- The ability of the image reproducing system to represent the fine structure of an object is known as its resolving power or resolution.
- It is necessary to consider this aspect separately in the vertical and horizontal planes of the picture.
Vertical resolution.
- The extent to which the scanning system is capable of resolving picture details in the vertical direction is referred to as its vertical resolution.
- It has already been explained that the vertical resolution is a function of the scanning lines into which the picture is divided in the vertical plane.
- Based on that discussion the vertical resolution in the 625 lines system can then be expressed as V r = N a × k where V r is the vertical resolution expressed in number of lines, N a is the active number of lines and k is the resolution factor (also known as Kell factor).
- Assuming a reasonable value of k = 0.69, V r = 585 × 0.69 = 400 lines
- It is of interest to note that the corresponding resolution of 35 mm motion pictures is about 515 lines and thus produces greater details as compared to television pictures.
Horizontal resolution.
- The capability of the system to resolve maximum number of picture elements along the scanning lines determines horizontal resolution.
- This can be evaluated by considering a vertical bar pattern as shown in Fig. 2.7(a).
- It would be realistic to aim at equal vertical and horizontal resolution and as such the number of alternate black and white bars that should be considered is equal to N a × aspect ratio = 585 × 4/3 = 780.
- Before proceeding further it must be recognised that as all lines in the vertical plane are not fully effective, in a similar way all parts of an individual line are not fully effective all the time.
- As explained earlier, it ultimately depends on the random distribution of black and white areas in the picture. Thus for equal vertical and horizontal resolution, the same resolution factor may be used while determining the effective number of distinct picture elements in a horizontal line.
- therefore, the effective number of alternate black and white segments in one horizontal line for equal vertical and horizontal resolution are : N = N a × aspect ratio × k = 585 × 4/3 × 0.69 = 533
- To resolve these 533 squares or picture elements the scanning spot must develop a video signal of square wave nature switching continuously along the line between voltage levels corresponding to black and peak white.
- This is shown along the bar pattern drawn in Fig. 2.7(a).
- Since along one line there are 533/2 ˜ 267 complete cyclic changes, 267 complete square wave cycles get generated during the time the beam takes to travel along the width of the pattern.
- Thus the time duration th of one square wave cycle is equal to
- Since the consideration of both vertical and horizontal resolutions is based on identical black and white bars in the horizontal and vertical planes of the picture frame, it amounts to considering a chessboard pattern as the most stringent case and is illustrated in Fig. 2.7(b ).
- Here each alternate black and white square element takes the place of bars for determining the capability of the scanning system to reproduce the fine structure of the object being televised.
- The actual size of each square element in the chess pattern is very small and is equal to thickness of the scanning beam.
- It would be instructive to know as an illustration that the size of such a square element on the screen of a 51 cm picture tube is about 0.5 mm2 only.
Colour resolution and
bandwidth.
- As explained above a bandwidth of 5 MHz (4 MHz inthe American system) is needed for transmission of maximum horizontal detail in monochrome.
- However, this bandwidth is not necessary for the colour video signals.
- The reason is that the human eye’s colour response changes with the size of the object.
- For very small objects the eye can perceive only the brightness rather than the colours in the scene. Perception of colours by the eye is limited to objects which result in a video frequency output up to about 1.5 MHz.
- Thus the colour information needs much less bandwidth than monochrome details and can be easily accommodated in the channel bandwidth allotted for monochrome transmission.