What's the Difference Between Frame Rate and Refresh Rate?

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What's the Difference Between Frame Rate and Refresh Rate?

Star-Trek-UHD-2.jpgWe recently received an email from a reader who had a question about our review of the Yamaha RX-A3050 AV receiver, in which writer Myron Ho explains that the receiver's HDMI 2.0 inputs allow it to accept a 4K/60 video signal. The reader owns a Samsung 120Hz 4K TV and wondered if, because the Yamaha only supports 4K/60, it is not "fast enough" to keep up with the 120Hz TV and is thus incompatible or at least limited in its performance. This is actually a topic that seems to confuse a lot of readers, so here's a quick primer to explain the difference between frame rate and refresh rate.

Let me stress off the bat, especially for the hardcore videophile readers, that my intent is to keep this explanation as simple as possible. I'm not going to delve into topics like interlaced fields versus frames, passing 4:4:4 versus 4:2:2 versus 4:2:0 subsampling rates, and other high-level concepts. I'm going to boil this topic down to the most basic principles to help everyone understand the fundamental difference. (In other words, there's no need to write a manifesto in the Comments section about all the complexities that I didn't cover. I know I didn't cover them. I did it on purpose.)

In talking about an AV receiver's ability to accept 4K/60 (aka 4K/60p) and a TV's ability to display 4K at 120Hz, we're talking about two different things: frame rate and refresh rate. "4K/60" refers to the source's frame rate. It means the content has a 4K resolution output at 60 frames per second. Most movies (except The Hobbit) are shot at 24 frames per second. On Blu-ray disc, a film will have a native frame rate of 1080p/24. On the new Ultra HD Blu-ray discs, films have a native frame rate of 4K/24 (more specifically, a 3,840 x 2,160 resolution at 24fps). The maximum frame rate that the new Ultra HD Blu-ray players (like the Samsung UBD-K8500) and other 4K media streamers (like the Roku 4) can output is 4K/60. There is no 4K/120 or higher output option. So, 4K/60 is the maximum frame rate that these new AV receivers like the Yamaha need to be able to accept and pass through.

On the TV side, we're talking about refresh rate: how many times per second the screen refreshes, or how many times it changes the image on the screen. In the past, all U.S. TVs had a refresh rate of 60Hz (which equals 60 times, or images, per second), and VCRs, DVD players, and other set-top boxes converted all content to a compatible 60Hz output in order to match what the TV could do. That was the only rate those TVs could accept. Nowadays, TVs are more flexible. They can have a refresh rate of 60Hz, 120Hz, or higher (although most 4K TVs max out at 120Hz), and many of them can accept either a 24fps or 60fps output signal from the source device. When the TV has a refresh rate of 120Hz, it converts the incoming signal (be it 24 or 60fps) to 120 by adding frames. It can do this by repeating frames, inserting black frames, or interpolating new frames. To understand why today's TVs have higher refresh rates, how they add those frames, and what effect it can have on picture quality, check out my story What Is Soap Opera Effect (and How to Make It Go Away).

For the purposes of this article, just understand that the conversion to 120 or higher happens inside the TV itself. Your 4K player outputs the 4K/24fps or 4K/60fps source to your AV receiver, which needs only to be able to pass it through to your display. Everything else will happen inside the TV. Certainly, there are plenty of other compatibility issues surrounding 4K that we can discuss, but a receiver's "speed" isn't one you need to worry about.

If you have other questions about 4K specs and compatibility concerns, feel free to send us an email or ask it in the Comments section below.

Additional Resources
• How to Choose the Right LCD TV at HomeTheaterReview.com. 
• What Is Refresh Rate at CNET.com.

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