Black Levels––0 IRE Or 7.5 IRE?
Dear Gary:
You know that on some DVD players (my Pioneer 868 for instance) you have the option of having black set to 0 IRE or 7.5 IRE. One is noticeably brighter than the other. Which one is the best for image quality? If I set the DVD player to the brighter setting (7.5 IRE), I guess I can turn the brightness down on the projector (might save a few hours?), but... If I am in ANY WAY affecting picture quality, blacks bandwidth, and white crush, then I would rather have it set to the darker player setting (0 IRE). What is your suggestion? What is best for picture quality? 0 IRE or 7.5 IRE? Are blacks and whites affected?
My system is a Pioneer 868 into a Sim2 RTX 55 DLP rear-pro via HDMI. I watch both PAL and NTSC DVDs equally.
Martin Jeeves
mailto:m.jeeves@kingsfund.org.uk
Video Technical Editor Greg Rogers Comments:
I’m confused by your question because setting the DVD player’s black level output to 0-IRE or 7.5-IRE should only apply to analog output signals––usually only analog YPbPr signals, but some DVD players also provide this option for S-video or composite video analog signals. However, you mention that your DVD player is connected to your DLP projector via HDMI digital signals, which shouldn’t be affected by a 0 IRE/7.5 IRE black level setting (technically referred to as 7.5 IRE black-level setup). So I’ll address the issue of 7.5 IRE setup for analog signals, and then I’ll discuss another option that applies only to DVI/HDMI digital signals.
If your DVD player and your display (or a video processor that receives the DVD player signals) are properly and fully calibrated, and they process the analog signals correctly, it makes no practical difference in video quality whether the signals include 7.5 IRE black-level setup or not. (Signals without 7.5 IRE setup can provide an insignificant increase in signal-to-noise ratio, which may or may not be achieved by the player, but won’t be visible regardless.) However, you must make sure that the black level (Brightness), Contrast, and Color saturation controls are accurately calibrated for either option. Most video processors and displays must be carefully recalibrated when switching the 7.5 IRE setup on or off. However, some DVD players and displays still don’t process signals correctly without 7.5 IRE black level setup. They may clip signals below black and compress signals (which become non-linear) slightly above black. Therefore, you may achieve better image quality by using 7.5 IRE black-level setup with some products.
There is a completely separate option (unrelated to 7.5 IRE setup) that applies only to digital RGB signal levels when using a DVI or HDMI interface. Some, but not all, DVI players provide the option of using digital RGB-video levels or digital RGB-PC levels. When using digital RGB-video levels, black is represented by digital level 16 (in 8-bit digital video standards) and reference white (corresponds to a 100 IRE analog level) is represented by digital level 235. Since 8-bit digital values range from 0 to 255, digital RGB-video levels can represent video below black (less than 16) and above reference white (greater than 235). However, digital RGB-PC levels represent black as digital level 0, and reference white as digital level 255. Therefore, RGB-PC levels can’t represent video below black or above reference white. It is quite common for video image content to go above reference white. For that reason, RGB-video levels should be used whenever possible. When YCbCr video is transferred over HDMI (or DVI), the 8-bit video levels are 16 to 235 for black to reference white, so YCbCr can also transmit below black and above reference white video.
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