As the conventional analog television picture is made up of lines, the line structure determines the definition or the fineness of detail which can be portrayed in the vertical axis. The limit is reached in theory when alternate lines show black and white. In a 625-line picture there are roughly 600 un-blanked lines. If 300 of these are white and 300 are black then there will be 300 complete cycles of detail in one picture height. One unit of resolution, which is a unit of spatial frequency, is C/ph or cycles per picture height. In practical displays the contrast will have fallen to virtually nothing at this ideal limit and the resolution actually achieved is around 70 percent of the ideal, or about 210 c/ph. The degree to which the ideal is met is known as the Kell factor of the display.
Definition in one axis is wasted unless it is matched in the other and so the horizontal axis should be able to offer the same performance. As the aspect ratio of conventional television is 4: 3 then it should be possible to display 400 cycles in one picture width, reduced to about 300 cycles by the Kell factor. As part of the line period is lost due to flyback, 300 cycles per picture width becomes about 360 cycles per line period.
In 625-line television, the frame rate is 25 Hz and so the line rate Fh will be:
Fh = 625 × 25 = 15625 Hz
If 360 cycles of video waveform must be carried in each line period, then the bandwidth required will be given by:
15 625 × 360 = 5. 625 Mega-Hertz
In the 525-line system, there are roughly 500 un-blanked lines allowing 250 C/ph theoretical definition, or 175 lines allowing for the Kell factor. Allowing for the aspect ratio, equal horizontal definition requires about 230 cycles per picture width. Allowing for horizontal blanking this requires about 280 cycles per line period, in 525-line video, Ft,-525 × 30 = 15750 Hz Thus the bandwidth required is:
15 750 × 280 = 4. 4 Mega-Hertz
If it is proposed to build a high-definition television system, one might start by doubling the number of lines and hence double the definition. Thus in a 1250-line format about 420 C/ph might be obtained. To achieve equal horizontal definition, bearing in mind the aspect ratio is now 16: 9, then nearly 750 cycles per picture width will be needed. Around 890 cycles per line period will be needed when allowing for horizontal blanking. The line frequency is now given by:
Fh = 1250 × 25 = 31250 Hz
And the bandwidth required is given by:
31250 × 890 = 28 Mega-Hertz
Note the dramatic increase in bandwidth. General the bandwidth rises as the square of the resolution because there are more lines and more cycles needed in each line. It should be clear that, except for research purposes, high-definition television will never be broadcast as a conventional analog signal because the bandwidth required is simply uneconomic. If and when high-definition broadcasting becomes common, it will be compelled to use digital compression techniques to make it economic.