Input and output devices

Overview

Recall questions on barcode readers, digital cameras, laser printers and RFID, how each works, and the situations each is and is not suited to.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a barcode reader for, and what does a barcode encode?

   Model answer

   A barcode reader is an input device that reads a barcode, a printed pattern of parallel dark bars and light spaces of varying widths, and converts it into the number it represents. It is used to identify an item quickly and reliably, most often at a point of sale or in stock control, where the number read is used as a key to look up the product's description and price in a database. The barcode itself carries only an identifying number, not the price.

2. 2

   Describe the principle of operation of a barcode reader.

   Model answer

   A light source, usually a red laser or an LED, is shone onto the barcode. The white spaces reflect the light strongly while the black bars absorb most of it and reflect weakly. A photodiode or light sensor detects the intensity of the reflected light as the beam sweeps across the code, in a laser scanner by means of a rotating or oscillating mirror, producing a varying analogue voltage. That signal is converted into a pattern of pulses whose widths correspond to the widths of the bars and spaces, and this pattern is decoded into the digits it stands for. A check digit, calculated from the other digits, is then verified so a misread is rejected rather than accepted.

3. 3

   Give the main characteristics and the suitability of a barcode reader.

   Model answer

   It is fast, cheap, accurate and simple to operate with little training, and the labels themselves cost almost nothing to print. Data entry that would take seconds and risk typing errors takes a fraction of a second and is checked automatically. It is therefore highly suitable for retail checkouts, warehouse and library stock control and parcel tracking. It is unsuitable where the label may be dirty, creased, damaged or hidden, because it needs a clear line of sight to a flat, undamaged code, it must be pointed at each item individually, and only a short identifying number can be stored in a one-dimensional code.

4. 4

   Describe the principle of operation of a digital camera.

   Model answer

   Light from the scene passes through a lens which focuses it onto an image sensor, a rectangular grid of millions of tiny light-sensitive elements. During the exposure each element accumulates an electrical charge in proportion to the amount of light falling on it, so brighter parts of the image produce larger charges. Because the elements respond only to brightness, a colour filter array is laid over the sensor giving each element a red, green or blue filter, and the missing colours for each position are later calculated from its neighbours. The charge from each element is converted to a voltage and then digitised by an analogue-to-digital converter, giving a binary value per pixel per colour channel. The camera's processor then applies white balance and sharpening, usually compresses the image, and writes the file to flash memory.

5. 5

   Give the main characteristics and the suitability of a digital camera.

   Model answer

   Its characteristics are its resolution, the number of pixels in the sensor grid; its colour depth, the number of bits used per pixel, which determines how many distinct colours can be recorded; the physical size and quality of the sensor and lens; the optical zoom range; and the capacity of its storage card. Uncompressed file size is the width in pixels times the height in pixels times the colour depth, so higher resolution and depth mean better quality but larger files, and this trade-off governs how many images fit on a card. It is suitable wherever images must be captured, reviewed immediately, copied without loss of quality and transferred electronically, which is why it has displaced film almost entirely. Note that image quality depends on the sensor size and lens at least as much as on the pixel count, so more megapixels do not by themselves mean better pictures.

6. 6

   Describe the principle of operation of a laser printer, in order.

   Model answer

   First, a rotating photosensitive drum is given an even electrostatic charge over its whole surface by a charging roller or corona wire. Second, a laser beam is swept across the drum by a rotating many-sided mirror and switched on and off as it goes, discharging the points it strikes, so that the areas corresponding to the image are left with a different charge from the rest: this is the latent electrostatic image. Third, the drum rotates past the toner cartridge, and the fine, oppositely charged toner powder is attracted to and sticks to the image areas only. Fourth, the paper is given a charge and passed close to the drum, so the toner is pulled off the drum onto the paper. Fifth, the paper passes between heated fuser rollers which melt the toner and press it so that it bonds permanently to the fibres. Finally the drum is discharged and any residual toner scraped off, ready for the next page.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.