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Old December 19th, 2008, 11:50 PM   #1
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HD to SD Downconversion

Ok,

So in the past week or so, there has been a lot of talk about the footage coming out of the EXx cameras not converting to SD very well. I tried the experiment for myself last night and mine worked ok. So tonight I thought I'd give something a go.

I took a reasonably clean 1080p interview, and cut a 5 second snippet out of it. I then resized it to 1280x720 and made an .mp4 file as a control. After doing that, I created a 16:9 720x405 file. From that, I created an mp4, and an mpeg2 file ready to drop into my authoring software.

I uploaded all three files on Youtube, and onto Vimeo as well. You can download all three files from Vimeo from here:

Perrone Ford on Vimeo

The Youtube site is here:
YouTube - perroneford's Channel


The workflow:

1. Load 1080p .MXF file into Sony Vegas adjust exposure.
2. Render 1080p uncompressed .AVI file
3. Open 1080p .avi in Virtualdub. Resize to both 1280x720 and 720x405 and save uncompressed versions
4. Open 720p file in Vegas and render .mp4 at 6 mbps.
5. Open 405p file and render .mp4 file
6. Open 405p file and render widescreen mpeg2 file
7. Upload files to youtube and vimeo

From my testing, I am able to VERY clean SD files from 1080p sources with a minimum of fuss. If virtualdub understood the MXF files, it would save a step, but it does not.

This also show me that the creation of SD footage is NOT tied to the camera or the XDCamEX codec. It is entirely possible to get lovely footage at SD sizes from the 1080p original.

I will do a file replacement on the Vimeo site for the 720p file so it can be seen there in HD. The Youtube one shows properly but one of the 405p files is not yet showing.

I hope this helps out.
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Old December 20th, 2008, 02:05 AM   #2
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Quote:
Originally Posted by Perrone Ford View Post
I took a reasonably clean 1080p interview, and cut a 5 second snippet out of it. I then resized it to 1280x720 and made an .mp4 file as a control. After doing that, I created a 16:9 720x405 file.
So I'm back at my assumption that you don't know what you're doing.
Resizing 16:9 to 720x405 for DVD-use just doesn't make much sense.
Quote:
Originally Posted by Perrone Ford View Post
6. Open 405p file and render widescreen mpeg2 file
What's exactly happening in this step?
Any resizing/resampling of the pixels?
If no: It will look distorted on your TV, because DVDs don't use square pixels, but your 405p-file does.
If yes: down to 360 or up to 480 lines?
If down to 360 lines: you're using letterboxing instead of true widescreen.
If up to 480 lines: You're interpolating 75 extra-lines from the 405 initial ones instead of going directly from 1080 to 480. That's sub-optimal.
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Old December 20th, 2008, 02:14 AM   #3
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Thanks for your detailed workflow, I am sure it must work otherwise you wouldn't have posted the formula.

On trawling through this site and other I have found many workarrounds to the problem of converting HD to SD - most seem to be long winded and rather defeat the object of working with Solid State memory. Over the last two days I have burnt over 20 test DVDs trying to find the answer, each DVD had its own set of problems, the main one being artifacts on fine detail. My main reason for choosing the EX3 was that I could shoot in NTSC or PAL format, for my DVD sales the North America market is very important, if this were not the case then I may have purchased the Z7.

The sollution came late last night. The EX3 has a built in downconverter, which will output files in the SD format. You do need to have a video capture card which will accept the Component video cable (RGB cable). I have a Matrox Rx2 card. From the Video Set menu select YPbPr/SDi OUT and select SD. From your NLE select Capture and set the EX3 to Media, select thumbnails and choose a video clip, press the play button and start recording the video to your computer. The footage will be added to the clip bin in much the same way as capturing from tape.

OK, so it isn't as convenient as being able to drag a file straight from the card, but at least you get to review the footage as it is being imported.

I will continue to find a better solution, I am sure you must be able to to downconvert a file from one memory card to another, if this is the case then you should be able to drag the files directly into the timeline.

Finally, the footage captured using my technique above is perfect, certainly as good and better than anything I previously captured using my Canon XH A1.

