Color resolution when downconverting to SD at DVinfo.net

Kristian Indrehus · January 31st, 2006, 12:11 PM

I have read several places that HDV color resolution, when downconverted to SD in the right codec, can hold 4:2:2 colors.

Can anyone verify this, and if so explain exactly how it works ?

Ben De Rydt · January 31st, 2006, 01:45 PM

1080i HDV (1440x1080 pixels, 4:2:0) contains 720x540 chroma values (Cr and Cb). Enough for 4:4:4 SD (NTSC). Your NLE shouldn't lose chroma values when downsizing. It will in all likeliness decompress the HDV material to uncompressed 4:4:4 or 4:2:2 before the downscale to SD.

Emre Safak · January 31st, 2006, 02:53 PM

Now that's a great reason to shoot in HD.

Ken Hodson · January 31st, 2006, 03:01 PM

When you are downsampling resolution, as long as it is into a codec that will not restrict colour(chroma) info, the downsample will not stay locked at the origional 4:2:0, but will condense the chroma info that it has available. If that is SD 4:4:4, it will take advantage of that space. You aren't gaining anything, you are just condensing what was already there.

For best results I have found using a codec that does intelligent upsampling to 4:2:2, such as AspectHD. Then perform FX work or do CC in 16bit rendered out uncompressed or other non-restrictive codec. When all is done downsample to SD uncompressed and then render to DVD. I see the point of having the prestine 4:4:4 SD downsample, is it allows super quality DVD's. Very pro looking. I see the pre-downsampled HD as the best to do FX manipulation on, as the final downsample "cleans" everything up. Thats my preferance.

Steven White · January 31st, 2006, 03:17 PM

Agreed with Ken on all accounts.

-Steve

David Taylor · January 31st, 2006, 03:49 PM

Well said Ken.... When SD is the final intended output, the "oversampling" afforded by the source HD signal produces very professional looking SD by increasing the effective chroma fidelity of the SD output compared to the 4:2:0 HD source.

Kristian Indrehus · February 1st, 2006, 12:50 PM

Quote:

Originally Posted by Emre Safak

Now that's a great reason to shoot in HD.

My point exactly :)

Thanks guys for the valuable information. Now the follow-up question:

Will the condensed color (now 4:2:2 or 4:4:4) be an advantage (compared to SD 4:2:0) when pulling keys and doing blue-screen work?

David Taylor · February 1st, 2006, 02:06 PM

Quote:

Originally Posted by Per Kristian Indrehus

My point exactly :)

Will the condensed color (now 4:2:2 or 4:4:4) be an advantage (compared to SD 4:2:0) when pulling keys and doing blue-screen work?

You bet.... But SD is not usually 4:2:0 unless it's been converted into or captured (less common) in MPEG. Most forms of DV are 4:1:1, but the answer of "you bet" still applies for 4:1:1 DV source.

David Heath · February 1st, 2006, 05:39 PM

Quote:

Originally Posted by David Taylor

But SD is not usually 4:2:0 unless it's been converted into or captured (less common) in MPEG. Most forms of DV are 4:1:1,........

That's true in 60Hz countries, but the previous poster is from Norway. For PAL, SD normally is 4:2:0 in the DV variants, at least as far as DV and DVCAM are concerned, the exception here being DVCPRO which is 4:1:1. (And that is far less common than the other two.) An obvious advantage is when a DVD is the end product - using DV or DVCAM means the same colour space (4:2:0)exists throughout the production process.

Alex Leith · February 3rd, 2006, 09:33 AM

Quote:

Originally Posted by David Heath

An obvious advantage is when a DVD is the end product - using DV or DVCAM means the same colour space (4:2:0)exists throughout the production process.

Almost... except the 4:2:0 sampling on a DVD is different in structure to the 4:2:0 sampling used in PAL DV...

