Two sets of factors greatly impact the perceived quality of streaming video. One set of factors is made up of key characteristics of the file containing a video. The other set of factors is made up of key characteristics of the display window, the portion of the screen or monitor within which the video program will be displayed on the end-user's computer or other device. Some of the key factors apply to both the video file and the display window. These are width, height, and aspect ratio. Key factors that are associated only with the video file, and not with the display window, are file size and bit rate.
A video file, and a display window within which the video will appear, each has a width and height. These are usually measured in pixels, which are tiny, square fractions of a computer monitor, television screen, or other display device. In theory, the width and height of a video file or of a display window can have any values. In practice, though, some measurement and ratios are much more common than others. Most video programs are rectangular, with a width that is greater than the height, resembling the screen in a movie theater.
The aspect ratio of a video file or a display window is simply the ratio of the width to the height. For example, a video program with a width of 960 pixels and a height of 720 pixels has a ratio of approximately 1.333, determined just by dividing 960 by 720. Some aspect ratios are more common among video programs, and the most widely used are often identified as a standard ratio, such as 16:9 or 4:3. 16:9 is an aspect ratio of 1.7778 (= 16/9), and 4:3 is an aspect ratio of 1.333 (=4/3).
Part of the perceived quality of a video experience depends on respecting the aspect ratio of the video source. The width and height of a program can often be scaled up or down with no ill effects, as long as the ratio of width to height remains unchanged. Altering the aspect ratio, however, is likely to distort the video image, either stretching the image or compressing it in one dimension or the other.
Another part of the perceived quality of the video experience is the relationship between the aspect ratio of the video file and the aspect ratio of the display window. Since video players (both hardware and software) usually scale a video program up or down to fit within the available display window, a big difference between the aspect ratio of the program and display window typically results in "black bars," areas of unused display space. The black bars are technically harmless, but may give an impression that the video content is smaller than it has to be, and may make the overall color tone of the video appear darker than it actually is. The black bars can often be reduced or eliminated by a better match between the aspect ratio of the video program and the aspect ratio of the window within which it is displayed.
The data-density of a video file is measured as the bit rate, also spelled bit-rate or bitrate. Conceptually, the bit rate of a video program can be thought of as the number of bits needed to describe a single frame of a video program, or as the number of bits needed to describe a single second of a video program (or of a single minute, etc.). If other factors are held constant, a video file with a higher bit rate will have a better display quality, appearing crisp and clear. It will also take up more storage space, will take up a lager portion of available Internet bandwidth while being transmitted, and will result in a larger total number of bits transmitted. The same video stored at a lower bit rate will take up less storage space, will use less of the available Internet bandwidth while being transmitted, and will result in a smaller number of total bits transmitted. However, the perceived quality of the video is likely to be lower, with blurring of fine details and with boxy "pixelation" effects in scenes with more motion or complex imagery.
Although bit rate is a large factor in determining the size of a file containing a video program, bit rate by itself is not very meaningful. Bit rate is best understood in comparison with the raw numbers (not the ratio) identifying the width and height of the video program. Since the width and height of a video program represent actual pixels, higher numbers require more bits of data. In other words, a file containing a video program that is 960 pixels wide by 720 pixels high simply MUST be larger than a file containing a very similar video that is 640 pixels wide by 480 pixels high. Setting the bit rate too low for a given width and height will dramatically reduce the quality of the video, causing blurring and pixelation. Setting a bit rate higher than needed for a given width and height will result in a file that is much larger than required. In practice, then, finding the right balance between file size and video quality often requires setting the width and height for a video file, as well as the bit rate.
What we typically think of as a "video" program actually contains two separate media streams. One of these is the video stream, consisting a series of visual images. The other is the audio stream that accompanies those images. Under most circumstances, the audio stream is much smaller than the video stream, and so it is often ignored when trying to balance file size and video quality. One factor, though, that is worth considering, is the number of channels in the audio stream. DVDs containing older material may have audio streams that are stereo (2 channels) or even mono (one channel). Increasingly, newer materials and newer DVDs have higher quality audio streams with 5 or more channels (Dolby Pro Logic, etc.). Of course, the additional channels require additional data, contributing to a larger file size.
Having more than two audio channels can be enjoyable when watching some kinds of DVDs, but is simply a waste with most forms of streaming video. Many streaming servers and players simply do not support anything other than one or two channels. When preparing a video program for streaming, reducing 5 or more channels to 2-channel stereo is likely to save file space and never be noticed.