I used footage that was shot at 1920 x 1080, 1440 x 1080, 1280 x 720 interlaced and progressive, they all worked perfectly. My tests were all done using NTSC, but I will repeat them using PAL footage at a later stage.
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Old December 20th, 2008, 02:36 AM   #4
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Quote:
Originally Posted by Vincent Oliver View Post
Over the last two days I have burnt over 20 test DVDs trying to find the answer, each DVD had its own set of problems, the main one being artifacts on fine detail.
ok, let's memorize for a minute, the problem are artifacts on fine details. Perhaps you're using a bad mpeg2-encoder with bad settings, but who cares...
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Originally Posted by Vincent Oliver View Post
The sollution came late last night. The EX3 has a built in downconverter, which will output files in the SD format. You do need to have a video capture card which will accept the Component video cable (RGB cable). I have a Matrox Rx2 card. From the Video Set menu select YPbPr/SDi OUT and select SD. From your NLE select Capture and set the EX3 to Media, select thumbnails and choose a video clip, press the play button and start recording the video to your computer. The footage will be added to the clip bin in much the same way as capturing from tape.
So this back-to-analog workaround doesn't show artifacts on fine details? Did you ask yourself the question: Why? Because there's analog bluring and sub-optimal downscaling of the ex3 happening which finally entirely destroys the fine details? Or noise and other analog artifacts cover the initial problem of artifacts on fine details?

Quote:
Originally Posted by Vincent Oliver View Post
I have found many workarrounds to the problem of converting HD to SD
Converting HD to SD isn't a problem.
Problems are:
-the halting problem
-a lot of people don't know the basics of digital media
-people who don't know those basics are answering questions in forums like these, so a lot of confusion gets into them.

Last edited by Dominik Seibold; December 20th, 2008 at 03:26 AM.
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Old December 20th, 2008, 02:36 AM   #5
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Quote:
Originally Posted by Dominik Seibold View Post
So I'm back at my assumption that you don't know what you're doing.
Resizing 16:9 to 720x405 for DVD-use just doesn't make much sense.

What's exactly happening in this step?
Any resizing/resampling of the pixels?
If no: It will look distorted on your TV, because DVDs don't use square pixels, but your 405p-file does.
If yes: down to 360 or up to 480 lines?
If down to 360 lines: you're using letterboxing instead of true widescreen.
If up to 480 lines: You're interpolating 75 extra-lines from the 405 initial ones instead of going directly from 1080 to 480. That's sub-optimal.
Domininik. I know you are trying to help (me and others) here. So I'll be brief. These files are going to youtube, or being tested on my PC. Which uses square pixels. When going to DVD for TV, I am not using square pixels. My tests are not designed to be pixel aspect ratio correct, only to be able to determine if we are losing resolution along the path.

I invite you to view this file which I uploaded some hours ago and I assume you have not seen:

Interview 405p Mpeg2 on Vimeo

That is a REAL render for DVD output from a 720x480 timeline with a 16:9 source.

Here is another I was working on as I was typing:
http://www.vimeo.com/2581948
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Old December 20th, 2008, 02:54 AM   #6
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Originally Posted by Perrone Ford View Post
That is a REAL render for DVD output from a 720x480 timeline with a 16:9 source.
Ok I believe you, that you know how to do it.
But then what's your posted workflow for? It's not for people who want to do DVDs, though the title "HD to SD Downconversion" let people suppose that, and it's not for people who want to upload youtube-videos, because youtube supports 720p.
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Old December 20th, 2008, 03:08 AM   #7
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Quote:
Originally Posted by Dominik Seibold View Post
Ok I believe you, that you know how to do it.
But then what's your posted workflow for? It's not for people who want to do DVDs, though the title "HD to SD Downconversion" let people suppose that, and it's not for people who want to upload youtube-videos, because youtube supports 720p.
This is why I said yesterday I didn't want to get into the whole pixel aspect ratio thing. This workflow is really just so people can compare what they see on the screen between their 1080p or 720p out of the camera, to a rendered mpeg2 file which would be the end of the workflow. Because these comparisons are done on the computer (and not burning a new DVD every time they want to compare) I thought it might be helpful.

So I'll say it here. If you are ACTUALLY BURNING A REAL DVD, keep your project at 720x480 for an NTSC SD DVD. PAL folks can adjust as necessary.
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Old December 20th, 2008, 03:13 AM   #8
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Quote:
Originally Posted by Dominik Seibold View Post
Ok I believe you, that you know how to do it.
But then what's your posted workflow for? It's not for people who want to do DVDs, though the title "HD to SD Downconversion" let people suppose that, and it's not for people who want to upload youtube-videos, because youtube supports 720p.
What I am hoping to glean, is whether people are losing quality in the step where the video gets scaled from 1080/720 to 480, or if they are losing the quality from their mpeg2 codec. In following this workflow, if it's the scaler, then step 5 is going to look bad. If step 5 looks solid, but step 6 does not, then it's the codec. If we look bad at step 4, then we have other issues like a possible interlace problem, or something else going on.

