In practice, a modem is connected to the serial port of the computer, or plugged in to the bus lines. Software is used to set up the modem and the port for the required data rate, the telephone number to use, and so on. When a communication is to be sent, the software will make the modem open the line and dial the number, and when the remote computer answers, the two confirm connection and the data is sent. At the other end, the modem has responded to the ringing tone and has opened the line. The signals from the transmitting modem are analyzed, the receiving modem automatically sets itself to use the same system. It then acknowledges contact, and receives the signals, which are usually stored as a file on the disk. The modem can be located inside the computer or externally.

Broadband works by using separate frequency ranges over the telephone lines. The lowest frequencies are used exclusively for telephone conversations. The higher frequencies are reserved for broadband signals, and at each telephone socket you need to use a microfilter that separates the frequencies. The microfilter plugs into the phone socket, and your telephone plugs into the telephone socket on the microfilter. Your computer is connected (by way of a modem or, more usually, a modem–router) to the other outlet on the microfilter. The system that is used for domestic broadband is called ADSL (asymmetric digital subscriber line), and the asymmetric part refers to the rates: fast for downloading data to your computer, but slow for uploading from your computer to a website (since you need downloading much more than uploading). The arrangement of frequency bands is illustrated in Figure 14.3.

The predominant system for modulation in the UK is called DMT ( discrete multitone). As the name suggests, the available bandwidth for ADSL is used by a set of sub-carriers that are spaced 4.3125 kHz apart. Each of these sub-carriers will be modulated by a portion of the digital data, and sub-carriers that are not required will be suppressed so as to save total bandwidth. This approach is similar in principle to the methods used for digital radio, and the usual digital processing methods are used to ensure high efficiency and signal integrity.

Given then that you can get the modulated broadband signals from the microfilter, what then? You could use a broadband modem, but most users nowadays go one step further and install a modem– router. As the name suggests, the router connects to the broadband socket of the microfilter and passes demodulated data signals to and from one or more computers. The advantage of this is that you can connect more than one computer to the broadband (and to each other), so setting up a home network. In addition, if the router is of the wireless type you have the choice of connecting by way of a WiFi signal as well as by cables.

The most significant advantage, however, is in security. Modems are not truly secure devices, but routers are much better and if you enable the firewall of the router and use the highest level of security for any WiFi connection then you can be reasonably relaxed about security (though Windows users should ensure that they have an effective virus detector such as AVG Free).

Though ADSL through existing telephone lines is the most favored system in the UK at the time of writing, delivering speeds up to 8 Mb/s in areas close to a telephone exchange, it is not the only method of obtaining broadband. In areas that are serviced by cable television (using fiberoptic cable with wired connection to each household) higher speeds and more reliable connection can be achieved. At the time of writing there is a proposal to upgrade all existing telephone lines, starting with a few selected towns, to fiberoptic cables that are laid in the same conduits as the existing copper lines. This should bring much higher speeds (50 Mb/s or more) for these areas, and the hope is that this will be extended over much of the UK.

Another broadband system uses the mobile phone network. The cost may based on the amount of data downloaded each month (ranging from 5 GB upwards) rather than on speed, or a combination of speed (typically 7 Mb/s) and download limit (0.5–5.0 GB typically) can be quoted. The number of providers and options makes it very difficult to decide what is a reasonable deal for any given user. The technology is simple: a universal serial bus (USB) fitting adapter (a dongle) contains the mobile phone SIMM chip (single in-line memory module; the data chip for the mobile phone) and is plugged in to your computer. All the software supply and setting up are then carried out for you and the security risks are less than for a fixed line system. This type of connection is intended primarily for those who must use the Internet on the move (despite the signal fluctuations from place to place) and the cost makes it less attractive to home users.

The last option is broadband by satellite, using the incoming satellite signals to carry the downloaded data and a fixed telephone line to upload. Prices are comparable to ADSL, with the advantage that the signal can be better in areas that are distant from a telephone exchange. For business users for whom cost is less of a factor, a transmitting satellite dish can be used to provide faster uploading.

The alternative to these on–off video signals is to use the analog type of signal that is used for television monitors, in which each signal for a color can take any of a range of sizes or amplitudes, not just on or off. This method allows you to create any color, natural or unnatural, by controlling the brightness of each of the primary colors individually. A monitor like this is much closer to television monitors in design, and some (but certainly not all) monitors of this type can be used to display pictures from sources such as television camcorders and video recorders. Monitors for a modern PC should allow the use of resolutions up to around 1600 × 1200. The color capabilities depend on the number of bits used to code each pixel, and the relationship is that the number of colors that can be displayed is 2 ^N, where N is the number of bits per pixel. Twenty-four-bit color is a common requirement for applications such as digital photography, and this corresponds to 16,777,216 colors, usually referred to as 16 million colors. Thirty-bit and 32-bit color systems are also used for critical applications.

