What Frame Rate Is
In video (both Analog and HD), just as in film, images are displayed as Frames. However, there are differences in the way the frames are transmitted and/or displayed on a television screen.
How Frames are Displayed in Analog Video
Lines and Pixels
A television or recorded video image is basically made up of scan lines or pixel rows. Unlike film, in which the whole image is projected on a screen at once, a video image is composed of lines or pixel rows displayed across a screen starting at the top of the screen and moving to bottom. These lines or pixel rows can be displayed in two ways. The first way is to split the lines into two fields in which all of the odd numbered lines or pixel rows are displayed first and then all of the even numbered lines or pixel rows are displayed next, in essence, producing a complete frame. This process is called interlacing or interlaced scan.
The second method, used in LCD, Plasma, DLP, OLED flat panel TVs and computer monitors, is referred to as progressive scan. Instead of displaying the lines in two alternate fields, progressive scan allows the lines or pixel rows to be displayed sequentially. This means that both the odd and even numbered lines or pixel rows are displayed in numerical sequence.
NTSC and PAL
The number of vertical lines or pixel rows dictates the capability to produce a detailed image, but there is more. It is obvious at this point that the larger the number of vertical lines or pixel rows, the more detailed the image. However, within the arena of analog video, the number of vertical lines or pixel rows is fixed within a system. The two main analog video systems are NTSC and PAL.
NTSC is based on a 525-line or pixel row, 60 fields/30 frames-per-second, at 60Hz system for transmission and display of video images. This is an interlaced system in which each frame is displayed in two fields of 262 lines or pixel rows, which is then combined to display a frame of video with 525 lines or pixel rows. NTSC is the official analog video standard in the U.S., Canada, Mexico, some parts of Central and South America, Japan, Taiwan, and Korea.
PAL is the dominant format in the World for analog television broadcasting and analog video display and is based on a 625 line or pixel row, 50 field/25 frames a second, 50HZ system. The signal is interlaced, like NTSC into two fields, composed of 312 lines or pixel rows each. Since there are fewer frames (25) displayed per second, sometimes you can notice a slight flicker in the image, much like the flicker seen on projected film. However, PAL offers a higher resolution image and better color stability than NTSC. Countries on the PAL system include the U.K., Germany, Spain, Portugal, Italy, China, India, Australia, most of Africa, and the Middle East.
For more background information on the PAL and NTSC analog video systems, check out my article: An Overview of Worldwide Video Standards.
DigitalTV/HDTV and NTSC/PAL Frame Rates
Although the increased resolution capability, digital format broadcasting, and high definition video software content standards are a step up for consumers, when comparing HDTV to analog NTSC and PAL standards, the fundamental common foundation of both systems is the Frame Rate.
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.
With the implementation of the Digital TV and HDTV, the foundation of how frames are displayed still have their roots in the original NTSC and PAL analog video formats. In soon-to-be former NTSC-based countries, Digital and HDTV are implementing the 30 Frame-per-second frame rate, while soon-to-be PAL-based countries are implementing a 25 Frame-per-second Frame rate.
NTSC-Based Digital TV/HDTV Frame Rate
Using NTSC as a foundation for Digital TV or HDTV, if the frames are transmitted as an interlaced image (1080i), each frame is composed of two fields, with each field displayed every 60th of a second, and a complete frame displayed every 30th of a second, using a NTSC-based 30 frame-per-second frame rate. If the frame is transmitted in the progressive scan format (720p or 1080p) it is displayed twice every 30th of a second. In both cases, a unique high definition frame is displayed every 30th of a second in former NTSC-based countries.
PAL-Based Digital TV/HDTV Frame Rate
Using PAL as a foundation for Digital TV or HDTV, if the frames are transmitted as an interlaced image (1080i), each frame is composed of two fields, with each field displayed every 50th of a second, and a complete frame displayed every 25th of a second, using a PAL-based 25 frame-per-second frame rate. If the frame is transmitted in the progressive scan format (720p or 1080p) it is displayed twice every 25th of a second. In both cases, a unique high definition frame is displayed every 25th of a second in former PAL-based countries.
In the final analysis, Digital TV and HDTV, although a leap forward in terms of what you actually see on the screen, with increased resolution and detail, still has roots in 60-plus year-old analog video standards. As a result, there are, and will be, for the foreseeable future, differences in Digital TV and HDTV standards in use throughout the World, which reinforces the barrier to true Worldwide video standards for both the professional and the consumer.
Also, let us not forget that there are still many NTSC and PAL-based video playback devices, such as VCRs, analog camcorders, and non-HDMI equipped DVD players still in use around the world that are plugged into, and viewed on, HDTVs.
For a more in-depth look at Video Frame Rate, as well as Refresh Rate, which is an additional function performed by a TV that also affects how the image looks on the screen, check out my article: Video Frame Rate vs Screen Refresh Rate.