Flicker-free
Flicker-free is a term given to
Televisions operating at these frequencies are often labelled as being 100 or 120 Hz without using the words flicker-free in the description.
Prevalence
The term is primarily used for CRTs, especially televisions in 50 Hz countries (PAL or SECAM) and computer monitors from the 1990s and early 2000s – the 50 Hz rate of PAL/SECAM video (compared with 60 Hz NTSC video) and the relatively large computer monitors close to the viewer's peripheral vision make flicker most noticeable on these devices.
Contrary to popular belief, modern LCD monitors are not flicker free, since most of them use pulse-width modulation (PWM) for brightness control. As the brightness setting is lowered, the flicker becomes more noticeable, since the period when the backlight is active in each PWM duty cycle shortens. The problem is much more pronounced on modern LED backlit monitors, because LED backlights reacts faster to changes in current.
Implementation
The goal is to display images sufficiently frequently to exceed the human flicker fusion threshold, and hence create the impression of a constant (non-flickering) source.
In computer displays this consists of changing the frame rate of the produced signal in the
On television, this is more involved, as the source material has a fixed frame rate (and is also traditionally interlaced video, in which one-half of the scan lines of each frame are broadcast at a time). Most simply, the frame rate can be doubled by simply displaying the same broadcast image twice in rapid succession, as is done with movie projectors (which display each frame of 24 fps film two or more times) – either displaying each field twice or alternating fields.
Alternatively, this can involve motion interpolation, where rather than displaying the original fields twice, creates interpolated images between the original frames. This may be combined with deinterlacing, converting the image to progressive scan (attempting to create a full picture from the two half images).
Higher refresh rates, while they reduce flicker, may cause other problems. Simply redisplaying the fields may cause
See also
Notes
- Pulse Width Modulation Effects of PWM used in LCD brightness control