picture frames. The techniques were primarily developed for digital data transmission but have been adopted as a means of reducing broadcast transmission bandwidths and storage requirements on disks and VTRs. A number of compression techniques are in regular use for moving images. These include ETSI, JPEG, Motion JPEG, JPEG 2000, DV, MPEG-1, MPEG-2, MPEG-4, AVC Intra, Windows Media and Real. Where different techniques are used in the same stream, problems can occur and picture quality can suffer more than if the same method is used throughout. The MPEG-2 family of compression schemes, which was originally designed for program transmission, has been adapted for studio use in Betacam SX and IMX recorders. While there is much debate, and new technologies continue to be developed, it remains true that the best compressed results are produced from the highest quality source pictures. Poor inputs do not compress well. Noise, which may be interpreted as important picture detail, is the enemy of compression. See also: Compression ratio, Concatenation, Digital Betacam, ETSI, JPEG, JPEG 2000, MPEG-2, MPEG-4, Windows Media
Compression ratio
A B C D E F G H I J
K L
M N O P Q R S T U V
W X Y Z
The ratio of the amount of data in the non-compressed digital video signal to the compressed version. Modern compression techniques start with component television signals but a variety of sampling systems are used, 4:2:2 (‘Studio’ MPEG-2), 4:2:0 (MPEG-2), 4:1:1 (NTSC, DVCPRO), etc. The compression ratio should not be used as the only method to assess the quality of a compressed signal. For a given technique, greater compression can be expected to result in lower picture quality but different techniques give widely differing quality of results for the same compression ratio. See also: Concatenation, DV, ETSI, JPEG, JPEG 2000, MPEG
Concatenation
The linking together of systems in a linear manner. In digital television this often refers to the concatenation of compression systems which is a subject of concern because any compression beyond about 2:1 results in the removal of information that cannot be recovered. As the use of compression increases, so too does the likelihood that material will undergo a number of compressions between acquisition and transmission. Although the effects of one compression cycle might not be very noticeable, the impact of multiple decompressions and recompressions – with the material returned to baseband in between – can cause considerable damage. The damage is likely to be greatest where different compression schemes are concatenated in a particular signal path.
Conform
Cutting together recorded material according to a prepared scheme such as a rough cut or EDL (Edit Decision List). EDLs can be used to directly control conforming in an online edit suite (auto-conforming). The time to conform varies widely, from a tape-based suite which takes much longer than the finished program’s running time, to a nonlinear online suite with true random access to all material. This reduces time by loading material in C-mode and the conforming itself takes only a moment and still allows any subsequent adjustments to be easily made. Note that with in-server editing, material may be loaded onto the server as an independent task, rather than involving the edit equipment itself. This circumvents the loading time so further reducing the total time to produce the finished program. The same is also true of nonlinear edit systems with the bandwidth to support loading as a background task. There are a number of established ways, or modes, of approaching video editing. A-mode is a simple sequential
method, starting from the beginning of the program and going through to the end. B-mode uses all shots from one particular source before the next source is used (i.e. checkerboarding). C-mode is similar to B-mode but works sequentially through the playback source rather than the record tape. D-mode is similar to A-mode except that dissolves and other transitions are performed last, at the end of the conform. E-mode is similar to C-mode but with transitions at the end, as in D-mode. Note that the logic of the above modes is based on the characteristics of videotape. As tape is increasingly replaced by other recording media, network conform is becoming increasingly used. Part of the work of the Advanced Media Workflow Association (AMWA) is to provide new industry standards for this important area. See also: Background loading, Delta editing, In-server editing, Uncommitted editing Website:
www.aafassociation.org
Consolidation
Clearing continuous space on a disk store to allow consistent recording. This generally involves the moving of data on the disks to one area, leaving the remainder free so that recording can proceed track-to-track – without having to make random accesses. The larger the amount of data stored, the longer consolidation may take. Careful consideration must be given to large-capacity multi-user systems, such as video servers, especially when used for transmission or on-air. The need for consolidation arises because of the store’s inability to continuously record television frames randomly at video rate. See also: Defragmentation, True random access
Constant bit rate (CBR) compression Compression systems that are used to create a fixed rate of output data. This is usually to fit within a given bandwidth such as that available on a video tape recorder or a constant bit rate transmission channel. With video, the amount of useful information contained in the material varies widely both spatially and temporally – with movement. For example, a football match with crowds and grass texture as well as fast panning cameras typically contains far more information than a largely static head-and-shoulders shot of a newsreader. Using constant bit rate means that the quality is altered to reduce the information to fit the fixed bit rate. In the football case, the grass may go flat during a camera pan, with the texture suddenly reappearing when the camera is still. As overflowing the available bit rate could have disastrous results with bits being lost, the aim is always to use just under the available bit rate. The degree of success in almost filling the available space (not easily done live) is a measure of the quality and efficiency of the compression system. See also: Variable bit rate
Content
Pictures, sound, text, graphics, etc., that are edited and ready for delivery to customers – typically as programs for television. See also: Essence
Contouring An unwanted artifact similar to posterization. Digital video systems exhibit contouring when insufficient quantizing levels or inaccurate processing are used, or poor truncation occurs. The result is that the picture’s brightness changes in steps that are too large and become visible over relatively even-brightness areas – like the sky. See also: Dynamic Rounding
Control track A linear track recorded onto videotape at frame frequency as a reference for the running speed of a VTR, for the positioning or reading of the video tracks and to drive a tape counter. It is a magnetic equivalent of sprocket holes in
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GLOSSARY OF TERMS
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