Category Archives: l

Latency (of data)

The delay between requesting and accessing data. For disk drives it refers to the delay due to disk rotation only, even though this is only one of several factors that determines time to access data from disks. The faster a disk spins the sooner it will be at the position where the required data is under the replay head. As disk diameters have decreased so rotational (spindle) speeds have tended to increase but there is still much variation. Modern 2.5-inch drives typically have spindle speeds of between 7,200 and 15,000 RPM, so one revolution is completed in 8 to 4 ms respectively. This is represented in the disk specification as average latency of 4 or 2 ms.

For solid-state ‘drives’ (SSD) the latency is much less resulting in faster access to data, particularly if the data is scattered around the store; here, SSD can be over 100 times faster than HDD, but just a few times faster for large unscattered data files.

Latitude

Latitude is the capacity of a camera to capture information over a wider brightness range than is needed for the final print. This provides a degree of freedom in post production for adjustment to match other shots, or for creating keys or adding digital effects.

See also: HDR

Layer(ing)

A collection, or ‘pack’ of video clip layers can be assembled to form a composite layered clip. Layers may be background video or foreground video with their associated matte run. The ability to compose many layers simultaneously means the result can be seen as it is composed and adjustments made as necessary.

LCOS

Liquid Crystal On Silicon. An imaging chip technology that has been likened to a cross between LCD and DLP. Like LCDs this uses one liquid crystal per pixel to control the light, but whereas LCD is transmissive, the light travels through the crystals, LCOS appears as a reflective chip, like DLP. LCOS is the basis for many imagers, JVC’s implementation is D-ILA and Sony’s SXRG appear to use some similar ideas – though many refinements are used.

Letterbox

A method used to show higher aspect ratio (e.g. 16:9) images on a low aspect ratio (e.g. 4:3) display. While all the contents of the pictures can be seen there are strips of (usually) black above and below the picture which some people do not like. Now that nearly all viewers have 16:9 screens, the use of letterbox is passing into history.

Letterbox

See also: 14:9, Anamorphic, ARC

Linear (editing)

The process of editing footage that can only be accessed or played in the sequence is was recorded. Tape and film are linear and they have to be spooled for access to any particular material and can only play pictures in the order they are recorded.

With spooling, jogging and pre-rolls, so called ‘mechanical considerations’, absorbing upwards of 40 percent of the time in a VTR edit suite, linear editing is slow for everyday editing. The imposition of having to record items to an edit master tape in sequence limits flexibility for later adjustments: e.g. inserting shots between existing material may involve either starting the job again or re-dubbing the complete piece. For simple changes however, linear suites are still fast for tape-based material, but random access storage, solid state chips or hard disk drives, provide a far faster and more flexible platform for editing.

See also: C-mode, Digital disk recorder, True random access

Linear (keying)

In linear keying the ratio of foreground to background pictures at any point on the screen is determined on a linear scale by the level of the key (control) signal. This form of keying provides the best possible control of key edge detail and anti-aliasing. It is also essential for the realistic keying of semi-transparent effects such as transparent shadows, through-window shots and partial reflections.

See also: Keying

LSB

Least Significant Bit. In a binary number, this is the final digit – 0 or 1 – and is the least significant because changing it has the smallest effect on the whole number represented compared with changing any of the other digits in the binary number. However, even with their relative insignificance, LSBs can have a significant impact if not cared for properly, particularly when multiplying two binary numbers together – which is why Quantel invented Dynamic Rounding.

See also: Dynamic Rounding, Binary, Dither, Truncation, Digital Mixing, MSB

LTC

Longitudinal Timecode. Traditionally, timecode has been recorded along a linear track on videotape. It was recorded and read by a static head on videotape recorders so it could be easily read when the tape is moving forwards or backwards, but nothing was read during a freeze frame – when VITC, timecode recorded with the picture material, was still working. Today, with tape use now falling, actual linear TC is not so often used. However timecode is still in wide used to identify and access video frames.

See also: VITC

LTO-5, LTO-6

Linear Tape-Open. An open magnetic tape data storage technology started in the late 1990s. The standard form factor is called ‘Ultrium’ and uses linear, not helical , recording. In 2000 an LTO-1 Ulitrium cartridge could store 100 GB. There is a development plan guided by the Linear Tape-Open (LTO) Program Technology Provider Companies HP, IBM and Quantum. In 2012 it reached LTO-6, offering 2.5 GB storage per cartridge, with a maximum data speed of 160 MB/s. There is also a lossless 2.5:1 LTO-DC data compression scheme available (not suitable for video). In the video industry LTO is used for video archive and transfer.

LTO-7 is immanent, offering 6.4 TB native capacity before using the 2.5:1 compression, and 315 MB/s maximum date speed. The LTO development plan continues, stretching to LTO-10.

See also: SAIT-2

Website: www.quantum.com

Luminance

A component of video: the black and white or brightness element, of an image. It is often represented as Y, so the Y in Y,B-Y,R-Y, YUV, YIQ and Y,Cr,Cb is the luminance information of the signal.

In a color TV system the luminance signal is usually derived from the RGB signals originating from cameras, by a matrix or summation of approximately:

Y = 0.3R + 0.6G + 0.1B (based on ITU-R BT.601)

There are other similar equations from different TV standards. The precise values depend on the color primaries of the display standard used.

See also: RGB, Y (B-Y) (R-Y), Y,Cr,Cb, YUV

LUT

LUT is the shortened description of the term ‘Look-Up-Table’. In its simplest form it consists of a finite number of  positive integer input values which map to new output values. Typically a LUT will take 2n (64, 128, 256…) input values from 0 to 2n – 1 and map the input to the new output. Although they can be any size required. For example, a small LUT of size 16 might look like this when describing a math function where the output = input2 :

InputOutput
00
11
24
39
416
525
636
749

etc.

The example shows positive integer address (input) producing an integer Output. This need not be the case; the output could be floating point or any number form needed.

Although the sample LUT above is a mathematical model this does not have to be the case as the input can refer to any value the user desires so LUTs are very convenient where there may be no distinct or a very complex connection between the input values and the output.

LUTs are very common in hardware based systems as they can be used to simplify a mathematical process to map Input address to Output very efficiently. With today’s high performance GPU math processes the developer needs to consider whether the LUT is as efficient as just doing the math on each value.

Inputs to LUTs are typically positive integers. It is possible to engineer a more complex form of LUT which takes a floating point input. Here the LUT is used to look up integer values above and below the Input value and the final result is interpolated from there. For example:

Input value = 3.4

For LUT Input value 3 the Output = 27.5 (these values come from some other complex process)

For LUT Input value 4 the Output  = 29.5

The final output will be 27.5 + (29.5 – 27.5) * 0.4 = 28.3.

Thus a complicated math process can be approximated by a LUT and a simple interpolation.

This process can be expanded further into multi-dimensional LUTs where the Output is a result of multiple Input values.

In image processing systems LUTs can be used for gamma correction, color space conversion (rgb to XYZ, yuv to rgb) and color correction for example.

See also: Color cube