DA boosting black levels on playback?

entilza72 wrote on 10/2/2010, 5:17 AM

Hi team,

I've noticed some unexpected behaviour recently, and wanted to confirm if others are seeing the same thing.

I'm working on a project with a lot of dark scenes. The dark scenes will often fade to or from black. The file has been graded to ensure levels are correct and look nice.

What I'm seeing on DVD Architect's preview screen is a much lighter image than if I play the video file back to the computer screen. The blacks have been boosted to a murky grey/black color. Further, any solid black gets boosted even more, so much so that when the solid black fades up into the dark image, the image actually "jumps" from a solid grey color, back to a blacker color before finishing its fade up and settling down with a more washed out look. It's as if a threshold is reached and the image is being turned back "down".

The original file exhibits none of these problems when played back on the same monitor.

To confuse matters, the burned DVDs and Blu-Rays exhibit the same black problem when played back on a Sony Playstation. Yet, when playing the same DVD back on a simple DVD player with the same monitor, its not there. Admittedly, the monitor's Playstation input has a higher brightness than the DVD input, so that will not be helping.

Any suggestions? Is DA playing with the video levels on its preview? Or playing with them overall?

Or is it something else?

Cheers,
Jason

Comments

musicvid10 wrote on 10/2/2010, 6:28 AM

What "files?" What are their properties? How were they "graded?" Can you upload an example somewhere?

Tests with m2ts of a calibrated gray scale show no difference whatsoever in levels reproduction between the DVDA preview and any other viewer.

"a much lighter image than if I play the video file back to the computer screen. "
Does this mean you are previewing on an external monitor? DVDA "may" do a colorspace conversion for conventional screens, but I haven't tested this.

How the blacks are reproduced on any particular player and screen combination is a total crapshoot. The variations in equipment default and user settings that occur are totally unpredictable, except in the rare cases where they have been professionally calibrated.

entilza72 wrote on 10/2/2010, 8:58 AM

Hi musicvid,

They're m2ts files. Glad to hear about your test - it gives me some hope. The files were graded in Vegas with a video scope using curves, color balance, and levels.

Playing back on Vegas on the same monitor, there is no sign of trouble. Nor does the scope twitch. When the image fades up, the scope fades up smoothly from just below 0. Yet play the same clip from the same file in DVDA and I get a big jump from a "bright" washed out black (should be 0 black) to a semi-grey-black that is half way between the bright washed out black and 0, as the image begins to fade up.

I know what you mean re: most monitors. The DVD authoring and editing are both being done on the same PC LCD monitor. The same monitor gives great results for Vegas and Windows Media Player playback.

I'm not sure how to post an example - posting a clip from the file will be pointless because it is the playback software that we are discussing. Perhaps if I can get the same grey card you have tested with, I can post side-by-side frame grabs? Can you advise where to get them?

Cheers,
Jason

musicvid10 wrote on 10/2/2010, 9:20 AM

"posting a clip from the file will be pointless because it is the playback software that we are discussing.
If you can upload an original sample somewhere (sorry, thought I was clear), I have the same software, so maybe I can see what you are talking about. DVDA preview may be dropping frames on your system to keep up with playback.

m2ts (HDV) should be REC 709, not cRGB, but I don't know how much that has to do with your preview problem, since I fed it full range when I tested.

Here is my gray scale:

entilza72 wrote on 10/2/2010, 4:51 PM

Hi Musicvid,

Thanks for posting that dude. I appreciate it.

Firstly a question: where do you set REC 709 instead of cRGB?

OK, here's what I found:

1. I am saving m2t, not m2ts. If I try m2ts I get a "the error could not be determined" error in Vegas before rendering output starts.

2. Using the greyscale wedge, Vegas displays the greyscale properly. Both in the editor, and on playback of the m2t file in Win Media Player.

3. With the same monitor, using the rendered output, DVDA shows a noticable difference in the wedge with a milkier black (100%). Full screen helps illustrate.

So, at least I am getting the same results. The question is: is this just a harmless color profile change in display? Or is it impacting the outputted media? My DVD player suggests no, but my Blu-Ray suggests yes.

Here's a clip that comes up from 0% (black) to a dark image (I'll only leave it online for a few days):
http://www.redsector.net/eg/example.m2v
(7Mb file)
When it is played back properly, it simply fades up to a dark-ish image from black. When it is played back improperly, it presents with a solid milky black, that switches to a darker (but still milky) black as it begins to fade up.

Cheers,
Jason

entilza72 wrote on 10/2/2010, 6:05 PM

I have captured the difference between Vegas and WMP, vs DVD A in simple screen caps. This is an accurate depiction of the differences in blacks.

http://www.redsector.net/eg/grey-fail.png

musicvid10 wrote on 10/2/2010, 6:46 PM

After the fade-in, the luminance black levels in your clip are 22. The range is 22-89.
They start at 16, drop slightly, then come up to 22 as the clip fades in.

Studio RGB blacks are 16. Computer RGB blacks are 0. So you've got gray.

The difference you see in the DVDA preview isn't apparent to me. They all look flat to me. Checked in DVDA, WMP, VLC, and SMPlayer, also Vegas after muxing to TS.

I don't believe DVD Architect does any colorspace conversions during Prepare. What you give it is what you get, afaik.

musicvid10 wrote on 10/2/2010, 8:07 PM

Since your clip was so short, I copied it multiple times.
First three are your original clip levels, second three are Studio blacks (recommended), and third set are Computer blacks (just for reference).
Your highlight levels were not changed.

http://www.mediafire.com/?c7t0geqkqc47xia

entilza72 wrote on 10/2/2010, 9:18 PM

Thanks Musicvid. Playing back your clips, there was no sign of the "flash".

