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Glossary / Term Explanation!

AC-3 / Dolby Digital - is Dolby's third generation audio coding algorithm. It is a perceptual coding algorithm developed to allow the use of lower data rates with a minimum of perceived degration of sound quality.

Aspect Ratio - Easiest way to explain aspect ratio is to say that it means the ratio between the width of the picture and the height of the picture. Normal TV's aspect ratio is 4:3 (1.33:1), HDTV's aspect ratio is 16:9 (1.85:1) and CinemaScope movies' aspect ratio is 2.35:1.For extremely good explanation of aspect ratios, please visit this URL

ASPI - Advanced SCSI Programming Interface is an old high-level interface written for accessing data stored on SCSI devices. It was developed by Adaptec, and welcomed by developers frustrated with complexity of the SCSI hardware layer. ASPI layer is also used to control ATAPI CD-ROM devices connected to an ATA (IDE) bus.

Authoring means, in video world, a process where already-encoded video files are transferred into a specific format that describes how the data should be kept on storage media, such as CD or DVD. Most common use of the term is when speaking of DVD authoring, using a separate DVD authoring software that allows users to create menus, chapter selections, etc to the disc's layout in addition to the actual video file(s) and transfer this structure to filesystem that can then be burned to a DVD. Other authoring processes include VCD authoring and SVCD authoring. In both processes, authors can create menus and chapter selections and various other tasks before the structure is ready for CD burning, pretty much like in DVD authoring (just in more limited scale).

AVI - stands for Audio Video Interleave. This is a container video format that specifies certain structure how the audio and video streams should be stored within the file. AVI itself doesn't specify how it should be encoded (just like the streaming format ASF), so the audio/video can be stored in very various ways. Most commonly used video codecs that use AVI structure are M-JPEG and DivX ;-). AVI contains code called FourCC which tells what codec it is encoded with.

Bitrate - Very often used when speaking of video or audio quality -- defines how much physical space one second of audio or video takes in bits (note: not in bytes). For example: 3 minutes of MP3 audio in 128kbit/sec CBR bitrate takes 2.81 megabytes of physical space (1,024 x 128 / 1,024 / 1,024 x 180 / 8).

CBR - stands for Constant Bitrate. Basically it is a term that describes how video or audio is encoded -- constant bitrate means that the bitrate doesn't vary during the video or audio at all, but is same through the clip. CBR bitrates are very easy to use in calculations -- if you have an MP3 file that has CBR of 128kbit/sec and it lasts for 3 minutes, the amount of HDD space it takes can be calculated easily:      128kbit = 128 x 1024 bits   1 byte = 8 bits   1 kilobyte = 1,024 bytes   1 megabyte = 1,024 kilobytes   3 minutes = 180 seconds      180 x 128 x 1024 / 8 / 1024 / 1024 = 2.81MB

Codec - stands for Coder/Decoder. Basically it is a piece of software or a driver that adds a support for certain video/audio format for your operating system. With codec, your system recognizes the format the codec is built for and allows you to play the audio/video file (=decode) or in some cases, to change another audio/video file into that format (=(en)code).

Decode - Virtually all video and audio files have been encoded with some compression method, like DivX or MP3 and when we want to "open" these files, we need to decode that compression. Decoding basically means opening a compressed file, whether its done by playing the file or by storing it in an uncompressed format to HDD.

Demuxing / Demultiplexing - basically means, when speaking of video formats, splitting the file that contains both audio and video data (and possible other data streams as well, like subtitles), into separate files, each containing one element of the original file. Demuxing file doesn't weaken the video nor audio quality, it doesn't do anything for these data streams, it just simply saves them into separate files.

DVD - stands for Digital Versatile Disc. It is very often used as a replacement acronym for DVD-Video, which is one standard based on DVD format.

Encode - Encoding is opposite of decoding. Encoding means that a file, whether it is an audio, video or picture file, is compressed to another format that normally takes up less physical drive space than the previous format. Common video encoding methods are DivX, MPEG-1, MPEG-2 and MPEG-4 -- most common audio encoding method or format is MP3.

fps / Framerate - Defines how many pictures eg. frames one second of video or audio contains, normally used acronym for framerate is fps - frames per second. Human eye can't see picture changes after the framerate is more than ~24fps. In American TV system NTSC the framerate is appx. 29.97fps and in European PAL system the framerate is 25fps.

Frame - In video and audio terminology, frame means one still picture -- by changing still pictures eg. frames quickly, human eye "thinks" that the video is smooth and can't separate pictures from each others and instead sees smooth video.

