Category Archives: G

G.fast

A high-speed digital subscriber line (DSL) standard for short local loops (the connection between the customer’s premises and the telecom’s network) that is expected to deliver from 150 Mb/s up to 1 Gb/s data rates over copper, and is slated as matching fiber at distances up to 400 metres. This means that many consumers can receive fast internet without running fiber to the house. The protocol is defined in Recommendation ITU-T G.9701. Such data rates are ample to support live 4K and even 8K UHD streaming to viewers’ homes. The first consumer installations of this technology are expected in late 2015.

Gamma

Gamma has several meanings. In the video world a television screen’s brightness is not necessarily linearly proportional to amplitude of the picture signal. This is approximated by a power law, the power being referred to as gamma. For a CRT (cathode ray tube) the gamma is generally calculated to be 2.6. This is compensated for in TV cameras by a gamma of 0.45 giving an overall gamma of 0.45 x 2.6 = 1.17 – adding overall contrast to help compensate for domestic viewing conditions. Today, most viewers have panel screens that use one of several different technologies. Usually there is a menu somewhere for selecting a gamma setting to suit your screen for the correct gamma and gamut.

In film, gamma describes the average slope of the D/Log E curve over its most linear region. For negative stocks this is approximately 0.6, for intermediate stocks this is 1.0 and for print stocks 3.0. This gives a system gamma of 0.6 x 1 x 3 = 1.8. This overall boost in contrast is much reduced due to flare and auditorium lighting conditions of cinemas.

With video now available on a wide range of devices there may be a need to re-visit the gamma settings. For example, a digital film master is versioned for mobile phones, and for home TV (where viewers have, LED, plasma and a few CRT displays) as well as for digital and film cinemas. This can be achieved by applying suitable LUTs for each version.

See also: Internegative, Interpositive

Website: www.myperfectpicture.tv

Gamut

In image media this usually refers to the available range of colors from a display device, as in Color Gamut. This varies according to the color space used; YCrCb, Rec709 (HDTV), Rec2020 (UHD), DCI-P3 (digital cinema) and CMYK (paper print colors) all have different color gamuts.

See also: Illegal colors

Gateway

A device connecting two computer networks. For example, a gateway can connect a local area network (LAN) to a storage area network (SAN). This way a PC connected on an Ethernet LAN may have access to files stored on the SAN even though the PC is not SAN aware.

Generation loss

The signal degradation caused by successive re-recordings (copying). Freshly recorded material is first generation; one re-recording, or copy, makes the second generation, etc. This was of major concern in analog linear editing but much less so in a digital suite. Non-compressed component DVTRs should provide at least twenty tape generations before any adverse artifacts become noticeable, most likely due to dropouts on the tape. Even better multi-generation results are possible with disk-based systems. These can re-record millions of times without causing dropouts or errors. This means that the number of generations are effectively limitless, which is very useful when building multi-layer video, or movie, effects.

Other possible recording media candidates include solid state memories such as DRAM, which is very reliable over a huge number of read/write cycles; provided the power stays on. Static RAM, such as SD cards, is not suitable as it is not as fast as DRAM and is known to degrade over many read/write cycles.

Besides the limitations of recording, the action of processors such as decoders and coders will make a significant contribution to generation loss. The decode/recode cycles of NTSC and PAL are well known for their limitations. Caution is also needed for digital video compression (coding) systems, including MPEG, DV and JPEG, not to mention the vast number of proprietary variants, as well as the color space conversions that typically occur between computers handling RGB and video equipment using Y,Cr,Cb.

See also: Concatenation, Error detection concealment and correction

Ghosting (Stereoscopic)

Artifacts typically caused by signal leakage (crosstalk) between the two ‘eyes’. A secondary ‘ghost’ image can be seen. There are several possible causes that can introduce the problem during acquisition, post production and display. One reason can be high contrast levels between an object and its background.

Gigantism (Stereoscopic

Confusing visual cues in a stereoscopic scene that can make an object appear to be the ‘wrong’ size, i.e. the impression of strangely enlarged size of objects. This is due to the choice of interaxial distance relative to the focal length of the camera lenses, e.g. shooting with an interaxial distance much less than the interocular distance of adult human eyes can make a figure appear to be a giant.

