Thursday, 10 April 2008

History of LCD

Liquid crystal was discovered by the Austrian botanist Fredreich Rheinizer in 1888. "Liquid crystal" is neither solid nor liquid (an example is soapy water).

In the mid-1960s, scientists showed that liquid crystals when stimulated by an external electrical charge could change the properties of light passing through the crystals.

The early prototypes (late 1960s) were too unstable for mass production. But all of that changed when a British researcher proposed a stable, liquid crystal material (biphenyl).

Today's color LCD TVs and LCD Monitors have a sandwich-like structure (see figure below).

tft lcd tv structure 1

What is TFT LCD?

TFT LCD (Thin Film Transistor Liquid Crystal Display) has a sandwich-like structure with liquid crystal filled between two glass plates.

tft lcd monitor structure 2

TFT Glass has as many TFTs as the number of pixels displayed, while a Color Filter Glass has color filter which generates color. Liquid crystals move according to the difference in voltage between the Color Filter Glass and the TFT Glass. The amount of light supplied by Back Light is determined by the amount of movement of the liquid crystals in such a way as to generate color.

TFT LCD - Electronic Aspects of LCD TVs and LCD Monitors

Electronic Aspects of AMLCDs

The most common liquid-crystal displays (LCDs) in use today rely on picture elements, or pixels, formed by liquid-crystal (LC) cells that change the polarization direction of light passing through them in response to an electrical voltage.

As the polarization direction changes, more or less of the light is able to pass through a polarizing layer on the face of the display. Change the voltage, and the amount of light is changed.

There are two ways to produce a liquid-crystal image with such cells: the segment driving method and the matrix driving method.
The segment driving method displays characters and pictures with cells defined by patterned electrodes.

The matrix driving method displays characters and pictures in sets of dots.

Direct vs. multiplex driving of LCD TVs.

direct vs. multiplex driving of lcd tv

The segment drive method is used for simple displays, such as those in calculators, while the dot-matrix drive method is used for high-resolution displays, such as those in portable computers and TFT monitors.

Two types of drive method are used for matrix displays. In the static, or direct, drive method, each pixel is individually wired to a driver. This is a simple driving method, but, as the number of pixels is increased, the wiring becomes very complex. An alternative method is the multiplex drive method, in which the pixels are arranged and wired in a matrix format.

To drive the pixels of a dot-matrix LCD, a voltage can be applied at the intersections of specific vertical signal electrodes and specific horizontal scanning electrodes. This method involves driving several pixels at the same time by time-division in a pulse drive. Therefore, it is also called a multiplex, or dynamic, drive method.

Passive and Active Matrix LCDs

There are two types of dot-matrix LCDs.

Passive-matrix vs. active-matrix driving of LCD Monitors.

passive-matrix vs. active-matrix driving of lcd monitors

In passive-matrix LCDs (PMLCDs) there are no switching devices, and each pixel is addressed for more than one frame time. The effective voltage applied to the LC must average the signal voltage pulses over several frame times, which results in a slow response time of greater than 150 msec and a reduction of the maximum contrast ratio. The addressing of a PMLCD also produces a kind of crosstalk that produces blurred images because non-selected pixels are driven through a secondary signal-voltage path. In active-matrix LCDs (AMLCDs), on the other hand, a switching device and a storage capacitor are integrated at the each cross point of the electrodes.

The active addressing removes the multiplexing limitations by incorporating an active switching element. In contrast to passive-matrix LCDs, AMLCDs have no inherent limitation in the number of scan lines, and they present fewer cross-talk issues. There are many kinds of AMLCD. For their integrated switching devices most use transistors made of deposited thin films, which are therefore called thin-film transistors (TFTs).

The most common semiconducting layer is made of amorphous silicon (a-Si).
a-Si TFTs are amenable to large-area fabrication using glass substrates in a low-temperature (300°C to 400°C) process.

An alternative TFT technology, polycrystalline silicon - or polysilicon or p-Si-is costly to produce and especially difficult to fabricate when manufacturing large-area displays.

Nearly all TFT LCDs are made from a-Si because of the technology's economy and maturity, but the electron mobility of a p-Si TFT is one or two orders of magnitude greater than that of an a-Si TFT.

This makes the p-Si TFT a good candidate for an TFT array containing integrated drivers, which is likely to be an attractive choice for small, high definition displays such as view finders and projection displays.

