Film-Tech Forum ARCHIVE: Digital show starts without one color...

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Posted by Mark Lensenmayer (Member # 134) on 08-29-2008, 01:39 PM:

The digital screening I saw this morning started with only red and green images. Needless to say, the trailers looked very weird. I went to the lobby to tell them, and it was corrected when I got back to the room.

This was a 3-D show but it happened during the 2-D trailers. It was the first show of the day. Theatre was an AMC.

Can someone explain what happened?

Posted by Thomas Pitt (Member # 4322) on 08-30-2008, 01:30 AM:

Either the blue signal to the projector (the colors are sent through separate wires) wasn't connected properly - loose wire or similar - or the actual DLP chip for the blue channel wasn't responding. Since they were able to fix it so quickly, I'd say it wasn't anything complex, like the DLP mirrors getting jammed.

Sometimes a projector can go wrong and start up without one of the color chips responding. Usually a power-cycle will sort that out, as it'll re-initialize everything.

Posted by Pete Naples (Member # 761) on 08-30-2008, 03:08 AM:

quote: Thomas Pitt
(the colors are sent through separate wires)

If this was a D Cinema show, that statement is not true. The signal to a D Cinema projector from the server is carried by dual link SDI.

The point about the projector needing rebooted is valid, I have seen this once or twice.

Posted by Josh Kirkhart (Member # 1046) on 08-30-2008, 05:22 AM:

No restart was needed or it would not have been onscreen when he returned to the theatre.
The projector was simply in the wrong Format.
Various formats are preset or customized with defined color/crop/aspect ratio/frame rate/ etc.
They were more than likely using two different format 2d/3d buttons and manually changed it.
Why it did not start in correct format is a whole different question(bad showlist build/operator error/automation cues not reset before show etc.).
Showlist could be built so that the 2D content plays at 48fps in the 3D format so all content conforms to the 3D masking crop, so no Format, lens or zoom changes are needed depending on the house setup(anamorphic/single lens/zoom lens/flat/scope feature etc.)

Posted by Stephen Furley (Member # 1277) on 08-30-2008, 07:36 AM:

quote: Pete Naples
If this was a D Cinema show, that statement is not true. The signal to a D Cinema projector from the server is carried by dual link SDI.

It would be possible if the trailers were run form some sort of alternative content system, via a DVI link, though it's not likely since all of the signals are carried on one connector. If it was a broken wire in a cable it seems unlikely that it would have been diagnosed and fixed that quickly. It would be more likely with analogue signals on 3, 4 or 5 separate BNC cables if one of them was incorrectly connected.

A re-boot would be likely to take more than a few seconds, and I can't think of any format/system mis-match which would simply drop the blue. There hadn't been a technician working on the system before the first show that had turned off the blue channel for some reason and forgotten to turn it back on, had there? Even that seems unlikely, since he/she would be likely to check the picture when they finished, and unless they were still there, and realised what had happened it probably wouldn't have been fixed that quickly.

How are the bits shared between the two HDSDI links in a digital cinema system? does each cable carry half the bits for each pixel, or half the pixels for each frame, or do the two cables carry alternate frames, or what? I've never tried disconnecting one cable to see what would happen, but I suspect that you'd lose the entire picture. Does one link carry the clock for both, in the same way that the first AES pair for the audio carries the clock which is used by all four pairs?

Posted by Mark J. Marshall (Member # 1409) on 08-30-2008, 09:28 AM:

quote: Josh Kirkhart
Why it did not start in correct format is a whole different question(bad showlist build/operator error/automation cues not reset before show etc.).

I'm really confused by this. Does a DLP projector NOT have the ability to look at a movie file, determine its format, and configure itself correctly before playing the file?

Posted by David Zylstra (Member # 4230) on 08-30-2008, 09:39 AM:

quote: Pete Naples
carried by dual link SDI

Which is 2 wires - basically bandwidth is shared across both, one wire is fully used and the other carries the remainder of the information. I've seen it many times where the secondary one comes loose there is picture but it looks like colors are missing, if the primary comes loose there is no picture.

It is also very possible that the playlist was started in the wrong colorspace.

Posted by Chris Slycord (Member # 4239) on 08-30-2008, 09:45 AM:

quote: Mark Lensenmayer
Can someone explain what happened?

Since you said it was with a 3D movie, similar stuff to what you're describing can happen when the server's playlist triggers the projector to switch to a 3D format (either a 3D Flat or 3D Scope) but for whatever reason the projector stayed in the non-3D format.

quote: Mark J. Marshall
I'm really confused by this. Does a DLP projector NOT have the ability to look at a movie file, determine its format, and configure itself correctly before playing the file?

