Black Level, 7.5 IRE, 0 IRE, | Blu

Note: Because this type of discussion sometimes gets archived. I have edited this first posting to be more complete even when read out of the context of this and another discussion thread. I have incorporated some information from Jim Doolittle and Stacey Spears so we can have a better stand alone piece. Consequently, some comments and clarifications appearing later in this thread may seem to have been already covered in this first posting. No prescience really took place. GK

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I've seen people confused or posting confusing errata about 0 and 7.5 IRE black level settings in DVD players and displays. The concepts are actually quite simple but laborious to explain. After seeing one too many confused answer and explanation about the difference between American NTSC (black at 7.5 IRE) and Japanese NTSC (black at 0 IRE) and how that affects the picture, I couldn't take it any more. Here is my attempt at clearing some of the mess up......

Unless a display or processing system has insufficient ability to adjust its black level (brightness), one can use either a 0 or 7.5 IRE setup as black. The concepts of IRE, voltage, and black seem to create an undying amount of confusion and misinformed posting on the boards.

One could describe the video signal in volts, but IRE units are more convenient to talk about than 0.7 volts from porch to white. By definition there are 100 IRE units from zero (the porch) to the top of a white signal. The sync pulses are at -40 IRE.

In both the North American and Japanese NTSC systems, these levels stay the same. What changes is the voltage or IRE level at which black is represented. In North American NTSC production practice, black is represented in the video signal at 7.5 IRE. In Japanese production and equipment, black is represented as 0 IRE. In any case, max white is at 100 IRE.

As Jim Doolittle points out....

'.... The black level switch on a DVD player will add set up (bring the black level up to 7.5 IRE) if the end user wishes to use 'S' video or composite video outputs on the player, or if the display device only has 1 memory for brightness level, the end user could choose the 7.5 output level for component video so that it matches all his/her other sources.

To simplify, if using component the player should be set to output 0 IRE. If using 'S' or composite the player should be set to 7.5 IRE. This regardless of what the picture looks like after making a change. After making a change the display must be recalibrated.

Now, not all players offer the choice and if your not sure which one to use, you will have to have a waveform monitor or a lot of time and knowledge to figure it out. The players that do offer a choice have chosen marketing terms to describe the difference, such as 'dark', or 'enhanced'.

end quote.



Not every DVD player or device implements black at the IRE or voltage level expected. However, black in the picture is intended to be just as black, and white just as white no matter the levels implemented. The key is your display must represent the incoming signals correctly through proper calibration of black and white levels.

Again, if your display is adjusted correctly for the type of incoming video signal, black is no darker or lighter than black with one setup level than another. The output dynamic range should stay the same.

I can hear the guffaws now, 'But, my the picture looks different in brightness and shadow detail when I switch my DVD player between 0 and 7.5 IRE ('enhance black' or 'darker' on/off) for black!'

And I would patiently ask, 'Did you recalibrate the display's brightness and contrast controls each time you switched between sources to compensate for the changes in the video signal?'

This is where the disconnect seems to happen. In order to properly compare the effects of having a 0 vs 7.5 IRE setup black level you must adjust the display so it creates black at 0 IRE for an 0 IRE setup signal. You also have to readjust it to create black at 7.5 IRE for a 7.5 IRE signal. You also have to adjust the contrast so the max white is at the same light output level for each one. In other words, the picture ends up looking the same.

People sometimes calibrate their display for black at 7.5 IRE and then switch their DVD player to 'enhanced black.' Naturally, that will darken the image. Indeed anything which is black and up to about 7.5 IRE above black will turn black. That clips off shadow details. You can do that, but you should realize you have now set your system to intentionally alter the image and make everything darker than intended and hidden shadow details. Set the display so it displays 0 IRE as black, matches the incoming type of video signal, and you find yourself back at the same type of picture.

In short, if the video signal has a 0 IRE black (aka 'enhanced black' or Japanese NTSC, 0 IRE setup), the display's brightness control will need to be set higher than if the signal has a 7.5 IRE black.

There are caveats. It is possible that a display or video processor is unable to pass or display signals below a certain threshold. If a projector or scaler simply is unable to accept any signals below 7.5 IRE and you feed it a video signal which uses 0 IRE for black, no matter how far up you turn the brightness control, shadow details will be clipped off. For such a device, it is imperative that the video signal have a 7.5 IRE black level to ensure that the entire signal gets through.

One might run into a display device which has a very limited range in brightness adjustment ability and also expects all its signals to have 0 IRE as black. Feed such a device a 7.5 IRE black leveled signal and no matter how much you turn down brightness you simply cannot correctly adjust the projector to make black in the video signal be the darkest black possible on projector. Even black in the signal would cause the display to glow. Images would appear foggy. If one has such a limited projector, then it would be important to always feed it a 0 IRE black leveled signal. Fortunately, such a severely crippled range of black level (brightness) control should be very rare. A properly designed display should always be able to properly calibrate its brightness to render 7.5 IRE as black.

