Different cameras and resolutions
Standard Definition (SD) video was created for Standard Definition TVs. The normal resolutions are:
- 720 x 576 pixels (PAL, for Europe)
- 720 x 486 pixels (NTSC, for the USA)
Nowadays, with the almost universal use of High Definition TVs, professional video cameras and most reflex cameras record video in High Definition (HD). The following resolutions are used:
- 1440 x 1080 pixels (HDV)
- 1280 x 720 pixels (NTSC, for the USA)
- 1920 x 1080 pixels (Full HD)
Aspect ratio is the video’s ratio of width to height. It is calculated by dividing the width from the height of the image and is usually written as “X: Y”.
The aspect ratio of SD clips is 4:3. The aspect ratio of HD clips is 16:9. The final user can switch from one ratio to the other by zooming and cropping the image, stretching or distorting (highly unlikely that they choose to) or by filling in with black bands or design elements.
Frames per Second(fps)
The fps is the number of frames (i.e. images) shown per second. The brain perceives these images projected in consecutive order as continual movement because of the slight difference in time from frame to frame.
It is measured as frames per second (fps) or hertz (Hz).
The most common frame frequencies are the following:
- Cinema = 24 fps
- TV European (PAL & SECAM) = 25 fps
- TV USA (NTSC) = 29.97 fps
Interlaced vs. Progressive
When displaying or transmitting video, there are two ways to make the transition from frame to frame: interlaced or progressive.
The interlaced system was developed to solve the flickering caused by the cathode ray tubes in the old analogue TVs. For each frame, first the odd lines (of pixels) and then the even lines are drawn.
In the progressive system, each line is drawn from left to right consecutively. The progressive system produces a better definition and a cleaner image.
Material filmed with the interlaced system can be “desinterlaced” and converted to progressive.
Compression & Bit Rates
A video is made up of a series of images or frames that are shown every second. A 10’ video clip in Full HD contains 25 frames per second and each of those frames measures 1920 x 1080p. That means 250 images of nearly 6 MB each, which would add up to an uncompressed file of 1.4 GB.
That is why a compression algorithm or a “codec” (short for compressor-decompressor) is used in video to compress the information and reduce the file size. Of course, quality is important too, which is why it’s important to use a codec that effectively reduces the file size without a great loss of quality.
Two of the most common codecs are MPEG4 and H.264. The latter (H.264 from Advanced Video Coding) offers sharp and bright video quality, together with bit rates* that are lower than other formats which offer a comparative level of quality and compression
* The bit rate is the amount of information per unit of time of transmission. A high bit rate indicates a low compression of information.
Video File Formats
Video clips can be saved in different file formats. The most common ones are:
- AVI (Audio Video Interleave) is a video and audio container format used on the Windows operating system.
- MPEG is a group of methods defining compression of audio and visual (AV) digital data which was introduced by the "Moving Picture Experts Group".
- MOV is a multimedia container format developed by Apple which allows for multiple tracks, each of which stores a particular type of data: audio, video, effects, or text, which is why it’s an ideal format for use in the edition process.
- FLV or Flash video file developed by Adobe is a container format used to transmit video on internet with the Adobe Flash player. A number of 2.0 webs use this format.
The file format or codec of a video clip can be converted to another with a software conversion tool. There are numerous convertors; we recommend a free one called Mpeg-Streamclip that you might know of.
All of the video clips displayed on our web conform to certain standard formats and codecs. You can see the following information, which appears with each clip.