What the numbers with letters at the end mean
You’ll see numbers like 720p or 1080i used to either describe a camera specification, a video file or an option when saving a video. The numbers relate to the height of the picture frame in pixels:
480 = 720 x 480 pixels and is normal for NTSC type standard definition (SD) video.
576 = 720 x 576 pixels and is normal for PAL type SD video.
720 = 1280 x 720 pixels and is a high definition (HD) format.
1080 = 1920 x 1080 pixels. This is also high definition and is usually described as Full HD.
2K, Digital Cinema, 4k and UltraHD bring a whole new combination of frame rates and sizes – we’ll stick with HD video resolutions for now.
The letter at the end tells you whether or not the video is progressive scan ( p ) or interlaced ( i ):
p = Progressive scan. This means the picture is recorded in full frames in one go.
i = Interlaced video. This means the pictures is recorded in alternate odd and even scan lines or “fields”.
Online content is always progressive scan. If you shoot interlaced then you will need to de-interlace the content at some point for online delivery. This is usually best left to the end of the edit at the rendering stage especially when the content is being sent to multiple destinations e.g. Blu-ray, DVD, broadcast and online.
Most standard SD video and many HD recordings including broadcast TV are interlaced. Using interlaced images has a couple of advantages. It reduces the amount of data needed to store or transmit the video. The alternate lines (fields) are recorded sequentially so with a moving subject each field shows a slightly different image. When interlaced video is viewed on screen the human eye doesn’t notice the image is based on fields and is fooled into blending them into full frames thus creating smoother looking motion at lower frame rates.
Old TVs with a tube (CRT screens) were all interlaced. Modern flat screen TV panels are progressive scan. You can play interlaced video on progressive scan TVs. There’s no need for the viewer to adjust anything because the TV can work out what to do with the image (de-interlacing). Computer graphics card adapters and displays are progressive. Interlaced footage viewed on computer displays can make the footage look jagged or coarse if the software cannot de-interlace the files correctly. If you are editing video for interlaced delivery then you should preview the footage on a proper video output with editing hardware such as a Matrox or BlackMagic card to ensure that what you see is what you are actually going to get.
Frames rates? What’s all that about?
The frame rate of a video describes how many times per second the video picture is updated. The higher that number, the smoother the motion in the video will appear to the human eye. Video with lower frame rates can, in some circumstances seem to flicker or judder especially during motion such as when the camera pans across a scene.
The reason we ended up with different frame rates in different parts of the world is because TV cameras, TVs and everything else in the broadcast chain were connected to the mains supply. Here in the UK the mains supply runs at 50Hz whereas in the other countries the mains runs at 60Hz. It therefore made sense to base the refresh rate of TV equipment around the frequency of the mains supply for any given country. In 60Hz countries they started out using 60Hz for TV but when colour pictures came along they slightly altered the refresh rate to to 59.94Hz for technical reasons. The odd number creates a bit of confusion – 59.94Hz is often rounded up to 60Hz. Half of 59.94 is 29.97 but is often rounded up to 30 for convenience.
These are some common frame rates or “frames per second” (abbreviated to fps) and where they usually apply:
24fps – this is the frame rate of film. In video it tends to be 23.976fps that’s used to mimic the “film look”.
25fps is 50Hz halved & used in SD or HD. Usually PAL countries like the UK, Australia and most of Europe.
29.97fps is 59.94Hz halved – Used in SD or HD. Usually NTSC countries incl. USA, Canada, Japan.
30fps – Some cameras do actually record 30fps but often it might really mean 29.97fps rounded-up.
50fps – Used for HD recordings in PAL countries – gives a really smooth motion pictures.
60fps (usually 59.94 rounded-up) – Used for HD recordings in NTSC countries – again gives smooth motion.
Joining the numbers, letters and numbers together
Now we know what the different bits mean we can hopefully understand the full thing easier. Here goes:
720p/50 = Frame size 1280 x 720, progressive scan with 50 frames per second.
1080p/25 = Frame size 1920 x 1080, progressive scan with 25 frames per second.
With a bit of luck that makes perfect sense. It would be great if things were this simple and when describing progressive scan video it pretty much is that easy.
We’ve already explained that interlaced video images contain alternate lines or fields. Each field contains half the information needed to draw a full picture. The numbers for frame rates should only describe full frames. So an interlaced video with 50 fields (or half frames) per second actually only has 25 full frames of information per second. Hence it’s written as 25.
1080i/25 – Frame size 1920 x 1080 Interlaced with 50 Fields Per Second.
480i/30 – Frame size 720 x 480 interlaced with 59.94 fields per second.
It’s worth noting that all 1280 x 720 video is progressive scan – there isn’t an interlaced version.
Forward slash frame rate
You may see video described as 50i. Most people would accept this is 50 interlaced fields per second but when it’s shown in context next to a frame size the EBU preference is to always describe the frame rate 1080i/25 and NOT the field rate.
So 1080i/25 does mean exactly the same thing as 1080i50 that you may see elsewhere. The forward slash in the middle indicates that it’s EBU preferred terminology.
Odd sizes and frame rates
Most video cameras and TVs show a landscape picture. The picture is wider than it is tall. But these days, especially with mobile phones video can be shot in portrait mode so the image is taller than it is wide. So when we shoot a full HD video portrait the picture might be 1080 wide x 1920 tall at 30 frames per second. It’s still a 1080p/30 video.
Some recordings, such as in HDV video camcorders, and many HD TV signals are stored or broadcast at 1440 x 1080i. This is another bit of trickery used to reduce the amount of data needed to record or broadcast the footage. The pixels in the video are oblong (non-square). The pixel aspect ratio is 1.33:1. Essentially this means the pixels are stretched horizontally to fill a 1920 x 1080 frame during playback. This is hardly noticeable to the average viewer. You may also see 960 x 720 pixel video from some cameras. Again these employ non-square pixels to generate a 720p image.
Sometimes cameras can run at low, variable or selectable frame rates. An example of this might be a low end web cam that might run at 15fps because that’s good enough for a talking head shot or to fit in with a slower internet connection. On a faster connection the frame rate might be increased to 30fps. Note: The camera itself possibly runs at 30fps all the time but for technical reasons it’s picture is only recorded or transmitted at lower frame rate.
It’s sometimes necessary to deliver progressive video with equipment that expects (or was designed for) interlaced video. This is achieved by using Progressive segmented frame or PsF. It’s similar to interlaced but the two fields are identical.
Very high or low frame rates are employed to produce a special effect such as ultra-smooth slow motion or time-lapse videos.
We hope you find this quick guide helpful and easy to understand. Feel free to ask a question or comment below. We’ll do our best to reply.