The development of
televisions in the 1930s was determined by a number of technical limitations. The
AC power
line frequency was used for the vertical refresh rate for two reasons. The first reason was that the television's vacuum tube was susceptible to interference from the unit's power supply, including residual ripple. This could cause drifting horizontal bars (hum bars). Using the same frequency reduced this, and made interference static on the screen and therefore less obtrusive. The second reason was that television studios would use AC lamps, filming at a different frequency would cause
strobing. Thus producers had little choice but to run sets at 60 Hz in America, and 50 Hz in Europe. These rates formed the basis for the sets used today: 60 Hz System M (almost always used with
NTSC color coding) and 50 Hz System B/G (almost always used with
PAL or
SECAM color coding). This accident of chance gave European sets higher resolution, in exchange for lower frame rates. Compare System M (704 × 480 at 30i) and System B/G (704 × 576 at 25i). However, the lower refresh rate of 50 Hz introduces more flicker, so sets that use digital technology to double the refresh rate to 100 Hz are now very popular. (see
Broadcast television systems) Another difference between 50 Hz and 60 Hz standards is the way motion pictures (film sources as opposed to video camera sources) are transferred or presented. 35 mm film is typically shot at 24
frames per second (fps). For PAL 50 Hz this allows film sources to be easily transferred by accelerating the film by 4%. The resulting picture is therefore smooth, however, there is a small shift in the pitch of the audio. NTSC sets display both 24 fps and 25 fps material without any speed shifting by using a technique called
3:2 pulldown, but at the expense of introducing unsmooth playback in the form of
telecine judder. Similar to some computer monitors and some DVDs, analog television systems use
interlace, which decreases the apparent flicker by painting first the odd lines and then the even lines (these are known as fields). This doubles the refresh rate, compared to a progressive scan image at the same frame rate. This works perfectly for video cameras, where each field results from a separate exposurethe effective frame rate doubles, there are now 50 rather than 25 exposures per second. The dynamics of a CRT are ideally suited to this approach, fast scenes will benefit from the 50 Hz refresh, the earlier field will have largely decayed away when the new field is written, and static images will benefit from improved resolution as both fields will be integrated by the eye. Modern CRT-based televisions may be made
flicker-free in the form of 100 Hz technology. Many high-end LCD televisions now have a 120 or 240 Hz (current and former NTSC countries) or 100 or 200 Hz (PAL/SECAM countries) refresh rate. The rate of 120 was chosen as the
least common multiple of 24 fps (cinema) and 30 fps (NTSC TV), and allows for less distortion when
movies are viewed due to the elimination of
telecine (3:2 pulldown). For PAL at 25 fps, 100 or 200 Hz is used as a fractional compromise of the least common multiple of 600 (24 × 25). These higher refresh rates are most effective from a
24p-source video output (e.g.
Blu-ray Disc), and/or scenes of fast motion. ==Displaying movie content on a TV==