Common Terms
The following terms are commonly used in the area of forensic audio and video analysis. These definitions are provided by the Scientific Working Group on Digital Evidence (SWGDE) and the Scientific Working Group on Imaging Technology (SWGIT). A full comprehensive glossary is also available online.
Archive Copy - A copy of data placed on media suitable for long-term storage, from which subsequent working copies can be produced.
Artifact - A visual/aural aberration in an image, video or audio recording resulting from a technical or operational limitation. Examples include speckles in a scanned picture or “blocking” in images compressed using the JPEG standard.
Capture - The process of recording data, such as an image, video sequence or audio stream.
Compression - The process of reducing the size of a data file. (See also, “Lossy Compression” and “Lossless Compression”.)
De-interlacing - Separating an interlaced frame into two discrete fields.
Field - An element of a video signal containing alternate horizontal lines. For interlaced video, the scanning pattern is divided into two sets of spaced lines (odd and even) that are displayed sequentially. Each set of lines is called a field, and the interlaced set of the two sets of lines is a frame.
Frame - Lines of spatial information of a video signal. For interlaced video, a frame consists of two fields, one of odd lines and one of even lines, displayed in sequence. For progressive scan (non-interlaced) video, the frame is written through successive lines that start at the top left of the picture and finish at the bottom right.
Image Analysis - A sub-discipline of Digital & Multimedia Evidence, which involves the application of image science and domain expertise to examine and interpret the content of an image and/or the image itself in legal matters.
Image Comparison - The process of comparing images of questioned objects or persons to known objects or persons or images thereof, and making an assessment of the correspondence between features in these images for rendering an opinion regarding identification or elimination.
Image Content Analysis - The drawing of conclusions about an image. Targets for content analysis include, but are not limited to: the subjects/objects within an image; the conditions under which, or the process by which, the image was captured or created; the physical aspects of the scene (e.g., lighting or composition); and/or the provenance of the image.
Interlaced scan - A technique of combining two television fields in order to produce a full frame. The two fields are composed of only odd and only even lines, which are displayed one after the other but with the physical position of all the lines interleaving each other, hence interlace3.
Interpolation - A method of image processing whereby one pixel, block, or frame is displayed or stored based on the differences between the previous and subsequent pixel, block or frame of information. [Taken from the Encyclopedia of Photography 3rd Edition] This is often done to increase the apparent clarity of an image.4
Multiplexer/Demultiplexer - A device used to combine multiple video signals into a single signal or separate a combined signal. These devices are frequently used in security and law enforcement applications for recording and/or displaying multiple camera images simultaneously or in succession.
Nominal resolution - The numerical value of pixels per inch as opposed to the achievable resolution of the imaging device. In the case of flatbed scanners, it is based on the resolution setting in the software controlling the scanner. In the case of digital cameras, this refers to the number of pixels of the camera sensor divided by the corresponding vertical and horizontal dimension of the area photographed.
Sharpening - A process used to emphasize edge detail in an image by enhancing the high frequency components.
Time lapse video recording - Process by which images are recorded at less than the standard rate of frames per second (NTSC–29.97; PAL–25.00) thus extending the period of time that can be covered by the storage medium.
Video stabilization - The process of positioning individual frames so that a selected object or person will remain in the same location as the video is played.
Work copy - A copy or duplicate of a recording or data that can be used for subsequent processing and/or analysis.
3 Damjanovski, V. CCTV Netorking and Digital Technology, Butterworth-Heinemann: Waltham, MA, 2000.
4 Zakia, R. D. The Focal Encyclopedia of Photography, 3rd Ed., Focal Press; Butterworth-Heinemann: Waltham, MA, 1993.