What Frames Are
In video (both analog and high definition), just as in film, images are displayed as Frames. However, there are differences in the way the frames are displayed on a television screen. In terms of traditional video content, in NTSC-based countries there are 30 separate frames displayed every second (1 complete frame every 1/30th of a second), while in PAL-based countries, there are 25 separate frames displayed every second (1 complete frame displayed every 25th of a second). These frames are either displayed using the Interlaced Scan method or the Progressive Scan method.
However, since film is shot at 24 frames per second (1 complete frame displayed every 24th of a second), in order to display film on a typical television screen, the original 24 frames must be converted to 30 frames by a process known as 3:2 pulldown.
What Refresh Rate Means
With today's television display technologies, such LCD, Plasma, and DLP, and also disc-based formats, such as Blu-ray Disc (as well as the now discontinued HD-DVD), another factor has entered into play that affects how frames of video content are displayed on a screen: Refresh Rate. Refresh rate represents how many times the actual TV, video display, or projected screen image is completely reconstructed every second. The idea is that the more times the screen is "refreshed" every second, the smoother the image is in terms of motion rendering and flicker reduction.
In other words, the image looks better the faster the screen can refresh itself. Refresh rates of televisions and other types of video displayed are measured in "hz" (Hertz). For example: A Television with a 60hz refresh rate represents complete reconstruction of the screen image 60 times every second. As a result, this also means that each video frame (in a 30 frame per second signal) is repeated twice every 60th of a second. By looking at the math, one can easily figure out how other frame rates relate to other refresh rates.
Frame Rate vs Refresh Rate
What makes things confusing is the concept of how many separate and discreet frames are displayed every second, verses how many times the frame is repeated every 1/24th, 1/25, or 1/30th of a second to match the refresh rate of the Television display.
TVs have their own screen refresh capabilities. A television's screen refresh rate is usually listed in the user manual or on the manufacturer's product web page.
The most common refresh rate for today's Televisions are 60hz for NTSC-based systems and 50hz for PAL-based systems. However, with the introduction of some Blu-ray Disc and HD-DVD players that can actually output a 24 frame per second video signal, instead of the traditional 30 frame per second video signal, new refresh rates are being implemented by some television display makers to accommodate these signals in the correct mathematical ratio.
If you have a TV with a 120hz refresh rate that is 1080p/24 compatible (1920 pixels across the screen vs 1080 pixels down the screen, with a 24 frame per second rate). The TV ends up displaying 24 separate frames every second, but repeats each frame according to the refresh rate of the TV. In the case of 120hz each frame would be displayed 5 times within each 24th of a second.
In other words, even with higher refresh rates, there are still only 24 separate frames displayed every second, but they may need to be displayed multiple times, depending on the refresh rate.
To display 24 frames per second on a TV with a 120hz refresh rate, each frame is repeated 5 times every 24th of a second.
To display 24 frames per second on a TV with a 72hz refresh rate, each frame is repeated 3 times every 24th of a second.
To display 30 frames per second on a TV with a 60 hz refresh rate, each frame is repeated 2 times every 30th of a second.
To display 25 frames per second on a TV with a 50 hz refresh rate (PAL Countries), each frame is repeated 2 times every 25th of a second.
To display 25 frames per second on a TV with a 100 hz refresh rate (PAL Countries), each frame is repeated 4 times every 25th of a second.
NOTE: The above explanation is with pure frame rates. If the TV also has to do a 24 frame per second to 30 frame per second or vice versa frame rate conversion, then you also have to deal with 3:2 or 2:3 Pulldown, which adds more math. 3:2 pulldown can also be performed by a DVD or Blu-ray Disc player, or other source device, before the signal reaches the TV.
How TVs Handle 1080p/24
If a TV is 1080p/60 or 1080p/30 - only compatible, it would not accept the 1080p/24 input. Currently, only Blu-ray Discs and HD-DVD discs are the main sources of 1080p/24 material. However, most Blu-ray Disc and HD-DVD players convert the outgoing signal to either 1080p/60 or 1080i/30 so that the information can be processed by a TV properly for screen display if it is not compatible with 1080p/24.
NOTE: Although 1080p/60-only TVs cannot display 1080p/24 - 1080p/24 TVs can display 1080p/60 via video processing.
The whole thing boils down to the concept of separate frames vs repeated frames. In the case of frame rate vs refresh rate calculations, repeated frames are not considered separate frames as the information in the repeated frames is identical. It is when you move to a frame with different information that you count it as a new frame.
However, in addition to screen refresh rate, another technique that some TV manufacturers use that can enhance motion response and reduce motion blur is referred to as Backlight Scanning. In other words, let's say a TV has a 120Hz screen refresh rate. It is possible that it may also incorporate a backlight that flashes on and off rapidly at an additional 120Hz every second (in between the screen refresh rate repeated frames). This technique delivers the effect of having a 240Hz screen refresh rate by effectively cheating the system.
On the TVs that use this technology, it can be enabled or disabled separately from the screen refresh rate setting, if the effect of the backlight scanning technique is not preferred. Also, while some TV manufacturers implement backlight scanning, some do not, or just use it in some models and not others.
The Soap Opera Effect
Even though all this frame rate, refresh rate, and backlight scanning trickery is designed to deliver a better viewing experience for consumer, it doesn't always turn out that way. On the one hand, the issues of motion lag are diminished or eliminated, but what can happen as a result of all this processing is what is referred to as the "Soap Opera Effect". The visual result of this effect is that film-based content looks like it was shot on video, which gives movies an eerie, video tape or stage production look, like a soap opera or live or live-on-tape TV broadcast. If you find that this effect bothers you, fortunately, most TV makers provide a setting that can adjust the amount of, or turn off, the added refresh or backlight scanning features.
With more sophisticated technologies being employed in today's HDTVs, it is important that consumers arm themselves with the knowledge of what is important and what isn't. With HDTV, the concept of Screen Refresh Rate is indeed important, but don't get bogged down with the numbers, and be aware of possible visual side effects.
The important thing to take into consideration is how the increase in refresh rate and/or the added implementation of backlight scanning improves, or doesn't improve, the perceived screen image quality for you, the consumer. Let your own eyes be your guide as you comparison shop for your next television.