Broadcast Television Systems - Line Count

Line Count

As interlaced systems require accurate positioning of scanning lines, it is important to make sure that the horizontal and vertical timebase are in a precise ratio. This is accomplished by passing the one through a series of electronic divider circuits to produce the other. Each division is by a prime number.

Therefore there has to be a straightforward mathematical relationship between the line and field frequencies, the latter being derived by dividing down from the former. Technology constraints of the 1930s meant that this division process could only be done using small integers, preferably no greater than 7, for good stability. The number of lines was odd because of 2:1 interlace. The 405 line system used a vertical frequency of 50 Hz (Standard AC mains supply frequency in Britain) and a horizontal one of 10,125 Hz (50 × 405 ÷ 2)

• 2 × 3 × 3 × 5 gives 90 lines (non interlaced)
• 2 × 2 × 2 × 2 × 2 × 3 gives 96 lines (non interlaced)
• 2 × 2 × 3 × 3 × 5 gives 180 lines (non interlaced) (used in Germany in mid-1930s before switch to 441-line system)
• 2 × 2 × 2 × 2 × 3 × 5 gives 240 lines (used for the experimental Baird transmissions in Britain )
• 3 × 3 × 3 × 3 × 3 gives 243 lines
• 7 × 7 × 7 gives 343 lines (early North American system also used in Poland and in Soviet Union before WW2)
• 3 × 5 × 5 × 5 gives 375 lines
• 3 × 3 × 3 × 3 × 5 gives 405 lines (used in Britain, Ireland and Hong Kong before 1985)
• 2 × 2 × 2 × 5 × 11 gives 440 lines (non interlaced)
• 3 × 3 × 7 × 7 gives 441 lines (used by RCA in North America before the 525-lines NTSC standard was adopted and widely used before WW2 in Continental Europe with different frame rates)
• 2 × 3 × 3 × 5 × 5 gives 450 lines (non interlaced)
• 5 × 7 × 13 gives 455 lines (used in France before WW2)
• 3 × 5 × 5 × 7 gives 525 lines System M (a compromise between the RCA and Philco systems. Still used today in most of the Americas and parts of Asia)
• 3 × 3 × 3 × 3 × 7 gives 567 lines (used for a while after WW2 in the Netherlands)
• 5 × 11 × 11 gives 605 lines (proposed by Philco in North America before the 525 standard was adopted)
• 5 × 5 × 5 × 5 gives 625 lines (576i) (developed independently by German and Soviet engineers during the mid-late 1940s. Still used today in most parts of the world)
• 2 × 3 × 5 × 5 × 5 gives 750 lines at 50 frames (used for 720p50 )
• 2 × 2 × 2 × 2 × 3 × 3 × 5 gives 750 lines at 60 frames (used for 720p60 )
• 3 × 3 × 7 × 13 gives 819 lines (737i) (used in France in the 1950s)
• 3 × 7 × 7 × 7 gives 1029 lines (proposed but never adopted around 1948 in France)
• 3 × 3 × 5 × 5 x 5 gives 1125 lines at 25 frames (used for 1080i25 but not 1080p25 )
• 3 × 3 × 5 × 5 x 5 gives 1125 lines at 30 frames (used for 1080i30 but not 1080p30 )

Notes

1. The division of the 240-line system is academic as the scan ratio was determined entirely by the construction of the mechanical scanning system used with the cameras used with this transmission system.
2. The division ratio though relevant to CRT-based systems is largely academic today because modern LCD and plasma displays are not constrained to having the scanning in precise ratios. The 1080p high definition system requires 1126-lines in a CRT display.