Standard Definition

Before I even start talking about high definition, let’s cover the basics and look at the standard definition television systems that we’re all used to. Here in the UK we employ the PAL (Phase Alternating Line) television system, while North America, for example, uses NTSC (National Television System Committee). The two main differences between these two systems is the amount of lines of image data they provide and the speed at which frames are displayed. A PAL television signal will provide 576 lines, displayed at 25fps, while an NTSC signal offers only 480 lines of data, but at 30fps.

Now, the frame rates quoted for both PAL and NTSC don’t really tell the whole story, because in reality a PAL image is drawn 50 times a second, while an NTSC image is drawn 60 times a second. The reason for this is that each full frame takes two scans – the television will draw the even and odd numbered lines on separate passes to create a solid image. This is known as an interlaced image. Of course this happens so fast that it should be indistinguishable to the human eye, but that depends entirely on the content being displayed. Problems can occur when there is very fast movement being displayed as this can result in the two halves of a frame not lining up perfectly and thus looking somewhat blurred.

As if the inherent problems with interlaced moving images wasn’t enough, there is another big issue that has historically afflicted our television systems. While PAL runs at 25fps and NTSC runs at 30fps, actual movies are shot at 24fps. Obviously for PAL the difference is pretty minimal and the traditional solution was to simply speed the film up by around four per cent, thus creating a 25fps frame rate. The downside of this method is that the pitch of dialogue and music rises as a result.

The pitch increase when transferring film to PAL video can be avoided by using a different, although more complex method. The original film can be transferred on a one-to-one basis to PAL video, but every twelfth frame is repeated, thus creating 25fps, rather than the original 24fps, without the need to speed up the playback.

Transferring film to NTSC video is far more complex and involves a somewhat infamous technique called 3:2 pull down. This process is far more complex than either of the PAL solutions and I don’t have the time (or inclination) to explain it fully here. However, it is fair to say that the processing involved in the 3:2 pull down method results in degraded image quality compared to the original footage, with smooth camera pans often appearing jerky when watched as NTSC video. In its favour though, it doesn’t result in the increased audio pitch seen with the traditional PAL transfer method.

So, traditional standard definition television systems rely on an interlaced image creation, where each complete image is drawn in two passes – this coincided with the way a cathode ray tube television worked. The maximum amount of resolution on offer is 576 lines using PAL and only 480 lines using NTSC, and the frame rate can be either 25fps (50Hz) or 30fps (60Hz). But just how much does high definition TV improve on this?

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