Frames & Fields (Pt.1)

The first silent films used a frame rate of 14 to 24 frames per second (fps). Most films still use 24fps (25 in some countries outside the US) which projects 24 still images in succession. Your brain translates these stills into a fluid moving picture. There are 3 main frame rates used today being 24p, 25p, and 30p with variations of these being 50i & 60i. The “p” stands for progressive where each frame consists of one complete image just like film. And “i” denotes Interlaced, where each frame is made up of 2 fields interwoven together. Interlace scanning displays each line or row of pixels, the odd rows on the first field and the even on the second field. Think of two combs representing 2 fields and layer them over each other so the bristles interlock with each other leaving no spaces between them. They are now one complete image. Interlacing was originally instituted to lower bandwidth required for sending out TV signals to households. Interlacing cuts the bandwidth in half by sending only half the image at a time. Learn more…

However, the industry has moved away from interlaced content in favor of progressive in order to display better image quality for larger screens. Interlacing can cause many problems with video if it is converted improperly. Even if everything is correct, interlaced video can sometimes cause problems within a scene for lets say thin horizontal lines on a sign or shirt. If these (real world) lines are too similar to the size of the lines (or pixels) that make up the interlaced video image, the result can create a flicker called “interline twitter”. In some cases, if the frame rate is changed incorrectly there can be noticeable motion problems as shown in the following image examples.

interlaced_cutA cut where the fields of each scene overlap.

interlaced_video_artefactsImproperly de-interlaced footage

interlace2Ghosting effect

In part 2 I will go deeper into the timing of frame rates and introduce conversions. Stay tuned…


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