DVD Encoding

DVD-Video can be encoded to either MPEG-1 or MPEG-2, as summarised in the table below:

PAL/ SECAM resolutions 720 x 576

704 x 576

352 x 576

352 x 288

352 x 576

352 x 288

NTSC resolutions 720 x 480

704 x 480

352 x 480

352 x 240

352 x 480

351 x 240

Variable Bit Rate (VBR), Constant Bit Rate (CBR) VBR or CBR CBR
PAL/SECAM frame rate 25fps
NTSC frame rate 24 or 30fps

Variable Bit Rate encoding allows higher image quality at a lower average bit rate by using more data to encode those parts of a video sequence which are more complex and do not compress well. Using Constant Bit Rate encoding, the video data rate must be high enough to encode all the video well.

Early DVD-ROM drives used one of two strategies for delivering MPEG-2 video. Some used a technique called analogue overlay, also referred to as video overlay or simply overlay. Others employed the VGA-inlay approach, sometimes referred to as VideoInlay. Both methods display video in a window or at full screen, but they take different approaches. VideoInlay relies on the PC’s graphics adapter to scale the video and output it to a monitor. Overlay boards, by contrast, provide their own hardware scaling and output the video themselves, overlaying it with graphics output passed through from the VGA card. With these boards, an included cable runs from the VGA connect of the display adapter to an input on the bracket of the decoder board.

The major drawback of the VGA-inlay approach is the load it places on a system. While pushing 30 frames of video per second might not saturate the PCI bus, it does keep bus utilisation high. When playing scenes encoded at a high bit rate, VGA-inlay boards can overwhelm older, slower display adapters with too much data, requiring a reduction in horizontal resolution to produce an acceptable picture.

Requiring a bit more work to install and configure than VGA-inlay boards, video-overlay boards demand less of a system and tolerate a wider variety of hardware. While video output may be less sharp than that displayed by VGA-inlay boards, the video-overlay approach has the advantage of being capable of decent performance with just about any video card.

The original audio encoding format specification for European DVD discs was MPEG-2 surround sound, creating a degree of confusion since Dolby Digital AC-3 had become established as a mainstream format with the rest of the DVD world. The situation was clarified in January 1998 when the DVD Forum agreed on a dual standard which allowed both encoding formats.

The launch of DVD-Video has been severely impacted by technical issues, and not least those concerning encryption. DVD-Video players finally came to market in Japan in November 1996 and in the USA in March 1997. In Europe, the major launch was delayed until the autumn of 1998.