connections existed; they were seen as being too complicated/too much bother/why use four cables when one will do?

SCART enabled a single cable, and unlike the old EIAJ 8-pin connector often found on U-Matic etc. handled stereo sound. If you connect a signal source and a display together with a fully-wired SCART cable they will generally automatically work in the best mode of which both are capable.

On most televisions which have two SCART sockets only one is RGB capable, the other often has s-video, so you do need to know which to connect to, which goes against the original design concept. The Philips idea of colour coding the sockets blue or orange to indicate whether they handled RGB never seemed to catch on, and I can never remember which colour was which. As I've said before, it was a good idea, but badly implemented. Also, some of the cables are of very poor quality.

Do you ever get SCART over there; I've never seen it on any American equipment?

Posted by Paul Mayer (Member # 355) on 05-22-2006, 06:42 PM:

I've never seen it offered for home use over here, and wonder what Joe is working with that uses it.

Didn't realize that SCART cables and switches aren't necessarily fully populated. That seems dumb to me, but what do I know? The few times I've used it (industrial shows here using EU equipment suppliers like Barco and Philips), the switches and cables had all pins wired up, so I never had to worry (much) about what signals needed to go where. I just assumed all SCART cables were fully wired, allowing the source and destination machines to dictate what the signals would be. Guess I got lucky on those shows.

Posted by Joe Redifer (Member # 3) on 05-22-2006, 08:36 PM:

I am using it to hook up several older videogame systems (Genesis/Mega Drive, Super Nintendo, Neo-Geo, Sega Saturn and soon NEC TurboGrafx-16) which ouput RGB. I then transcode that RGB into component (Y, Pb, Pr) and that goes into my super kick ass NTSC TV which can handle 240p. Doing it this way looks MUCH better than composite or S-video, and many of the older systems don't even offer S-video. I like component because the reds don't bleed, and it looks as close to true RGB as you're gonna get. Everything works fine now, but I was looking for something that would enable me to be a bit more lazy and not have to switch the cable myself.

Posted by Stephen Furley (Member # 1277) on 05-23-2006, 07:24 AM:

Most cables and devices are fully-wired these days, but not all are. In the past composite only ones were quite common, as few people had any use at home for RGB signals; about the only thing that used them were a few early home computers, such as the BBC micro, which output a basically broadcast standard 625/50 interlaced signal, but could do so in RGB form if required. This was the original reason for the inclusion of RGB in the SCART spec, and the reason why separate input and output pins were nor provided; computers were always outputs, and televisions were always inputs.

The RGB connections which most televisions had provided for some years were seldom used until things like DVD players and digital set-top boxes started to appear, when they proved to be useful. Because most televisions in Europe had SCART connections, which could handle RGB, but not Y, Pr, PB, DVD players for the European market tended to have RGB (via SCART only) and component connections, but not Y, Pr, Pb, unlike those in NTSC countries. Recently, Y, Pr, Pb connections have started to appear, on three phono sockets, though most televisions do not have the inputs they are provided on most projectors these days. Oddly, it tended to be on the cheaper DVD players, of Chinese origin, where Y, Pr, Pb component outputs were first provided.

More recently, DVD and hard disk recorders which can record RGB signals have created a need for bi-directional RGB signals; I'm not sure how they do this with only one set of pins, I assume they switch them between being outpus and inputs somehow.

Joe, what are you using to transcode <-RGB Y, Pr, Pb->, and will it do both NTSC and PAL? I've found a couple of units over here that do it, but they're ridiculous prices.

Posted by Joe Redifer (Member # 3) on 05-23-2006, 03:00 PM:

It's a unit I ordered from Australia (not from that site, I don't think). It was around $100 or so. It just transcodes, it doesn't have anything to do with NTSC or PAL. If a 59.94Hz signal goes in, that's what comes out. So it'll work with PAL or NTSC depending on the timing and resolution of the input signal. I've had zero problems with it. I also modified it to output sound.

