Monitor Interlacing

Back in the 1930s, TV broadcast engineers had to design a transmission and reception system that satisfied a number of criteria:

  • that functioned in harmony with the electricity supply system
  • was economic with broadcast radio wave bandwidth
  • could produce an acceptable image on the CRT displays of the time without undue flicker.

The mains electricity supply in Europe and the USA was 50 Hz and 60Hz respectively and an acceptable image frame rate for portraying motion in cinemas had already been established at 24fps. At the time it was not practical to design a TV system that operated at either of the main electricity rates at the receiver end and, in any case, the large amount of broadcast bandwidth required would have been uneconomical. Rates of 25fps and 30fps would reduce the broadcast space needed to within acceptable bounds but updating images at those rates on a phosphor type CRT display would produce an unacceptable level of flickering.

The solution the engineers came up with was to split each TV frame into two parts, or fields, each of which would contain half the scan lines from each frame. The first field – referred to as either the top or odd field – would contain all the odd numbered scan lines, while the bottom or even field would contain all the even numbered scan lines. The electron gun in the TV’s CRT would scan through all the odd rows from top to bottom, then start again with the even rows, each pass taking 1/50th or 1/60th of a second in Europe or the USA respectively.

This interlaced scanning system proved to be an effective compromise. In Europe it amounted to an effective update frequency of 50Hz, reducing the perception of flicker to within acceptable bounds whilst at the same time using no more broadcast bandwidth than a 25fps (50 fields per second) system. The reason it works so well is due to a combination of the psycho-visual characteristics of the Human Visual System (HVS) and the properties of the phosphors used in a CRT display. Flicker perceptibility depends on many factors including image size, brightness, colour, viewing angle and background illumination and, in general, the HVS is far less sensitive to flickering detail than to large area flicker. The effect of this, in combination with the fact that phosphors continue to glow for a period of time after they have been excited by an electron beam, is what creates the illusion of the two fields of each TV frame merging together to create the appearance of complete frames.

There was a time when whether or not a PC’s CRT monitor was interlaced was as important an aspect of its specification as its refresh rate. However, for a number of years now these displays have been designed for high resolution computer graphics and text and with shorter persistence phosphors, making operation in interlaced mode completely impractical. Moreover, by the new millennium display many alternative display technologies had emerged – LCD, PDP, LEP, DLP etc. – that were wholly incompatible with the concept of interlaced video signals.