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Transcoding to DNxHD decreases quality

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Thomas Schöbinger
Transcoding to DNxHD decreases quality
on Nov 3, 2015 at 1:07:03 pm

I started editing on Adobe Premiere CS6 but later switched to Avid Media Composer 6.5 for several reasons, most importantly the big set of useful shortcuts, file management and the flawless playback when dealing with transcoded material. I was always told (by teachers at my university as well as when browsing forums on the web) that the generation loss during the import is negleable and that DNxHD is a codec that preserves a lot of image quality.

I work with a lot of DSLR footage and when comparing an original frame with a transcoded frame, the difference really isn't too noticeable. But when I take a frame which is rather dark and add some color correction, the issues come to light. In the below picture the frame on top was AMA-linked, while the frame at the bottom was transcoded to DNxHD 185. The source material is from a Canon 5D Mark3, 1080p25, which matches the project settings. I put the same color correction effect (black/white and some curves) on both clips, than exportet both as DNxHD 120 and put them together in After Effects.

It's needless to say that the above picture looks much more grisp and has far less issues with banding and compression artefacts. The transcoded clip looks washed out and you can see serious artefacting and banding (for example on the woman's forehead). I've tried many different Avid Codecs, even 1:1 10bit MXF and DNxHD 365, but none looks anything as good as the original file when it comes to color correction.

I guess there is no solution to my problem except for switching to another software, but I wonder: If importing footage into Avid obviously reduces the possibilities of color correction, not to say that it decreases your over all image quality, how come nobody seems to care? Most companies I know use Avid.

Does this loss of quality bother any other Avid editor out there? How do you deal with it? Is an offline-online workflow via some other software like Davinci Resolve really the only way to get the most of your footage, while still using Avids great editings features? I mean, I'm not talking about some ARRi raw footage or something like that, just some simple h.264-compressed DSLR-footage.



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Shane Ross
Re: Transcoding to DNxHD decreases quality
on Nov 3, 2015 at 5:54:12 pm

I don't think I've noticed quality this bad before. Then again, I don't transcode to DNx185, and then to DNx120 on export. I either stick with DNxHD 185 throughout...exporting a QT in that codec...or output to tape. Wondering if the transcode to 185, and then again to 120 is the cause. Because that's two levels of transcoding, where your AMA H.264 only had one at that point...H.264 to DNx120.

Why not export the transcoded footage SAME AS SOURCE...DNx185. And then the H.264 footage, export as DNx185...and then compare?

Shane
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Read my blog, Little Frog in High Def


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Thomas Schöbinger
Re: Transcoding to DNxHD decreases quality
on Nov 3, 2015 at 6:29:02 pm

No, the transcoding while export surely isn't the issue. I just tried exporting both the transcoded frame and the AmA-linked one separately and it looks exactly the same. The difference is already visible inside Avid, the export doesn't really change anything.

I'm not saying that the transcoding itself makes the footage look as horrible as in the picture. The difference only really becomes visible when color correction or other effects like keying are applied. Both frames that I posted where at some point transcoded to DNxHD, since I exported them from Avid, the only difference is that in the above picture it happened after making the picture black and white and modifying the luma curve.


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John Pale
Re: Transcoding to DNxHD decreases quality
on Nov 3, 2015 at 6:42:01 pm

If you are on a Mac, you can also transcode to the Apple ProRes MXF. They are considered "native" for Avid and should have identical performance. I'd be curious to see if there is any quality difference.


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John Pale
Re: Transcoding to DNxHD decreases quality
on Nov 3, 2015 at 6:46:11 pm

And yes. You can also use DaVinci Resolve to do your transcodes and see if that helps.

You probably have an Avid support contract if you own the software or are a subscriber. You should contact Avid support and let them know what you are seeing. This should not be happening.


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Thomas Schöbinger
Re: Transcoding to DNxHD decreases quality
on Nov 3, 2015 at 7:08:30 pm

No, I'm on a PC so I guess importing ProRes won't work. Still, I tried importing as XDCAM HD which looks somewhat different. More noisy, therefore less banding.


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Thomas Schöbinger
Re: Transcoding to DNxHD decreases quality
on Nov 3, 2015 at 8:21:54 pm

Okay, apparently the issue goes away if I do not import the footage, but transcode the AMA-linked clips. Looks way better. What exactly is the difference between these two methods of ingest? They both convert the footage to DNxHD, don't they?


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Juan Juig
Re: Transcoding to DNxHD decreases quality
on Nov 4, 2015 at 12:22:18 am

I am not sure whether this has anything to do with it but.. perhaps you are importing the footage with wrong colour space. For example importing 601/709 footage as RGB results in lower contrast. Try out different import options and let me know what happens.


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Thomas Schöbinger
Re: Transcoding to DNxHD decreases quality
on Nov 4, 2015 at 6:24:55 am

The color levels also aren't the issue. It's DSLR footage, import settings are RGB. If I select Rec709 when importing, the blacks are crushed but besides that the image looks just as bad, with heavy blocking. Transcoding via AMA is also a lot faster. Retranscoding my whole footage wasn't really a big deal.

There seems to be another difference between import and Ama-link-transcode:


The Image shows the RGB-Parade of a black to white gradient I created in AE. As you can see, importing changes the gamma, while transcoding keeps everything as is. The gradient is RGB, thus 0-255. Why is it that the AMA-linked clip is still displayed as 16-235?


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Juan Juig
Re: Transcoding to DNxHD decreases quality
on Nov 4, 2015 at 7:15:21 pm

Avid's workspace by default is 16-235, but if you import the footage correctly that won't have an effect on footage. You can still export as RGB and everything will be fine.

So the conclusion is that it is better to AMA link and then transcode the footage?


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John Pale
Re: Transcoding to DNxHD decreases quality
on Nov 5, 2015 at 2:31:30 am

Wait....you were importing DSLR footage? Good grief...don't do that. AMA link then transcode. Its more modern, accurate..and also faster.


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Thomas Schöbinger
Re: Transcoding to DNxHD decreases quality
on Nov 5, 2015 at 6:51:32 am

Yeah, so that's the conclusion: Always AMA-link and transcode.

What's the deal with that? If Avid has a newer method of converting the footage, which preserves more quality, doesn't mess with the gamma (as you can see in the waveform-picture I posted) and even goes a lot faster, why is there still the import option, or, why don't they change the import option so that it is as good as AMA-linking and transcoding? If you are not familiar with those facts it seems much more convenient to always click on import when you are planning to transcode the footage anyway.

Or is this some particular issue only occuring with h.264/DSLR footage?


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John Pale
Re: Transcoding to DNxHD decreases quality
on Nov 5, 2015 at 1:33:12 pm

Agreed, it's kind of a mess for new users. I think Avid fails to appreciate how utterly weird the ingest process is for someone unfamiliar with what's been going on in Avidville for the past decade.

AMA (now simply called Link) has been around a fairly long time now. When it was first introduced, it wasn't very capable and people still used Import for most things. Gradually, it's gotten to the point where you can use it for most everything. Import needs to stay there for backwards compatibility with old projects at the very least. Personally, I still use Import for audio files and graphic files with alpha channels, as I find the behavior more reliable. Unfortunately Import still uses the old QuickTime engine to do its thing (relying on tech that Apple itself has deprecated). It's been largely unchanged for years. It's not just a DSLR issue.


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Thomas Schöbinger
Re: Transcoding to DNxHD decreases quality
on Nov 5, 2015 at 4:02:28 pm

Okay, at least now I know. Thank you all a lot for the help.


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