Digital Technology IB 12. Decimal Numbers. Example: 452 = 4 x x x 1. Symbols: 0-9. Place Values: multiples of ten.

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1 Technology Decimal Numbers Symbols: 0-9 Place Values: multiples of ten Binary Numbers Symbols: 0 and 1 Place Values: multiples of two Least-significant bit (LSB) Most-significant bit (MSB) Example: 452 = 4 x x x 1 Examples: = 1 x x x x x 1 = 19 0 = 0 1 = 1 2 = 10 3 = 11 4 = = = = = = = = = = = = = = = = = signal: potential difference is either High (1) or Low (0) signal: potential difference varies continuously with time Method of storage Typical information Overview of process or Photocopying Text or pictures Optics and electrostatics used to fix powder to paper LPs (vinyl) Music or speech Sound variations stored as grooves in vinyl Cassette tapes Music or speech Sound variations stored in magnetic fields on tape Floppy disks Hard disks All forms Bits stored as variations in magnetic fields on disk CD, DVD All forms Bits stored as series of optical bumps to be read by laser 1

2 Storing Information on CDs A CD is a fairly simple piece of plastic, about four one-hundredths (4/100) of an inch (1.2 mm) thick. Most of a CD consists of an injection-molded piece of clear polycarbonate plastic. During manufacturing, this plastic is impressed with microscopic bumps arranged as a single, continuous, extremely long spiral track of data. Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc, covering the bumps. Then a thin acrylic layer is sprayed over the aluminum to protect it. The label is then printed onto the acrylic. A cross section of a complete CD (not to scale) looks like this: The elongated bumps that make up the track are each 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers high. (A nanometer is a billionth of a meter.) Looking through the polycarbonate layer at the bumps, they look something like this: You will often read about "pits" on a CD instead of bumps. They appear as pits on the aluminum side, but on the side the laser reads from, they are bumps. The incredibly small dimensions of the bumps make the spiral track on a CD extremely long. If you could lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide and almost 3.5 miles (5 km) long. Reflection from bump or pit: Constructive interference Interpreted as 0 Reflection from edge of pit: Destructive interference Interpreted as 1 Condition for destructive interference: height of bump/depth of pit = λ/4 So path difference between two light beams is λ/2 Example: What wavelength of laser light should be used to read the data shown encoded at right? 600 nm 2

3 Advantages of digital storage of information over analogue storage Quality Reproducibility Retrieval speed Portability of stored data Manipulation of data (editing) Output can be virtually indistinguishable from input Use of laser ensures that each retrieval is virtually identical since light does not damage surface Very high speed different section can be accessed randomly Miniaturization techniques ensure that large quantities of data can be stored in a small device (eg. flash drives) Easily achieved with little corruption of data (eg. Photoshop) Output can be virtually indistinguishable from input but is more liable to damage or corruption (eg. scratches on LPs) Process of retrieval often affects quality of future retrievals (eg. needle may scratch LP) Slow retrieval speed data needs to be retrieved in sequential order Storage devices usually take up much more space All manipulation increases possibility of data corruption Implications for society of ever-increasing data storage capabilities Moral/Ethical Information that is potentially problematic can easily be shared (eg. terrorism, crime) Issues concerning ownership of electronic data (eg. piracy) Privacy concerns Social Use in documenting abuses of human rights Unequal access to the Internet Control of information and opinions Economic Quicker access to information needed to make economic decisions (eg. price comparisons) Rise of new businesses and decline of older ones Environmental Reduction in use of paper and other materials used traditionally to stored information Recycling of electronic junk is problematic due to more dangerous materials used in their manufacture 3

4 Data Capture using CCDs Charge-Coupled Device (CCD): a silicon chip divided into small area called pixels. Each pixel can be considered to behave as a capacitor (a device that stores charge). The CCD is used to electronically record an image focused onto its surface. Image capture: Incident light causes charge to build up within each pixel due to the photoelectric effect. An electrode then measures the potential difference developed across each pixel and converts it into a digital signal. The position of the pixel is also stored. Image retrieval: Since both the location and potential difference of each pixel are recorded, all the information needed to store and reconstruct the image is saved. Each p.d. can be converted to a digital signal and the digital signals can be converted to an image. The intensity of the light at each pixel, and thus the image, can be reconstructed. Capacitance: the ratio of charge to potential difference Formula: C = Q/V Units: C/V or farad (F) Type: Scalar 1. The potential difference measured across a 100 pf pixel is 25 mv. Determine the charge and number of electrons stored in the pixel. Quantum Efficiency: ratio of the number of photoelectrons emitted to the number of photons incident on the pixel 2. If the quantum efficiency of the pixel described above is 90%, how many photons were incident on it? 4

5 Magnification ratio of the length of the image on the CCD to the length of the object 3. A digital camera is used to photograph an object that is 3.0 x 10-1 m 2 in area. The image that is focused onto the CCD is 4.5 x 10-3 m 2. What is the magnification of the camera? Resolution: the ability to distinguish between two sources of light Applied to CCD: two points on an object may be just resolved on a CCD if the images of the points are at least two pixels apart 4. The CCD of a digital camera has a square image collection area that measures 25 mm on each side and a resolution of 5.0 megapixels. An object that is photographed by the camera has an area of 4.6 x 10-3 m 2. The image formed on the CCD has an area of 1.0 x 10-4 m 2. a) Calculate the magnification. b) Estimate the length of a pixel on the CCD. c) Two small dots on the object are separated by a distance of 0.20 mm. Deduce whether the images of the dots will be resolved. 5

6 Effect on quality of processed image Quantum efficiency The greater the quantum efficiency, the greater the sensitivity of the CCD. Magnification A greater magnification means that 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 clarity of the image and the amount of detail recorded. Some advantages of using CCDs compared with the use of film: a) Each photo does not require film and is thus cheaper and uses less resources/produces less waste. b) Traditional film has a quantum efficiency of less than 10% while CCDs can have quantum efficiencies of over 90%. This means that very faint images can be photographed with CCDs. c) The image is digital and can be stored and edited more easily. d) Images can be viewed immediately with no processing time delay. e) Storage, archiving, and retrieving a large number of photos is easy and efficient. CCDs are used for image capturing in a large range of the electromagnetic spectrum (not just visible light). cameras Video cameras Telescopes Medical X-ray imaging Very convenient to take and share photographs, but image quality can be less than that of traditional film unless the camera is of high quality (more expensive). Digitized images are usually better quality than analogue images stored on magnetic videotape and are easier to store and transport. It is possible to continuously record video without interruption during playback. Searches are faster and easier to perform. storage is fast and utilizes re-usable media, an advantage for security cameras. Sensitivity of CCDs is better than traditional film and allows for detailed analysis over a range of frequencies. CCDs also allow for remote operation of telescopes, both ground-based and in orbit, like the Hubble space telescope. X-rays have better contrast and can be processed, allowing for enhancements and detailed study. Information can be quickly shared between hospitals and more easily stored and retrieved. 6