The bubblejet system, devised by Canon, uses a tiny heating element in each jet tube, and an electric current passing through this element will momentarily vaporize the ink, creating a bubble that expels a drop of ink through the jet. The piezoelectric system, devised by Epson for its Stylus™ models, uses for part of the tube a piezoelectric crystal material which contracts when an electrical voltage is applied, so expelling a drop of ink from the jet. This latter system has been developed to allow for high-resolution printing of 720 dots per inch or more, and bubblejet models of high resolution are also available.

Both types of inkjet can print in color (using colored inks in addition to black), with resolution of 600 dots per inch or more, and at speeds of, typically, 12 pages per minute of text. Color printing is always much slower than monochrome. Inkjet printers are cheap, but the manufacturers recover their investment by charging very high prices for the ink cartridges. Combination inkjet printer/scanner machines (often referred to as all-in-one) can be used as copiers.

The heart of a laser printer is a drum which is wider than the width of the paper the machine will use and made from light-sensitive material. This drum is an insulator, so that it can be electrically charged, but the electric charge will leak away in places where the drum has been struck by light. The principle of the laser printer (and the photocopier) is to charge the drum completely and then make the drum conductive in selected parts by being struck by a laser beam. The beam is switched on or off and scanned across the drum as the drum rotates, all controlled by the pattern of signals held in the memory of the printer, and enough memory must be present to store information for a complete page. A schematic for a typical laser printer is shown in Figure 14.5.

This system requires about 0.5 Mbyte of memory as a minimum for text work, and 2 Mbyte or more if elaborate high-resolution graphics patterns have to be printed. Color laser printers can use typically 64 MB maximum. Once the scanning process is complete, the drum will contain on its surface an electrical voltage ‘image’ corresponding exactly to the pattern that exists in the memory, which in turn corresponds to the pattern of black dots that will make up the image. Finely powdered resin, the toner, will now be coated over the drum and will stick to it only where the electric charge is large, i.e. at each black dot of the original image.

The coating process is done by using another roller, the developing cylinder, which is in contact with the toner powder, a form of dry ink. The toner is a light dry powder which is a non-conductor and also magnetic (some machines use a separate magnetic developer powder), and the developing cylinder is magnetized to ensure that it will be coated with toner as it revolves in contact with the toner from the cartridge. A scraper blade ensures that the coating is even. As the developing cylinder rolls close to the main drum, toner will be attracted across where the drum is electrically charged, relying on the electrical attraction being stronger than the magnetic attraction. Note that two forms of attraction, electrostatic and magnetic, are being used here.

Rolling a sheet of paper over the drum will now pass the toner to the paper, using a corona discharge to attract the toner particles to the paper by placing a positive charge on to the paper. After the toner has been transferred, the charge on the paper has to be neutralized to prevent the paper from remaining wrapped round the drum, and this is done by the static-eliminator blade. This leaves the toner only very loosely adhering to the paper, and it needs to be fixed permanently into place by passing the paper between hot rollers which melt the toner into the paper, giving the glossy appearance that is the mark of a good laser printer. The drum is then cleared of any residual toner by a sweeping blade, recharged, and made ready for the next page.

The main consumables of this process are the toner and the drum. The toner for laser printers is contained in a replaceable cartridge, avoiding the need to decant this very fine powder from one container to another. The resin is comparatively harmless, but all fine powders are a risk to the lungs and also carry a risk of explosion. Drum replacement will, on average, be needed after each 80,000 copies, and less major maintenance after every 20,000 copies. Paper costs can be low because any paper that is suitable for copier use can be used; there is little advantage in using expensive paper, and some heavy-grade paper may cause problems of sticking in the rollers.

Some models use a separate developer powder (a set of negatively charged and magnetic beads) in addition to toner, and the developer will have to be replenished at some time when the toner is also exhausted. The Hewlett-Packard LaserJet machines use a cartridge which contains both the photoconductive drum and the toner in one package, avoiding the need for separate renewal. The life is typically quoted at about 3500 sides at the average print density of word-processed text, but this figure will be drastically reduced if you print a lot of dense graphics and fonts. Long-life cartridges are available.