REPORT: Perfect Picture Enjoyment: It All Depends on the Frame Rate!

By Denis Freund
17.10.2007

Table of Contents:

Introduction
TFTs are hold-type displays
Available display formats and their characteristics
Fair material feeding – therefore everything so simple?
Get rid of the jerking
The judder test
Everything done right and nonetheless still jerking?
Even more fluent, more sharp – 100Hz and intermediate pictures
Conclusion

Introduction

The heading may at first be confusion. This is because other than with the good old tube screen (CRT), a Liquid Crystal Display (LCD) always offers a flicker free picture due to its construction. In the computer area the 60Hz has prevailed as the control. That leads to the assumption, that this topic also plays no more role for LCD TVs.

It is indeed a necessity to show the film material with the correct video refresh rate, something not focused upon by general attention until relatively recently. For a long time only insider groups and technology freaks were occupied with it. Even the manufacturers have in the meantime more intensively and more clearly offensively faced up to this problem. Just because of that it is important to shed light on this underestimated topic.

TFTs are hold-type displays

To begin with, a short overview of a crucial characteristic of TFT monitors: This concerns the so-called "Hold-Type Displays", that means that the condition of a pixel remains unchanged for the duration of a picture period, or the display of a single picture. Exactly this is what leads to the flicker free depiction, however, it also brings with it the disadvantage of the well-known movement blurriness of TFTs.

It is based on the perception characteristics of the human eye. Also faster pixel switching times (response time) cannot prevent this. The image repetition frequency determines the frequency with which the picture is again developed. Visible advantages can naturally exist here only in the context of the response time.

The in the computer branch normal 60Hz are a useful value, a good compromise as well for bandwidth needs, technical characteristics and operability. The last can be quite limited by a too small video refresh rate, for example, that the mouse cursor can not be so exactly positioned.

This limitation however, only happens by very high resolution displays and certain control variations, because the available broadcasting bandwidth for the picture is also limited. For the home user this problem is irrelevant, since even 30 inch WOXGA displays with 2,560 x 1,600 pixels can be run on commercially available graphics cards at 60 Hz.

The increasing spreading of desktop TFTs, widescreen formats and the enormous growth have brought with it that the devices used for some years now are increasingly being used for the playing of videos.

This circumstance did not directly contribute however, to increase the consciousness for the importance of a fair material feeding image repetition frequency and rather than that provided for a certain "hardening" of the viewer for jerks and other picture impairments.

Available display formats and their characteristics

This directly leads us to a central question: What is a „fair material feeding”? In order to answer that we first have to determine which of the different materials is present. At the same time it is very important, to make a difference between the picture signal and the transferred content.

For further simplification we can still differentiate in the following between "broadcast material" (television/ radio broadcasting), DVD and the new HD media, also Blu-ray and HD-DVD. The used trade terms and basics are explained in detail in our deinterlacing article.

PAL-Broadcast

In the European area or the PAL drawing is this thing still very clear. Here we have to deal almost exclusively with a 50Hz interlaced signal. These can either actually contain interlaced material (half-images) which consist accordingly of 50 fields/s or however, it is on the basis of progressive pictorial material (frames).

Since LCD TVs are based on full image depiction, at any rate a deinterlacing has to take place. During this it should not matter on this place, whether the procedure happens in the TV itself, in the DVD player or other external devices. Important is the end result: That is made out of 50fps (frames per second).

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