Structure of Color TFT LCD TVs and LCD Monitors

A TFT LCD module consists of a TFT panel, driving-circuit unit, backlight system, and assembly unit.

Structure of a color TFT LCD Panel:

Structure of a color TFT LCD Panel
  1. LCD Panel
    - TFT-Array Substrate
    - Color Filter Substrate
  2. Driving Circuit Unit
    - LCD Driver IC (LDI) Chips
    - Multi-layer PCBs
    - Driving Circuits
  3. Backlight & Chassis Unit
    - Backlight Unit
    - Chassis Assembly

It is commonly used to display characters and graphic images when connected a host system.
The TFT LCD panel consists of a TFT-array substrate and a color-filter substrate.

The vertical structure of a color TFT LCD panel.

vertical structure of a color TFT LCD panel

The TFT-array substrate contains the TFTs, storage capacitors, pixel electrodes, and interconnect wiring. The color filter contains the black matrix and resin film containing three primary-color - red, green, and blue - dyes or pigments. The two glass substrates are assembled with a sealant, the gap between them is maintained by spacers, and LC material is injected into the gap between the substrates. Two sheets of polarizer film are attached to the outer faces of the sandwich formed by the glass substrates. A set of bonding pads are fabricated on each end of the gate and data-signal bus-lines to attach LCD Driver IC (LDI) chips

Driving Circuit Unit

Driving an a-Si TFT LCD requires a driving circuit unit consisting of a set of LCD driving IC (LDI) chips and printed-circuit-boards (PCBs).

The assembly of LCD driving circuits.

assembly of LCD driving circuits

A block diagram showing the driving of an LCD panel.

block diagram showing the driving of an LCD panel

To reduce the footprint of the LCD module, the drive circuit unit can be placed on the backside of the LCD module by using bent Tape Carrier Packages (TCPs) and a tapered light-guide panel (LGP).

How TFT LCD Pixels Work

A TFT LCD panel contains a specific number of unit pixels often called subpixels.
Each unit pixel has a TFT, a pixel electrode (IT0), and a storage capacitor (Cs).
For example, an SVGA color TFT LCD panel has total of 800x3x600, or 1,440,000, unit pixels.
Each unit pixel is connected to one of the gate bus-lines and one of the data bus-lines in a 3mxn matrix format. The matrix is 2400x600 for SVGA.

Structure of a color TFT LCD panel.

Structure of a color TFT LCD panel

Because each unit pixel is connected through the matrix, each is individually addressable from the bonding pads at the ends of the rows and columns.
The performance of the TFT LCD is related to the design parameters of the unit pixel, i.e., the channel width W and the channel length L of the TFT, the overlap between TFT electrodes, the sizes of the storage capacitor and pixel electrode, and the space between these elements.
The design parameters associated with the black matrix, the bus-lines, and the routing of the bus lines also set very important performance limits on the LCD.

In a TFT LCD's unit pixel, the liquid crystal layer on the ITO pixel electrode forms a capacitor whose counter electrode is the common electrode on the color-filter substrate.

Vertical structure of a unit pixel and its equivalent circuit

Vertical structure of a unit pixel and its equivalent circuit

A storage capacitor (Cs) and liquid-crystal capacitor (CLC) are connected as a load on the TFT.
Applying a positive pulse of about 20V peak-to-peak to a gate electrode through a gate bus-line turns the TFT on. Clc and Cs are charged and the voltage level on the pixel electrode rises to the signal voltage level (+8 V) applied to the data bus-line.

The voltage on the pixel electrode is subjected to a level shift of DV resulting from a parasitic capacitance between the gate and drain electrodes when the gate voltage turns from the ON to OFF state. After the level shift, this charged state can be maintained as the gate voltage goes to -5 V, at which time the TFT turns off. The main function of the Cs is to maintain the voltage on the pixel electrode until the next signal voltage is applied.

Liquid crystal must be driven with an alternating current to prevent any deterioration of image quality resulting from dc stress.
This is usually implemented with a frame-reversal drive method, in which the voltage applied to each pixel varies from frame to frame. If the LC voltage changes unevenly between frames, the result would be a 30-Hz flicker.
(One frame period is normally 1/60 of a second.) Other drive methods are available that prevent this flicker problem.