I have quite a few questions now though! Appologies in advance!

Re: black luminance levels, where are you getting your numbers from? Looking at the Vectorscope in Vegas, I am getting below 0 for the solid black (-8% according to the scope) and then rising to -3% for blacks after the fade up.

I'm guessing your readings are more accurate because they match the symptoms I am seeing: but where are you getting them from?

And how do you force "Studio Blacks" for the overall image (in Vegas I presume - but how)? I know there are "Computer RGB -> Studio RGB" filters under Levels in Vegas, but I have never used these. Is it related to these?

Finally, I believe you are saying that "in a Studio RGB color space, I am wanting my blackest black to be 16". Is that correct? Less than 16 is "off the scale", and above (eg 22) starts to turn grey.

About the only ref to Studio RGB I can find in DVD A is under "Options -> Prefs -> Video Device" is "Use Studio RGB", which is ticked and greyed out (cannot be changed).

Thanks,
Jason

musicvid10 wrote on 10/2/2010, 9:31 PM

I really don't have time to answer all your questions, but in brief:

1) If your scopes aren't showing your clip's blacks at 22, they are set up incorrectly. Forget the vectorscope for now and learn the basics by using the histograms.
2) You should start with Wikipedia and Glenn Chan's basic articles to begin to understand the differences between Computer RGB and Studio RGB in Vegas. 0-255 is absolute, 16-235 leaves some "wiggle room." HD expects 16-235 RGB from Vegas. That's oversimplified, but it'll get you started down the right track.
3) Use the Levels, Scopes and your eyes to set the black levels optimally. Practice, practice, practice. Best of luck.

entilza72 wrote on 10/2/2010, 10:58 PM

Thanks dude.

I think I've figured out the issue after looking up some on Glenn's work.

The project is in Studio RGB format (16-235). Glenn notes this: "If the project is for DVD or broadcast output, we should do something about the default background color (0 0 0 RGB). This is an ILLEGAL black and can cause the picture to roll on very old TV sets."
http://www.glennchan.info/articles/vegas/v8color/vegas-9-levels.htm

What I am seeing on some playback devices is a false black being inserted on the Vegas generated blacks, until the image fades up. This *may* be the device overriding an illegal (sub 16) black image. Its just a guess, but it would also explain why I never see it on playback on the PC (except in DVD A), because the PC is happy with Computer RGB space.

I'm going to add a "16" minimum black, rerender, rebuild, burn, test, and report back.

Cheers,
Jason

entilza72 wrote on 10/3/2010, 2:26 PM

I can now confirm adding a "16 black" instead of the default Vegas "0 black" fixed the problem.

It would appear some playback devices (or monitors?) were overriding the "illegal" 0 black signal they were getting (when the entire image consisted of nothing but 0 black), boosting it to 16. When the image starts to come into the right range, it stops messing with the image, making the noticable dip.

The "minus 0" levels on the vectorscope were also a result of using Computer RGB space on a Studio RGB project. Simply, 16 is represented as 0 on the vectorscope for such projects, making 15 and less be in the -0 range.

Cheers,
Jason.

musicvid10 wrote on 10/3/2010, 2:40 PM

"16 is represented as 0 on the vectorscope"
You need to keep both bias boxes in the scopes settings unchecked. They are not used for HD.

How again are you checking levels on the vectorscope? Seems impossible to me since grayscale is represented as a dot. Something I'm missing here?

entilza72 wrote on 10/5/2010, 2:39 PM

Sorry musicvid - I must have missed something. What do you mean by "greyscale represented as a dot"? I don't recall saying or implying that.

Checking levels on the 'scope simply by looking at the range from 0 to 100.

BTW - thanks again for the time you've taken in this thread. You allowed me to find and solve the issue, learn something, and then learn again with the 'scope settings.

Cheers,
Jason

musicvid10 wrote on 10/5/2010, 8:23 PM

"Sorry musicvid - I must have missed something. What do you mean by "greyscale represented as a dot"? I don't recall saying or implying that."

Good gollies, entilza,

Here is the Vectorscope representation of the video clip you provided for me to examine.

You keep using the term "Vectorscope" to identify the means by which you are arriving at the video levels.

And I keep trying to guide you to the conclusion that the Vectorscope does not represent Levels at all, but chroma displacement instead.
"The vectorscope monitor in the Video Scopes window allows you to monitor the chroma values (color content) of your video signal. The monitor plots hue and saturation on a color wheel."

The Video Scopes in Vegas come in four flavors: Vectorscope, Waveform, Histogram, and RGB Parade. One of these gives no indication of luminance or composite levels whatsoever. So which scope are you really using?

Habitually using imprecise language is considered kind of ineffective here, and all of us including me have needed to have it pointed out at one time or another.
Because of time constraints, this will need to be my last post in this discussion. And I hope I've helped a little bit. Good luck to you.

entilza72 wrote on 10/7/2010, 6:01 AM

Whoops sorry to have wasted your time re: nomenclature. I was in fact reading levels off a Histogram, not a Vectorscope. I use the display mode that has the vectorscope, waveform luminance, and histogram display. Slackly, I was refering to the whole window as a vectorscope, when I should have refered to the particular monitor I was using.

Possibly this is why I completely overlooked your gentle guidance re: vectorscope use! As you say, being precise with language is important. Again, appologies.

And yes, you've been very helpful and I am grateful. For starters, your guidance allowed me to find the cause of my flashing black problem! And I'll never refer to a collection of video scopes as a vectorscope again, unless it really is a vectorscope. ;-)

Thank you!