MPEG-2 Reference Information

The following sections contain useful information for creating MPEG-2 output media files.

About Choosing the Bit Rates

When determining average and maximum bit rates for standard DVD playback, remember to consider the bit rate of your audio track as well as the MPEG-2 bit rate.

You must keep the total of both average and maximum audio and video bit rates under 10.08 Mbps, the maximum guaranteed transfer rate from standard DVD players. Because DVD-compatible audio formats are constant bit rate (CBR), there is no maximum audio bit rate to worry about.

For example, if you are using AIFF audio at 1.5 Mbps, you should keep both the average and maximum video bit rates under 8.5 Mbps. Typically, your average bit rate will be lower than this (for example, 3.5 Mbps for 2 hours of footage on your DVD). However, your maximum bit rate must also stay below this number. A maximum bit rate of 8.0 Mbps is recommended to provide an extra margin for error (for example, to accommodate subtitle streams). If you are using one of the DVD-compatible compressed audio formats such as Dolby Digital or MPEG-1/Layer-2, your audio bit rate may be as low as 0.2 to 0.4 Mbps, in which case you can set your maximum bit rate about 1 Mbps higher.

Also, as a general rule, set your maximum bit rate at least 1 Mbps higher than your average bit rate, to allow for bit-rate variability in achieving the goal of constant quality.

MPEG-2 Video Frame Sizes and Formats

Since MPEG-2 uses fixed video frame sizes, Compressor enters the output frame size in the fields in the Geometry pane based on your video format selection.

The video format you choose in the Video Format pop-up menu determines the options for the associated characteristics such as frame size and rate, aspect ratio, and field dominance. For more information, see Video Format Tab.

Video format
Frame size (pixels)
Frame rate (fps)
Aspect ratio
Scanning method
NTSC
720 x 480
23.98 (progressive only), 29.97
4:3 or 16:9
Interlaced, progressive
PAL
720 x 576
25
4:3 or 16:9
Interlaced, progressive
720p
1280 x 720
23.98, 25, 29.97, 50, 59.94
16:9
Progressive
HD 1440 x 1080
1440 x 1080
23.98 (progressive only), 25, 29.97
16:9
Interlaced, progressive
HD 1920 x 1080
1920 x 1080
23.98 (progressive only), 25, 29.97
16:9
Interlaced, progressive
640 x 480 (1.33)
640 x 480
23.98, 25, 29.97
4:3 or 16:9
Interlaced, progressive
640 x 360 (1.78)
640 x 360
23.98, 25, 29.97
4:3 or 16:9
Interlaced, progressive
640 x 352 (1.82)
640 x 352
23.98, 25, 29.97
4:3 or 16:9
Interlaced, progressive
640 x 384 (1.67)
640 x 384
23.98, 25, 29.97
4:3 or 16:9
Interlaced, progressive
640 x 320 (2.00)
640 x 320
23.98, 25, 29.97
4:3 or 16:9
Interlaced, progressive

Understanding GOPs and Frame Types

A major feature of MPEG-2 encoding is its ability to remove redundancy, not only within a frame, but also among a group of frames. MPEG-2 uses three frame types (I, P, and B) to represent the video. A group of pictures (GOP) setting defines the pattern of the three frame types used. These three picture types are defined in the following ways.

  • Intra (I): Also known as the key frame. Every GOP contains one I-frame. The I-frame is the only MPEG-2 frame type which can be fully decompressed without any reference to frames that precede or follow it. It is also the most data-heavy, requiring the most disk space. If you want to place an I-frame at a scene change or some other specific frame location, you need to manually set it using the Preview window. This is known as a forced I-frame. See Working with Markers and Poster Frames for more information.
  • Predicted (P): Encoded from a “predicted” picture based on the closest preceding I- or P-frame. P-frames typically require much less disk space than do I-frames because they reference a preceding I- or P-frame in the GOP.

    Note: Both I-frames and P-frames are also known as reference frames, because a B-frame may refer to either one or both frame types.

  • Bi-directional (B): Encoded from an interpolation of succeeding and preceding reference frames, either I-frame or P-frame. B-frames are the most storage-efficient MPEG-2 frame type, requiring the least amount of disk space.

The use of B- and P-frames is what allows MPEG-2 to remove temporal redundancy, contributing to its ability to compress video efficiently.

Things to Consider When Choosing a GOP Setting

You need to consider the following factors when choosing a GOP setting.

