MPEG-2 is a standard for "the generic coding of moving pictures and associated audio information". It describes a combination of lossy video compression and lossy audio compression (audio data compression) methods which permit storage and transmission of movies using currently available storage media and transmission bandwidth.

Main characteristics

MPEG-2 is widely used as the format of digital television signals that are broadcast by terrestrial (over-the-air), cable, and direct broadcast satellite TV systems. It also specifies the format of movies and other programs that are distributed on DVD and similar disks. As such, TV stations, TV receivers, DVD players, and other equipment are often designed to this standard. MPEG-2 was the second of several standards developed by the Moving Pictures Expert Group (MPEG) and is an international standard (ISO/IEC 13818). Parts 1 and 2 of MPEG-2 were developed in a joint collaborative team with ITU-T, and they have a respective catalog number in the ITU-T Recommendation Series.

While MPEG-2 is the core of most digital television and DVD formats, it does not completely specify them. Regional institutions can adapt it to their needs by restricting and augmenting aspects of the standard. See Profiles and Levels.

MPEG-2 includes a Systems section, part 1, that defines two distinct, but related, container formats. One is the Transport Stream, designed to carry digital video and audio over possibly lossy media, such as broadcasting, examples of which include ATSC and DVB. MPEG-2 Systems also defines Program Stream, a container format designed for reasonably reliable media such as optical disks, DVDs and SVCDs. MPEG-2/System is formally known as ISO/IEC 13818-1 and as ITU-T Rec. H.222.0.

The Video section, part 2 of MPEG-2, is similar to the previous MPEG-1 standard, but also provides support for interlaced video, the format used by analog broadcast TV systems. MPEG-2 video is not optimized for low bit-rates, especially less than 1 Mbit/s at standard definition resolutions. However, it outperforms MPEG-1 at 3 Mbit/s and above. All standards-compliant MPEG-2 Video decoders are fully capable of playing back MPEG-1 Video streams. MPEG-2/Video is formally known as ISO/IEC 13818-2 and as ITU-T Rec. H.262.

With some enhancements, MPEG-2 Video and Systems are also used in some HDTV transmission systems.

The MPEG-2 Audio section, defined in part 3 of the standard, enhances MPEG-1's audio by allowing the coding of audio programs with more than two channels. This method is backwards-compatible, allowing MPEG-1 audio decoders to decode the two main stereo components of the presentation.

Part 7 of the MPEG-2 standard specifies a rather different, non-backwards-compatible audio format. Part 7 is referred to as MPEG-2 AAC. While AAC is more efficient than the previous MPEG audio standards, it is much more complex to implement and somewhat more powerful hardware is needed for encoding and decoding. Advanced Audio is also defined in Part 3 of the MPEG-4 standard.

History

Video coding (simplified)

Main article: Video compression

An HDTV camera generates a raw video stream of more than one billion bits per second. This stream must be compressed if digital TV is to fit in the bandwidth of available TV channels and if movies are to fit on DVDs. Fortunately, video compression is practical because the data in pictures is often redundant in space and time. For example, the sky can be blue across the top of a picture and that blue sky can persist for frame after frame. Also, because of the way the eye works, it is possible to delete some data from video pictures with almost no noticeable degradation in image quality.

TV cameras used in broadcasting usually generate 50 pictures a second (in Europe and elsewhere) or 59.94 pictures a second (in North America and elsewhere). Digital television requires that these pictures be digitized so that they can be processed by computer hardware. Each picture element (a pixel) is then represented by one luma number and two chrominance numbers. These describe the brightness and the color of the pixel (see YCbCr). Thus, each digitized picture is initially represented by three rectangular arrays of numbers.

A common (and old) trick to reduce the amount of data is to separate the picture into two fields: the "top field," which is the odd numbered rows, and the "bottom field," which is the even numbered rows. The two fields are displayed alternately. This is called interlaced video. Two successive fields are called a frame. The typical frame rate is then 25 or 29.97 frames per second. If the video is not interlaced, then it is called progressive video and each picture is a frame. MPEG-2 supports both options.

Another trick to reduce the data rate is to thin out the two chrominance matrices. In effect, the remaining chrominance values represent the nearby values that are deleted. Thinning works because the eye is more responsive to brightness than to color. The 4:2:2 chrominance format indicates that half the chrominance values have been deleted. The 4:2:0 chrominance format indicates that three quarters of the chrominance values have been deleted. If no chrominance values have been deleted, the chrominance format is 4:4:4. MPEG-2 allows all three options.

MPEG-2 specifies that the raw frames be compressed into three kinds of frames: intra-coded frames (I-frames), predictive-coded frames (P-frames), and bidirectionally-predictive-coded frames (B-frames).

