TV levels vs. PC levels

[dansrfe]

Ok so I am aware that this topic has been talked about many times but I still have some questions and I thought it would be best that I asked them once and for all.

1) What is the purpose of TV levels? LCD's, LED's, and Plasma TVs all can output the entire range so what's the reason to have TV levels in the first place?

2) Since most content played back on pc's and tv's (dvd's, bd's etc.) all have TV levels, a conversion has to take place in order for the output to not look dull and washed out. If we have to expand to PC levels anyway then once again I don't understand the reason for TV levels.

I hope some answers can clear up the confusion I have on this topic. Thanks in advance!

[Ghitulescu]

The TV was invented long time before the PC. It was a time when people didn't have computers, nor Nintendos, not even cellphones.

TV level is an incorrect term to describe some of the NTSC/PAL system restrictions (pedestal and white clipping). Now google yourself ....:search: and you'll find a nice PDF explaining the whole issue.

Where's the confusion? That two different system use two different units?

[dansrfe]

The confusion is that today's "tv's" are LCD, LED backlit, and plasma screen tv's. I really don't know how many people are still using CRT tv's therefore I can't guesstimate that. AFAIK LCD's, LED backlit, and plasma displays all use PC levels. So I still don't get the use for "TV or 16-235 clamping". btw where can i find that PDF :D

[Ghitulescu]

Sorry, I forgot you can't search :p

Nevertheless, I told you and I repeat you once more: it's not a TV limitation but a standard limitation. In PAL it is possible to have a blacker black as the pedestal is not required (so for NTSC in Japan). When someone had the idea of putting computer images on TV the technology couldn't solve both ends in an economical way so they did it in software, was easier.

But why is so important?

[2Bdecided]

1) What is the purpose of TV levels? LCD's, LED's, and Plasma TVs all can output the entire range so what's the reason to have TV levels in the first place?

2) Since most content played back on pc's and tv's (dvd's, bd's etc.) all have TV levels, a conversion has to take place in order for the output to not look dull and washed out. If we have to expand to PC levels anyway then once again I don't understand the reason for TV levels.
TV levels: 16=black, 235=white
PC levels: 0=black, 255=white

The PC levels are kind of obvious, so the question becomes: why were the TV levels defined with headroom below black and above white?

This dates back to 1982 in ITU Rec 601 (http://en.wikipedia.org/wiki/Rec._601). It's simply that, in a studio environment, levels can't be quite as carefully controlled as in a PC, and processing like sharpening and band limiting create overshoots. If you clip these overshoots, you can create even more overshoots further down the line - so to allow for not-quite-right levels, and overshoots that don't spread out in a hideous way, a little headroom was left at either end of the scale.

These are the standard levels for YUV video. Everyone knows what they are. Changing them would only create confusion, without any benefit.

Cheers,
David.

[Midzuki]

The confusion is that today's "tv's" are LCD, LED backlit, and plasma screen tv's. I really don't know how many people are still using CRT tv's therefore I can't guesstimate that.

Many many millions of people,

— until the analog && interlaced broadcast is dead-and-buried

«all over the world».

[Ghitulescu]

The confusion is that today's "tv's" are LCD, LED backlit, and plasma screen tv's.

Aren't those "LED backlit" not LCD? Maybe is here your confusion. If you wanted to mention another new TV technology, that's OLED (still very expensive) but there are commercial models on sale.

[dansrfe]

LED Backlight is LCD with an White LED's used as backlight instead of a CCFL, so yes you are correct. OLED is another technology as well.

[TinTime]

TV levels: 16=black, 235=white
PC levels: 0=black, 255=white

The PC levels are kind of obvious, so the question becomes: why were the TV levels defined with headroom below black and above white?

This dates back to 1982 in ITU Rec 601 (http://en.wikipedia.org/wiki/Rec._601). It's simply that, in a studio environment, levels can't be quite as carefully controlled as in a PC, and processing like sharpening and band limiting create overshoots. If you clip these overshoots, you can create even more overshoots further down the line - so to allow for not-quite-right levels, and overshoots that don't spread out in a hideous way, a little headroom was left at either end of the scale.

