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Topic: Digital show starts without one color...

Jon P. Inghram
Expert Film Handler

Posts: 124
From: Wichita, KS USA
Registered: Jan 2007

posted 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.

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Mike Renlund
Film Handler

Posts: 71
From: San Francisco
Registered: Feb 2008

posted 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.

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Hendarto Ciputra
Film Handler

Posts: 8
From: Jakarta / Indonesia
Registered: Jul 2008

posted 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)

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Steve Guttag
We forgot the crackers Gromit!!!

Posts: 12814
From: Annapolis, MD
Registered: Dec 1999

posted 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

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Stephen Furley
Film God

Posts: 3059
From: Coulsdon, Croydon, England
Registered: May 2002

posted 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.

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Jon P. Inghram
Expert Film Handler

Posts: 124
From: Wichita, KS USA
Registered: Jan 2007

posted 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."

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