Reference black vs darker-than-black confusion (16-235 issue) - AVS Forum
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Old 09-25-2013, 11:22 PM - Thread Starter
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From this article:
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ALL video stored on modern discs, be it DVD or Blu-ray, are stored as YCbCr with a range of 16-235. But, the content is mastered such that 0-255 is present in the tape domain (the SDI stream - D5 archive tape, etc.) It is then captured into a computer and then the computer will decide what to do with it - preferably retaining the extended info (all data from 0-255). This entire range is then compressed into the 16-235 range accepted by DVDs, Blu-ray Discs, etc. When you play that back on a system set to 16-235, you get all the information as it was intended with no gaps. If you mismatch and play back with RGB (0-255) settings, you'll clip off the black and white levels and black will look "dark gray". Additionally, you won't be able to view the Blacker-than-Black images found on most video test discs.


This makes it seem as if what was originally the darkest possible black (RGB 0), is now represented as 16 in the YCbCr format.

So suppose when a disc is mastered, there's a scene where everything fades to black. The individual(s) mastering the disc decides that this black should have a value of 0 (see bolded part of above quote). So, when the information is compressed into the 16-235 range, this scene will be associated with a value of 16, and this will instruct the blu-ray to instruct the TV to output the darkest black possible.

So how on earth can there be darker than black patterns that you're NOT supposed to see after proper calibration?
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In order to see Blacker-than-Black test patterns, you'll need one of the discs below which will give you the ability to calibrate your display or projector to the correct levels. Remember, the proper level is such that the below black information, while present in the system, is NOT visible. TYhis means that you will want to make sure that the Below Black information comes through by cranking up your Brightness (black Level) until you see the below black pattern, and then decreasing Brightness until it is completely hidden, but leaves the next brightest pattern or line barely visible.

The second quote (taken from same article) implies that when the mastering was done, the original 0-255 range was not COMPRESSED to 16-235, but rather CLIPPED to 16-235.

This clipping idea would also be in line with something I remember Michael Chen said about how it's important to calibrate properly so that you don't see stuff like boom mic's and things that are mastered as blacker than black.


So I'm pretty confused here. Any insights would help!


edit: after studying the issue some more, it seems as if Reference black is not 16, but rather somewhere above 16 (that somewhere being 7.5 IRE). See top right of this image, taken from same article.

So it appears that while during the mastering process, compression is what occurs, not clipping.

But apparently, we're supposed to manually crush the blacks so that everything up to 7.5 IRE is rendered as complete black. And this is done just so that boom mics become invisible? That sounds like a really crappy way of doing things, as it reduces the levels of luminance even more! At least I think I understand what's going on (even though it seems silly to set black levels above 16).
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Old 09-26-2013, 12:16 AM
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Quote:
Originally Posted by spacediver View Post

But apparently, we're supposed to manually crush the blacks so that everything up to 7.5 IRE is rendered as complete black. And this is done just so that boom mics become invisible? That sounds like a really crappy way of doing things, as it reduces the levels of luminance even more! At least I think I understand what's going on (even though it seems silly to set black levels above 16).

Stay Calm.

7.5 IRE is digital level 16, at least that's how they translate for US NTSC.

Video content has black at 16, Computers use 0.

Computers have always been digital and bit depth is valuable, we started off with only 1 bit.

Video was analog and used voltages with infinite adjustability, but required headroom and footroom for broadcasting on high powered radio towers. IRE is strictly a term for dealing with analog voltages. When they switched to using digital video we were already at 24bit color so they decided they had enough bits to waste preserving the footroom that was in use for analog.

Expanding and compressing between these two ranges is extremely common. Clipping should never happen, if you think you read that somewhere you were mistaken or they were incorrect in the way they phrased it.

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Old 09-26-2013, 01:06 AM - Thread Starter
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If I'm understanding you correctly, then this diagram is incorrect:



Also, Michael Chen's tutorial on setting brightness actually instructs you to crush your blacks manually so that the darker bars in the test pattern blend in together.
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Old 09-26-2013, 05:09 AM
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You don't crush black.
You just set it right so that 16 ist black and not dark gray.
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Old 09-26-2013, 08:51 AM
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Quote:
Originally Posted by spacediver View Post

If I'm understanding you correctly, then this diagram is incorrect:



Also, Michael Chen's tutorial on setting brightness actually instructs you to crush your blacks manually so that the darker bars in the test pattern blend in together.

That graphic is wrong.

3.5 and 7.5 IRE do not belong on a chart with digital 16.

They are two separate concepts and should not be intermixed.


