AC3: Also known as Dolby Digital, it’s a digital audio format from Dolby Labs™. It can support up to 5.1 channels of audio.

ASF: Advanced Streaming Format: Microsoft\'s answer to Real Media and streaming media generally.

AVI: Audio Video Interleave: The video format most commonly used on Windows PCs. It defines how video and audio are attached to each other, without specifying a codec.

Bitrate: In a bit stream, the number of bits occurring per unit time, usually expressed in bits per second.

CBR/VBR: Constant Bitrate Variable Bitrate. Constant bitrate gives equal bits to each frame of a movie. Variable Bitrate gauges which frames need more bits to maintain a better image. Note: high motion scenes require more bits than low motions scenes.

Decoding: Decoding refers to the process of converting a compressed file to a viewable image.

Demultiplexing: The opposite of multiplexing. In this process, a combined audio/video stream will be separated into the number of streams it consists of (a video stream, at least one audio stream, and a navigational stream). Every VOB encoder demultiplexes the VOB files before encoding (FlasKMPEG, MPEG-2avi, DVD2MPG, ReMPEG-2) and every DVD player does the same (audio and video are being treated by different circuits, or decoded by different filters on a PC).

Digital Video: Digital video is usually compressed because it takes Terabytes (thousands of Gigabytes or for the mathematicians among you: 10¹² Bytes) to store an uncompressed movie. Because standard lossless compression is insufficient for video, the video codecs have to get rid of irrelevant information - stuff the human eye won't see or is unlikely to see. Since that is still not enough modern compression, algorithms use keyframes and I and P frames to save space.

DirectX: DirectX is an application program interface developed by Microsoft, first for Windows 95, for creating and managing graphic images and multimedia effects in applications such as games and active Web pages. It includes the DirectDraw API for direct access to video memory.

DivX^TM: DivX^TM is a video technology that compresses digital video so it can be downloaded over DSL or cable modems in a relatively short time with no reduced visual quality.

Encoding: The process of taking a raw uncompressed file and compressing it to an encoded form while maintaining the quality.

Frame: The basic source of a movie; one frame represents one image. A movie usually runs at 24 frames per second, equaling 24 different images per second. Imagine 24 images with a bird on it. In the first image, the bird is on the left, gradually moving to the right. On the 24th frame the bird is on the right end of the image. When these 24 images are played in sequence fast enough, the human eye sees a bird flying from left to right.

Frameserving: Frameserving is using one utility to decode or read a video file and after it has been decoded, sending it straight to any other video editing or encoding application.

Hue: Hue is the overall gradation of color.

I and P Frames: Frame describing only the differences to the frame before (this is less than accurate but I think you'll get the picture that way). Say we have a keyframe with a bird before a cloudy sky. Then we can use I frames which say something like this: move the bird an inch to the left and one inch to the bottom.

iDCT & Wavelet: The video information inside MPEG files is stored in the frequency domain rather than in the spatial domain (the images we see). That way, the information is compacted and that compactation can be used to compress (reduce) the amount of information you have to send over the transmission channel. MPEG uses the DCT (Discrete Cosine Transform) to translate spatial information into frequency information. To bring back the spatial information from the MPEG stream you have to apply the iDCT, that is, the Inverse Discrete Cosine Transform, that undoes the DCT that was used during encoding. DCT and iDCT are basically the same as DFT (discrete fourier transforms) but the results are integers rather than complex reals you get in i/DFT. For more info please refer to a university-level book about DSP, communication systems, or similar. Wavelets are an alternative basis space. There are infinitely many wavelet bases (Daubechies, Haar, Mexican Hat, "Spline", Zebra, etc.), but their primary feature is that they are localized. Fourier basis functions span all space (from negative to positive infinity). Wavelets are basically individual pulses of waves (at various positions and scales). Their value in compression stems from factors like the grouping, which generally shows that a good 90% of the data is modeled by the low-pass filters, with the high-pass filters generally showing very small values that are mostly details. (Of course, this is not true if the source is noisy in the first place). For images, the greatest value comes from localization of the basis, which means that we can model discontinuities (e.g. edges) VERY well with wavelets. You will NOT get those weird JPEG halos if you use wavelets.

