DVD-Video discs use MPEG-2 video encoding for maximum quality and flexibility
DVD-Video is a flexible standard using MPEG video which provides excellent picture quality for NTSC or PAL/SECAM and long playing times for any movie. This page also describes the frame rates used for different source material and TV standards.
MPEG Video Encoding
Video can be encoded to either MPEG-2 or MPEG-1 for DVD-Video. These are summarized in the following chart. Note that NTSC refers to 525-line TV systems and PAL to 625 lines.
(horizontal x vertical)
|720/704 x 480
352 x 480/240
|352 x 480
351 x 240
(horizontal x vertical)
|720/704 x 576
352 x 576/288
|352 x 576
352 x 288
|VBR or CBR||VBR or CBR||CBR|
|PAL/SECAM frame rate||25 fps|
|NTSC frame rate (on disc)||24 or 29.97 fps|
VBR, 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 CBR, constant bit rate, encoding, the video data rate must be high enough to encode all the video well. For short videos where disc capacity is not an issue CBR, at as high a bit rate as possible, is the better choice.
MPEG-2 encoding was designed to encode full CCIR 601 digital signal at the full resolution of 720 x 480 (NTSC) or 720 x 576 (PAL/SECAM). MPEG-2 provides for both VBR and interlaced display as used for broadcast television signals and allows a compression ratio of up to 40:1.
MPEG-1, also used for Video CD, gives even longer playing time but at a lower quality. Since MPEG-1 does not allow for interlaced display, the image resolution available is reduced to 352 x 240 (NTSC) or 352 x 288 (PAL/SECAM). MPEG-1 is also restricted to constant bit rate (CBR).
For most purposes, MPEG-2 will be used for maximum quality. MPEG-1 is available where playing time is more important than quality.
PAL and NTSC Frame Rates
DVD-Video can comprise either PAL/SECAM (625-line) or NTSC (525-line) video material at frame rates of 24, 25 or 30 fps.
PAL/SECAM video uses a frame rate of 25 fps (frames per second). Source video material will be at this frame rate. Film material will have a frame rate of 24 fps and is usually played back at 25fps (to avoid frame rate conversion) and the audio is pitch corrected before the encoding begins.
NTSC video uses a frame rate of 30 (actually 29.97) fps which is identical to NTSC video material. Film material is usually converted from 24 to 30 fps by a '3/2 pull down' whereby frames are repeated to convert the 24fps film to 30fps video. However this is not necessary for DVD since the player can carry out the frame rate conversion. Therefore the video can be stored on disc at 24fps and displayed by the player at 30 fps. The encoder embeds MPEG-2 repeat_first_field flags into the video stream to make the decoder perform 3/2 pull down.
The result is that both PAL and NTSC versions of the same movie will comprise the same number of frames but as PAL frames are larger than NTSC frames they are likely to require more data rate for the same quality.
Some players may convert from PAL to NTSC or NTSC to PAL, but most NTSC players only play NTSC. PAL/SECAM players will play NTSC but require a NTSC TV/video monitor or will convert the NTSC to PAL. Therefore NTSC titles (with no region coding set) will play on PAL (ie European) and NTSC players, while PAL titles will play only on PAL players.
A single sided (DVD-5) DVD-Video disc will hold nominally 133 minutes of high quality MPEG-2 encoded video, together with three surround sound audio channels and four subtitle channels. (Without video compression one DVD-5 disc would hold only about 3 minutes of video.) For a dual layer disc (DVD-9) this increases to 240 minutes. A double sided disc (DVD-10) will hold slightly more at 266 minutes, but the disc needs to be turned over to play the other side.
Unlike audio CDs the playing time is not fixed but can vary. Longer playing times mean lower average bit rates and lower video quality, while shorter playing times allow higher bit rates and higher quality. The use of MPEG-1 video allows more playing time at the expense of quality. More or fewer audio channels or different audio bit rates also affect the video bit rate and/or playing time.
The maximum bit rate is 9.8 Mb/s for video, audio and sub pictures (the overall maximum, including control information, being 10.08 Mb/s). For a playing time of 133 minutes, the average bit rate is 4.7 Mb/s. The average video bit rate available depends on the number of audio streams and the encoding used, but should be close to 4 Mb/s for high quality results.
Quality and resolution are better than Laserdisc and Video CD and playing time is much longer than SVCD as shown below.
|Encoding format||Analogue composite||MPEG-1 (CBR) component||MPEG-2 (VBR) component||MPEG-2 (VBR) component|
|Image size (max)||352 x 240/288||480 x 480/576||720 x 480/576|
|Video bit rate||-||1.15 Mb/s||2.60 Mb/s (ave)||3.50 Mb/s (ave)|
|Audio channels||2 to 5.1||2||4||5.1|
|Languages||1||1||2 stereo or
|up to 8|
|Playing time||60 mins||74 mins max||37 mins at max bit rate||133 mins nominal per layer|
Laserdisc stores analogue video which is composite encoded, so will suffer from noise and other defects due to imperfections on the disc, but otherwise can offer high quality video that is close to the best that can be achieved from broadcast TV. Playing time is 60 minutes maximum per side.
Video CD is a CD-ROM format that uses MPEG-1 so is limited in playing time and video quality. Movies have been released on Video CD, but need 2 discs for a two hour movie.
SVCD (Super Video CD) makes use of VBR MPEG-2 to increase the video quality, but playing time is reduced so that a movie will require three discs not one.