Moving Image Formats - Presentation

Presentation

There are two kinds of displays on the market today: those which "flash" a picture for a short part of the refresh period (CRT, cinema projector), and those which display an essentially static image between the moments of refreshing it (LCD, DLP).

The "flashing" displays must be driven at least 48 Hz, although today, a rate significantly below 85 Hz is not considered ergonomic.

For these displays, the 24–30 Hz material is usually displayed at 2x, 3x, or 4x the capture rate. 50 and ~60 Hz material is usually displayed at its native rate, where it delivers a very accurate motion without any smearing. It can also be displayed at twice the capture rate, although moving objects will look smeared or trailed, unless intermediate frames are calculated using the motion analysis and are not just simply duplicated.

The "continuous" display can be driven at any integer multiple of the capture rate - it won't matter for the viewer, nor can it be visually discriminated. However, in general, "continuous" displays show noticeable smear over quickly-moving objects in 50 and ~60 Hz video material (even if their response time is instant). However there are two emerging techniques to combat smearing of the video-based material in LCD display: it can be effectively converted into the "flashing" display by appropriately modulating its backlight; and/or it can be driven at double the capture rate while calculating intermediate frames using the motion analysis (see LCD television).

Obviously, when presentation rate is not an integer multiple of the capture rate, the "fluidity" of the motion on the screen will suffer to a varying degree (terribly for video-, unpleasantly for film-based material). This is usually the case with computer-based DVD players and PAL PC TVs, where the user does not switch the refresh rate either out of ignorance, or due to technical constraints; which sometimes are, in fact, artificial, made by manufacturers counting on that user's ignorance. For instance some laptop LCD panels cannot be (easily) switched to anything but a 60 Hz refresh rate, and some LCD displays with DVI input refuse to accept digital input signal if its vertical refresh rate does not fit between 58 and 62 Hz.

Most software DVD players do not assist with switching display modes, and even if it is switched manually, they hardly synchronize frame updating with the display's vertical retrace periods. (There is only soft synchronization using hardware double buffering, which is not enough to match hardware players in the stability of playback.)