Interlace - Method of smoothening the video picture moving by having double the amount of frames than the video's fps value suggests. Basically, how the interlacing is done, let's say in PAL format, is that each frame is split into two separate pictures, which both are missing half of the picture information. The split is done by its horizontal lines. The PAL video contains 576 horizontal lines -- let's say that the numbering of these lines begins from the top of the frame being the line number 1. The frame is being split to half-frame A and half-frame B -- the half-frame A would contain lines 1, 3, 5, 7, ... and the half-frame B would contain horizontal lines 2, 4, 6, 8, 10, ...    So, when the video is shown with PAL that has frame frequency (fps) of 25fps, there are 50 "half frames" per second that are being updated. First comes the first full frame's "half A" that fills horizontal lines 1, 3, 5, .. leaving other ones blank (assuming that we start from a "zero state" that output doesn't have anything shown before we start counting this). Then comes first full frame's "half B" that fills the missing lines 2, 4, 6, ... and leaves the existing "half A"'s lines showing as well -- now we have the frame no.1 fully shown on screen. Then comes frame no.2's "half A" that then wipes off the frame no.1's "half A", but leaves the frame no.1's "half B" showing. So, now we have a mixture where lines 1, 3, 5, 7, .. are being filled by frame no.2 and lines 2, 4, 6, .. are being filled by frame no.1.

Inverse Telecine - Process where video editing tools reverse telecine process. Basically inverse telecine (or IVTC as it is also called) brings back movie's original framerate from NTSC's 29.97fps to 24fps.

Letterbox - Movies, for instance, are almost always made in widescreenformat. A widescreen movie needs to be displayed on a screen with different aspect ratio from the original film, the picture frame needs to be letterboxed. Letterboxing means adding black borders above and below the original frame. The size of the borders depends on the difference between aspect ratios. For instance displaying a 2.35:1 widescreen picture on a conventional 4:3 TV screen requires borders that cover 40 per cent of the screen.

Macrovision - American company that develops various copy-protection schemes for audio and video purposes. Is best known to have their copy-protection mechanism for VHS tapes bundled with virtually every sold pre-recorded VHS tape in the world. Also, every DVD player sold in the U.S. has Macrovision copy-protection mechanism built-in which prevents DVDs to be recorded to VHS cassettes without hacking either the DVD player or the VCR. They have recently also developed various audio CD protection mechanisms and record labels have already introduced few commercial audio CDs that have Macrovision's CDDA copy-protection mechanism.

MPEG - Moving Pictures Expert Group. A working group of ISO/IEC in charge of the development of standards for coded representation of digital audio and video. MPEG is not an acronym for any standard, it is the acronym for the group who develops these standards! For various standards, take a look at MPEG-1 or MPEG-2.

Multipass Encoding - Using multipass technique when encoding video into another format means basically that the video encoder analyzes the video many (multi :-) times from the beginning to the end before the actual encoding process. While scanning the file, encoder writes information about the original video to its own logfile and uses that log to determine the best possible way to fit the video within the bitrate limits user has set for the encoding process -- this is why multi-pass encoding is only used in VBR encoding (the CBR encoding doesn't offer any flexibility for the encoder to determine the bitrate for each frame). Best way to understand why this is used is to think of a movie -- when there are shots that are totally, absolutely black, like scene changes, normal 1-pass CBR encoding uses the exact same amount of data to that part as it uses for complex action scene. But by using VBR and multi-pass, encoder "knows" that this piece is Ok with lower bitrate and that bitrate can be then used for more complex scenes, thus creating better quality for those scenes that require more bitrate.

Multiplexing or muxing, when speaking of video and video editing, means basically a process where separate parts of the video (or 'streams' as they're called in video terminology) are joined together into one file. Typical process of multiplexing is where we have MPEG-2 video file (extension normally .m2v), an AC3 audio file and a subtitle file and we want to combine into MPEG-2 system stream file in order to burn that file to SuperVideoCD disc. Opposite process is called as demultiplexing.

Progressive or progressive scan is basically an opposite to interlaced picture.    In standard, interlaced analog TV, the picture on TV is changed 50 or 60 times (50Hz in Europe/PAL systems and 60Hz in most NTSC systems, including the American one), but the picture contains only every other horizontal line and the line between is left "empty" -- and the next frame then contains only the horizontal lines missed in the last frame. So, therefor in interlaced picture, at 50Hz frequency, the picture actually changes only 25 times per second (so its framerate is 25fps). But in progressive scan technology, every picture contains everything, so therefor 50Hz progressive scan video changes the picture 50 times per second, having the framerate of 50fps.