Interlaced: Devices that utilize interlaced scanning paint half of the scan lines (referred to as a field) and then paint the second half. Each field is updated 60 times per second. And a frame, made up of the two fields, is completely refreshed 30 times per second. While interlacing works well on smaller sets, on the bigger TV screens that are out today, the refreshing process may be visible to the eye, so that some people notice a perceptible flickering.

Interleaving: Describes the process of gluing together the audio and the video track at defined points. The player will recognize the interleave points and make sure that both audio and video are played in a manner that the "glued" points match throughout the movie.

Inverse Telecine: The inverse of Telecine. This process is performed to extract the original 24fps of a 29.97fps source.

Keyframe: A complete frame but heavily compressed.

MPEG: MPEG means Motion Picture Expert Group and it's THE resource for video formats in general. This group defines standards in digital video, among it the MPEG-1 standard (used in Video CDs), the MPEG-2 standard (used on DVDs and SVCDs), the MPEG-4 standard (used in DivX video technology) and several audio standards - among them MP3 and AAC.

MPEG-4: MPEG-4 is a standard defined by the Working Group 11 (Moving Picture Expert Group) of ISO (International Standard Organization) in October 1998 (date of the first draft of the standard). It is the standard for the mature digital era. With its added features, MPEG-4 offers better compression, interactivity, and universal Internet/wireless access to the medium.

Multipass encoding: True multipass encoding is currently available only for WM8 and MPEG-2 (SVCD & miniDVD). An encoder supporting multipass will, in a first pass, analyze the video stream to be encoded and write down a log about everything it encounters. Let's assume we have a short clip that starts out in a dialog scene where we have few cuts and the camera remains static. Then it leads over to a karate fight with lots of fast cuts and a lot of action (people flying through the air, kicking, punching, etc.). In regular CBR, encoding every second gets more or less bitrate (it's hard to stay 100% CBR but that's a detail) whereas in multipass VBR mode the encoder will use the bitrate according to his knowledge about the video stream, i.e. the dialog part gets the available bitrate and the fighting scene gets allotted more bitrate. The more passes, the more refined the bitrate distribution will be. In single pass VBR, the encoder has to base his decisions on where to use how much bitrate solely on the knowledge of the stuff it previously has encoded.

Multiplexing: Video and audio are usually encoded separately. You have to join them to make a movie you can play (you can play audio and video separately in two players but to get synch would be rather hard). During multiplexing, the audio and video tracks are combined into one audio/video stream. The audio and video stream will be woven together and navigational information will be added so that the player can, for example fast forward/backward and still retain synch audio/video.

Normalizing: Normalizing consists of finding the volume peak of a soundtrack and increasing the rest of the soundtrack to that level. This is usually done at WAV level, i.e. all variables are integers so the precision is not as good as in DRC. DRC is preferred over Normalizing.

PCM: Pulse Code Modulation is the most simple binary representation for digital audio. The audio signal is converted in samples according to the frequency of the signal. Every sample is then written in the stream without using any smart compression techniques.

Progressive: Progressive scanning paints all of the scan lines onto the screen at the exact same time, 60 times per second. Progressive scanning was developed for computer monitors in order to decrease flickering. Progressive scan has fewer jagged edges and can show movement in better detail with fewer trails (called artifacts).

Saturation: Saturation is the strength of the colors in a video.

SBC: Smart Bitrate Control. A new way to encode DivX;-). Using Nandub, it can modify many internal codec parameters on the fly during compression, giving you better quality and more control over the encoding.

Telecine: A process to bring a 24fps source (usually a movie is shot at that speed) to 29.97fps or 29.97x2 interlaced fields per second.

VOB Files: All DVD movies are stored in so-called VOB files. VOB files usually contain multiplexed Dolby Digital Audio and MPEG-2 video. VOB Files are called as follows: vts_XX_y.vob where XX represents the title and Y the part of the title. There can be 99 titles and 10 parts, although vts_XX_0.vob never contains any video, usually just menu or navigational information.

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