Interlaced or progressive scan  ... ?

Horizontal scan rate is the number of individual picture lines per second.

Vertical scan (refresh) rate is the number of pictures per second.

In the UK system the picture is made up of  625 horizontal lines. If we display every line for every picture we would end up with  50 X 625 picture lines per second or 31,250 lines and our horizontal scan rate would be 31,250Hz. In the early days of TV this would have been a lot of information to transmit requiring a large bandwidth so a scheme was introduced to deal with this hence the interlaced system When colour was introduced, the system adopted in the U.K. for dealing with this was PAL. PAL stands for Phase Alternate Line. (The phase of one of the colour difference signals was inverted on alternate T.V.lines)

In an interlaced picture the horizontal scan rate is 50 *  625 / 2 i.e. 15,625 Hz. Again this is the vertical scan rate times the number of lines in the picture but now it is divided by 2 because in an interlaced pictures only half the information is sent for each alternate field. Odd lines are sent on one pass followed by even numbered lines on the next pass and so on.

In an interlaced picture there is 50 fields per second but only 25 full frames (NTSC 60 fields and 30 frames)

Each field consists of the full number of picture lines divided by 2 that is 312.5 for PAL and 262.5 for NTSC there is a half line scan at the bottom of the picture.

Interlaced pictures were introduced as a compromise i.e. so as  not to have a too large bandwidth but only displaying half of the frame content on each scan of the TV tube. Use was made of the way the human eye and brain respond to give a picture that appeared detailed with a low flicker rate but at the same time could be squeezed into the available transmission bandwidth.

There are problems with interlaced pictures and as technology improves,  non interlaced or progressive scan systems are now becoming more popular. Jagged edges to sloping lines and less than perfect brightness and detail are among the (albeit very slight) defects in the interlaced system.

Current SyncBlaster products are designed to work with a horizontal scan rate of 15,625Hz.

The Keene Electronics RGB2C converter will handle 31.250Hz signals and consequently can be used to convert progressive scan component video signals into RGB H V. This signal can then be displayed on almost any monitor capable of displaying a VGA signal. ie most computer monitors, LCD monitors, projectors and plasma screens.