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Today's Show:
We get a handful of emails every week asking about Refresh Rates and Response Times: what are they? are they different? do I care? While refresh rate applies to any TV technology, response time is something an LCD buyer needs to know about. And with the rapid growth of LCD sales, it looks like just about everyone these days is an LCD buyer. So what do they mean and why do you care? We're glad you asked...

Refresh Rate and Response Time, What's up with That?
Response Time

Let's start with response time. Simply put, this measures the amount of time it takes for a single pixel on the screen to change color. Technically it is the time it takes for one pixel to change from black to white and back to black again (or gray to white to gray). Obviously as pictures on the screen change, the pixels have to change. If the pixels can't change fast enough, the old image stays on the screen and you see blurry or "smeared" images.

Early LCD screens had significant problems with response times. They are measured in milliseconds, where the smaller the time the better it is. Typical TVs today have 8ms or 4ms response times. Either one should be sufficient for normal TV and gaming. Of course all other specs being equal, you should opt for the 4ms set.

Imagine response time like the shutter on a camera. To take a picture the shutter has to go from closed to open and back to closed again. If this process takes too long you either won't get the picture you wanted to get (if it takes too long to open), or the picture you do capture will be blurry or smeared (if it takes too long to close). LCD pixels are the same. They have to change fast enough to show you only what you want to see on screen at that exact instant.

Refresh Rate

That gets us to refresh rate, which defines what should be on screen at any exact instant. Refresh rate is the number of times per second that the TV puts a new image on screen. Again, to be really simple, the more times the image is refreshed, the smoother motion will look on your set. Imagine watching a movie where the screen only refreshed 5 or 6 times per second. Not very pleasant.

Of course there is some ability for overkill. The ATSC spec for HDTV has a maximum of 60 frames per second, so why would a TV need to refresh faster than that? Two reasons. First, your eye prefers a higher refresh rate. Most computer screens refresh at or faster than 70 Hz because your eyes get tired staring at 60 Hz. Second, if the TV has the ability to interpolate between frames it can actually make fast motion look smoother. Of course if the TV simply shows the same frame twice (doubling the 60 fps to match 120 Hz), there isn't much benefit.

However, the one really big benefit of 120 Hz LCD is with content that was originally shot on film. Film is often recorded at 24 fps. To get this to 30 or 60 fps, the TV has to use a process called 3:2 pull-down. It can produce a side effect that makes some frame transitions look really choppy. This is often referred to as judder. Since 120 Hz is evenly divisible by 24 fps, no 3:2 pull-down is necessary, thus no judder.

Back to the camera analogy, if we may. Think of refresh rate as the number of times per second that your finger can click the button to take a picture. If you want to string all the pictures together, the more of them you capture per second, the smoother the transition will be from picture to picture. Of course if your finger can click faster than your shutter can keep up with, you get no benefit from the extra frames.

Conclusion

What it all comes down to it is that both specs work together to produce a smooth picture on screen. You can't have one without the other. Response times will effect everything you watch. If they're too slow, you'll see the smearing, especially with video games and fast action. While you may not see much difference with 120Hz for normal day to day viewing, if judder in 24 frame film content bothers you, 120 Hz is the way to go.

Posted by The HT Guys, March 9, 2009 11:53 PM

About The HT Guys

The HT Guys, Ara Derderian and Braden Russell, are Engineers who formerly worked for the Advanced Digital Systems Group (ADSG) of Sony Pictures Entertainment. ADSG was the R&D unit of the sound department producing products for movie theaters and movie studios.

Two of the products they worked on include the DCP-1000 and DADR-5000. The DCP is a digital cinema processor used in movie theaters around the world. The DADR-5000 is a disk-based audio dubber used on Hollywood sound stages.

ADSG was awarded a Technical Academy Award by the Academy of Motion Picture Arts and Sciences in 2000 for the development of the DADR-5000. Ara holds three patents for his development work in Digital Cinema and Digital Audio Recording.

Every week they put together a podcast about High Definition TV and Home Theater. Each episode brings news from the A/V world, helpful product reviews and insights and help in demystifying and simplifying HDTV and home theater.