Macroblock
A macroblock is a portion of image that consists of 16x16 picture elements (pixels or pels).
At the macroblock layer motion compensation and prediction are performed and it's possible to change the quantisation step.
It must be noticed that, if the picture is an interlaced frame picture, the odd lines of the macroblock belong to the first field and the even lines to the second field.

Video decoding process at the macroblock layer
Decode the macroblock mode and the possible quantiser_scale_code.
If it's an Intra macroblock:
Decode the blocks which the macroblock consists of.
If it's a Non-Intra macroblock:
Decode the prediction mode and the motion vectors.
Produce the suitable prediction for the macroblock.
Decode the blocks which the macroblock consists of, obtaining the prediction errors values.
Add the prediction errors values to the prediction.



Macrovision
An analog video copy protection scheme that alters the unseen part of a video signal such that a VCR or other macrovision enabled device may not record the video signal properly. There are several types:
- Automatic Gain Control
- 2-line color stripe
- 4-line color stripe

Symptoms of this include picture fading in and out or color banding of the signal.


Matroska
A new video and audio container format similiar to AVI but with several new features like support for OGG audio, Variable Framerate Video.
Matroska File Formats:
.mkv : Generally video files, as well those containing audio ( movies ) or video only
.mka : audio only files, can contain any supported audio compresion format, such as MP2, MP3, Vorbis, AAC, AC3, DTS, PCM and soon MPC ( musepack )
.mks : a so called 'elementary' matroska stream containing any subtitles stream

Mini DV
Mini DV is a video cassette designed for use in MiniDV digital camcorders. The picture quality of digital video (DV) recorded on a Mini DV cassette is basically identical or better to the quality of DV recorded on a Hi8 or 8mm cassette by a Digital8 camcorder. Mini DV can have up to 530 lines of video resolution for some camcorder models. However, Mini DV tapes are smaller which allows for smaller camcorders. Mini DV tapes are available in lengths of 30 and 60 minutes (plus, recording in LP mode lets you extend total recording time with a 60-minute tape to 90 minutes).



miniDVD
miniDVD is a DVD video written onto a CD-R(W) instead of a DVD disc. miniDVD is also sometimes called cDVD. A miniDVD only fits about 15 minutes of DVD quality video on a 650 MB CD-R(W). Not many DVD players will play miniDVD - see the DVD Players miniDVD list for compatible players. DVDRhelp.com miniDVD page

MJPEG
Moving JPEG. A moving image which is made by storing each frame of a moving picture sequence in JPEG compression, then decompressing and displaying each frame at rapid speed to show the moving picture.M-JPEG does not use interframe coding as MPEG does. Sometimes called Motion JPEG.

MLP
Meridian Lossless Packing. A lossless compression technique (used by DVD-Audio) that removes redundancy from PCM audio signals to achieve a compression ratio of about 2:1 while allowing the signal to be perfectly recreated by the MLP decoder.

Motion compensation and prediction
Motion compensation and prediction are performed at the macroblock layer.
The goal of motion compensation is to provide a good prediction for the macroblock. Actually, in the macroblocks where prediction is applied, the DCT is performed to the prediction errors instead of to the image samples and more the prediction errors are low and more the entropy coding is effective. Therefore, with good predictions it's possible to have low bit rate and good quality.
In nearly still pictures it's quite easy to have very good predictions using the pixels just in the same position of those to predict, but in motion pictures it's necessary to take movements into account.

Both the coder and the decoder have two frame-memories where they store the decoded pictures used as references.
Where do the predictions come from ?
It depends on the kind of the picture.

Predictions for a P_picture
If it's a frame picture they may come from the previous I or P frame.
If it's a field picture they may come from the two I or P fields coded most recently.
Predictions for a B_picture
They may come from the previous (in display order) I or P frame as from the next (in display order) I or P frame and there may be an interpolation between predictions coming from both directions.
See also the sequence layer.

What's a motion vector ?
A motion vector is a bi-dimensional pointer that tell the decoder how much left/right and up/down, from the position of the macroblock, is located the prediction macroblock in the reference frame or field.
Motion vectors have an half-pel resolution, that means that an interpolation process is necessary to get the prediction (for MPEG-1 is also possible the simpler pel resolution selected at the picture layer). It must be noticed that the same motion vector is applied both to luminance and, after being scaled, to chrominance.

What's motion estimation ?
Motion estimation is the process, perfomed by the coder, that should find the motion vector pointing to the best prediction macroblock in a reference frame or field.
The goodness of a prediction macroblock is in general evaluated minimizing a cost function that may be the absolute error or the mean squared error. The cost function is applied to the macroblock (or a part of it), such technique is called block matching and is the most used in video coding. In general every possible prediction in a given range is evaluated, so we speak about full search. Unfortunatly the computation's complexity is proportional to the search area and can be quite heavy, and on the other hand the search area has to be wide enough to include every movement.
It must be noticed that the capability to perform a good motion estimation is a key point for the quality of a coder.

The prediction menu for frame pictures.
Frame-based prediction.
A single motion vector for the whole macroblock. It's used when movements between the two fields are unsignificant. It's the only possible choice for progressive images.
Field-based prediction.
Two different motion vectors. One for the samples belonging to the first field and one for those belonging to the second field. It's used when movements between the two fields are important.