Polarity-inversion driving methods.

Polarity-inversion driving methods

In an active-matrix panel, the gate and source electrodes are used on a shared basis, but each unit pixel is individually addressable by selecting the appropriate two contact pads at the ends of the rows and columns.

Active addressing of a 3x3 matrix

Active addressing of a 3x3 matrix

By scanning the gate bus-lines sequentially, and by applying signal voltages to all source bus-lines in a specified sequence, we can address all pixels. One result of all this is that the addressing of an AMLCD is done line by line.

Virtually all AMLCDs are designed to produce gray levels - intermediate brightness levels between the brightest white and the darkest black a unit pixel can generate. There can be either a discrete numbers of levels - such as 8, 16, 64, or 256 - or a continuous gradation of levels, depending on the LDI.

The optical transmittance of a TN-mode LC changes continuously as a function of the applied voltage.
An analog LDI is capable of producing a continuous voltage signal so that a continuous range of gray levels can be displayed.
The digital LDI produces discrete voltage amplitudes, which permits on a discrete numbers of shades to be displayed. The number of gray levels is determined by the number of data bits produced by the digital driver.

Generating Colors

The color filter of a TFT LCD TV consists of three primary colors - red (R), green (G), and blue (B) - which are included on the color-filter substrate.

How an LCD Panel produces colors.

How an LCD Panel produces colors

The elements of this color filter line up one-to-one with the unit pixels on the TFT-array substrate.
Each pixel in a color LCD is subdivided into three subpixels, where one set of RGB subpixels is equal to one pixel.
(Each subpixel consists of what we've been calling a unit pixel up to this point.)

Because the subpixels are too small to distinguish independently, the RGB elements appear to the human eye as a mixture of the three colors.
Any color, with some qualifications, can be produced by mixing these three primary colors.

The total number of display colors using an n-bit LDI is given by 23n, because each subpixel can generate 2n different transmittance levels.

LCD vs CRT

The table below shows the main factors differentiating LCD and CRT displays from a users' perspective.

Liquid Crystal Display (LCD)(CRT) Cathode-Ray Tube display
compactbulky
lightweightheavy
low power (c.20W)high power (c.150W)
perfectly sharplimited sharpness; tend to blur more at high brightness, and with age
perfect image geometrytend to suffer from geometric distortions, which may be picture (brightness) dependent, and worsen with age
"consistent" tonal scalestrong bright areas can cause other regions of the picture to dim
excellent text contrastpoor text contrast (bandwidth limited)
do not normally flickerinherently flicker (although peoples sensitivity varies)
contrast/colour change with viewing angleconsistent image irrespective of viewing angle
poor black on dark imagesgood blacks (quality monitor, properly adjusted)
may cause motion-blurusually portray motion well
peak brightness limited by backlight;
photos/videos can appear "flat"
very high (small area) peak brightness possible;
gives "sparkle" and "life" to movies/video/photos
may have or develop "stuck" pixelsnot pixel-based, no problem
fixed inherent resolutionsupport multiple resolutions equally well
maturing technology; cost fallingmature technology; cheap
native interface would be digital (eg. DVI)naturally suited to analog interface
image can be sub-optimal with analog interfacenaturally suited to analog interface

On the basis of image-quality alone, in my opinion LCD is the monitor of choice for "office" and technical/CAD applications (largely text-based, or detailed but colour-non-critical graphics), while CRT still has the upper hand for high-end photographic/art work and for television displays.
Higher-priced LCDs (probably using "In-Plane Switching" "IPS" liquid crystal modes) marketed specifically for pre-press or photographic work should have colours which are less affected by viewing angles for that application (IPS tends to have a less- good black-state -lower contrast- however). "Vertically Aligned" eg "MVA" (Multidomain -VA) boast the darkest blacks, equivalently highest contrast, of any LCD technology, but response time and viewing angle are poorer than IPS.