David Heath · February 3rd, 2006, 05:47 PM

Quote:

Originally Posted by Alex Leith

........the 4:2:0 sampling on a DVD is different in structure to the 4:2:0 sampling used in PAL DV...

Could you elaborate? I'm aware that a DVD uses MPEG2, whereas DV is intra frame compression only, but my quote referred to colour space which I thought was the same in each case. Or am I missing something?

Alex Leith · February 4th, 2006, 02:57 AM

Quote:

Originally Posted by David Heath

Could you elaborate? I'm aware that a DVD uses MPEG2, whereas DV is intra frame compression only, but my quote referred to colour space which I thought was the same in each case. Or am I missing something?

Basically the matrix of colour samples are a different shape.

PAL DV uses co-sited 4:2:0. Cb and Cr are sampled with every second Luminance samples on alternating lines. It goes:

Line A: Y+Cb, Y, Y+Cb, Y
Line B: Y+Cr, Y, Y+Cr, Y

MPEG-2 uses inter-sited 4:2:0. Cb and Cr are still sampled on alternating lines, but they're offset from the Luminance samples (which is a little hard to depict without a graphic). It (sort of) goes:

Line A: Y, Cb, Y, Cb
Line B: Y, Cr, Y, Cr.

So basically the colour samples for MPEG don't directly align with the colour samples for DV (either PAL or NTSC).

January 31st, 2006, 12:11 PM	#1
Kristian Indrehus Regular Crew Join Date: Sep 2005 Location: Norway Posts: 123	Color resolution when downconverting to SD I have read several places that HDV color resolution, when downconverted to SD in the right codec, can hold 4:2:2 colors. Can anyone verify this, and if so explain exactly how it works ?

January 31st, 2006, 01:45 PM	#2
Ben De Rydt Regular Crew Join Date: Jan 2005 Location: Belgium Posts: 139	1080i HDV (1440x1080 pixels, 4:2:0) contains 720x540 chroma values (Cr and Cb). Enough for 4:4:4 SD (NTSC). Your NLE shouldn't lose chroma values when downsizing. It will in all likeliness decompress the HDV material to uncompressed 4:4:4 or 4:2:2 before the downscale to SD. __________________ Please keep in mind that English is not my native language.

January 31st, 2006, 03:01 PM	#4
Ken Hodson Trustee Join Date: Aug 2003 Location: Vancouver BC Canada Posts: 1,315	When you are downsampling resolution, as long as it is into a codec that will not restrict colour(chroma) info, the downsample will not stay locked at the origional 4:2:0, but will condense the chroma info that it has available. If that is SD 4:4:4, it will take advantage of that space. You aren't gaining anything, you are just condensing what was already there. For best results I have found using a codec that does intelligent upsampling to 4:2:2, such as AspectHD. Then perform FX work or do CC in 16bit rendered out uncompressed or other non-restrictive codec. When all is done downsample to SD uncompressed and then render to DVD. I see the point of having the prestine 4:4:4 SD downsample, is it allows super quality DVD's. Very pro looking. I see the pre-downsampled HD as the best to do FX manipulation on, as the final downsample "cleans" everything up. Thats my preferance. __________________ Damnit Jim, I'm a film maker not a sysytems tech.

January 31st, 2006, 02:53 PM	#3
Emre Safak Trustee Join Date: Feb 2004 Location: Brookline, MA Posts: 1,447	Now that's a great reason to shoot in HD.

January 31st, 2006, 03:17 PM	#5
Steven White Major Player Join Date: Nov 2004 Location: Canada Posts: 547	Agreed with Ken on all accounts. -Steve

January 31st, 2006, 03:49 PM	#6
David Taylor Major Player Join Date: Sep 2003 Location: Solana Beach, CA Posts: 853	Well said Ken.... When SD is the final intended output, the "oversampling" afforded by the source HD signal produces very professional looking SD by increasing the effective chroma fidelity of the SD output compared to the 4:2:0 HD source.