RPC-1 term is used to refer to PC's DVD-ROM drives that were built before 1st of January, 2000. Before that date, virtually all DVD-ROM drives permitted the user to change the region code settings of the drive freely when necessary, thus allowing watching/using DVD-Video movies from all over the world. This was changed when the MPAA forced hardware manufacturers to implement a new technology, dubbed as RPC-2 in their drives, that only allows max. 5 changes in the region code.

RPC-2 term is used to refer to PC's DVD-ROM drives that are built after 1st of January, 2000. Before that date, virtually all DVD-ROM drives permitted the user to change the region code settings of the drive freely when necessary, thus allowing watching/using DVD-Video movies from all over the world. Those older drives are called as RPC-1 drives. This was changed when the MPAA forced hardware manufacturers to implement a new technology, dubbed as RPC-2 in their drives, that only allows max. 5 changes in the region code.

SVCD - stands for Super Video CD (called also SuperVCD or Chaoji VCD). It is a new CD standard (actually it is currently undergoing official standardization process) developed in 1998 by Chinese consumer electronics manufacturers, Chinese government and VCD consortium (Sony, Philips, Matsushita and JVC). SVCD is a successor for extremely popular video format called VideoCD which was based on MPEG-1 video encoding. SVCD itself contains MPEG-2 video stream and MPEG-1 or MPEG-2 audio stream (MPEG-1 stereo audio layer II, MPEG-2 stereo audio layer II or MPEG-2 Multi-Channel 5.1 surround audio). It's video bitrate is normally higher than VideoCD's -- clear difference to VideoCD is the fact that SVCD doesn't specify a certain bitrate for video. Unspecified video bitrate also causes a situation where one SVCD disc can contain various amount of video -- normally one SVCD disc contains 35-40 minutes of video, but by reducing the bitrate, one CD can hold up to 74 minutes of video (which is the same amount what VCD disc contains).    As an addition, SVCD can also contain multiple audio streams (just like a DVD-Video can), subtitles, still images, multi-level hierarchical menus, chapters (for indexing), hyperlinks and playlists. Just like VCDs (and audio CDs), SVCDs require a specific way how they are burned on the CD -- just sticking all the required files into CD structure doesn't make disc a SVCD compatible. Most of the new CD burning applications support SVCD already, so authoring your own SVCDs should be relatively easy. SVCD's resolution is 2x higher than VCD's, in PAL the resolution is 480x576 and in NTSC it is 480x480. Framerates are 25fps and 29.97fps, just like in any other video format. SVCD's quality is somewhere between VideoCD and DVD-Video. Most of the DVD players can play SVCD discs which makes is perfect format for backing up your DVD movies and a very good alternative for DivX ;-) format. SVCD has also gained popularity among movie studios -- in Far East distribution, of course -- and most of the studios already release their movies in SVCD format in China, Hong Kong, Taiwan, etc..

Transcoding or more specifically Compressed-domain Transcoding means normally a re-encoding process that changes the video or audio features, such as resolution or bitrate, by changing parts of the a/v content, but not by reconstructing the content again (which is the case in encoding process). Compressed-domain transcoding also maintains the format of the file same as in the original file.

VCD - stands for VideoCD (version 2.0 to be more specific). VideoCD is a standard developed in early 1990's that allows regular CD to contain 74 minutes of video and audio. Both, video and audio, are encoded in MPEG-1 format and stored on the CD in specific format. VideoCDs can be played in most of the stand-alone DVD players, in all stand-alone VCD players and in all computers that have CD-ROM drive. This is the VCD's strong point against DivX format which is based on MPEG-4 audio/video encoding technology. VideoCD resolution is in PAL format 352 x 288 pixels with 25 frames/second. In NTSC format it is 352 x 240 pixels with 29,97 frames/second (except in NTSC film format, where the framerate is 23,976 frames/second. Audio is encoded with bitrate of 224 kbit/sec in MPEG-1 Layer2 format (in both PAL and NTSC versions). Video is encoded with bitrate of 1150 kbit/sec.    VideoCDs are pretty rare in western countries -- basically only VCDs you see in the Europe or in the U.S. (except in NYC's Chinatown, which is a true VCD paradise in middle of the western city :-) are illegal copies or porn movies. On the other hand, VCD is a very popular method for movie distribution in China, Hong Kong, Singapore, Malaysia, etc.. Some studios release some of their movies officially for VCD format in Asia. It has almost completely replaced regular VHS format in Asia, because cheap VCD recorders are widely available there. VideoCD's successor is called SuperVideoCD.

VOB - stands for DVD Video Object. It is basically one of the core files found on DVD-Video discs and contains the actual movie data.    Basically VOB file is just a basic MPEG-2 system stream -- meaning that it is a file that contains multiplexed MPEG-2 video stream, audio streams (normally AC3 format) and subtitle streams.