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  • Next few slides require a quick revision of interference of light.
  • Revision of photoelectric effect will be useful for this topic henceforth.
  • Revision of photoelectric effect will be useful for this topic henceforth.

14 14 Presentation Transcript

  • Digital technology 14.1 and 14.2
  • What are binary digits? Computers use binary numbers, and therefore use binary digits in place of decimal digits. The word bit is a shortening of the words " B inary dig it ." Whereas decimal digits have 10 possible values ranging from 0 to 9, bits have only two possible values: 0 and 1.
  • Decimal and binary numbers.
    • You can see that in binary numbers, each bit holds the value of increasing powers of 2.
    • That makes counting in binary pretty easy E.g 1011 means
    • (1 * 2 3 ) + (0 * 2 2 ) + (1 * 2 1 ) + (1 * 2 0 ) = 8 + 0 + 2 + 1 = 11
    • Some more examples
    • 10 = 1010 12 = 1100 13 = 1101 16 = 10000
  • Conversions
    • Change into binary
    • 14
    • 31
    • 46
    • 59
    • Change into decimal
    • 100
    • 11010
    • 10011
    • 101010
  • Bits and bytes.
    • Bits are rarely seen alone in computers. They are almost always bundled together into 8-bit collections, and these collections are called bytes .
    • With 8 bits in a byte, you can represent 256 values ranging from 0 to 255, as shown here:
    • 0 = 00000000 1 = 00000001 2 = 00000010 ... 254 = 11111110 255 = 11111111
  • Bits and bytes continued
    • CD uses 2 bytes, or 16 bits, per sample. That gives each sample a range from 0 to 65,535, like this:
    • 0 = 0000000000000000 1 = 0000000000000001 2 = 0000000000000010 ... 65534 = 1111111111111110 65535 = 1111111111111111
  • Analog at a glance As a technology, analog is the process of taking an audio or video signal (in most cases, the human voice) and translating it into electronic pulses. Digital on the other hand is breaking the signal into a binary format where the audio or video data is represented by a series of "1"s and "0"s.
  • A to D
    • Digital technology breaks your voice (or television) signal into binary code—a series of 1s and 0s—transfers it to the other end where another device (phone, modem or TV) takes all the numbers and reassembles them into the original signal. The beauty of digital is that it knows what it should be when it reaches the end of the transmission.
  • Is the duplication perfect?
    • But like any transferred technology, digital has a few shortcomings. Since devices are constantly translating, coding, and reassembling your voice, you won't get the same rich sound quality as you do with analog.
  • Can we use the digital phone using an analog line?
    • There are digital-to-analog adapters that not only let you use analog equipment in a digital environment, but also safeguard against frying the internal circuitry of your phone, fax, modem, or laptop. Some adapters manufactured by Konexx come designed to work with one specific piece of office equipment: phone, modem, laptop, or teleconferencer. Simply connect the adapter in between your digital line and your analog device.
  • Ancient way of recording the analog way
    • In the Beginning: Etching Tin
    • Thomas Edison is credited with creating the first device for recording and playing back sounds in 1877. His approach used a very simple mechanism to store an analog wave mechanically. In Edison's original phonograph , a diaphragm directly controlled a needle, and the needle scratched an analog signal onto a tinfoil cylinder .
    • http://communication.howstuffworks.com/analog-digital1.htm
  • An analog wave Analog Wave What is it that the needle in Edison's phonograph is scratching onto the tin cylinder? It is an analog wave representing the vibrations created by your voice. For example, here is a graph showing the analog wave created by saying the word "hello": Image from www.howstuffworks.com
  • Analog recording contd….
    • The waveform was recorded electronically rather than on tinfoil, but the principle is the same.
    • What this graph is showing is the position of the microphone's diaphragm (Y axis) over time (X axis). The vibrations are very quick -- the diaphragm is vibrating on the order of 1,000 oscillations per second.
    • Notice that the waveform for the word "hello" is fairly complex.
  • Getting in to the digital world
    • In a CD (and any other digital recording technology), the goal is to create a recording with :
    • very high fidelity (very high similarity between the original signal and the reproduced signal)
    • perfect reproduction (the recording sounds the same every single time you play it no matter how many times you play it).
