The technical side of video production has never been straight forward, and is often extremely confusing for video producers and clients alike. I thought I’d make a series of posts which address some of the basics to help people develop a better understanding of how things work.
Picture Aspect Ratios
The aspect ratio is simply the relationship between the width and height of the picture. In films this is generally expressed as a decimal ratio such as 1.85:1 or 2.35:1. With video and television the convention is to express this relationship using whole numbers, generally either 4:3 or 16:9.
4:3 was the ratio for standard definition television all around the world. Several television standards emerged, using different numbers of pixels to make up the pictures and showing the image at a different number of frames per second. A pixel is basically the individual dots which make up the picture.
Widescreen videos are typically shot in 16:9. This can be both in standard or high definition. Any modern flatscreen television will be a widescreen.
Standard Definition Systems
The UK and other countries use the PAL television system which uses 720×576 pixels to make up the image.
A system called NTSC is used in many countries including America, the picture is made from 720×480 pixels.
SECAM is the standard definition system used in France, and parts of Africa and Eastern Europe. There are several variants of SECAM but it generally uses the same resolution as PAL but manages colour in a completely different way.
Interlaced vs Progressive
These standard definition pictures were made up of interlaced fields in which every other line would be shown alternately creating the illusion of a complete image.
In PAL systems each field was shown 25 frames per second – giving a total of 50 interlaced fields per second, each comprising half of an image.
With NTSC each field was shown about 30 times per second (actually 29.97 times). This meant that 60 fields were actually shown per second.
Feature films shown in the cinema are projected at 24 frames per second.
Progressive whole frames are generally considered to be a major part of the ‘cinematic’ look of feature films and are far better to work with especially if the video is to include motion graphics or animation along with the video footage.
High definition technically means anything larger than standard definition but has really come to be known as a picture with either 1280×720 or 1920×1080 pixels.
HDV cameras typically shoot 1280×720 progressive frames (720p) or 1920×1080 interlaced frames (1080i).
Full HD typically refers to 1920×1080 progressive frames (1080p).
Pixel aspect ratios
Now as if this weren’t already confusing enough the pixels which make up the images in PAL and NTSC are not square and are all different. Newer screens, including all computer monitors and flat screen televisions however do use square pixels. This means that any standard definition video shown on a new television screen would be made slightly thinner because the rectangular pixels would be replaced by thinner square pixels. This distortion will make people look taller and thinner and can be quite off-putting and obviously is not desirable.
To compensate for this when exporting a standard definition video for use on a square pixel display it is necessary to add extra horizontal pixels. With a 4:3 PAL video this means using 768×576 pixels instead of 720×576. A widescreen PAL video would become 1024×576 pixels.