LCD tests

Clock/phasing for Analog inputs

LCD panels use discrete pixels and ideally should be connected to the image source via a digital interface such as DVI. In an analog video signal from a computer's "VGA connector" the luminance data for one pixel runs into the next in a time continuum. To properly recover the data back into the correct discrete pixels requires accurate synchronisation of a "sampling-clock" in the monitor to the "pixel clock" in the graphics card. Although possible, this can be technically tricky to get right (the VGA output was never designed for this!). Symptoms of incorrect clock/phase settings include blurry or shimmering text, and shimmering on fine crosshatch patterns.The "automatic" adjustment of many LCD monitors is not always successful, and manual tweaking may be needed. It is probably wise to allow the monitor (and PC) to warm up for 5- or 10minutes before running this test.
  • Clock/phasing test screen (for analog-input LCD screens)
For an illustrated explanation of what these controls do, and how to use them, see Clock/Pitch and Phase controls.

Inversion

In liquid crystal pixel cells, it is only the magnitude of the applied voltage which determines the light transmission (the transmission vs. voltage function is symmetrical about 0V). To prevent polarisation (and rapid permanent damage) of the liquid crystal material, the polarity of the cell voltage is reversed on alternate video frames. Unfortunately it is very difficult to get exactly the same voltage on the cell in both polarities, so the pixel-cell brightness will tend to flicker to some extent at half the frame-rate. If the polarity of the whole screen were inverted at once then the flicker would be highly objectionable. Instead, it is usual to have the polarity of nearby pixels in anti-phase, thus cancelling out the flicker over areas of any significant size. In this way the flicker can be made imperceptible for most "natural" images.

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Line-paired RGB sub-pixel
dot-inversion pattern
Row inversion (lower power)
used eg. on laptops

The following test patterns deliberately excite only one polarity-half of the inversion pattern for some common schemes, and one of them should cause your screen to flicker. This is not a fault with the screen, but enables you to find out which inversion scheme your screen uses.

Note: if using an analog-input monitor, the following tests will not be meaningful unless the clock and phasing settings are correct (see previous test).

Warning to anyone who suffers from epilepsy or other extreme flicker-sensitivity: one or more of the tests below is likely to make your LCD flicker at 30 to 40Hz.

  • Pixel dot-inversion
  • RGB sub-pixel dot-inversion
  • Line-paired pixel dot-inversion
  • Line-paired pixel dot-inversion (offset)
  • Line-paired RGB sub-pixel dot-inversion
  • Line-paired RGB sub-pixel dot-inversion (offset)
  • Row-inversion (also known as line-inversion) - likely to be used on laptops
  • Line-paired row-inversion - likely to be used on laptops
  • Line-paired row-inversion (offset) - likely to be used on laptops
  • Dot-inversion (green test)- will flicker with the (non line-paired) dot-inversion schemes
  • Line-paired dot-inversion (green test)- will flicker with either of the line-paired dot-inversion schemes
  • Line-paired dot-inversion (green test)- will flicker with either of the line-paired dot-inversion schemes (offset)

On Internet Explorer, you can switch to full-screen (and back) using F11. You might also want to set the toolbar to auto-hide (right-button menu when in full-screen mode).

Extreme tech:

The inversion pattern for any given screen will inevitably flicker to some extent and is not a fault. If it really flickers a great deal then it may indicate that the common-electrode voltage has not been set up properly. In that case you might also perceive a "dot crawl" effect on plain colours of medium brightness. A grossly mis-set common-voltage will also make your screen more susceptible to temporary 'image sticking' problems.
Common-electrode voltage can sometimes be adjusted by means of an internal preset, or on a manufacturers' configuration screen (adjust for minimum flicker on the inversion-pattern) ...but doing so would almost certainly invalidate your warrantee. Typically, with the optimum setting, the centre of the screen will have minimum flicker on the inversion pattern, while the flicker will increase somewhat towards the left and right edges. If there's a distinct minimum anywhere on the screen, then the setting is pretty close. Note also that the optimum setting is likely to drift over the life of your screen, and may be slightly affected by temperature and the greylevel of the test pattern.
Any adjustments are made at your own risk!

  • Dot-crawl / line-crawl test page

Some diagonal cross-hatch patterns used for shading by CAD programs can interact with inversion and cause objectionable flicker on some LCD monitors - this should only be a problem if the manufacturer has left the common-electrode voltage grossly mis-set. Having seen the severity of the effect on some new monitors (and how easily it could have been tweaked out at the factory), I'm beginnning to form the opinion [January 2005] that in some cases this flicker could argueably be classed as a "fault" with the product.