GOP Structure

This setting specifies whether there will be two, one, or no B-frames between the reference frames within a GOP. GOP structure, along with GOP size, determines the number of I-, P-, and B-frames that will be used during transcoding.

The GOP structure you choose depends on how far apart P-frames should be spaced. Since a P-frame is predicted from the previous reference frame (either an I-frame or a P-frame), if there are one or two B-frames in between, the prediction must cover the distance objects can move over the duration of two to three frames.

In principle, the less average motion there is from one frame to the next, the farther apart P-frames can be spaced, and the greater the compression can be. For most video material, the IBBP structure is a good choice. Material with unusually fast motion throughout the entire sequence may benefit from an IBP or IP structure, but in such cases a relatively high bit rate (6 to 8 Mbps for SD video) may be required for good quality.

GOP Size

This setting specifies the number of frames within a GOP. Because exactly one I-frame exists per GOP, longer GOP sizes generally provide greater compression, because B- and P-frames are smaller than I-frames.

For most media, spacing I-frames about 1/2 second apart gives good results. This equates to a GOP size of 15 frames for NTSC and 12 frames for PAL. The DVD-Video specification prohibits GOP lengths from being much longer than this. Generally, only material with scene changes occurring less than 1/2 second apart frequently throughout the video will benefit from shorter GOP sizes.

Open and Closed GOPs

Open GOPs are most efficient because they allow an extra B-frame in the GOP pattern. Open GOPs start with a B-frame that is able to look at the last P-frame from the preceding GOP as well as the first I-frame of its own GOP.

Figure. Diagram comparing Open GOP and Closed GOP.

By definition, closed GOPs cannot contain any frame that refers to a frame in the previous or next GOP. In contrast, open GOPs begin with one or more B-frames that reference the last P-frame of the previous GOP. Closed GOPs created by Compressor always begin with an I-frame.

Open GOPs generally provide slightly better compression than do closed GOPs of the same structure and size. The illustration above shows that a closed GOP contains one more P-frame than does an open GOP of the same length. Since P-frames generally require more bits than do B-frames, the open GOP achieves slightly better compression.

There are limitations to using open GOPs for DVD-Video discs that are created using a DVD authoring application. One limitation is that only closed GOPs are permitted within MPEG-2 streams that will be used for mixed-angle or multi-angle DVDs.

The other limitation is that DVD chapter markers can only be set at the beginning of a closed GOP. The best time to define chapter markers is before doing MPEG-2 transcoding. For example, if you specify your chapter markers in Final Cut Pro, you can set Compressor to do MPEG-2 transcoding with open GOPs. Compressor will then force a closed GOP to begin only at the specified chapter markers and will make all other GOPs open. You can also accomplish this by specifying “forced I-frames” in the Compressor Preview window and giving them a chapter name to be used by a DVD authoring application.

However, if you want to specify chapter markers at any GOP boundary after your video has been transcoded in the MPEG-2 format, you should use only closed GOPs. This freedom is limited, as it allows you to set chapter markers only at GOP boundaries, rather than at any video frame.

About 24p (23.98p)

For DVD authoring and encoding, 24p refers to a video sequence that contains 24 progressive (non-interlaced) frames per second, with NTSC-related standard definition frame dimensions (720 x 480 for MPEG-2). Film-based movies have a native frame rate of 24 fps, and because the MPEG-2 format is able to represent 24 fps video internally, many commercial movie DVDs are encoded in this way. But any time you use NTSC video in your project, the frame rate of film-transferred material will be slowed down from 24 fps to 23.976 fps (rounded to 23.98) and a 2:3:2:3 pull-down is added. So, the more accurate term is actually 23.98p.

Compressor can also do this for 24p source video files. For such material, the 23.98 frame rate option (in the Video Format tab) compresses each source frame one-for-one, without compressing repeated frames or fields in order to achieve a 29.97 fps display rate. This results in higher quality at a lower compressed bit rate than would be possible if the 24p material were converted to 29.97 fps prior to transcoding. Compressor also sets internal MPEG-2 frame flags correctly, so DVD players will properly apply the 3:2 pull-down process for display on 29.97 fps interlaced NTSC TV sets.

Note: If your source video has a frame rate of 24.00 fps rather than 23.98 fps, Compressor skips one out of every 1000 source frames. If the 24p source video is 23.98 fps, Compressor transcodes all source frames, without skipping (or repeating) any of them.