These are the standard levels for YUV video. Everyone knows what they are. Changing them would only create confusion, without any benefit.

Cheers,
David.

I guess another question then is why don't PCs use video levels? It would have saved a lot of problems, especially as more and more people are now plugging their PCs into their TVs.

[Ghitulescu]

They don't need them. The PC monitors were not designed to be interfaced with video equipment, except for CGA and VGA (with adapter). The VGA can be seen as a step back, since it dropped the digital interface in favour of an analog one, issue "solved" only recently by DVI (and laptops).

Concerning "those" people: they should not use the PC for playing, it was not designed this way (it can be used and it's extremely versatile), but for producing/editing multimedia. There are lots of issues to be taken care of when one decides to interface a PC with an analog monitor (less items when digital):
* electric and logic levels
* scanlines (480 vs. 481-486 for NTSC)
* PAR
* DAR
* overscan
* resolution (or how to fit 640x200 EGA pixels in 711x483 analog samples)
* color resolution (or how to properly subsample the colour info)
* interlacing the progressive content
* frame rate adaptation (or how to fit 72 Hz refresh rate into 59.94i)
* color space conversion (from RGB into Y/C)
and I think I can continue.

[2Bdecided]

I guess another question then is why don't PCs use video levels? It would have saved a lot of problems, especially as more and more people are now plugging their PCs into their TVs.This discussion is about digital levels.

For most of the last two decades+, connections to TVs have been analogue. In the analogue domain, there's no confusion. For both PCs and (PAL) TVs, 0V = black, 0.7V = white. (-0.3V = sync, giving 1Vpp). NTSC is fractionally different.

Why not use the conventional digital video levels of 16=black and 235=white inside PCs? Well, the computers of the day were developed separately from digital video standards, and didn't have "headroom" to spare in the early days.

More fundamentally, computers use RGB. The same TV standards define the conversion between YUV and RGB such that RGB is full range (0-255). So PCs do follow the TV standards.

There's no confusion anywhere - except maybe in things like Sony Vegas which allow 16-235 RGB! And AVIsynth which is flexible enough to let you get it "wrong" in any number of ways (sometimes for very good reasons, sometimes out of user ignorance).

Cheers,
David.

[rotty]

There is a good test to see if your PC output to your TV is set correctly.

Its software most of us use a great deal.

It's iTunes, startup iTunes, click on Music, make sure that the listing is in the standard format, i.e. list details with (altenating white and pale cyan horizontal bars with the tune names in black on them).

If your PC is set to output PC levels to the TV, the white and pale cyan bars are above peak white TV levels. The white and cyan bars will be clipped and both appear white on the TV.

In other words, if you are sending your TV PC levels instead of Studio levels then the entire music listing will be black tune names on white background (not alternating white and cyan lines)

Think we should call it "The iTunes test"

[dansrfe]

Once again. TV levels are only useful for CRT tv's. Everything else uses PC levels in this day and age. LCD, OLED, AMOLED, Plasma, Projector and DLP all use PC levels. I'm pretty sure even theatrical digital projection systems use PC levels.

[rotty]

Once again. TV levels are only useful for CRT tv's. Everything else uses PC levels in this day and age. LCD, OLED, AMOLED, Plasma, Projector and DLP all use PC levels. I'm pretty sure even theatrical digital projection systems use PC levels.


Im afraid that is not the case, Panasonic Plasma TV's with PC connected via HDMI require studio levels NOT PC levels.

[TinTime]

More fundamentally, computers use RGB. The same TV standards define the conversion between YUV and RGB such that RGB is full range (0-255). So PCs do follow the TV standards.

Im afraid that is not the case, Panasonic Plasma TV's with PC connected via HDMI require studio levels NOT PC levels.