Laptop LCD screens tend to be optimised for lower power, with some relaxation of image-quality criterion. As well as (often) lower brightness and less saturated colours, laptop screens usually use a 'row inversion' (aka 'line inversion') scheme rather than the dot inversion now universal in desktop screens. If you look closely at a row-inversion LCD, particularly if it is showing a fairly plain, mid-brightness colour, you may see a slight horizontal line interference pattern on alternate lines, which may appear to drift up or down the screen. This is also not uncommon on colour mobile-phone displays, or personal DVD players.

Cross-talk

Owing to the way rows and columns in the display are addressed, and charge is pushed around, the data on one part of the display has the potential to influence what is displayed elsewhere. This is generally known as cross-talk, and in matrix displays typically occurs in the horizontal and vertical directions. Cross-talk used to be a serious problem in the old passive matrix (STN) displays, but is rarely discernable in modern active-matrix (TFT) displays.

A fortunate side-effect of inversion (see above) is that, for most display material, what little cross-talk there is is largely cancelled out. For most practical purposes, the level of crosstalk in modern LCDs is negligible.

Certain patterns, particularly those involving fine dots, can interact with the inversion and reveal visible cross-talk. If you try moving a small Window in front of the inversion pattern (above) which makes your screen flicker the most, you may well see cross-talk in the surrounding pattern.

Different patterns are required to reveal cross-talk on different displays (depending on their inversion scheme). The following patterns may show cross-talk on your screen.

  • Crosstalk 1
  • Crosstalk 2
  • Crosstalk 3
  • Crosstalk 4
These patterns are not comprehensive and should not be used blindy to rate one screen against another. The appearance of a visible cross-talk from any of these patterns does not indicate a "fault condition" with your display!

Colour, and viewing-angle dependence

Colours displayed by LCD screens tends to vary with viewing angle. Please go to the following page to see this effect:
  • Colour and viewing-angle dependence
Most low-price 15" and 17" LCD monitors use the Twisted Nematic ("TN") liquid crystal mode; with these displays the image looks much lighter if looked down on from above, and much darker if looked at from below.

Newer liquid-crystal modes such as Vertical Alignment ("VA") or In-Plane Switching ("IPS") have less viewing- angle dependence, but may suffer slower response times and/or lower contrast.

Greyscale setup

Although not specific to LCD screens, I'm providing a greyscale test-page for completeness.
  • Greyscale alignment page

Refresh rate, response time, flicker and motion-blur

There seems to be a lot of confusion and mis-information on these topics on the web; here's my clarification...

Refresh rate is the rate at which the electronics in the monitor addresses (updates) the brightness of the pixels on the screen (typically 60 to 75Hz). For each pixel, an LCD monitor maintains a constant light output from one addressing cycle to the next (sometimes referred to as 'sample-and-hold'), so the display has no refresh-dependent flicker.
There should be no need to set a high refresh rate to avoid flicker on an LCD.

Response time relates to the time taken for the light throughput of a pixel to fully react to a change in its electrically-programmed brightness. The viscosity of the liquid-crystal material means it takes a finite time to reorientate in response to a changed electric field. A second effect (which has a rather more complicated explanation) is that the capacitance of the LC material is affected by the molecule alignment, and so if a step change is brightness is programmed, as the LC realigns the cell voltage changes and the brightness to which it settles is not quite what was programmed. Unless 'overdrive' (which tries to pre-compensate for this effect) is employed, it may take several refreshes before the light output stablises to the correct value. Response rate for dark-to-light is normally different from light-to-dark, and is often slower still between mid-greys. VESA and others define standard ways of measuring response time, but a single figure rarely tells the whole story.
Manufacturers 'response times' rarely tell the whole story.
Unless combined with a strobing backlight, response times much below 16ms are likely to be of only marginal benefit, owing to more-dominant 'sample and hold' effects (see below),

The visual effect of motion blur is self-explanatory and it is fairly intuitive to realise that a slow pixel response-time will cause this problem. What is less obvious, but at least as important in causing motion-blur, is the 'sample-and-hold' effect: an image held on the screen for the duration of a frame-time blurs on the retina as the eye tracks the (average) motion from one frame to the next. By comparison, as the electron beam sweeps the surface of a cathode ray tube, it lights any given part of the screen only for a miniscule fraction of the frame time. It's a bit like comparing film or video footage shot with low- and high-shutter speeds. Motion-blur originating from sample-and-hold in the display can become less of an issue as the frame (refresh) rate is increased... provided that the source material (film, video, or game) contains that many unique frames. For LCD TV there is significant interest in the industry in strobing (flickering!) the backlight deliberately so as to reduce sample-and-hold motion-blur!