An I-frame is a compressed version of a single uncompressed (raw) frame. It takes advantage of spatial redundancy and of the inability of the eye to detect certain changes in the image. Unlike P-frames and B-frames, I-frames do not depend on data in the preceding or the following frames. Briefly, the raw frame is divided into 8 pixel by 8 pixel blocks. The data in each block is transformed by a discrete cosine transform. The result is an 8 by 8 matrix of coefficients. The transform converts spatial variations into frequency variations, but it does not change the information in the block; the original block can be recreated exactly by applying the inverse cosine transform. The advantage of doing this is that the image can now be simplified by quantizing the coefficients. Many of the coefficients, usually the higher frequency components, will then be zero. The penalty of this step is the loss of some subtle distinctions in brightness and color. If one applies the inverse transform to the matrix after it is quantized, one gets an image that looks very similar to the original image but that is not quite as nuanced. Next, the quantized coefficient matrix is itself compressed. Typically, one corner of the quantized matrix is filled with zeros. By starting in the opposite corner of the matrix, then zigzagging through the matrix to combine the coefficients into a string, then substituting run-length codes for consecutive zeros in that string, and then applying Huffman coding to that result, one reduces the matrix to a smaller array of numbers. It is this array that is broadcast or that is put on DVDs. In the receiver or the player, the whole process is reversed, enabling the receiver to reconstruct, to a close approximation, the original frame.

Typically, every 15th frame or so is made into an I-frame. P-frames and B-frames might follow an I-frame like this, IBBPBBPBBPBB(I), to form a Group Of Pictures (GOP); however, the standard is flexible about this.

Macroblocks

P-frames provide more compression than I-frames because they take advantage of the data in the previous I-frame or P-frame. I-frames and P-frames are called reference frames. To generate a P-frame, the previous reference frame is reconstructed, just as it would be in a TV receiver or DVD player. The frame being compressed is divided into 16 pixel by 16 pixel macroblocks. Then, for each of those macroblocks, the reconstructed reference frame is searched to find that 16 by 16 macroblock that best matches the macroblock being compressed. The offset is encoded as a "motion vector." Frequently, the offset is zero. But, if something in the picture is moving, the offset might be something like 23 pixels to the right and 4 pixels up. The match between the two macroblocks will often not be perfect. To correct for this, the encoder computes the strings of coefficient values as described above for both macroblocks and, then, subtracts one from the other. This "residual" is appended to the motion vector and the result sent to the receiver or stored on the DVD for each macroblock being compressed. Sometimes no suitable match is found. Then, the macroblock is treated like an I-frame macroblock.

The processing of B-frames is similar to that of P-frames except that B-frames use the picture in the following reference frame as well as the picture in the preceding reference frame. As a result, B-frames usually provide more compression than P-frames. B-frames are never reference frames.

While the above generally describes MPEG-2 video compression, there are many details that are not discussed including details involving fields, chrominance formats, responses to scene changes, special codes that label the parts of the bitstream, and other pieces of information.

Audio encoding

MPEG-2 also introduces new audio encoding methods. These are

• low bitrate encoding with halved sampling rate (MPEG-1 Layer 1/2/3 LSF)
• multichannel encoding with up to 5.1 channels
• MPEG-2 AAC

Video Profiles and Levels

MPEG-2 video supports wide range of applications from mobile to high quality HD editing. For many applications, it's unrealistic and too expensive to support the entire standard. To allow such applications to support only subsets of it, the standard defines profile and level.

The profile defines the subset of features such as compression algorithm, chroma format, etc. The level defines the subset of quantitative capabilities such as maximum bit rate, maximum frame size, etc.

A MPEG application then specifies the capabilities in terms of profile and level. For example, a DVD player may say it supports up to main profile and main level (It's often written as MP@ML). It means the player can play back any MPEG stream encoded as MP@ML or less.

The table below summarizes the limitations of each profile and level.

Applications

DVD

The DVD standard uses MPEG-2 video, but imposes some restrictions:

• Allowed Resolutions
  • 720 × 480, 704 × 480, 352 × 480, 352 × 240 pixel (NTSC)
  • 720 × 576, 704 × 576, 352 × 576, 352 × 288 pixel (PAL)
• Allowed Aspect ratios (Display AR)
  • 4:3
  • 16:9
  • (1.85:1 and 2.35:1, among others, are often listed as valid DVD aspect ratios, but are actually just a 16:9 image with the top and bottom of the frame masked in black)
• Allowed Frame rates
  • 29.97 frame/s (NTSC)
  • 25 frame/s (PAL)

