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How They Work - Which One to Choose
    The basic computing circuits of a portable work identically to desktop machines, although portables traditionally run at somewhat lower clock speeds to reduce power consumption and heat build-up.
    The biggest difference is the display system. Almost all available portables currently use one of two popular LCD technologies for their display screen: passive matrix (also known as dual-scan or DSTN) and active matrix (also known as thin-film transistor or TFT).
    In both types, a light source behind the screen provides a source of white light. Color filters turn the white into the three colors needed to form full color images. The LCD acts as an array of tiny pixels (picture elements) that block or pass the light through to the viewer. Typically, sizes range from about 640 by 480 pixels for smaller models up to 1024 by 768 for premium models with 13-inch or larger screens.
    With a passive matrix display, the signal needed to switch the state of each pixel is assembled from the row and column signals that meet at that point in the array. This results in signal strength and switching speed that are limited both because it's hard to drive any sharp change in power and by the necessity to keep other pixels from responding as well. As a result, the display may not be able to follow very fast changes in the display content - this is particularly noticeable when you try to move the on-screen cursor rapidly. Another side effect is a imitation in viewing angle. Unless you sit directly in front of the screen the image will seem to fade out.
    With an active matrix display, a transistor is built into the display circuitry at each pixel position, and the signal in each row and column circuit only needs to be powerful enough to turn that transistor on and off. Because that small signal can be changed much more quickly than the larger signal needed for passive matrix, the result is a display that can keep up with video signals and fast cursor movement and that also has higher contrast and a wider viewing angle.
    Older PC-style notebooks required you to add your own pointing device if you wanted to use a point-and-click interface such as Windows. Now you'll find a mouse-equivalent of some sort built in to most models (Macintosh portables have always had pointing devices).
    The most popular current format seems to be the pressure-sensitive touchpad, which uses a resistive membrane to produce a signal corresponding to where you touch the pad (or press it with a stylus). Another alternative is a joystick-like device built into the keyboard. Some models still use trackballs, but the mechanical rotation sensors have been replaced by optical encoders of various sorts(unfortunately, that still leaves various rollers to get clogged up).
    Even larger portables don't have enough space for all the disk drives you might want to use, so many employ a drive bay system that allows you to swap in hard disk drives, CD-ROM drives, floppy-disk drives and optical drives. As with many other portable subsystems, the specifics of the design are unique to each manufacturer, so you'll have to get a drive packaged especially for that particular machine.
    Most smaller portables and some of the larger models have one or more PC Card slots for add-in cards (they used to be called PCMCIA slots, and you may see them referred to this way in some manufacturers literature). You can use these cards to add dozens of different functions ranging from extra memory to exotic machine control interfaces.
    The slots and cards come in three "Types" that vary by their thickness: Type I is 3.3 mm, Type II is 5.0mm and Type III is 10.5mm. The thinner types are usually used for solid-state add-ins such as memory cards. Modems and network interfaces are usually Type II cards, and Type III slots are wide enough for miniature hard-disk drives.
    Some newer portables may also support the CardBus version of the standard, with a 32-bit pathway through the interface and heightened performance specifications.
    Another addition on many notebooks is an infrared communications port. These ports can communicate with printers or other computers with similar ports, either for wireless connection to the printer or to synchronize the files on two computers. So far, the version standardized by the IrDA (Infrared Data Association) offers only limited channel capacity, but in thefuture this pathway may become more important.
    With some notebooks, you'll have a choice of batteries, while others are limited to a single technology. Nickel cadmium (often known as Nicads, although that's actually a brand name) is the least expensive of the popular varieties, and offers moderate performance. Most manufacturers have moved to the nickel hydride variety, which provides more power for the same weight and size and at a slightly higher cost. Premium models use lithium ion, with a still-higher power density and cost.

    Important Features
    If you're using a portable for extensive computing rather than as a simple memo and communications device, you'll want to evaluate the processor and memory capacity much as you would a desktop machine.
    For machines built on the Intel architecture you can already get "Pentium with MMX" and processors running at over 200 MHz, but you'll have to wait until sometime later this year to get a Pentium II designed for portable use. A few machines use Intel's "Mobile Module" CPUs, which include a Pentium MMX chip, cache memory and PCI interface in a single module--which Intel says should be replaceable at some future date with a faster model. Although you don't need the MMX features for most current office applications, it will help out with most new and future multimedia programs.
    Apple claims that its new "G3" PowerPC chip is even faster, with clock rates that match or exceed Intel's fastest Pentiums. But as with desktops, clock speed comparisons within a line are fairly indicative, but comparisons between different chip architectures don't necessarily reflect relative performance. You can also get more modest PowerBook portables with CPU's comparable to Intel's medium-speed chips.
    Most portables require their own format for memory, and you usually pay a premium for it compared to the same chips configured for a desktop machine.
    Internal hard disk drives are slightly more standardized, with some exchange between units. Capacities have already passed 6 gigabytes on some portables, although 1 to 3 gigabytes is more common. But the tiny 2-inch and smaller drives used in most portables aren't the commodity items of their larger desktop cousins, and consequently cost more for the same storage capacity.
    If you're carrying a portable around in a shoulderbag all day, the most important feature by far will be weight. A few full-function portables weigh in under 2 pounds, but most weigh between 2 and 5 pounds. So-called "ultraportables" with reduced-size keyboards and small screens can run under 1 pound.
    In other applications where you plan to use a portable machine away from mains power, battery life can be the most important characteristic. Typically, manufactures claim from 1 to 6 hours of running time.
    In many offices a network connection is essential. Some portables have Ethernet built in, some have an Ethernet connection in a docking station, and Macintosh models have built-in LocalTalk. If you have a free PC Card slot, you can add a card for most common networks.
    Modem links are provided in a similar assortment of internal, add-in, dock-based and PC Card-based configurations. Some portables even offer mounting brackets or cable connections to make it easy to connect their modem to a cellular phone or to use a wireless modem for truly portable connectivity.

