EX1 Color Space

[Craig Seeman]

Juan Martinez, Senior Manager Technology, Sony, posted the following in another forum regarding the EX1, color space and keying. I thought it was so important given the 4:2:2 vs 4:2:0 arguments I've seen, I'm posting it here. Juan, if you're looking in I'm sure you won't mind it. It's important to get this info out there.

"Actually, the Ex1 outputs honest to goodness 10-bit 4:2:2 at all resolutions via its built-in HDSDI/ SDI interface."

"Now, a word about "color space". In the old SD days, 4:4:4, 4:2:2 and 4:2:0 had absolute meaning and direct correlation to the picture quality (luminance and color resolution). In the case of HD, 4:4:4, 4:2:2 and 4:2:0 are relative. Thus, these ratios have no "real" meaning anymore. Therefore, it would be possible for an HD format with small luminance and color spatial resolution to claim 4:2:2... so, the only way to compare is to count the actual captured pixel resolution. Once, you know the exact number of luminance pixels stored, you can apply the ratio to compare apples to apples (for a given aspect ratio)."

"For compositing, all that is necessary is for color sampling to be tight enough for the keyer to effectively detect the edges. You will find that Ex captures 960 color pixels in the horizontal direction capturing a higher color resolution than some HD formats. The color sampling is also halved in the vertical to 540, which is still suitable for good keying. Lastly, at the core of modern compositing engines are sophisticated keying algorithms that even look at luminance transitions to more accurately detect the edges. Please try keying with Ex 4:2:0. I believe you will be surprised how good keys you will be able to achieve."

"Sorry for the long winded reply. I hope it helps."

"Juan Martinez
Senior Manger Technology
Sony Electronics"

[Mike Barber]

Originally Posted by Craig Seeman

In the old SD days, 4:4:4, 4:2:2 and 4:2:0 had absolute meaning and direct correlation to the picture quality (luminance and color resolution). In the case of HD, 4:4:4, 4:2:2 and 4:2:0 are relative. Thus, these ratios have no "real" meaning anymore.

Can anyone elaborate on that for me? Why is it absolute in SD but not HD?

[Steve Mullen]

Originally Posted by Mike Barber

Can anyone elaborate on that for me? Why is it absolute in SD but not HD?

Here's my take. In the SD world any camera that was considered "pro" would have 720 to 960 CCD elements. The larger number provided true over-sampling of an SD image OR it supported widescreen operation. (In other words, 960 was considered an SD CCD resolution.) The bottom-line was that SD was captured by about 800-pixels no matter the company making the camera. With 4:2:2 sampling, the two chroma components has an effective bandwidth of about 400-pixels.

Today in the HD world, we have companies using 960-pixel CCDs for HD while other companies use 1920-pixel CCDs. Once we record both kinds of HD we can compare the number of luma samples with the number of chroma samples from the two types of cameras AFTER the two types of chroma "compression."

Bottom-line, unless a 960-pixel camera uses 4:2:2 sampling it would be totally unusable. Or, put another way, even with 4:2:2 sampling the actual number of chroma samples it provides (480) only matches (if that) those offered by a 1920-pixel camera with 4:2:0 recording. Going with a "4:2:2" format doesn't get you more chroma information in a frame when keying.

Thus, the claim that "4:2:2 supports better keying" must be viewed in the context of the absolute -- not pixel-shifted -- CCD resolution provided by the camera.

PS: the same is true on the vertical axis. So any H. advantage provided by the higher rez sensor is multiplied by the V. advantage. One may be able to make the case the EX1 provides better keying despite 4:2:0 sampling.

Moreover, since the augmentation provided by pixel-shift is color dependent, the EX1 may also support more "stable" keying results.

Hopefully, I've not messed-up Juan's thoughts.

[Christopher Barry]

Originally Posted by Mike Barber

Can anyone elaborate on that for me? Why is it absolute in SD but not HD?

Did D1 PAL or NTSC ever get encoded as 540x576 or 540x486 with a 1.33x horizontal stretch on playback from the deck or cam? 540x1.33 takes you to 720. Go as far as 480x486 if you apply a 1.5x stretch, as one codec below does.

Does Juan Martinez comments have anything to do with most HD codecs are not pixel for pixel encoded? Such as HDCam records 1440x1080, plays back with a 1.33x horizontal stretch. DVCProHD in 720p is 960x720, with a 1.33x stretch, similarly DVCProHD 1080p24 is 1280x1080 (1.5x stretch) and 1080p25 is 1440x1080 (1.33x stretch), and so on.

The EX1 is the first camera in this price range to provide pixel for pixel encoding, 1920x1080. Other high end camera, such as the Sony F23, Panasonic HPX-3000, etc, have full 1920x1080 encoding.

Granted the EX1 is 4:2:0, however, what exactly are the above codecs when you factor in the 1.33x and 1.5x horizontal stretch on playback of the file? Existing information is being spread to fill the gap, so what really are some of these 4:2:2 codecs above, once you slot them into a true 1920x1080 viewing frame? Are they really 4:2:2? Interesting how things are now evolving, and quickly.

[Barry Green]

Originally Posted by Christopher Barry

The EX1 is the first camera in this price range to provide pixel for pixel encoding, 1920x1080. Other high end camera, such as the Sony F23, Panasonic HPX-3000, etc, have full 1920x1080 encoding.