Well, no the DLP doesn't have the ability to look at the file since the file is on the server. And in the end, the problem is an issue of proper communication between DLP and server.

Posted by Mark Gulbrandsen (Member # 72) on 08-30-2008, 10:00 AM:

quote: Mark Lensenmayer
This was a 3-D show but it happened during the 2-D trailers. It was the first show of the day. Theatre was an AMC.

I'm assumming this was not a 4K Sony location but a 2K location.... If this was a Sony System then disregard the below comments since the Sony is a 10 bit system.

Actually it is possible to unplug the B channel HD-SDI cable and see this effect on some projectors. Remember the B channal only carries the extra two bits of data to make this a 12 bit system. Christie SB's will still produce an image with the B channel unplugged and they do look like there is a color missing although I can't recall which one. Now I seriously doubt that anyone was messing with the cables at this theater but ys never know....

Mark

Posted by Joe Elliott (Member # 4006) on 08-30-2008, 11:40 AM:

At our location, we have a Doreme server with a Barco, (I know, Marks favorite combination), and if you try to play 2-D content in 3-D mode the colors are off. You have to switch modes in between, or in your server you set the 2-D content to play at 48fps. Then everything plays correctly.

Posted by Mark J. Marshall (Member # 1409) on 08-30-2008, 06:30 PM:

Maybe I wasn't clear in my question, so let me rephrase...

Does the software which accesses the file on the server and sends the signal to the projector have the ability to read the format of the file and then set the proper configuration on the projector? Or does the user have to tell the projector what it's supposed to be doing? (Which lens to use, 2D or 3D, etc.)

Posted by David Zylstra (Member # 4230) on 08-30-2008, 08:48 PM:

currently not in our case with DoReMi and Christie - our booth staff has to create a playlist with the right format cues that tell the projector what presets to use (and the projector preset chosen determines which lens to use).

Posted by Mike Renlund (Member # 4675) on 08-30-2008, 11:41 PM:

In 2D digital cinema, the video information is carried on both HD-SDI cables.

Cable 1:
Red - 4 bits
Green - 4 bits
Blue - 2 bits

Cable 2:
Blue - 2 bits

When you run 3D, with the increased frame rate, the information is much more intense, and the video data changes:

Cable 1 (Left Eye)
Red - 4 bits
Green - 2 bits
Blue - 2 bits

Cable 2 (Right Eye)
Red - 4 bits
Green - 2 bits
Blue - 2 bits

When your show started, they had it in the 2D format on the projector, and obviously the color info would be read incorrectly.

Hope this helps.

[ 08-31-2008, 09:25 PM: Message edited by: Mike Renlund ]

Posted by Stephen Furley (Member # 1277) on 08-31-2008, 04:33 AM:

quote: Mike Renlund
Cable 1 (Left Eye)
Red - 4 bits
Green - 2 bits
Blue - 2 bits

Cable 1 (Right Eye)
Red - 4 bits
Green - 2 bits
Blue - 2 bits

Did you mean 'Cable 2 (Right Eye)', otherwise cable 2 doesn't seem to be doing anything?

Why 4 bits for R and only 2 for G and B? The RGB colour space normally uses the same bandwidth for each channel, unlike the various colour difference systems.

What does this 4 and 2 bits, mean anyway? This obviously isn't enough to transmit the full range of colours, is this 4 bit, 2 bit, 2 bit pattern repeated several times to build up te full pattern of bits for each pixel? If so, then why; why not just transmit all of the red bits, then all of the green, then all of the blue? What is the total bit rate of a HD-SDI interface anyway?

Posted by Tristan Lane (Member # 1169) on 08-31-2008, 01:11 PM:

quote: Mark J. Marshall
Does the software which accesses the file on the server and sends the signal to the projector have the ability to read the format of the file and then set the proper configuration on the projector? Or does the user have to tell the projector what it's supposed to be doing? (Which lens to use, 2D or 3D, etc.)

The settings are not automatic. User specified macros (formats) need to be selected (by use of a cue or automation setting) to tell the projector which macro to use during playback of a specific feature.

Posted by Jon P. Inghram (Member # 4125) on 08-31-2008, 01:55 PM:

quote: Stephen Furley
What is the total bit rate of a HD-SDI interface anyway?

It's 1.485 Gbits/s for a single link, double that for a dual link. And according to the DCI spec the maximum bitrate for the video is 250 Mbits/s. The max bitrate for video, audio, and subtitles combined is 307 Mbits/s.

Also, I can't believe they use the XYZ color space as it's overkill for both digital AND film photography and only wastes bits on imaginary "colors" the human eye can't even see.

Posted by Mike Renlund (Member # 4675) on 08-31-2008, 09:34 PM:

Fixed that cable 2 thing.