Posted by Stephen Furley (Member # 1277) on 05-23-2006, 05:35 PM:

quote: myself
Because most televisions in Europe had SCART connections, which could handle RGB, but not Y, Pr, PB, DVD players for the European market tended to have RGB (via SCART only) and component connections, but not Y, Pr, Pb, unlike those in NTSC countries.

I just read what I wrote, and it's wrong. What I meant to say was:

Because most televisions in Europe had SCART connections, which could handle RGB, but not Y, Pr, PB, DVD players for the European market tended to have RGB (via SCART only) and composite connections, but not Y, Pr, Pb, unlike those in NTSC countries.

Posted by Steve Guttag (Member # 268) on 05-23-2006, 07:39 PM:

It should also be understood that the sync for the RGB in Europe is not on green but on the composite signal. Some people refer to it is RGBCvS. One can not just merely hook the composite signal to an "S" terminal of a 4-wire system either...the sync must be taken from the composite line and then fed to the "S" terminal. There are also devices in the states that will take the RGBCvS signal directly if it knows that is the format.

Posted by Stephen Furley (Member # 1277) on 05-24-2006, 04:47 AM:

SCART does use what is normally the composite video pin for the sync when in RGB mode; I think all RGB monitors that I've ever seen with BNC connections can use either four cables (separate sync), or three cables (sync on green). They either have a switch or auto detect. Many these days have five connectors, so they can also work with positive horizontal and vertical syncs, so they can also be used as VGA computer displays. Over here separate sync is certainly far more common; I'm not sure if I've ever actualy used sync on green. Do you use sync on green over there?

Y,Pr,Pb always seems to be just three cables, I assume that it's sync on Y.

Getting back to SCART, another advantage of it is that a VCR can automatically switch a television into video mode when play is pressed, and switch it back to off-air mode when the tape is stopped.

I would guess that the days of SCART are now numbered; many new high end televisions over here are now 'HD ready', using either DVI or HDMI inputs, with HDCP. There is no HD broadcasting yet, but it's due to start this year. Analogue broadcasts are due to shut down within a few years, and VHS recorders are rapidly going out of fashion. Most of the higher-end (and many of the cheaper) new DVD players and recorders now have digital connections.

Posted by Joe Redifer (Member # 3) on 05-24-2006, 03:55 PM:

In Y,Pb,Pr (rolls right off of the tongue) the sync is indeed on Y. It is a crazy format. I really can't tell any difference between it and RGB, though. Does anyone notice any visual differences between the two? Component does not transmit green at all, and it does not compress blue or red to my knowledge. I think it figures everything out via math even though it's analog. Crazy!

Posted by Stephen Furley (Member # 1277) on 05-24-2006, 05:14 PM:

quote: Joe Redifer
Component does not transmit green at all, and it does not compress blue or red to my knowledge.

RGB uses the same bandwidth for all three signals, but Y,Pr,Pb uses reduced bandwidth for the colour difference signals, so the total babdwidth required for the signal is less. Also, of course, the Y and colour difference signals are a necessary intermediate step in encoding to just about any of the composite formats, PAL, NTSC, SECAM, FAM and various oher experimental ones which never caught on.

It would be possible in theory to use any two colour difference signals; I believe that the reason that green is not used in practice is that it is the smallest signal, and therefore the most vulnerable to noise.

You can do all sorts of odd things with colours; some of the early single Vidicon tube colour cameras used a stripe filter which was not red, green and blue, as might be expected, but white, green and cyan. the green stripes provided the green signal, cyan-green gave the blue, nd white-cyan gave the red. The reason for doing it this way was that the stripe filter transmitted roughly twich as much light as a RGB one would have done.

Posted by Paul Mayer (Member # 355) on 05-24-2006, 05:54 PM:

[Edit: Oops, doubled with Stephen but I'll leave it anyway.]

