Refresh Rate - Televisions

Televisions

When the first televisions were developed in the 1920s and 1930s, the limitations of vacuum-tube electronics made it difficult to run them at anything other than a multiple of the AC line frequency used to power the set. Thus producers had little choice but to run sets at 60 Hz in America, and 50 Hz in Europe. Using the AC power frequency for the vertical sync rate also ensured that any residual power supply ripple (which would be visible as horizontal bands) was stationary. If a different frequency is used the "hum bars" move and thus are more distracting. These rates formed the basis for the NTSC (60 Hz) and PAL & SECAM (50 Hz) sets used today. This accident of chance gave European sets higher resolution, in exchange for lower frame-rates. Compare NTSC (704x480 NTSC at 30i) and PAL/SECAM (704x576 at 25i). However, the lower refresh rate of PAL/SECAM introduces more flicker, so sets that use digital technology to double the refresh rate to 100 Hz are now very popular.

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 frame/s. 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 frame/s and 25 frame/s 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.

Unlike 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 exposure - the 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 frame/s (cinema) and 30 frame/s (NTSC TV), and allows for less distortion when movies are viewed due to the elimination of telecine (3:2 pulldown). For PAL at 25 frame/s, 100 or 200 Hz is used as a fractional compromise of the least common multiple of 600 (24 x 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.