Final Cut
Video Signals What’s in a Name?
BY JILL HEINERTH
Relative frame
dimensions of several standard video formats
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very issue of DIVER gives me a wonderful opportunity to wax on about the joy of shooting video of a massive manta ray or the revelations associated with turning on a great set of video lights in a decorated underwater cave. There’s no fireside story in this issue. Instead, I’m going to delve into the nitty gritty of video geekdom: file formats. You’ve likely come across .avi and .mov, but what about terms like codec, bit rate, aspect ratio and sample rate? Even if you own a seemingly simple GoPro, you’ll find 1080P and 4k listed in the resolution menu. Which one should you use? What do they mean? This short article begins to unravel some of the tricky concepts found in video file formats and hopes to help you gain a better understanding of how they all fit together. By understanding file formats, you will be able to select the right resolution for shooting and playback on different devices. A large amount of data is embedded in a given video signal. It is a whole lot more
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complicated than a simple still photo file. The information contained in a video signal includes visual material, audio attributes, dimensional information and frame rates. The data is put together in a structured container and that is called a file format. Examples are .mov or .avi. The problem with video file formats is that they don’t tell you much about the quality of the product inside the container. The .mov file format might contain low quality web content or it might be far more sophisticated and hold 3D big screen resolution with six channels of surround sound. To understand quality we need to look deeper into nomenclature. To understand the details inside the file format container, we can review the anatomy. The file can be divided into parts including video signal, audio data and something called a codec, which refers to the software language that is used to describe the video signal. Video applications have to call on that codec to read and write the file itself.
A Question of Character Every video file also has certain characteristics. These attributes include things like frame size, aspect ratio, frame rate, bit rate and audio sample rate. Let’s look at these individually.
Frame Size refers to the actual dimensions of pixels that make up the image. This is commonly called ‘resolution’, since a lot of pixels are needed to make up the sharpest, high resolution image. Your GoPro camera offers resolutions up to 4K, which is actually 3940×2160 pixels. The popular 1080 resolution is made up of a frame size of 1920x1080 pixels.
Aspect Ratio is the proportional width and height of the visual material. Our older televisions used to offer up an image that was 4 x 3 in dimension, expressed as 4:3. Now HD sets appear in a different shape. This is a ratio of 16:9. Your DSLR camera might shoot still photos in 3 x 2 aspect ratio. When you switch to shooting video on the same camera, you may see a dark area on the top and bottom of the 3 x 2 viewfinder on the back of the camera. It looks different than
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