Info about... Frames, Fields, Frame Rate and their Transmission Standards

What are Frames, Fields and Frame Rate?

Basically, a video can be thought of as being made up of a number of still images, called Frames. Which when played at speed and in succession, create the appearance of an moving image. The quantity of frames the viewer sees each second is known as, the Frame Rate which is also referred to as, Frames Per Second (fps).

As you are aware the most common ways of viewing video is via, Film (at the cinema), TV (Television) or PC (Personal Computer).

Film

Film runs through a projector at a constant speed of 24 fps. It’s methodology is simple, every 1/24 th of a second, the viewer sees a new frame. And because each new frame is seen after the old frame. his technique is also referred to as, progressive scanning. And is a world-wide standard.

TV

Analogue TV however, does not deal with video in terms of frames. Instead, it displays video using half-frames, called fields. Each frame contains exactly two fields.

One field is made up of the odd horizontal lines in a frame. This is called the odd field or the top field (since it contains the top line of the image).

The other field is made up of the even horizontal lines in a frame. This is called the even field or bottom field.

This technique of broadcasting video is referred to as, interlaced scanning.

Currently there are three common TV broadcasting standards. PAL, SECAM and NTSC. And all of them use, interlaced scanning to produce a moving image.

PAL Colour System

PAL is the most common European TV transmission standard.

It is also used for DVD, VCR, VCD, Mpeg4 (DivX and XviD), LaserDisc and with both analogue and digital home camera equipment. PAL material is played at 25 fps (or 50 fields per second).

But here is where the problems start, because as you may have already noticed, PAL TV uses one frame per second more than film.

So how can we play a 24fps movie on a 50 fields per second PAL TV?

Well we could present each movie frame twice in consecutive fields. Making the movie play at 48 fields per second. But playing the movie like this would lead to jerky playback because 2 fields would be missing in every second of playback.

So to get round this, the movie is simply speeded up by 1 frame a second (2 fields). Which means a typical  60 min film actually runs for 57 min 36 sec. And this is why the same movie when watched at home on your PAL TV, will always be a bit shorter than when viewed at the cinema!

SECAM Colour System

SECAM is a transmission standard developed by the French and can also found in Europe.

It too uses 25 fps (or 50 fields per second). But differs to PAL due to the way the colour and brightness is handled, prior to broadcasting.

But for what we are talking about here. They are effectively the same and follow the same frame speed/field rules.

NTSC Colour System

NTSC is most commonly found in the USA, Canada and Japan.

It is also used for DVD, VCR, VCD, Mpeg4 (DivX and XviD), LaserDisc and with both analogue and digital home camera equipment. But unlike PAL it plays at somewhere near 29,97 fps (to be exact it is 4,5 MHz/286/525).

But as you can see there's a much bigger problem here when it comes to converting films 24 fps to NTSC's 29,97 fps

So how can we play cinema movies on NTSC TV's. Accelerate them? Unfortunately no. Because there would be a noticeable speed difference. For example a typical  60 min film would last just 48 min 3 sec.

So NTSC engineers came with a solution called telecine or 3:2 pull down which uses quite a complicated process to convert a film that runs at 24 fps to run at 29.97 fps.

The first step, is to slow down the video by 0.1% to 23.976 fps. Then approximately 6 frames are added to the video each second, bringing the frame rate to 29.97 fps.

This is done by adding 1 extra frame to each group of 4 film frames.

And although it would be quite simple to duplicate 1 out of every 4 frames to produce the extra frame. This method is not used. Because the duplication of one frame would cause that frame to be displayed for twice as long as the other 3 frames. Which leads to jerky playback.

Fortunately, film producers can make use of the field-based nature of video to more gradually introduce the extra frame. Instead of adding a whole new frame at once, 2 fields are introduced separately to each group of 4 film frames. And since 2 fields make up 1 frame, this method is equivalent to adding 1 new frame.

However, since the 2 duplicated fields are not added at the same time, this reduces the jerkiness of the video.

And technically, because NTSC TV's typically operate at 60Hz (to match the incoming electricity supply frequency), which is not a multiple of 29,97fps, there will always be a negligible amount of jerkiness during video playback.

PC's

PC's and their monitors are great when it comes to displaying video. Because they can handle it progressively, just like film.

They can also be configured to produce many more frame rates than an PAL TV's 50 or 100Hz or an NTSC's TV's 60Hz. In fact, even the most basic a PC monitor/video card can be configured to scan (or refresh) at 60Hz, 75Hz, 85Hz or 100Hz

To obtain a smooth playback on a PC you just have to make sure that your video card uses a refresh rate that is an exact multiple of the movie you play.

We are therefore able to surmise, that PAL material on a PC can be played perfectly at a frequency of 50 Hz, 75 Hz or 100 Hz.

However, NTSC material will never yield perfect playback results (even in progressive mode) for exactly the same reasons it's not possible on a TV.

HDTV (High Definition Television)

At the moment, there are around 20 different HDTV broadcasting formats being used around the world. The differ mainly in the scanning (interlaced or progressive) and the resolution (quantity of pixels) of the images.

The US is currently the most advanced market with over 6 million HD television. This is because US Digital Terrestrial Television is compelled to provide HDTV. NBC and CBS are using 1080i (1920 wide x 1080 high interlaced). ABC uses 720p (1280 wide x 720 high progressive) and Fox uses 480p (720 wide x 480 high progressive), a bit like DVD.

As is so often the case, no one can agree on one 'definitive' standard. But without doubt 1080i and 720p are the most popular. At first glance it seems clear that the 1080i must offer a superior picture to 720p. However, the reality is not quite so simple.

You see a 1080i transmission contains two image fields of 1920x540 (2,073,600 total) pixels at 30fps and a 720p transmission displays two full frames of 1280x720 (1,843,200 total) pixels at 60fps. So the total pixel rate is almost the same.

However, interlacing images produces unwanted artefacts, together with a whole bunch of other problems associated when trying to combine interlaced image fields (especially NTSC ones). And while NHK in Japan was researching HDTV for its MUSE analogue service (around 25 years ago) they concluded that the picture quality of an progressive scan image could look every bit as good as an interlaced scan image when using 60% less scan lines. More scan lines means more bandwidth, more bandwidth costs more money to broadcast. And TV's built to display more scan lines cost more money.

So progressive is surly the way to go!

Digital Displays (A quick note)

Also, because we are now in the digital age, we can now have LCD and plasma screens that display the picture using pixels (instead of scan lines). And given that such displays have started to appear offering 1280x768 pixels (WXGA), complete with DVI input connectors, which is perfect for 720p HDTV, it will soon be possible to maintain a digitized signal right up to the screen. No more fields, no more scan lines, no more analogue!

Last Updated

Mon 09 Nov 04 @ 18:15