Colour-temperature

Monitors, both flat-panel and CRT, often come with the ability to adjust the colour-temperature, perhaps to 3200K, 5000K, 6500K, or 9300K.
The sRGB standard for PC graphics (along with western television standards) assumes a display colour temperature of 6500K. Under normal circumstances you will get most accurate colour-reproduction using the 6500K setting on your display. Setting too high a colour temperature (eg 9300K) will result in images which are rather blue ("cold", artistically-speaking), while too low a colour temperature (eg 3200K) will make the display look very yellowy.

Common pixel-array sizes, or 'resolution'

NamePixel arrayAspect ratioComment
VGA640×4804:3
SVGA800×6004:3
XGA1024×7684:3
WXGA1365×76816:9 Wide-XGA; used for widescreen LC TV displays (beware: sometimes 1280×768 is called WXGA)
SXGA1280×10245:4This format is "squarer" than the others
WSXGA+1680×105016:10Wide-SXGA (plus a bit more)
UXGA1600×12004:3
WUXGA1920×120016:10Wide-UXGA
QXGA2048×15364:3

Note that, in general, LCD panels in LC-TV products typically have much lower resolution (i.e. fewer, and bigger pixels) than similarly-sized LCD computer monitors.

Television-standard based pixel-array sizes

In 1981, the 'Rec.601' standard was finalised for the digitisation of television signals (for studio use). It is based upon a 13.5MHz-sampling rate of existing analog television signals. From this, follow a number of oft-quoted resolutions (note the number of active-lines is less than the system number of lines; "625-line" standards have 576 active lines, and "525-line" standards have 480 active lines):

NamePixel arrayAspect ratioComment
PAL (625/50)720×5764:3 (or 16:9)NB. Pixels are not 'square'
NTSC (525/60)720×4804:3 (or 16:9)NB. Pixels are not 'square'
CIF (PAL-based)352×2884:3"Common Intermediate Format" - essentially half the lines and columns of full-res PAL
CIF (NTSC-based)352×2404:3"Common Intermediate Format" - essentially half the lines and columns of full-res NTSC
QCIF (PAL-based)176×1444:3Quarter-CIF
QCIF (NTSC-based)176×1204:3Quarter-CIF
QCIF+176×2204:5Portrait

Some mobile phone or PDA-type displays are specified from the CIF family. Whether CIF/QCIF formats have square pixels or not probably depends on the application in mind!

For interest, modern cinema films are made either in 'widescreen' 1.85:1 or 'scope' 2.35:1 aspect ratios. Widescreen television standardised on 16:9 (1.78:1), so 'scope' films still have narrow black bars at the top and bottom when presented in a TV 16:9 frame. Widescreen-films (1.85:1) are usually shot with sufficient excess head- and footroom to fill a 16:9 TV frame when required.

DVDs are mastered in either 4:3 or 16:9 frames, in both cases PAL-format DVDs have 576 active lines vertically (NTSC DVDs will have 480 active lines). The horizontal pixel-count for DVDs is universally 720 pixels (NTSC and PAL) for both 4:3 and 16:9 ratios; television and DVD pixels are not square!
In principle, a PAL 16:9 DVD should display without need for vertical scaling on a square-pixel display 1024 pixels wide (576×16/9 = 1024).
Owing to non-square pixels, and the fact that analogue processing does not define (or maintain) horizontal resolution of video-material precisely, it may be less worthwhile trying to match horizontal pixel-count for display.

HDTV pixel-array sizes

Modern HDTV standards are based upon square pixels, and 16:9 aspect ratio. We have "720" formats, based upon 720 active scan lines and 1280 pixels per line (1280×720) and "1080" formats based upon 1080 active scan lines (1920×1080). Interlaced and progressive versions exist. Both 50Hz and 60Hz refresh variants are standardised, as is a 24Hz progressive scan (directly compatible with film). There are many good engineering reasons to avoid interlaced systems, but they seem to live on anyway. It's looking as though most equipment, both professional and domestic, will handle most of the common variants natively.

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