The Truth About Contrast Ratio

With numbers ranging from 250:1 to 100,000:1, it's tough to really understand what the heck contrast ratio is, what it measures, or how to set it.  Can we even trust what a manufacturer states, or do we need to take it all with a grain of salt?  Of course our gut tells us to pick the biggest one, but that might not always be the best answer.  How its measured is every bit as important as what the measurement is.

At a very basic level, contrast ratio is the difference in brightness between the brightest image and the darkest image a display device, such as a TV, can show.  In simpler terms, you measure how bright a black box is on screen, then put a white box on the screen and measure how bright it is.  The difference in that measurement yields the contrast ratio.  Based on that definition, yes, you were correct before, the bigger the better.  Some display devices, like CRT and Plasma, actually turn off the black pixels or areas, producing a pure black image.  In those cases, it's the brightest image we're more concerned with.  Other technologies, like LCD, DLP and LCoS, that use a constantly lit bulb, both the darkest and the lightest measurements are very important.

But that isn't the whole story.  There are two ways for a TV company to measure contrast ratio: the full on / full off method and the ANSI contrast ratio method.  The full on/off method simply turns the whole screen white then turns the whole screen black.  Measurements are taken at each one and recorded, then combined to for an overall contrast ratio.  This is the most common measurement used by marketing and manufacturong because it tends to yield better results.  The ANSI method, on the other hand, uses a screen with alternating black and white squares, resembling a chess board.  The average light output of the white rectangles is compared to the average light output of the dark rectanges and there you have it.  It is more accurate, but produces smaller numbers, and is thus not used often.

When you cut through all of the techno-jargon, contrast ratio is really a measurement of gray-scale.  It is crucial to get it set correctly to make sure you don't lose detail in dark scenes or also in very bright scenes.  Since gray-scale is used to determine the hue and intensity of a color, the contrast ratio on a television is crucial to good HDTV viewing.  It also makes sure that the color black is, in fact, black.  But contrast ratio isn't a setting all its own.  It works very closely with the brightness control to present an overall experience.  For example, a perfectly set contrast ratio will be completely washed out if the brightness is set too high.

 

We've begun to see a new measurement in contrast ratio called 'dynamic contrast'.  It's typically used when measuring LCD sets and covers some of those issues we've mentioned about the bulb always being on.  When the TV needs to show a dark image or scene, it actually reduces the power to the bulb to reduce the overall brightness of the set. This gets you much closer to the plasma's ability to actually shut off certain areas on the screen.  The problem with this method is that any bright areas on the screen get washed out, so you get good darks at time, good brights at times, but not at the same time.  It has become more and more common for manufacturers to report on dynamic contrast because it is typically better than static contrast.  Never compare between the two.  A plasma display with a static contrast ratio of 3000:1 will certainly outperform an LCD with an equivalent dynamic contrast ratio, especially since that typically implies a lesser static contrast ratio of perhaps 1000:1 or worse.

Notes on HDMI as a control protocol HDMI-CEC

• We talked about HDMI way back on Episode #15  (July 7, 2005)

• There are so many control protocols out there, does HDMI CEC stand a chance vs MoCA, HANA , DLNA, etc?

• Products are starting to ship from Panasonic and Philips - this is a great sign.

• However, there are proprietary implementations on top of CEC, potentially destroying the interoperability promise of CEC.
• Panasonic calls their control protocol EZSync
• LG calls their control protocol Simple Link
• Samsung's brand of HDMI-CEC is called Anynet+

• If all the AV devices are moving to HDMI, doesn't CEC make the most sense?  Why do we need proprietary versions of it?  How does that help consumers?

• The best part is that HDMI-CEC takes us, like Ara said back in July '05, full circle.  It's going to be as easy as buying a TV, DVD player, receiver, etc, and plugging in one cable.  Then any of your remotes should work as a universal remote.  Cool stuff.