• Audio+video bitrate
  • Video peak 9.8 Mbit/s
  • Total peak 10.08 Mbit/s
  • Minimum 300 kbit/s
• YUV 4:2:0
• Additional subtitles possible
• Closed captioning (NTSC only)
• Audio
  • Linear Pulse Code Modulation (LPCM): 48 kHz or 96 kHz; 16- or 24-bit; up to six channels (not all combinations possible due to bitrate constraints)
  • MPEG Layer 2 (MP2): 48 kHz, up to 5.1 channels (required in PAL players only)
  • Dolby Digital (DD, also known as AC-3): 48 kHz, 32–448 kbit/s, up to 5.1 channels
  • Digital Theater Systems (DTS): 754 kbit/s or 1510 kbit/s (not required for DVD player compliance)
  • NTSC DVDs must contain at least one LPCM or Dolby Digital audio track.
  • PAL DVDs must contain at least one MPEG Layer 2, LPCM, or Dolby Digital audio track.
  • Players are not required to play back audio with more than two channels, but must be able to downmix multichannel audio to two channels.
• GOP structure
  • Sequence header must be present at the beginning of every GOP
  • Maximum frames per GOP: 18 (NTSC) / 15 (PAL), i.e. 0.6 seconds both
  • Closed GOP required for multiple-angle DVDs

DVB

Application-specific restrictions on MPEG-2 video in the DVB standard:

Allowed resolutions for SDTV:

• 720, 640, 544, 480 or 352 × 480 pixel, 24/1.001, 24, 30/1.001 or 30 frame/s
• 352 × 240 pixel, 24/1.001, 24, 30/1.001 or 30 frame/s
• 720, 704, 544, 480 or 352 × 576 pixel, 25 frame/s
• 352 × 288 pixel, 25 frame/s

For HDTV:

• 720 x 576 x 50 frames/s progressive (576p50)
• 1280 x 720 x 25 or 50 frames/s progressive (720p50)
• 1440 or 1920 x 1080 x 25 frames/s progressive (1080p25 – film mode)
• 1440 or 1920 x 1080 x 25 frames/s interlace (1080i25)
• 1920 x 1080 x 50 frames/s progressive (1080p50) possible future H.264/AVC format

ATSC

Allowed video resolutions:

• 1920 × 1080 pixel, 30 frame/s (1080i)
• 1280 × 720 pixel, 60 frame/s (720p)
• 720 × 576 pixel, 25 frame/s (576i, 576p)
• 720 or 640 × 480 pixel, 30 frame/s (480i, 480p)

Note: 1080i is encoded with 1920 × 1088 pixel frames, but the last 8 lines are discarded prior to display.

MPEG-2 audio was a contender for the ATSC standard during the DTV "Grand Alliance" shootout, but lost out to Dolby AC-3.

ISO/IEC 13818

Part 1

Part 2

Part 3

Part 4

Part 5

Part 6

Part 7

Part 9

Part 10

(Part 8: 10-bit video extension. Primary application was studio video. Part 8 has been withdrawn due to lack of interest by industry.)

Patent holders

Approximately 640 patents worldwide make up the "essential" patents surrounding MPEG-2. These are held by over 20 corporations and one university. Where software patentability is upheld, the use of MPEG-2 requires the payment of licensing fees to the patent holders via the MPEG Licensing Association. The patent pool is managed and administered by MPEG Licensing Authority, a private organization. The development of the standard itself took less time than the patent negotiations.

• Alcatel-Lucent
• Canon Inc.
• Columbia University
• France Télécom (CNET)
• Fujitsu
• General Electric Capital Corporation
• General Instrument Corp. (now the broadband division of Motorola)
• GE Technology Development, Inc.
• Hitachi, Ltd.
• KDDI Corporation (KDDI)
• LG Electronics Inc.
• Matsushita
• Mitsubishi
• Nippon Telegraph and Telephone Corporation (NTT)
• NXP
• Philips
• Robert Bosch GmbH
• Samsung
• Sanyo Electric Co., Ltd.
• Scientific Atlanta
• Sharp
• Sony
• Thomson Licensing S.A.
• Toshiba
• Victor Company of Japan, Limited (JVC).

According to the MPEG-LA Licensing Agreement MPEG-LA, any use of MPEG-2 technology is subject to royalties.

• Encoders have a $0.50 charge for each product.
• Decoders have a $0.50 charge for each product.
• Royalty-based sales of encoders and decoders are subject to different rules and $2.50 per unit.
• Also, any packaged medium (DVDs/Data Streams) is subject to licence fees according to length of recording/broadcast.

In the case of free software such as VLC media player (which uses the ffmpeg library) and in which the software is not sold, the end-user bears the royalty.

See also

• MPEG transport stream
• MPEG-1 Audio Layer II
• MPEG-4
• MP2, MP3
• AAC
• DVD
• Theora

References

1. ^ ISO/IEC 13818 MPEG-2 at the ISO Store.
2. ^ ^{a b} ITU-T Rec. H.222.0
3. ^ ITU-T Rec. H.262
4. ^ Richard M Stallman, Patents - Barriers to development Theora Video and Vorbis Audio
5. ^ MPEG-2 Systems PATENT PORTFOLIO LICENSE
6. ^ MPEG-2 PATENT PORTFOLIO LICENSE