Downconverting XDCAM HD to SD - problems?

[Andy Mees]

no argument with that. hence why I use a software downconvert ... the story needs feeding asap !

don't forget though that the F70 for example does a high quality downconvert to uncompressed SD, so I'm not suggesting using the downconvert via DV per se.

[Greg Boston]

Originally Posted by Andy Mees

don't forget though that the F70 for example does a high quality downconvert to uncompressed SD, so I'm not suggesting using the downconvert via DV per se.

Absolutely. I keep forgetting that the deck can output SD SDI. Until recently, I sincerely believed the camera was capable of the same.

-gb-

[Greg Boston]

Originally Posted by Paul Gale

I don't understand much about colour space issues and how this is improved by downconverting - is there a primer or other good info out there that I can read up on?

First you need to understand the basics of chroma subsampling. Color space is a different issue (RGB vs. YUV) and I used that term mistakenly in my post above.

Chroma subsampling refers to how much color information is thrown out in the interest of reducing bandwidth of the signal so that it can be reliably recorded on the medium. It's a cost decision on the part of the mfg as well as market segmentation.

4:4:4 means as you go across one horizontal line a distance of 4 pixels, you have 4 luma samples, 4 red samples, and 4 blue samples.

4:2:2 means in the same 4 horizontal pixels, you would still get 4 luma samples, but the red would repeat for two pixels, as would the blue. This reduces the color resolution, but saves on data volume.

4:1:1 means in the same 4 horizontal pixels, you still get 4 luma samples, but the red repeats for 4 pixels, as does the blue. This further reduces the amount of color resolution, but saves even more space.

4:2:0 is a different animal. It employs an alternating line strategy where as you have 4 luma samples then 2 red samples, and no blue samples ON ONE LINE. On the next line, it would give 4 luma samples, then 2 blue samples, and no red samples. The pattern alternates the red and blue going down each line through the frame.

Both HDV and XDCAM HD use 4:2:0. When you down convert, you squeeze all that information into a smaller space so the color resolution has an 'apparent increase' as Nate says.

Hope this helps,

-gb-

[Brett Sherman]

Originally Posted by Nate Weaver

If you do that same oversampling trick by shooting HD, and downconverting later (in your computer) to a format better than DV (like uncompressed SD), then the 4:2:0 color resolution of XDHD becomes an effective color resolution of 4:2:2 or better, a side effect of the image resizing. You also skip the DV compression. If you see this often enough, you get to the point where you can spot it quite easily; DV has a "coarseness" that you can spot after you get a good look at something better.

Does the 350 output a downsampled SD signal via the HD-SDI output (or component output on the 330)? If so, that might be another workflow. Of course you'd loose you timecode reference and clip organization, but in select circumstances could be a way to avoid conversion within the computer.

[Nate Weaver]

Originally Posted by Brett Sherman

Does the 350 output a downsampled SD signal via the HD-SDI output (or component output on the 330)?

No to downconvert out on the HD-SDI of the 350. Dunno about the 330. Something makes me think that WOULD work.

That's the problem with hardware downconvert on the camera, on the 350 you only get composite which is no good for post.

I kind of also disagree about hardware being better than software for downconverts. Hardware would be better in the case of a Terenex box with HD-SDI, but not as good if we're talking about the composite out on the 350.

With the software tools I have, I have a lot of control over the process, including fine control over putting edge enhancement back in.

Not to mention that software is just the most convenient place for it to be for a lot of my work. If I had to run HD-SDI out in real time to tape, and back in again just to author the DVD project I'm working on, I'd go insane.

If I need to run off a Digibeta or SP, all I have to do is export my timeline and go to Starbucks (er, and have a deck rental delivered :-)

[Paul Gale]

I'm interested in the SDI out of the 350 as well - what EXACTLY does it give you - compressed or uncompressed?

Also I'm led to believe it's AFTER the compression sectionof the camera i.e. you don't get anything better than the original 4:2:0 - or is that not true?