I've got a Panasonic plasma which is why I wasn't sure about 2Bdecided's reply. I pass it RGB over HDMI and the black level assumes video levels on the default brightness (50%) setting. So my PC (aside from video converted to RGB without level expansion) doesn't follow my TV's standards.

I suppose the major users of RGB connections to TVs will be games consoles - I wonder what levels they use?

[rotty]

I've got a Panasonic plasma which is why I wasn't sure about 2Bdecided's reply. I pass it RGB over HDMI and the black level assumes video levels on the default brightness (50%) setting. So my PC (aside from video converted to RGB without level expansion) doesn't follow my TV's standards.

I suppose the major users of RGB connections to TVs will be games consoles - I wonder what levels they use?


Yes, if I use the Sony VAIO with the Panny I set the ATI Radeon setting to studio levels and then everything is fine.

BTW changing your brightness setting wont change the clip point, so wont help.

Like I said previously, use iTunes display to test.

[TinTime]

BTW changing your brightness setting wont change the clip point, so wont help.

That's exactly what it does :)

It clips everything to black below whatever level you choose with the brightness setting.

[dansrfe]

This discussion is very confusing for me. Where TV levels (16-235) are used and where PC levels (0-255) are used needs to be clarified in a table or something. Also i would like to say that my Nvidia Control Panel does have an option to send a YCBCR or RGB signal to my DLP TV. Also does YCBCR always indicate clipped (16-235) levels and does RGB always indicate (0-255) levels? Should I be sending YCBCR or RGB to my DLP TV? Which levels should I choose? Does level output also depend on the connection mechanism? Do different TV's have different specifications as to what they "should" be getting even if they can convert between standards themselves? I used to think CRT TV's are the only ones that still use clipped (16-235) levels but if other modern day tv's still use them then is there any proof of that being so? Don't all modern TV's display in RGB (0-255) just as a PC does?

To whoever intends to reply, please leave a clear-cut answer so that there isn't any more confusion. Some outside sources or links on this topic to prove some of this stuff would be nice as well.

[dansrfe]

PS: 2Bdecided's replies seem to be in the right direction to me. Can't tell about all the other information in the thread however.

[rotty]

That's exactly what it does :)

It clips everything to black below whatever level you choose with the brightness setting.


I think you misunderstood me. Adjusting the brightness down will collapse near blacks to black, the further down you take the brighness the more grey levels will turn to black.
Greyscale bars would show this effect perfectly.

However, the brightness control will bring the white level down but the whites that have been clipped will still be clipped.

[TinTime]

I think you misunderstood me. Adjusting the brightness down will collapse near blacks to black, the further down you take the brighness the more grey levels will turn to black.
Greyscale bars would show this effect perfectly.

Yes, I realise this.

However, the brightness control will bring the white level down but the whites that have been clipped will still be clipped.

No, they shouldn't be. This (in theory) is what the display's contrast setting is for.

[Lyle_JP]

Not all NTSC devices use TV Scale though. I seem to recall that in Japan, their NTCS standard included the whole 0-255 range. I know that many import laserdisc players would output signals at a different brightness than their domestic counterparts.

[rotty]

Yes, I realise this.



No, they shouldn't be. This (in theory) is what the display's contrast setting is for.

If the contrast (gain control) is the first stage in v.amp then yes, but clipping is done normally at least one stage back and the user gain control is usually well after this.

[Ghitulescu]

Not all NTSC devices use TV Scale though. I seem to recall that in Japan, their NTCS standard included the whole 0-255 range. I know that many import laserdisc players would output signals at a different brightness than their domestic counterparts.

AFAIK NTSC Japan differs from NTSC USA in that no pedestal (setup) is present.

Most NTSC gear also has the possibility of eliminating the pedestal, which slightly alters the brightness, and only the digital gear with digital connections to digital TVs (eg HDMI) can be set to full scale or TV scale.

The best tests are always the test disks many providers publish (pick the gray scale test or similar), like Burosch or HQV.