2D digital cinema playing JPEG 2000 content is described as 4:4:4 meaning that red, green, and blue are all sampled at the same rate.

When you move to 3D, the framerate goes up, and you have separate info for the left and right eyes. The TI Light Engine cannot handle that bitrate at the full chip. So, in machines with 1.2 inch chips, the image size is reduced. .98 inch chips have the new FFIBS, and can run at full resolution.

Unfortunately, with regard to the color, the reason we use 2 HDSDI cables is that only so much color information can be sent on the cable. So each eye is getting the full red, but green and blue are sampled at half. Its the only way to get the 3x framerate and color information for each eye.

Posted by Hendarto Ciputra (Member # 4868) on 09-01-2008, 04:45 AM:

@Mark J Marshall,
The GDC servers that we used here in Singapore are able to have metadata inserted in each files(be it main feature/trailers/preshows)

Posted by Steve Guttag (Member # 268) on 09-01-2008, 10:28 AM:

Oh come on...when you don't know something do you just make it up? What is worse, do you then pass it along as factual knowledge to then confuse others?

4:2:2 as well as the other color sampling definitions refer to a component color space where the first digit is the Luminance "Y" and the other two are the component colors Cb and Cr. 4:2:2 is a form of data compression where the fact humans are MUCH less sensitive to chroma than luminance. The chroma in 4:2:2 has half the resolution of Luma in the horizontal but the same resolution in the vertical.

4:4:4 both Luma and Chroma have the same resoution.

On HDSDI cables, for 4:4:4 they are split up so one cable has 4:2:2 while the other cable has 0:2:2. Thus, if the source is encoded 4:4:4...with just one cable connected, you should get a picture with wacked out color...with just the other cable connected, you won't get a picture.

For more research into the subject try looking up CCIR REC. 601 and related documents.

While I don't profess to be an expert in this subject...at least I'm not trying to just make things up to come up with an explanation.

Steve

Posted by Stephen Furley (Member # 1277) on 09-01-2008, 11:33 AM:

Now I'm confused; Jon wrote:

quote: Jon P. Inghram
It's 1.485 Gbits/s for a single link, double that for a dual link. And according to the DCI spec the maximum bitrate for the video is 250 Mbits/s. The max bitrate for video, audio, and subtitles combined is 307 Mbits/s.

If these figures are correct, then a single link HD-SDI interface has more than enough bandwidth, even if you double it for 3-D. Surely this can't be the case, or projectors wouldn't be using two cables.

Then Steve wrote:

quote: Steve Guttag
4:2:2 as well as the other color sampling definitions refer to a component color space where the first digit is the Luminance "Y" and the other two are the component colors Cb and Cr. 4:2:2 is a form of data compression where the fact humans are MUCH less sensitive to chroma than luminance. The chroma in 4:2:2 has half the resolution of Luma in the horizontal but the same resolution in the vertical.

4:4:4 both Luma and Chroma have the same resoution.

On HDSDI cables, for 4:4:4 they are split up so one cable has 4:2:2 while the other cable has 0:2:2. Thus, if the source is encoded 4:4:4...with just one cable connected, you should get a picture with wacked out color...with just the other cable connected, you won't get a picture.

That sounds fair enough. So, what colour space are digital projectors using? Are they RGB, or 4:4:4 Y,Cr,Cb, or what? If there isn't enough bandwidth available, and it is necessary to reduce the number of bits in one or more channels then it would seem that Y,Cr,Cb would be a better choice than RGB, in much the same way as Y,Pr,Pb tends to be used for analogue component video, with RGB mainly being used for computer displays, other than with SCART connectors, which use RGB because this input was originally intended mainly for use with early home computers.

What is 4:2:0, as used by PAL DV? It obviously isn't what it sounds like. How does it compare to the 4:1:1 used by NTSC DV. Yes, I know I shouldn't be calling it 'PAL' and 'NTSC', but you know what I mean.

Posted by Jon P. Inghram (Member # 4125) on 09-01-2008, 03:01 PM:

A single link normally does 10 bit 4:2:2 YCbCr. To get the 12 bits that DCI uses the second link carries the extra 2 bits. The dual link system also supports 4:4:4 in YCbCr and RGB.

Actually, looking at it closer, I don't think the uncompressed video is carried over the HD-SDI cables anyway, it just sends sends the compressed video stream to the projector which decompresses it and displays it. The DCI spec says the decompressed video needs 10 Gbits/s which isn't going to fit over any version of HD-SDI.

They could have used 10 or 1 Gbit Ethernet between the server and the projector and it would have worked just as well or better and been cheaper. Instead, Hollywood, with their severe paranoid personality disorder, went with a restrictive physical interface that, due to the way it's licensed, isn't available on consumer equipment and thus is considered more "secure."