14.1 Analogue and digital signalsEdit
14.1.1 Solve problems involving the conversion between binary numbers and decimal numbersEdit
In the binary system, only 0 and 1 are used to describe a number. Each digit, from the right end, corresponds to 20, 21, 22, and so on.
e.g. 101001(2) = 1×25 + 0×24 + 1×23 + 0×22 + 0×21 + 1×20 = 41.
The first nonzero digit in a number given in a binary form is called MSB (Most Significant Bit), and the last digit in a number given in a binary form is called LSB (Least Significant Bit).
14.1.2 Describe different means of storage of information in both analogue and digital formsEdit
- Cassette tape
- Floppy disk
- Hard disk
- CD (Compact Disk)
- DVD (Digital Versatile Disk)
14.1.3 Explain how interference of light is used to recover information stored on a CDEdit
The inner working of a CD is composed of pits and bumps - or sometimes called islands and bumps. A laser is used to read these pits and bumps (the rings of the CD). As the laser hits a pit, light reflects off it, returning a signal of 1 (constructive interference). However, when the laser hits the edge of the bump, destructive interference takes place (signal of 0) and the incident and reflected ray have a phase difference of wavelength/2. Therefore, the pit depth/bump height can be approximated to wavelength/4.
14.1.4 Calculate an appropriate depth for a pit from the wavelength of the laser lightEdit
pit depth = 1/4*wavelength
14.1.6 Discuss advantage and disadvantage of the storage of information in digital rather than analogue formEdit
- Capacity of data storage is huge
- The access to particular storage data is fast
- The retrieval of data is fast
- The storage is more reliable
- Data can be processed and manipulated by a computer
- Stored data can be copied or erased easily
A serious error with a digital storage is usually catastrophic (data may never be recoverable), while analogue data degrades slowly.
14.1.7 Discuss the implications for society of ever-increasing capability of data storageEdit
Possible considerations are:
- Piracy: Data can be copied without any degradation in the quality of the information, leading to a large market of illegal music and film discs.
- Privacy: Governments and corporations can store a great deal of information about citizens, and this can be accessed easily. For example, police records, mobile phone records, etc.
- Since storage capacity of data is increasing, more information is available to society. This has led to much greater efficiency in many areas including banking and clinical medicine, and has also facilitated communication on a domestic and global level.
- Waste: Digital storage devices are disposed of on a regular basis, especially because of constant technological upgrades. Many are disposed even though they contain materials that can be reused in new discs, like the polycarbonate in CDs.
14.2 Data capture; digital imaging using charge-coupled devices (CCDs)Edit
14.2.1 Define capacitanceEdit
Capacitance (C) is the ability of storing charge. Its unit is F (Farad). Q=CV, where Q is charge flows through the capacitor, and V is voltage.
14.2.2 Describe the structure of a charge-coupled device (CCD)Edit
CCD is a silicon chip with surface area equals to approximately 20x20mm2 to 60x60mm2 covered with light-sensitive elements called pixels (picture elements). Size of each pixel is 5-25x10-6m. Each pixel can be considered to behave as a capacitor.
14.2.3 Explain how incident light causes charge to build up within a pixelEdit
As mentioned before, the pixels behave like a capacitor, which in reality is designed to store charge over a potential difference. When light hits a pixel (of course assuming the light has the minimum frequency), an electron is emitted-the photoelectric effect-and a potential difference is created. Each pixel stores a certain amount of charge and "passes" on the charge in the array of pixels. The intensity of the incident light affects the amount of charge that can be held by each pixel.
14.2.4 Outline how the image on a CCD is digitizedEdit
The resulting change in potential caused by the electron pairs migrating to the relevant electrodes is converted into a digital signal.
14.2.5 Define quantum efficiency of a pixelEdit
Quantum efficiency is defined as the ratio of the number of photoelectrons emitted to the number of photons incident on the pixel; not every incident photon will lead to emission of photoelectron. Usually quantum efficiency is between 70 to 80%.
14.2.6 Define magnificationEdit
Magnification of a CCD is the ratio of the length of the image on the CCD to the length of the object.
Two points on an object will be resolved on a CCD if the images of the points are more than two pixels apart.
14.2.8 Discuss the effects of quantum efficiency, magnification and resolution on the quality of the processed imageEdit
Quantum efficiency; The greater the QE, the greater the sensitivity of the devices. Magnification; a greater magnification means more pixels are used for a given section of the image. the image will be more detailed. Resolution; The greater the resolution the greater the amount of detail recorded. An improvement in resolution will mean a given image will occupy more memory
14.2.9 Describe a range of practical uses of a CCD, and list some advantages compared with the use of filmEdit
Used in... - imaging devices such as photocopiers, fax machines, mail sorters and bar code readers - closed circuit television cameras and video cameras - astronomy (to detect faint/distant objects) - detecting low levels of radiation - testing the effectiveness of drugs binding to their target - X-ray imaging
Also CCD's have QE of 25%-95%, while for film it is only 5-20%
CCD less noisier than film so generates less dark current
14.2.10 Outline how the image stored in a CCD is retrievedEdit
- light from an object is brought to a focus on the collection area - the light incident on the collection area varies in intensity and wavelength - the number of electrons ejected from each pixel will vary from pixel to pixel - the potential change associated with each pixel varies from pixel to pixel - the potential changes across the collection area is a "map" of the image of the object on the collection area - each change of potential difference associated with a given pixel is converted to a digital signal