16 should produce your minimum light output

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Old 09-26-2013, 09:10 AM
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I've got Comcast/Xfinity HD Cable and I've noticed that when I watch On Demand movies from the FearNet channel, the STB starts using PC levels and I need to switch my Samsung TV from HDMI Black Level Low to Normal to retain shadow detail (yet if I use this setting for any other channels On Demand or in general the pictures washes out badly confirming the STB is using Video levels). Any reason why FearNet uses PC levels?
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Old 09-26-2013, 09:24 AM - Thread Starter
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Originally Posted by AV-Freak View Post

You don't crush black.
You just set it right so that 16 ist black and not dark gray.

That's crushing blacks isn't it? By doing this, the first 16 bars all become black. Isn't this exactly what crushing black means?
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Old 09-26-2013, 09:32 AM
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Quote:
Originally Posted by spacediver View Post

That's crushing blacks isn't it? By doing this, the first 16 bars all become black. Isn't this exactly what crushing black means?

No. Crushing would mean that areas just above black would merge into black...

...Royce...

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Old 09-26-2013, 09:33 AM
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Quote:
Originally Posted by spacediver View Post

That's crushing blacks isn't it? By doing this, the first 16 bars all become black. Isn't this exactly what crushing black means?

16 is black and the background of the test pattern is also black... so they should blend together perfectly as they will both be 16 when brightness is set right
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Old 09-26-2013, 09:41 AM
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Quote:
Originally Posted by spacediver View Post

That's crushing blacks isn't it? By doing this, the first 16 bars all become black. Isn't this exactly what crushing black means?

Digitial:
0% - 16
1% - 18
10% - 38
100% - 235

Analog:
0% - 7.5 IRE
1% - 8.425 IRE
10% - 16.75 IRE
100% - 100 IRE


Anything lower than 7.5 IRE or Digital 16 is encoding noise. It equates to a negative light percentage. Effectively it would be a black hole sucking in light. I don't know about you but I've never seen a black hole up close and personal.

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Old 09-26-2013, 09:45 AM
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Quote:
Originally Posted by PlasmaPZ80U View Post

I've got Comcast/Xfinity HD Cable and I've noticed that when I watch On Demand movies from the FearNet channel, the STB starts using PC levels and I need to switch my Samsung TV from HDMI Black Level Low to Normal to retain shadow detail (yet if I use this setting for any other channels On Demand or in general the pictures washes out badly confirming the STB is using Video levels). Any reason why FearNet uses PC levels?

Because a lot of people responsible for providing content care a lot less about quality than you do.

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Old 09-26-2013, 09:55 AM - Thread Starter
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Quote:
Originally Posted by sotti View Post

Digitial:
0% - 16
1% - 18
10% - 38
100% - 235

Analog:
0% - 7.5 IRE
1% - 8.425 IRE
10% - 16.75 IRE
100% - 100 IRE


Anything lower than 7.5 IRE or Digital 16 is encoding noise. It equates to a negative light percentage. Effectively it would be a black hole sucking in light. I don't know about you but I've never seen a black hole up close and personal.

In Michael's video, I'm assuming those first 16 dark bars start at 16. Afterall, they're a blu-ray test disc, and I'm assuming the system is working within a 16-235 range, where 16 is pure black.

So on the test pattern, the bars labeled 0,1,2,3, etc. actually correspond to Digital 16, Digital 17, Digital 18, Digital 19, etc.

So it would appear he's actually instructing us to crush everything from Digital 16 to Digital 32 together
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Old 09-26-2013, 10:33 AM
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No you still can encode below 0%. An 8 bit code always CAN encode 0-255 and voltages can always drop to 0.

Test patterns intentionally encode below black so you can determine the correct black level.

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Old 09-26-2013, 11:30 AM - Thread Starter
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I see, so those first 16 bars really are digital 0-15. I suppose this is yet another reason why it's always good to calibrate using test patterns generated on the same source you are going to be viewing. I can imagine it would be pretty tedious to encode below black using a PC, and have it output properly on a TV.

thanks again Sotti, I owe a lot of my learning to your patience.
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Old 09-26-2013, 12:53 PM
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Quote:
Originally Posted by spacediver View Post

I see, so those first 16 bars really are digital 0-15. I suppose this is yet another reason why it's always good to calibrate using test patterns generated on the same source you are going to be viewing. I can imagine it would be pretty tedious to encode below black using a PC, and have it output properly on a TV.

thanks again Sotti, I owe a lot of my learning to your patience.

Well this is where things get a bit tricky when you mix in PCs and video. What a PC says a level is on screen or in paint or via dvd playback software may be very different than what it actually puts on the wire.

You can configure a PC to output reference black from a video file at 0, 16, or 30, so it's an easy thing to get wrong. One of the reasons I don't recommend people even trying to hook up a computer to their home entertainment equipment unless they are already extremely comfortable with all of this.

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Old 09-26-2013, 03:31 PM
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This page of info 'may' help...

http://www.lightillusion.com/data_tv_levels.html

Steve

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