Frame (video)

A frame is an electronically coded still image in video technology.

Line and resolution

The frame is composed of picture elements just like a chess board. Each horizontal set of picture elements is known as a line. The picture elements in a line are transmitted as sine signals where a pair of dots, one dark and one light can be represented by a single sine. The product of the number of lines and the number of maximum sine signals per line is known as the total resolution of the frame. The higher the resolution the more faithful the displayed image is to the original image. But higher resolution introduces technical problems and extra cost. So a compromise should be reached in system designs both for satisfactory image quality and affordable price.

Viewing distance

The key parameter to determine the lowest resolution still satisfactory to viewers is the viewing distance, i.e. the distance between the eyes and the monitor. The total resolution is inversely proportional to the square of the distance. If d is the distance, r is the required minimum resolution and k is the proportionality constant which depends on the size of the monitor;

Since the number of lines is approximately proportional to the resolution per line, the above relation can also be written as

where n is the number of lines. That means that the required resolution is proportional to the height of the monitor and inversely proportional to the viewing distance.

Moving picture

In moving picture (TV) the number of frames scanned per second is known as the frame rate. The higher the frame rate, the better the sense of motion. But again, increasing the frame rate introduces technical difficulties. So the frame rate is fixed at 25 (System B/G) or 29.97 (System M). To increase the sense of motion it is customary to scan the very same frame in two consecutive phases. In each phase only half of the lines are scanned; only the lines with odd numbers in the first phase and only the lines with even numbers in the second phase. Each scan is known as a field. So the field rate is two times the frame rate.

Example (System B)

In system B the number of lines is 625 and the frame rate is 25. The maximum video bandwidth is 5 MHz.^[1] The maximum number of sine signals the system is theorically capable to transmit is given as follows:

The system is capable to transmit 5 000 000 sine signals in a second. Since the frame rate is 25, the maximum number of sine signals per frame is 200 000. Dividing this number by the number of lines gives the maximum number of sine signals in a line which is 320. (Actually about 19% of each line is devoted to auxiliary services. So the number of maximum useful sine signals is about 260.)

References