AC-3

See Dolby Digital Accommodation (Stereoscopic)

The ability of our eyes to refocus at a new point of interest. In normal vision, the processes of focusing on objects at different distances (accommodation) and convergence/divergence (the angle between the lines of sight of our eyes) are linked by muscle reflex. A change in one creates a complementary change in the other. However, watching a stereoscopic film or TV program requires the viewer to break the link between these different processes by accommodating at a fixed distance (the screen) while dynamically varying eye convergence and divergence (something we don’t do in life and can quickly lead to headaches if over- used in stereo3D) to view objects at different stereoscopic distances.

Active line The part of a television line that actually includes picture information. This is usually over 80 per cent of the total line time. The remainder of the time was reserved for scans to reset to the start of the next line in camera tubes and CRT screens. Although the imaging and display technologies have moved on to chips and panels, there remains a break (blanking) in the sampling of digital TV as in ITU-R BT.601 and ITU-R BT 709. These ‘spaces’ carry data for the start of lines and pictures, as well as other information such as embedded audio tracks. See also: Active picture

Active picture

The area of a TV frame that carries picture information. Outside the active area there are line and field blanking which roughly, but not exactly, correspond to the areas defined for the original 525- and 625-line analog systems. In digital versions of these, the blanked/active areas are defined by ITU-R BT.601, SMPTE RP125 and EBU-E.

ADC or A/D Analog to Digital Conversion. Also referred to as digitization or quantization. The conversion of analog signals into digital data – normally for subsequent use in digital equipment. For TV, samples of audio and video are taken, the accuracy of the process depending on both the sampling frequency and the resolution of the analog amplitude information – how many bits are used to describe the analog levels. For TV pictures 8 or 10 bits are normally used; for sound, 16, 20 or 24 bits are common. For pictures the samples are called pixels, which contain data for brightness and color. See also: AES/ EBU, Binary, Bit, Into digits (Tutorial 1), Pixel

ADSL

Asymmetrical Digital Subscriber Line – working on the copper ‘local loop’ normally used to connect phones, ADSL provides a broadband downstream channel (to the user) of maximum 8 Mb/s and a narrower band upstream channel (from the user) of maximum 128-1024 kb/s, according to class. Exactly how fast it can run ultimately depends on the performance of the line, often dictated by the distance from the telephone exchange where the DSLAM terminates the line. See also: Broadband, DSL

AES/EBU

The Audio Engineering Society (AES) and the EBU (European Broadcasting Union) together have defined a standard for Digital Audio, now adopted by ANSI (American National Standards Institute). Commonly referred to as ‘AES/EBU’ and officially as AES3, this digital audio standard permits

a variety of sampling frequencies, for example CDs at 44.1 kHz, or DATs and digital VTRs at 48 kHz. 48 kHz is widely used in broadcast TV production although 32-192 kHz are allowed. One cable and connector, usually an XLR, carries two channels of digital audio. See also: Word clock Website: www.aes.org

Aliasing

Undesirable ‘beating’ effects caused by sampling frequencies being too low to faithfully reproduce image detail. Examples are: 1) Temporal aliasing – e.g. wagon wheel spokes apparently reversing, also movement judder seen in the output of standards converters with insufficient temporal filtering. 2) Raster scan aliasing – twinkling effects on sharp boundaries such as horizontal lines. Due to insufficient filtering this vertical aliasing and its horizontal equivalent are often seen in lower quality DVEs as detailed images are compressed. The ‘steppiness’ or ‘jaggies’ of poorly filtered lines presented at an angle to the TV raster is also referred to as aliasing. See also: Anti-aliasing, Interpolation (temporal & spatial), Into digits (Tutorial 1)

Aliens

A familiar term for alias effects, such as ringing, contouring and jaggy edges caused by lack of resolution in a raster image. Some can be avoided by careful filtering or dynamic rounding.

Alpha channel

Another name for key channel – a channel to carry a key signal. See also: Keying, 4:4:4:4

Anaglyph (Stereoscopic)

A type of stereoscopy in which the left eye and right eye images are separated by color filtering and then superimposed as a single image rather than two separate images. Each eye sees only the required image through the use of complementary colored filters (e.g. red and green or red and cyan). Anaglyph glasses have been popular over the years for viewing 3D comics and some 3D films (particularly on VHS and DVD). See also: 3D

Anamorphic

Generally refers to the use of 16:9 aspect ratio pictures in a 4:3 system. See also: Aspect ratio – of pictures

Answer print

The answer print, also called the first trial print, is the first print made from edited film and sound track. It includes fades, dissolves and other effects. It is used as the last check before running off the release prints from the internegatives.

Anti-aliasing Smoothing of aliasing effects by filtering and other techniques. Most, but not all, DVEs and character generators contain anti-aliasing facilities. See also: Aliasing, Interpolation (spatial), Interpolation (temporal)

API

Application Programming Interface – a set of interface definitions (functions, subroutines, data structures or class descriptions) which provide a convenient interface to the functions of a subsystem. They also simplify interfacing work by insulating application programmers from minutiae of the implementation.

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