Early French `HD' Television.

[dtuomi]

The choice of number of lines has to do with frequency divider circuits. Before the advent of IC's you could only do odd numbers (usually primes like 3,5, & 7) of divisions of a main clock frequency to obtain the line and field rates.

So lets take B&W 525 lines at 60hz. The vertical frequency is 60hz. The clock division would be 3 × 5 × 5 × 7 to get 525 lines. And if you multiply by 60 and then divide by 2 you get the horizontal frequency of 15,750hz. When NTSC color was introduced the vertical frequency was reduced to 59.94Hz and the horizontal frequency to ~15,734Hz, but the math still works.

So at 50hz the values of 5 * 5 * 5 * 5 give us 625 line video and the values of 13 x 7 x 3 x 3 give 819 lines.

The number of lines in a TV signal can't be any old arbitrary number. But it does work for a large number of television lines, including hi-def. The Japanese analog standard for hi-def is 1125 lines (1080 active, sound familiar?) at 60hz and its division is 5 x 5 x 5 x 3 x 3.

David