DVD Black Level Confusion

Posted by kent h on October 01, 1998 at 23:59:34:

In Reply to: DVD Black Level Confusion posted by Robin Luckey on September 30, 1998 at 13:40:51:

:
: Can someone explain to me why DVD makers are advertising "expanded" black range, which pushes the black level to as low 0 IRE?

: I understand that a standard color bar screen contains both 0 IRE and 7.5 IRE black, and that a player must be able to reproduce 0 IRE if you want to use a DVD like Video Essentials as a monitor calibration source. However, in the normally visible picture, 7.5 IRE is standardized as the darkest possible black, and your monitor should be calibrated for this. So why would I ever want my DVD player to output levels lower than 7.5 during normal playback?

: Is this expanded black range merely a trick which "crushes" the video black normally at 7.5 IRE down to 0 IRE to "improve" the apparent quality or expand the contrast range? This seems to be an undesirable modification of the signal. Or is this just the manufacturer's way of stating that the device is capable of correctly outputing 0 IRE when required, and that some other DVD players are thus incapable of this?

: Does the image storage encoding on DVD even accomodate the fact that there is a range of black possible below 7.5 IRE?

: I am considering a purchase of a high-quality DVD player and I am wondering if there is anything I need to be worried about in all of this, and whether "expanded black range" should be a feature I care about.

: Thanks,
: Robin

I will try and explain this to the best of my ability. I not an expert who works in the video field.
>>>>I understand that a standard color bar screen contains both 0 IRE and 7.5 IRE black,>>>

Actually, the PLUGE stripes on a DVD are in digital units, which correspond to about +4 IRE above black and
- 4 IRE below black.
Depending on your player, this would translate to 7.5 +4 IRE and 7.5 - 4 IRE or +4 IRE and -4 IRE if the player was set for black to be 0 IRE.
Black in Digital form (CCIR 601 standard) on the DVD is 16 digital and the player can translate that on its analog output to 7.5 IRE or 0 IRE.

>>>>>So why would I ever want my DVD player to output levels lower than 7.5 during
normal playback?
>>>>>>
No, unless you live in Europe. PAL TV uses 0 IRE for black. If you have a DVD player that uses 7.5 IRE for black, your TV will be mis-calibrated for one of the inputs.

>>>>>Does the image storage encoding on DVD even accomodate the fact that there is a
range of black possible below 7.5 IRE?
>>>>>
Yes, but indirectly. DVD is encoded digitally with 8 bit words. 16 is black and less than this is below black. If the DVD player translates dig 16 to 7.5 IRE as black, below black will be less than 7.5 IRE.

You are not really getting an expanded range with players that put out 0 IRE. For players that use 0 IRE for black, they will go between 0 and 100 IRE. For players that use 7.5 IRE (the standard for consumer NTSC displays) they will output 7.5 to 100 IRE. However, If you have calibrated your set, black at either 0 IRE or 7.5 IRE will be essentially 0 light, and 100 IRE will be just below your point of blooming of your phosphors. So in either case you are seeing the full dynamic range of the picture.

The problem with many DVD players is not that they would set black level to 7.5 IRE but that they would not output any signal below 7.5 IRE (except sync pulses, which go way below black) , so they did not display the "blacker than black" PLUGE stripe. I have a Sony DVP 7000, which does display blacker than black and sets black to 7.5 IRE. The only thing that not having the blacker than black be displayed really does is make it harder to calibrate your set.

hope this helps,

- kent h

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