[Greg Boston]

Originally Posted by Paul Gale

I'm interested in the SDI out of the 350 as well - what EXACTLY does it give you - compressed or uncompressed?

Also I'm led to believe it's AFTER the compression sectionof the camera i.e. you don't get anything better than the original 4:2:0 - or is that not true?

Wrong! SDI is ALWAYS uncompressed 4:2:2. Dual HD SDI can give you 4:4:4. However, to capture HDSDI, you need to have a capture board and a RAID array to be able to handle all the data coming in at that speed.

Think about it, Paul. What good would an SDI connection be if it were compressed and chroma subsampled like the other options.

-gb-

[Paul Gale]

OK cool - so provided I had a PC with suitable capture card and RAID array, I could capture to disk in the studio and get a better key with chromakey than with the XDCAM recorded material?

I was confused as I asked this elsewhere and was told I wouldn't get a better result.

[Paul Gale]

Oh, and what kind of data rate do you need for HD SDI - in MB per sec?

[Rob Stiff]

Compressor 2 from Apple's Final Cut Studio does a fantastic
job downconverting HD XDCAM footage to SD. If you are editing
in HD XDCAM and your final goal is to put it out on DVD, then
do a straight downconversion to SD MPEG-2. Progressive HD video
downconvered to progressive SD looks great. Adjusting the different filters will yield different results. This advice was given to me by Nate Weaver and it has worked well.

I have had XD HDCAM footage tested/converted with high-end Terenex eqipment. Although impressive, not needed unless doing any live stuff.

[Greg Boston]

Originally Posted by Paul Gale

OK cool - so provided I had a PC with suitable capture card and RAID array, I could capture to disk in the studio and get a better key with chromakey than with the XDCAM recorded material?

I was confused as I asked this elsewhere and was told I wouldn't get a better result.

The confusion may be coming from whether or not you take a live feed from the camera, or play back the disc via HDSDI. In the latter, the 4:2:0 subsampling and Long GOP MPEG2 compression has taken place already. In order to fit the HDSDI spec, the disc playback would be upsampled back to 4:2:2 but won't be as good as recording it natively via a live camera head feed.

-gb-

[Paul Gale]

Yes that makes sense thanks.

[Derek Prestegard]

You need OVER 150MB/s for 1080p uncompressed :)

It's quite impractical, but that's where lossless codecs like Sheer and Cineform come into play.

Also, the comment about blurring before downscaling seems awfully misguided to me.

If you're getting aliasing or other resizer related artifacts when doing a large downconversion, then the answer is to switch the resize algorithm. I don't know what algorithm your method uses, but it's probably Bicubic (sharp?), as that's the standard.

A bilinear resize would probably be a little bit softer without all the aliasing, but much better than blurring out all the details before you downconvert! Especially a dumb blur like the one FCP has.

I hate to chirp in and add some Windows workflow, but if you have a Windows system available, and want to do the best possible down conversion possible (in software, without something like a Snell & Willcox Alchemist), then AviSynth is your best friend. It has a vast selection of resizers all the way from a primitive point resize, to the more common bilinear and bicubic, to the more advanced lanczos3, lanczos4, spline16, spline36, gauss, etc... The consensus seems to be that for large scale resizing, spline resizers maintain detail without introducing sharpening halos.

If you want a sharper still result, you can sharpen the HD input first with something like LimitedSharpenFaster, and then downconvert.

It's also got some wonderful deinterlacers, and denoisers. forum.doom9.org is the unofficial 'home' for AviSynth usage and development. Everyone should really give it a read. It's a forum mostly targeted towards DVD backup, but they really know their stuff about processing, and the pros could really learn a thing or two from the open-source guys :)

~Derek

[Uli Mors]

@Greg:

Thanks for the Color Sampling information - this was the best (and easiest) explanation I´ve ever read.

Uli

[Greg Boston]

Originally Posted by Uli Mors

@Greg:

Thanks for the Color Sampling information - this was the best (and easiest) explanation I´ve ever read.

Uli

Thanks for the kind words, Uli. We're here to help each other and that's the best compliment one can receive.

-gb-