8.7 Digital versatile disc (DVD)

Digital versatile/video disc (DVD) is a new storage medium for MPEG-2 coded high quality video. DVD discs with 9 Gbytes storage capacity (in two tracks of 4.5 Gbytes) are introduced to replace the 648 (or 700) Mbytes CD-ROMs. The main reason for the introduction of this new product is that viewers' expectations of video quality have grown over time. CD-ROMs could only store MPEG-1 compressed video of SIF format at about a target rate of 1.2 Mbyte/s. When SIF pictures are enlarged to the standard size (e.g. 720 pixels by 576 lines), to be displayed on TV sets, for certain scenes the enlarged pictures look blocky. This is usually not sufficient for home movies or HDTV programmes.

In DVD, video of CCIR-601 standard size is MPEG-2 compressed. Considering the double-track DVD discs of total capacity of 9 Gbytes, the nominal movie of 90 minutes long can be coded at an average bit rate of 6–12 Mbit/s, depending whether one or both tracks are used.

To increase the video quality and at the same time to optimise the storage capacity, the MPEG-2 encoder is set to encode the video at a variable bit rate (VBR). This is done by fixing the quantiser step size at a constant value, producing video of almost constant quality over the entire programme, irrespective of scene complexity. Due to constant quantiser step size, during high picture activity, the instantaneous bit rate of the encoder can be very high (e.g. 30 Mbit/s). However, these events only occur for a short period, and there are occasions when the scenes might be very quiet, producing lower bit rates (e.g. 2 Mbit/s). Depending on the proportions of the scene activities in the video, its peak-to-mean bit rate ratio, even smoothed over a GOP can be of the order of 3–5 (peak/mean ratio smoothed over one frame can easily rise above 10). Thus had the video been coded at a constant bit rate (CBR), then for the same picture quality as VBR, the target bit rate would have to be set to the peak bit rate. Hence for quiet scenes, the storage capacity of the disc can be wasted. In fact the advantage of VBR over CBR is the saving in storage capacity by the ratio of the peak bit rate to the mean bit rate, which can be considerable.

The main problem with VBR is that the chunk of compressed data read from the disc decodes a variable number of pictures per given time unit (e.g. seconds). For a uniform and smooth display (e.g. 25 pictures per second), the read data from the disc has to be smoothed. This is done by writing it into a random access memory (RAM) and reading it at the desired rate of the decoder. Considering that today electronic notebooks are equipped with 256-512 Mbytes RAM, they are not too expensive to be included in the DVD decoders. These are sufficient to store about 5–10 minutes of the programme, well over what is needed to produce pictures without interruptions.