I've never looked closely but yeah, there should be no visible difference between an image maintained in RGB through the entire signal path versus the same image in component, at least when using real-world sources. I'm sure there are some CG test images out there capable of revealing a difference.

Component does indeed take advantage of reduced bandwidth for the two color-difference signals though. That's why it was created - to save bandwidth over RGB. Many years ago the NTSC did studies which showed that the human eye sees fine detail only in B&W. That's what lead to the component system - transmit the luminance and detail using full bandwidth just once; the color can get away with half bandwidth since the eye will never notice the lack of detail in color.

In a way, video images are kinda like comic book or graphic novel drawings - detail is done with great effort using a fine black ink pen while the color can be applied using a brush.

Posted by Joe Redifer (Member # 3) on 05-24-2006, 07:26 PM:

Green is not used in component because it takes up far more bandwidth than red or blue. The "color difference" takes up less bandwidth than the actual color as well, since it is just the difference. Green is determined by the display.

Green is REALLY important. Check out the following uncompressed examples I made:

Here is the full uncompressed picture (sized down for the forum) I took of Red Rocks. This picture has never been compressed. Size = 399KB

The same picture, but the BLUE element has been reduced to 1/4 of its original size, and the green and red remain full size. Still looks fairly good, eh? Size = 352KB

Now the RED element has been reduced to 1/4 of its original size, but blue and green are both still full resolution. Looks a bit worse, but still almost acceptable to most people. Size = 337KB

Here the GREEN has been reduced to 1/4 its original size, but both the red and blue remain full resolution. Look how crappy it looks! The eye really depends on the color green. Size = 344KB

Just for fun I made this picture where the green is at full res, but the red is 1/3 of its original size and the blue is a miniscule 1/8 of its original size! Looks like crap but it STILL looks better than when only the green is compressed by a factor of four. Size = 221KB.

Moral of the story: Your eyes suck at blue.

Posted by Robert Minichino (Member # 3495) on 05-24-2006, 10:00 PM:

With component you actually do get the full color bandwidth (that's why it's better than Y/C (S-Video) and composite). Sets recover green because having Y, R-Y, B-Y and knowing that Y = 0.30R + 0.59G + 0.11B, one can then manipulate R-Y, B-Y to arrive at:

G-Y = -0.508(R-Y) - 0.186(B-Y)

However, NTSC composite and Y/C don't actually use R-Y and B-Y directly, but rather I and Q which are linear combinations of the former:

I = 0.74(R-Y) - 0.27(B-Y)
Q = 0.48(R-Y) + 0.41(B-Y)

This gives an I which ranges from red to cyan, and a Q that ranges from magenta to green, with I having 3 MHz of bandwidth and Q having 1 MHz (so the difference in bandwidth isn't simple red/blue/green).

The benefit of Y/C (S-Video) is circumventing the set's Y/C separator which separates luma from chroma in the composite signal for further processing. Normally in a composite signal the luma and chroma bands overlap, causing dot-crawl if the chroma isn't properly filtered out of the luma or loss of detail if they're just simply band-limited to separate them. The comb filter is the better, more expensive, more complex solution.

And then the benefit of component is circumventing the color decoder altogether. As in the equations above, YPrPb is a linear combination of RGB and they can be converted to the other with simple analog circuits, no bandwidth cuts necessary.

Posted by John Pytlak (Member # 331) on 05-25-2006, 08:50 AM:

A "rule of thumb" is that we get 60% of our visual information in the green, 30% in the red, and only 10% in the blue portions of the spectrum. That's why in color print film, the magenta dye layer is the topmost imaging layer (modulates green light), and the yellow dye layer (modulates blue light)is at the bottom. Also why in a "Bayer" filter array in a digital camera, you have more green sensitive pixels:

Patent for "Bayer Filter" Used in Digital Cameras

quote:
United States Patent 3,971,065
Bayer July 20, 1976
Inventors: Bayer; Bryce E. (Rochester, NY)
Assignee: Eastman Kodak Company (Rochester, NY)