Tradeoffs and Buying Advice
    If you're looking for a particular software environment, you can choose a portable computer built to run any of the popular operating systems. However, except for some very specialized applications that require another system, it's likely you'll want either a Windows or MacOS-based system because of their lower cost and greater variety of software and accessories.
    If you're matching a portable to a desktop machine, it's slightly easier to share files or set up networks between machines running the same operating system -- but popular programs for both types of machines usually include file translators.
    As with desktop machines, you'll pay significantly more for a unit with the latest and fastest processor than one that's a bit further from the cutting edge.
    While a fast processor speed has the same advantage in a portable machine as it does in a desktop system, it has a larger design tradeoff. Given the same basic design, running circuits faster consumes more electrical power and produces more heat. Since power is precious in a battery-operated machine and heat is hard to deal with in a compact box, a machine running at modest speeds may sometimes be a better fit. Consequently, if you're looking for a machine to use mostly on battery power, we suggest you go for one of the more moderate speed units, unless you need fast processing for particularly demanding applications and are willing to sacrifice running time. On the PC side, that might be a machine with a Pentium 133 or equivalent running under Windows (even a 33 MHz 486 will do fine for light duty  under DOS). Apple has abandoned the low-end of the portables market, so you won't be able to get a machine with less than a 133 MHz PowerPC processor. On both sides, if you need more power you can more than double the processor speed and more than double the cost.
    Similarly, extra RAM and a large hard drive both make it easier to use large and complex programs on a portable machine, but they also use extra power and cost more--although extra RAM can sometimes save power if it allows you to use the hard disk less frequently.
    An initial memory complement of 16 megabytes and a maximum memory capacity of 16 to 32 megabytes may be enough for light-duty machines. Machines for more challenging applications may need double that amount.
    Whether you want to pick the more economical passive matrix screen technology or the sharper but more costly active matrix approach is largely a matter of budget and personal preference. Some people perceive the difference as minor, while for others it's a major differentiation. If you're not sure, be sure to try out both technologies on the kind of files you're likely to be working with. You're most likely to notice the difference on moving images and pictures where you'll notice small color variations such as close-ups of faces.
    Screen sizes for portable computers range from under 6-inches to more than 12-inches. Because the difficulty of making a flat screen goes up much faster than the size, the larger sizes are significantly more expensive. They're also more breakable. But if you expect to use a portable for long periods at a time, they can be worth the extra cost and risk. In any case, if you expect to run standard software you should look for a screen that shows at least 640 by 480 pixels--while software for both Windows and  Macintosh is supposed to be independent of screen size, many programs do not display properly at lesser resolutions.
    How large a hard disk you'll need depends on your applications. A few hundred megabytes will do fine for wordprocessing and email, but you could need up to several gigabytes for multimedia presentations, mobile databases or graphics work.
    If you're going to be using your portable in the field, you'll want to pay particular attention to battery life. Unfortunately, there's an entire cult of batteries, battery chargers, "conditioners" and exotic battery technologies, but very little objective information. For example, although scientific tests generally find no evidence of a "memory effect" in modern nickel-cadmium batteries of this size range, many users swear they have to discharge this type of battery completely at regular intervals or it won't take a full charge.
    We suggest you consider specified battery life claims as only rough guides rather than specific promises -- running time values supplied by manufacturers usually turn out to be optimistic when compared to average results in the field. The actual time depends heavily on how often your programs access the disk drive, whether you're using a power-saver mode that sacrifices performance for running time, and many other variables. If you need to work for long period away from mains power, you might want to look for models that have easily replaceable batteries or even a model that can also run off ordinary "flashlight" style batteries in a pinch. If you're using a portable at just two locations and you need to use external resources at each location, you may want to look for a model that offers a reasonably-priced docking station. That way, you can leave a docking station connected at each location rather than having to connect cables each time.
    Only a few printers and portables have the IrDA system working yet, but it's a real convenience if you only need to print an occasional document and you don't want to dock the computer or hook up cables.
    Finally, as with buying an automobile, style, fit and finish mean a lot more to some people than others. A keyboard layout or case style that's fine for your coworker can turn out to be a major irritant for you. If you plan to spend much time typing away on  your portable or staring at the screen, make sure you either try out that model beforehand or buy where you can exchange a unit if necessary. Buying a unit that will provide you with the most pleasing working environment is crucial for long term satisfaction.

    * Este material foi copiado da página da PCMagazime.

    Contato: Marcos Dias de Moares