    • To accomplish these two goals, digital recording converts the analog wave into a stream of numbers and records the numbers instead of the wave.
    • The conversion is done by a device called an analog-to-digital converter (ADC). To play back the music, the stream of numbers is converted back to an analog wave by a digital-to-analog converter (DAC).
    • The analog wave produced by the DAC is amplified and fed to the speakers to produce the sound.
  • CDs and DVDs.
    • http://electronics.howstuffworks.com/cd.htm
    • Exploring Sound: Digital Sound
      • Laser discs such as CDs and DVDs carry digital information, which is represented by the binary code -- combinations of 1s and 0s. Any number can be represented in binary code.
  • APPLICATION -COMPACT DISCS (CD’S)
    • A CD is a fairly simple piece of plastic about 1.2 mm thick.
    • The CD consists of a moulded piece of plastic that is impressed with microscopic bumps arranged as a single, continuous spiral track of data.
    • A thin, reflective aluminium layer is placed onto the top of the disc, to cover the bumps.
  • APPLICATION -COMPACT DISCS (CD’S)
    • A thin acrylic layer is sprayed over the aluminum to protect it.
    • The label is then printed onto the acrylic.
  • APPLICATION -COMPACT DISCS (CD’S)
    • A CD has a single spiral track of data circling from the inside of the disc to the outside.
    • The data track is incredibly small. It is about 0.5 microns wide, with 1.6 microns separating one track from the next.
  • APPLICATION -COMPACT DISCS (CD’S)
    • Due to the extreme thinness, the total length of the track squeezed onto this small disc is about 8 km.
    • The information on the disc is read by shining a laser beam from the underside of the compact disc.
    • Thus the laser is seeing the “bumps”, not the “pits”.
  • APPLICATION -COMPACT DISCS (CD’S)
    • The diagram below gives you some idea of how small a CD “bump” is compared to a human hair.
  • APPLICATION -COMPACT DISCS (CD’S)
    • The sequence of bumps, the length of the bumps and the length of the spaces between the bumps provides the information that the CD player decodes.
  • APPLICATION -COMPACT DISCS (CD’S)
    • The laser that is used is an infrared laser emitting light at a wavelength of 780 nm.
    • The laser passes through the plastic and is reflected off the aluminum coating on the bumps and the land between them.
  • APPLICATION -COMPACT DISCS (CD’S)
    • A very important point is that the height of the “bumps” is approximately one quarter the wavelength of the laser light.
    • When the laser light is passing over the “land”, all of the light is reflected off and it travels back to photoelectric cell.
    • The photoelectric cell then produces an electric current.
  • APPLICATION -COMPACT DISCS (CD’S)
    • This electric current then goes on to generate sound in a loudspeaker (see loudspeaker application).
    • Now lets look at what happens when the laser light approaches a “bump”.
    • When the light reaches a bump, half of the light is reflected off the “bump” and half of the light is reflected off the “land”.
  • APPLICATION -COMPACT DISCS (CD’S)
  • APPLICATION -COMPACT DISCS (CD’S)
    • Because the bump is ¼ of a wavelength in height, the light being reflected off the land travels one half a wavelength further .
    • The light reaching the photoelectric cell coming from the “land” and the “bump” is out of phase . This leads to partial cancellation and a decrease in intensity.
    • This leads to decreased current being produced.
  • APPLICATION -COMPACT DISCS (CD’S)
    • As the laser moves along the track the intensity of the light falling on the photoelectric cell changes every time it comes into approaches or leaves a bump.
    • It is this change in intensity which causes the fluctuation in electric current, which causes the movement of the loudspeaker and ultimately the fluctuation in sound.
  • APPLICATION -COMPACT DISCS (CD’S)
    • USING INTERFERENCE TO KEEP A LASER ON TRACK
    • The musical data on the CD is read from the inside out.
    • The CD spins above the laser.
    • After one revolution, the laser must move to the outside exactly 1.6 microns to remain on track.
  • APPLICATION -COMPACT DISCS (CD’S)
    • This requires a very precise tracking mechanism and an accurate correction mechanism to move the laser back on track if it should stray off the line.
    • The tracking correction is achieved by first passing the laser beam through a diffraction grating, before it reaches the CD.
  • APPLICATION -COMPACT DISCS (CD’S)
    • When the monochromatic light passes through the diffraction grating a central beam and a first order diffracted beam will land on the CD.
