gamma, luminance, IRE ... what is the relationship ?

[lordsmurf]

IRE: IRE (Institute of Radio Engineers) is a means of measuring brightness as a relative percentage of total brightness. A 1V p-p video signal subdivides into 140 IRE units, each 0.00714V in size. Absolute black is 0 IRE, and peak white is 100 IRE, or 714.29 mV; synchronization signals extend below absolute black to –40 IRE. In 1953, NTSC (National Television Standards Committee) specifications redefined absolute black from 0 IRE (used in early black-and-white television) to 7.5 IRE, because early black-and-white transmitters couldn't manage a color signal with a 0V black level. ATSC (Advanced Television Systems Committee) specifications return absolute black to its original 0 IRE definition.

UNDERSTANDING AS OF NOW: So this essentially sets your boundaries. IRE sets top and bottom values, and nothing else. Your ability to tweak the internal gamma settings is not affected.

Gamma: A term that describes the tonal reproduction characteristics of a video signal.

UNDERSTANDING AS OF NOW: Tone can also mean shade, or the varying darkness/brightness level. Gamma can be adjusted to "lighten" or "darken" the video, however it is an ability separate from IRE min/max. You can adjust within the confines of the IRE settings. This is not really the same as luminance. It can be compared to LEVELS in Photoshop, where gray shades can be "lightened" but the white and black remain unchanged.

Luminance: The brightness information in the television picture. The luminance signal amplitude varies in proportion to the brightness of the televised scene and is therefore capable of producing a complete monochrome picture.

UNDERSTANDING AS OF NOW: Unlike adjusting the tones/shades of an image, luminance controls the master brightness/darkness level. TRUE light and dark. It also stores most of the "green" portion of the signal when mixed with color subcarriers (Y) of the image (at least in traditional 4:1:1, 4:2:2, 4:2:0, etc signals). Some of this may be slightly off, but I can only blame others, as no two people are able to quite give the same definition of some of these concepts. Not online, in books, or in person. The important issue is this stores the brightness information in the video signal. IRE and GAMMA are attributes of the stored info. I've also heard luminace called gamme.


This is a point I want to make about certain pieces of hardware and software. Some errors are never REALLY fixed, but merely augmented by changing another property. While similar, still not a TRUE fix.

It would also be nice to try and sort out some common terms that have obvious overlap.

[edDV]

Some comments and references for discussion.

IRE:

"UNDERSTANDING AS OF NOW: So this essentially sets your boundaries. IRE sets top and bottom values, and nothing else. Your ability to tweak the internal gamma settings is not affected."

7.5 IRE is still alive and with us during the capturing of any NTSC source be it an analog TV tuner, cable box (with exceptions), VHS/SVHS tape, broadcast analog tapes, composite digital broadcast tapes, etc.

Many pro NLE systems are configured with 7.5 IRE black, others (including most DV PC NLE's) use zero IRE black.

So when capturing to DV or MPeg2, it is necessary to map incoming 7.5 IRE to Zero IRE (16/255 digital) for NTSC sources.


Gamma:

a bit of reading needed
http://www.poynton.com/notes/colour_and_gamma/GammaFAQ.html
http://www.poynton.com/notes/Timo/index.html <-- less technical translation
http://www.techonline.com/community/ed_resource/37487

more:
http://www.poynton.com/ColorFAQ.html
http://www.poynton.com/papers/Discreet_Logic/index.html
http://www.adamwilt.com/pix-sampling.html


Luminance:

yes, master brightness where white is 100 IRE = digital 235/255 for 8bit luminance.
Headroom is provided above white to handle minor overloads and transients that extend above 100 IRE. This is important to prevent hard edges (cliping) that results in analog ringing or digital aliasing.

Select "NTSC Video Measurements" for basics
http://www.tek.com/Measurement/cgi-bin/framed.pl?Document=/Measurement/App_Notes/index...Set=television
or use this link http://www.tek.com/Measurement/App_Notes/NTSC_Video_Msmt/25W_7247_1.pdf

More Video Measurement references
http://www.tek.com/Measurement/cgi-bin/framed.pl?Document=/Measurement/App_Notes/index...Set=television


Many of the basic formulas are here
http://www.secs.oakland.edu/~srodawa/courseware/cse549/fa02/Ch02rjs.pdf

[lifesajrny]

You two guys rule
That's quite a bit of info, well worth reading.
Thanks

[gshelley61]

If you play back a commercial NTSC DVD video signal through a luminance meter, you will quickly discover that the setup (black level) is at 0 IRE, not 7.5 IRE as has been the standard for NTSC broadcast, VHS, laserdisc, etc. all these years. Modern displays are capable of handling 0 IRE (and even "blacker than black" signals). At 7.5 IRE, blacks look dark grey, not really black.

This is the primary reason I use a proc amp with a luminance/black level meter on all my video sources prior to capture. Each video source varies widely (even commercially produced video), and I often must adjust the black level and other signal characteristics to correct and optimize the image.

[edDV]

Yes the DVD standard has black at zero IRE but some players still add setup at the final stage on the NTSC output. Y generally has zero setup.

Consumer DV camcorders usually have zero setup on the NTSC outputs. Broadcast DV, DVCAM, DVCPro camcorders have setup on the NTSC and S-Video out but not on Y over IEEE-1394.

Mid level editing programs (Premiere, Vegas, FCF, AVID, ...) all have internal test signals and scopes now so you can see and adjust setup, white and chroma levels.

[lordsmurf]

Then what explains the IRE errors on Toshiba and Panasonic (and other) DVD recorders?

[edDV]

Possibly, I have no experience with those units. A waveform monitor is a great tool to analyze these issues. I use my Canopus ADVC-100 as a probe and my Vegas waveform monitor as the scope.

What I have determined so far is analog output levels (black and white) for various DVD and VCR players vary significantly so input filters must be used in Vegas to match levels to DV.

[gshelley61]

What I have determined so far is analog output levels (black and white) for various DVD and VCR players vary significantly so input filters must be used in Vegas to match levels to DV.

Amen. This goes for laserdisc players, too. Every piece of video gear, and every video source that can be played back on that gear, has variations. To properly transfer to a digital format like DV or MPEG2, video black level and maximum luma should be adjusted (if necessary) to stay within the standard 0 to 100 IRE limits.