Downconverting XDCAM HD to SD - problems?
 

I've heard that there can be problems downconverting HD material to SD, especially if there's a lot of fine detail in a scene. Does anyone do this on a regular basis - have you, too, found problems?

I guess if I know for certain that the material will be used in the SD realm, I'll just shoot in DVCAM mode - but it would be nice to have the option and not to worry about conversion issues.

I guess it's the same issue that faces other conversions i.e. how do you represent single pixel detail in less than a pixel! ;)

Thanks,

Paul.

Well, simply put, you cannot retain all of the information and detail of a pristine HD source in an SD downconversion.

If you use a very good resizer / deinterlacer, you can get excellent results, but SD will never be HD.

An ideal workflow IMHO would involve AviSynth - Load up the HD source, if it's interlaced, deinterlace it with TDeint() or MVBob.SelectEven(), and then use a sharp resize like LanczosResize(720,480) or Spline36Resize(720,480).

If you need to deliver 480i, then you can use TDeint(mode=1) to bob, or omit the SelectEven() from MVBob to get SD 59.94p output, which can be re-interlaced to 29.97. Doom9 has lots of good info regarding up/down conversion

Thanks Derek.

Please always remember, that there are users from all over the world in this forum. The original poster comes from Great Britain, and so he need to transfer his material to a PAL target: 25 Frames, 576 Lines and so on.

There is a world outside NTSC... :-)

Matthias

>how do you represent single pixel detail in less than a pixel?

Blur (apply 1 pixel, or even only 0.5 pixel, of blur to your image in the NLE of your choice) then downscale.

mmm - ok.

How does that work for you in comparisson to a straight HD to SD convert? Do you find the result much softer?

I'd have to say not. Any percieved softening is counteracted by the sharpening implicit in the downscale process.

That said, I offer it only as a suggested workflow which you may want to try.


You don't mention your target SD format, or your platform, NLE or other details, so I went for a very basic one word answer for you!

I work in FCP, and use a 0.5 pixel channel blur then export a QT Movie w/ Current Settings. (ie I keep the QT export as HD)
Following that I use MPEG Streamclip to downscale the QT to a DV PAL AVI file, I enable the "Better Downscaling" option in Streamclip's AVI exporter options.

The results work well for me. Without the blur, fine detail can cause the image to "sparkle".

OK thanks Andy - some good things to try there :)

I must say - I don't currently have an XDCAM camera but am seriously thinking of going that route shortly, hence the questions.

I'll also wait to see what's announced at NAB before jumping in too! I've heard various rumours...

Rumors are rumors, Paul.

I am in the NTSC world, so I cannot speak for the issues in PAL land but, I find that the downconverts from HD to SD on my F350 look better than material generated as SD (DVCAM). That may not be the case in all situations.

Paul, people in the film production world have been shooting HD for SD since 2001 or so, when the F900 and Varicam became available en masse.

There's a whole business of national and regional commercials, most of which used to be shot on 35 and 16 only, but because of shrinking budgets and the availability of HD 24p, started looking to HD. The F900 was the first video camera to give these folks the 24p they needed, so making good, high quality downconverts became the norm. I jumped from the film business to the video business as this was going on.

HD to SD downconverts (good ones) are better than shooting SD in the first place because of the concept of oversampling. If you take a huge, high-resolution image, and shrink it, you minimize a lot of the undesirable compression/colorspace artifacts that came with the original HD format. By the way, this is also why telecined 35mm looks so good as well...it virtually becomes grainless when all that information is shrunk to NTSC or PAL resolution.

So in the case of the 350, you're much better off shooting HD and downconverting to SD. The resolution might be the same in SD mode (because the image off the chips is still oversampled down to 720x480), but then that oversampled image is compressed to DV, with it's 4:1:1 color resolution.

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.

One other thing, mentioned by Andy above that often doesn't get mentioned (except when I'm rattling on about it). A lot of cameras don't re-introduce edge enhancement to the downscaled image on their downconverts...I have yet to see an HDV camera that does this. The XDHDs do, and you have 3 steps of adjustment. HDCAM and D5 decks have this as well, as well as standalone boxes like a Terenex. The in-camera edge enhancement gets averaged out in the image re-size, so often it's necessary to re-inroduce it on downconverts. Edge enhancement is something that's added to even a lot of film material coming out of a telecine these days, so it's important to keep it in mind in your downconverts.

I use the camera to downconvert to SD. There are menu settings for SD detail (high/mid/low) and Cross color (on/off) which are applied to the downconverted signal.



-gb-

I'd certainly agree that a hardware downconvert, including those available in your camera or deck, is almost always better than anything acheived in software.

Thanks Nate/Greg :)

How about chromakey? - If I wanted to end up with web based material, say at 400x300 for instance, would I be better keying in HD, SD or 400x300? (I'm thinking especially of what you said about the colour sampling).

Thanks,

Paul.

Yes, but Nate makes a good point about doing NLE downconversion to a higher color space SD, like uncompressed or DVCPRO50.

Part of it depends on how big of a rush you are in to get the material out.

-gb-

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?