The prediction menu for field pictures.
Field prediction.
A single motion vector for the whole macroblock. It's used when the whole area has a single movement.
16x8 prediction.
Two different motion vectors. One for the top half and one for the bottom half of the macroblock. It's used when the macroblock includes different objects with different movements and the macroblock area is to large to have a good prediction.

Motion Estimation
In video encoding, the process of analyzing previous or future frames to identify blocks that have not changed or have only changed location. Motion vectors are then stored in place of the blocks. This is very computation-intensive and can cause visual artifacts when subject to errors.

MP3
MP3 is an acronym for MPEG-1 Layer 3 audio encoding (it is not an acronym for MPEG3). MP3 is a popular compression format used for audio files on computers and portable devices.

The compression in MP3 works on the basis of a "psychoacoustic model" which means that parts of the audio that human ears cannot detect are discarded by the encoder. Although this is a LOSSY process, it can yield very high quality audio files are relatively high compression rates.

A typical MP3 file encoded at 128 kbit/s (12:1 compression) is near CD quality.

MP3 audio is increasingly being used in video production coupled with various MPEG-4 video codecs like divx. The audio may be encoded with a constant or variable bitrate.



MP3 ID3 Tag
An MP3 ID3 Tag is information stored at the end of an MP3 file. The tag can contain information about the Title/Songname, Artist, Album, Year, Comment, and Genre in version 1 and also Track in version 1.1. A proposed Version 2 is out which would be extendable to include more information and picture(s). ID3.org



MPA
Shorthand for MPEG Audio elementary stream(no video). Also called MP2 for MPEG Audio Layer2 but MP2 could also be MPEG2 Audio.

MPEG Audio
MPEG Audio is a family of generic standards for low bit-rate audio coding. There are three different encoding standards:

• Layer I - low complexity, good for consumer recording
• Layer II - high efficiency with medium complexity, good for professional recording and for broadcast
• Layer III - high complexity and high efficiency, suitable for very low bit-rates application
  

MPEG-1
An ISO/IEC (International Organization for Standardization/ International Electrotechnical Commission) standard for medium quality and medium bitrate video and audio compression. It allows video to be compressed by the ratios in the range of 50:1 to 100:1, depending on image sequence type and desired quality. The encoded data rate is targeted at 1.5Mb/s - this was a reasonable transfer rate of a double-speed CD-ROM player (including audio and video). VHS-quality playback is expected from this level of compression. The Motion Picture Expert Group (MPEG) also established the MPEG-2 standard for high-quality video playback at a higher data rates. MPEG-1 is used in encoding video for VCD. MPEG FAQ

MPEG-2
An encoding standard designed as an extension of the MPEG-1 international standard for digital compression of audio and video signals. MPEG-1 was designed to code progressively scanned video at bit rates up to about 1.5 Mbit/s for applications such as CD-i. MPEG-2 is directed at broadcast formats at higher data rates; it provides increased support for efficiently coding interlaced video, supports a wide range of bit rates and provides for multichannel surround sound coding such as PCM, Dolby Digital, DTS and MPEG audio.

MPEG-4
An ISO/IEC standard 14496 developed by the Moving Picture Experts Group (MPEG), the committee that also developed MPEG-1 and MPEG-2. These standards made interactive video on CD-ROM, DVD and Digital Television possible. MPEG-4 is the result of another international effort involving hundreds of researchers and engineers from all over the world. MPEG-4 was finalized in October 1998 and became an International Standard in 1999. The fully backward compatible extensions under the title of MPEG-4 Version 2 were frozen at the end of 1999, to acquire the formal International Standard Status early in 2000. Several extensions were added since and work on some specific work-items is still in progress.

MPEG-4 builds on the proven success of three fields:

Digital television
Interactive graphics applications (synthetic content)
Interactive multimedia (World Wide Web, distribution of and access to content)



MPEG-7
MPEG-7 is an ISO/IEC standard developed by the Moving Picture Experts Group. MPEG-7, formally named “Multimedia Content Description Interface”, is a standard for describing the multimedia content data that supports some degree of interpretation of the information’s meaning, Unlike previous MPEG standards aimed at encoding, MPEG-7 is not aimed at any one application in particular; rather, the elements that MPEG-7 standardizes support as broad a range of applications as possible.



mpegproperties
A compact program for Windows PCs that will give you all the parameters for a MPEG file.



MPV
MPEG-1 or MPEG-2 video elemantary stream(no audio). Also called M1V for MPEG-1 video and M2V for MPEG-2 video.



Multiplex
Joining video and audio to one file. Also called "Mux".



Multisystem
Describes a video component that can handle 2 or more types of broadcast video standards. Multisystem televisions, videocassette recorders, and DVD players are not found in all stores but are manufactired by many of the large electronics companies. Why buy multisystem? People who move between countries can play/watch video in both countries. Or if you have a relative in India and want to watch the latest from Bollywood in the US then multisystem works also. Multisystem components often cost more than the single system equivalents. Also some DVD players that are billed as single system VCD capable (NTSC / PAL) can play both.

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