Make sense?
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Old December 20th, 2008, 03:22 AM   #9
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What I am hoping to glean, is whether people are losing quality in the step where the video gets scaled from 1080/720 to 480, or if they are losing the quality from their mpeg2 codec.
If people would start to post stills from their (problematic) mpeg2-results instead of writing a thousand dubious words, we could immediately tell them where the problem in their workflow is. But posting stills from mpeg2-files seems to be even harder for them...
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Old December 20th, 2008, 03:30 AM   #10
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If people would start to post stills from their (problematic) mpeg2-results instead of writing a thousand dubious words, we could immediately tell them where the problem in their workflow is. But posting stills from mpeg2-files seems to be even harder for them...
Some have indicated that they don't see the problem on stills on the computer. So my next best thought was to take the thing all the way through mpeg2 render. If it looks good all the way through mpeg2 on the PC, then the problem is hardware / firmware related down the chain.

What I found mystifying was people saying that the CAMERA was not giving them good SD results. I just couldn't fathom that at all. If the camera is giving you stunning 1080p footage as everyone claims, then the fault is not with the camera or the codec. It's elsewhere in the chain.

Here is the 720p downres of that second interview clip: YouTube - iv2 720p mpeg4
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Old December 20th, 2008, 03:45 AM   #11
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Originally Posted by Perrone Ford View Post
Some have indicated that they don't see the problem on stills on the computer.
Perhaps we see. ;)
Quote:
Originally Posted by Perrone Ford View Post
So my next best thought was to take the thing all the way through mpeg2 render. If it looks good all the way through mpeg2 on the PC, then the problem is hardware / firmware related down the chain.
That's why I said "stills from the mpeg2-results", so the very end of the chain.
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Originally Posted by Perrone Ford View Post
What I found mystifying was people saying that the CAMERA was not giving them good SD results. I just couldn't fathom that at all. If the camera is giving you stunning 1080p footage as everyone claims, then the fault is not with the camera or the codec. It's elsewhere in the chain.
Absolutely. Btw, did you know that your choice of the lanczos-rescaler is a very good one, because the lanczos-rescaler is the best rescaler actually programmable? :)
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Old December 20th, 2008, 03:58 AM   #12
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Originally Posted by Dominik Seibold View Post

Converting HD to SD isn't a problem.
Problems are:
-the halting problem
-a lot of people don't know the basics of digital media
-people who don't know those basics are answering questions in forums like these, so a lot of confusion gets into them.
Surely that is why we come to forums such as this one, to find out what all the settings mean. I will be the first to admit that I don't know all the basics, but I am learning.


I have no problems with the camera, it is a fine instrument. All I want is to be able to produce decent quality as an end result - this may very well be down to the codecs, software etc. I am still finding out.

You are right my DVD has been downgraded in quality when compared to the original footage, but then I would expect this from SD. I am not 100% there yet, but nearer today than yesterday.
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Old December 20th, 2008, 04:33 AM   #13
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Which software do you use? Windows or Mac?
The first hint I can give you is to try to use some blur to make the edges softer. That will reduce mosquito-noise around edges and flickering on CRT-TVs. Also, if done before downscaling, it will reduce aliasing introduced by low-quality downscaling.
Please forget your analog workaround soon. It's far from being reasonable in any way.
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Old December 20th, 2008, 05:24 AM   #14
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I use Premierre Pro CS3 (Windows) and Encore CS3. I also have a Matrox RTX2 card with its own set of Codecs.

Up to now I have been bringing the video files directly into the timeline and then using the Adobe/Matrox Media encoder to output the file ready for burning in Encore. The Media encoder Transcodes the file on the fly. Premierre also has a XDCAM EX preset which does work, although it still relies on the Adobe/Matrox media encoder to produce the DVD ready file. I suspect the encoder may be at the root of the problem.
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Old December 20th, 2008, 06:47 AM   #15
 
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Perrone, maybe I missed something along the way in your explanation. When making DVDs, why not render straight from the HD timeline (mfx) using MainConcept MP2/DVD Architect NTSC Widescreen Video Stream?
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