So it comes down to the proverbial newbie question – Should I set my DVD player to 'enhanced black' or not?

For display systems which can handle either 0 or 7.5 IRE as black, the answer is that it really doesn't matter. You can select either standard for black. You just have to calibrate your display's brightness control to behave correctly with the type signal you are feeding it. The picture ends up the same either way because proper calibration removes the difference. You can of course decide to improperly calibrate the display and allow your picture to look different, but that is up to you. I personally, tend to recommend people stay with 7.5 IRE as black from the DVD player because that matches all their other video sources. That allows the same settings to be used with other video sources. Otherwise, one must remember that the brightness setting for everything else needs to be a bit lower than for DVD player.

Now as I have said, there is the possibility that you have the oddball display system which cannot differing signal levels well. Just make sure that black is black, and there is no clipping of shadow details while setting brightness. If you can't see the two moving near black bars in AVIA or Home Theater Tune-up, you ARE clipping shadows details.



This leads me to another misconception which keeps circulating on the forums --- the idea that black is encoded onto a DVD as being at a specific IRE level independent of the DVD player. That is not true. Not for Videl Essentials, Not for AVIA, Not for Home Theater Tune-up, Not for any theatrical release DVD. What is encoded onto the disc is true black in the video signal is at a specific digital level which I'll refer to as 'digital black'. All properly mastered material should have black in the image encoded at this same 'digital black.' The DVD player or HTPC takes the 'digital black' and outputs it as a voltage. If the player is set to output black at 0 IRE, then it comes out of the player as 0 IRE. If the player is set to output black at 7.5 IRE, then it comes out at 7.5 IRE. It is a small distinction, but is very important.

You understand the difference if you understand why the follow statement is false, 'VE has black encoded as 0 IRE but AVIA has black at 7.5 IRE.'

No, both discs encode black as the same digital 16 (digital black). There is no way for a disc to independently specify that a signal will come out of a DVD player at a specific voltage regardless of what the DVD player is set to do. The setting of the DVD player is what determines the actual output IRE or voltage which comes out when something 'black' on the disc is played. So again, if the player is one which outputs black as 7.5 IRE then black would be 7.5 IRE with either disc. If the player outputs black as 0 IRE then black comes out as 0 IRE on either disc.

Confusion happens when you take a disc like AVIA which labels the patterns as being a certain IRE. The labels are true if and only if the DVD player is itself NTSC accurate. In the cases, AVIA, VE, and HTT, the labeling is correct only if the player outputs digital black as 7.5 IRE and digital white as 100 IRE. If the player is set otherwise, the actual voltage or IRE level won't be exactly that said by the labeling. This is true for all three calibration discs. Fortunately, most DVD players you find in the store will come pretty close, so those labels are useful.

That means that if the DVD player is set to output digital black at 0 IRE, all the labels will be a bit higher than the actual measured IRE. It would be possible to create another set of patterns which were correctly labeled or signal leveled for a 0 IRE black player, but that would be of little real utility.

There is another more insidious problem. As Stacey Spears of Secrets of Home Theater points out, there is still variability in how well DVD discs encode black in the picture correctly at digital 16. Even on THX discs measured black is at, '… 20, 25, 14, etc.'

He goes on, ' The THX master plan was to feed test patterns in at the start so they would go through the same pipes as the video. Then they wanted to have THX DVD players look at special markers and would alter the levels on-the-fly on a disc-by-disc basis. They failed to get this second portion into any THX certified players. My problem is if they know the delta between their disc and what is correct, they should have fixed the disc or the end-to-end production chain.'

Variance in black and white levels is a mastering problem, not a sign that ones display is awry. The existing variability shows that even more work needs to be done to improve production standardization. That way the entire chain from production to home playback can have predictable results. Everybody gets to see the picture as intended. We take care to calibrate accurately. The production house should as well. We all see the same picture. If it looks good on their displays, it should look the same on ours.

We could simply give up and let everybody to follow their own 'standards' and ignore SMPTE's for black and white levels as well as saturation and hue for their production, but we'd be stepping back into the dark days of unpredictable results. The fight for everyone meeting standards needs to be an ongoing one. Otherwise, we would have some houses violate reserved headroom space in the digital levels to extend picture information. The result was severe clipping on displays which are correctly calibrated. That house could then claim their discs have a more dynamic image. Our whites are whiter, our blacks are blacker, colors more vivid, and sound effects louder! Do we really want a world in which discs vary greatly in brightness and loudness? That would make it impossible to accurately recreate the appearance and sound as originally intended without recalibrating for each production house's own 'standard.'