See also: Interaxial, Interocular, Miniaturization

Global (control)

The top level of control in a multi-channel DVE system. A number of objects (channels) can be controlled at one time, for example to alter their opacity or to move them all together relative to a global axis, one which may be quite separate from the objects themselves. This way the viewing point of all the assembled objects can be changed. For example, a cube assembled from six channels could be moved in 3D space as a single action from a global control.

See also: Axis

GPI

General Purpose Interface. This is a simple form of control interface typically used for cueing equipment, usually by a contact closure. It is simple, can be frame accurate and therefore can easily be applied over a wide range of video equipment.

GPU

Graphics Processing Unit. A chip or digital circuit designed specifically for processing graphics and generally providing the main processing power of a computer graphics card. Having much more graphics power and speed than central processor unit (CPU) chips, GPUs can take over many complex 2D and 3D image processing tasks from the CPU.

In the wider computer market, GPUs have transformed computer games, simulation and a whole host of complex digital displays. However, GPUs are not as versatile as CPUs and some graphics tasks may still need to go through CPUs or other specific processors.

In the media business, GPUs are used in a wide range of image processing equipment – such as picture color correctors and graders as well as the high-end television graphics equipment including that for live ‘3D’ graphics and virtual sets.

Grading (a.k.a. color timing)

Grading is the process of adjusting the color of a clip to get the best out of the material or to match shots perhaps taken at different times or in different lighting conditions. With film, grading was traditionally performed when going from internegative to print film by controlling the exposure of the film. In television it was traditionally done off the telecine for commercials or tape-to-tape for longform programs. Either way, both processes were, by their nature, linear.

The advent of non-linear grading systems (such as Quantel’s Pablo Rio) has changed the rules for color grading and correction. While there is still a requirement for an initial technical scan for film-originated material, from this point on grading can – and often does – happen at multiple stages in the post production process. For example, color correcting individual layers within multilayer composite shots (which may be shot under different lighting conditions) to ensure that the result is harmonious within itself. In addition, non-linear editing means that scene-to-scene comparisons and corrections can be made as the edit unfolds.

This eases the final grading process when the finished work is reviewed interactively with the director/client.

Secondary color correction is aimed at controlling a particular color or a narrow range of colors – such as those on a car or product. Here typically the hue, gain and saturation can be changed. There are also several methods available for defining the object, area or ‘window’ that requires color correction such as using wipe-pattern shapes, drawing an electronic mask by hand or a combination of automatic and by-hand methods. Some of the most sophisticated tools are provided by media workstations such as Quantel’s Pablo Rio.

See also: Film scanner, Telecine

Grain management

Controlling the amount of ‘film’ grain visible on a film or digital movie. Its appearance is considered by some to add a certain look to the production. Modern DI equipment can include grain management that can increase or decrease its visibility on film or digitally originated material. Aside from aesthetics, grain affects compression systems as they see it as extra movement and so can waste bandwidth by coding it – adding another reason for controlling the amount of grain according to the different coding requirements for, say, digital cinema and mobile reception.

Granularity

Term describing limits of accuracy or resolution. For example, in editing the granularity of uncompressed component video is one frame; it can be cut on any frame boundary. The granularity of long GOP MPEG-2 is about half a second; about 12 or 15 frames for 25 or 30 f/s television. In a digital imaging system the granularity of brightness is the minimum change per sample, corresponding to the effect of one LSB change.

GSN

Gigabyte System Network was developed in 1999 for the efficient movement of large quantities of data. Applied to video, it allows realtime transfers of larger-than-HD images on a network. GSN allows transfer speeds up to 800MB/s, has low latency, and is an ANSI standard compatible with HIPPI, Ethernet, and other standard networks, providing full-duplex data transmission over up to 200m. It was not widely used as there were competing standards. Today its speed is overtaken by the later versions of Fibre Channel and SCSI, which are part of the mass computer market, so low priced.

GUI

Graphical User Interface. A means of controlling or operating a system through the use of interactive graphics displayed on a screen. Examples in the computer world are the Apple Macintosh and Microsoft Windows, both designed for general-purpose use and usually operated with a mouse as the pointing device.

In 1981 Quantel introduced Paintbox with its on-screen menu system operated from a pressure sensitive pen and touch tablet. This purpose-built control has been further developed to cover a wide range of operations including DVEs, editing, VTR control, color grading and audio, and today is applied to the whole range of Quantel products. Besides its success in offering fast and effective control, the GUI also enables easy updates to accommodate new facilities.