Those aren't the only cams that have it though, there are some quite inexpensive recording formats that also offer full-raster 1920x1080. AVC-HD also offers 1920x1080 @ 4:2:0. And JVC offers an MPEG2 hard disk camcorder that does 1920x1080 recording. You can get 1920x1080 recording in a Panasonic HDC-SD5, Panasonic HDC-SX5, and JVC Everio GZ-HD7. I'd also not be surprised if the new Canon "dual flash" models, due to be introduced tomorrow at CES, offer full 1920x1080.

[David Lorente]

I think this can be easily understood (at least by me) by doing some maths:

In the SD world, all formats have been standardized to 720x576 or 720x480, depending on whether you use the PAL or NTSC standards (I'll do my maths just with PAL to simplify, you can easily do it for NTSC). That's it for the luminance signal, but the color difference are sampled at less resolution, so we effectively have less pixels stored to reduce bandwidth.

This means that in a 4:2:2 format (like D1, Digital Betacam, MPEG-IMX or DVCPRO50), the Y signal has 720x576 pixels, for a total count of 415 KP (KiloPixels, it may sound a bit odd, but I think this way it's easier to compare different resolutions). But the U and V signals will have just 360x576 pixels, or 207 KP.

If we go for a DVCAM or DVCPRO, which are encoded at 4:2:0 or 4:1:1, the U and V signals will be a quarter of the Y one:

4:2:0 - 360x288, 104 KP
4:1:1 - 180x576, 104 KP

Now, if SD resolutions are standardized, there's a mess of resolutions and encoding schemes in HD formats:

HDCAM SR: 1920x1080, 2074 KP, 4:2:2 or 4:4:4 encoding
HDCAM: 1440x1080, 1555 KP, 3:1:1 encoding
XDCAM HD, HDV: 1440x1080, 1555 KP, 4:2:0 encoding
XDCAM EX: 1920x1080, 2074 KP, 4:2:0 encoding
DVCPRO HD: 1280x1080, 1382 KP, 4:2:2 encoding
and so on...

These are the pixel count of the colour signals of these formats:

HDCAM SR: 960x1080 (when at 4:2:2), 1037 KP
HDCAM: 480x1080, 518 KP
XDCAM HD: 720x540, 388 KP
XDCAM EX: 960x540, 518 KP
DVCPRO HD: 640x1080, 691 KP

As a conclusion we can see that DVCPRO HD has not the double of color resolution than XDCAM EX, just 1.33 times of it... but the EX luminance resolution is 1.5 times the DVCPRO one. And it is interesting to point out than the EX color resolution is just the same as HDCAM, though sampled in a different way.

So one should be careful in comparing different recording formats because the sampling schemes are relative to each format's resolution, and it is clear that HDCAM SR performance will be far better than DVCPRO HD, though both systems have the same chroma to luma sampling relation.

[Christopher Barry]

Barry, you're correct. The EX1 is not the first, and there are other cameras you mentioned, that I know little about, that record full 1920x1080. I guess we will see more cams to follow soon. Thank you for apprising me.

[Christopher Barry]

David, very interesting analysis you presented, thank you.

[Steve Mullen]

Originally Posted by Christopher Barry

David, very interesting analysis you presented, thank you.

Yes -- thank you. I hope this will stop those 4:2:2 fans from claiming that DVCPRO HD offers better chroma. And with FCP finally supporting all the HDV and XDCAM EX formats using native editing (with Color renders to ProRes 422) -- the claims that long-GOP MPEG-2 is "hard to edit" will be quieted.

[Tom Roper]

Originally Posted by Barry Green

Those aren't the only cams that have it though, there are some quite inexpensive recording formats that also offer full-raster 1920x1080. AVC-HD also offers 1920x1080 @ 4:2:0. And JVC offers an MPEG2 hard disk camcorder that does 1920x1080 recording. You can get 1920x1080 recording in a Panasonic HDC-SD5, Panasonic HDC-SX5, and JVC Everio GZ-HD7. I'd also not be surprised if the new Canon "dual flash" models, due to be introduced tomorrow at CES, offer full 1920x1080.

Of those small cam formats you mentioned, I believe that all are single sensor, prismless designs. Full 1920x1080 in the case of the EX1 is still 3x as many photosites, and even HDV is still 2.3x as many photosites. As you know, it's the sensor(s), lens and internal processing that combines for the final resolution result. Full raster is part of that, but not all of it.

[Mike Barber]

Great informative responses, thank you everyone.

Seems I have been learning much lately about getting hung up on "numbers," which has been hindering me from seeing the forest for the trees.

[David Lorente]

Originally Posted by Steve Mullen

Yes -- thank you. I hope this will stop those 4:2:2 fans from claiming that DVCPRO HD offers better chroma. And with FCP finally supporting all the HDV and XDCAM EX formats using native editing (with Color renders to ProRes 422) -- the claims that long-GOP MPEG-2 is "hard to edit" will be quieted.

I'll tell you soon - I'm waiting my EX1 and a new FCP station (should be delivered by mid january). I'm planning to make some chroma key tests with airplane models and other freak things (remember the days when FX were done without computers?); this type of keying will be more difficult for the machines than a keying a human being, though I'll try also with one if I can find it :P

Anyway, I won't say if the EX is good or bad for this kind of job until I see the composed images.