MPEG-2 is the primary video format for DVD-VIDEO, and was originally launched in 1994 by the 'Moving Pictures Experts Group'.
MPEG-2 is also the basis of digital TV such as Freeview and Sky Digital, as well as current High Definition broadcasts.
MPEG-2 Video is very similar to MPEG-1 Video used for CD-ROMs and VideoCDs, except that MPEG-2 also supports interlaced video and full broadcast resolution as used by TV Broadcasters.
All MPEG Video is based on a 'Lossy' system, whereby only 'key frames' are stored in their entirety, with the frames in between consisting only of the differences. Frames can be encoded in three types: intra-frames (I-frames), forward predicted frames (P-frames), and bi-directional predicted frames (B-frames).
I-Frames are the single frames which do not refer to any other frame in their creation. The compression applied to these frames is very similar to the technique used for compression JPEG images for the Internet.
P-Frames are based upon a previous P or I Frames, while a B-Frame is based upon either previous reference frame, or a future reference frame. P and B Frames look at the blocks that make up the targeted reference frame, and notes which ones have moved and logs a 'motion vector'. It then logs the parts of the picture which have changed completely.
A collection of I, P and B frames is known as a GOP or Group Of Pictures. The order by which these 3 frame types are stored is known as the 'GOP Structure'. Typically GOP Structures for DVD are either 12 or 15 frames as this fits quite closely with PAL and NTSC frame rates giving you roughly 2 GOPs per second.
One example of a GOP structure would be:
I_BB_P_BB_P_BB_P_BB_P_BB_
The longer the GOP structure, the more efficient the encoding can be, because you are storing less full 'reference' frames, and more 'predictive' frames. |
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The downside is that your picture is more susceptible to picture break-up or 'macroblocking' because the image can change completely from the original reference frame.
There are an additional two flavours of GOP Structure: Closed and Open. Closed GOPs are the most common as they can only reference frames within their own GOP, while Open GOPs can reference any frame.
MPEG-2 Video as used on DVD has to follow a number of other requirements to be suitable for authoring: Supported resolutions are 720 × 576, 704 × 576, 352 × 576, 352 × 288 pixels at 25 frames per second for PAL, and 720 × 480, 704 × 480, 352 × 480, 352 × 240 pixels at 39.97 frames per second for NTSC. Both 4:3 and 16:9 FHA (Full Height Anamorphic) are supported, and should be flagged accordingly - see 'Aspect Ratios' for more information.
For multi-angle video, all of the video must be encoded as Closed GOP and to the same GOP structure - see 'Multiple Video Angles' for more information.
Finally the total bit rate of the combined Video, Audio and Subtitles cannot exceed 9.8Mbits / sec although 24-7 DVD would not recommend going above about 9.2Mbits / sec as you may experience problems on cheaper DVD players.
Audio takes up a constant bit rate, which is dependant on the audio format and the bit rate specified. Subtitles also take up a constant bit rate, which although small for an individual stream, can equate to quite a large amount of space when you are dealing with 10+ streams.
Once you have subtracted this from your 9.8Mbits/sec you will know what the maximum bit rate that your video stream can be. Note that this is the MAXIMUM bit rate; your AVERAGE bit rate will be dependant on the length of the footage you wish to include - see 'Bit Budgeting for DVD' for more information. |