  • APPLICATION -COMPACT DISCS (CD’S)
    • The central beam is focused on the track of the CD and passes over the bumps while the two first order diffracted beams are focused on the land on either side of the bumps.
    • One diffracted beam is slightly ahead of the other.
  • APPLICATION -COMPACT DISCS (CD’S)
    • The laser beam is tracking correctly when the central beam is varying in intensity from 35% to 100% and the two diffracted beams have a constant intensity of 100%.
  • APPLICATION -COMPACT DISCS (CD’S)
    • If the laser beam should stray to one side of its correct position, then the variation in intensity of the main beam is now reduced.
    • The leading tracking beam will also have a variation in the intensity because some of it is passing over the bumps.
    • The tracking mechanism “senses” that it must adjust the position of the laser down in order to put it back on track.
  • APPLICATION -COMPACT DISCS (CD’S)
  • APPLICATION -COMPACT DISCS (CD’S)
    • If the laser beam were to stray to the other side of its correct position, then the variation in intensity of the main beam is again reduced.
    • The TRAILING beam will now have a reduction in intensity.
    • The tracking mechanism “senses” that it must adjust its position up in order to get back on track.
  • CD’s and DVD’s
    • Data is stored digitally
      • A series of ones and zeros read by laser light reflected from the disk
    • Strong reflections correspond to constructive interference
      • These reflections are chosen to represent zeros
    • Weak reflections correspond to destructive interference
      • These reflections are chosen to represent ones
  • A CD’s pits and bumps
  •  
  • Reading a CD
    • As the disk rotates, the laser reflects off the sequence of bumps and lower areas into a photodector
      • The photodector converts the fluctuating reflected light intensity into an electrical string of zeros and ones
    • The pit depth is made equal to one-quarter of the wavelength of the light
  • Question
    • If a laser emits light of wavelength 760nm calculate how deep the pits would have to be for data to be stored appropriately?
  • Solution
    • The depth of the pit is d
    • From the bottom of pit the light travels an extra distance of 2d
    • For interference to occur the difference in path travelled needs to be λ/2
    • d = λ/4
    • d = 760/4 nm
  • DVD’s
    • DVD’s use shorter wavelength lasers
      • The track separation, pit depth and minimum pit length are all smaller
      • Therefore, the DVD can store about 30 times more information than a CD
      • If the pit depth on a DVD is 0.4micrometre what laser wavelength is used?
    • In the case of CD sound, fidelity (the similarity between the original wave and the DAC's output ) is an important goal, so the sampling rate is 44,100 samples per second and the number of gradations is 65,536. At this level, the output of the DAC so closely matches the original waveform that the sound is essentially "perfect" to most human ears .
  • Why is a CD’s capacity approximately 750 mb?
    • One thing about the CD's sampling rate and precision is that it produces a lot of data.
    • On a CD, the digital numbers produced by the ADC are stored as bytes , and it takes 2 bytes to represent 65,536 gradations.
    • There are two sound streams being recorded (one for each of the speakers on a stereo system). A CD can store up to 74 minutes of music, so the total amount of digital data that must be stored on a CD is:
    • 44,100 samples/(channel*second) * 2 bytes/sample * 2 channels * 74 minutes * 60 seconds/minute = 783,216,000 bytes (Convert to kbs and then Mbs.)
  • Why store digitally?
    • There are five main reasons we prefer storing data in a digital format rather than analogue
    • Quality
    • Reproducibility
    • Portability
    • Manipulation
    • Retrieval speed
  • Quality
    • Digital is less likely to be corrupted with time.
      • Consider a magnetic tape recording, the field strength will drop with time thus data can be lost. If it was digitally sorted as magnetic field and zero then the boundaries will still be readable even if strength diminishes.
  • Reproducibility
    • With digital data it is possible to assign locations
    • With analogue data it is usual to store alphabetically
  • Portability
    • Analogue storage is sequential and is often large
    • Digital storage allows for large amounts to be stored in pocket sized devices
    • What are the implications on society of the ever increasing storage capacity?
  • Manipulation
    • Digital data is less prone to error in calculations
    • Digital is easier to manipulate and process
  • Capacitance
    • Capacitance is the quantity of charge that can be stored per unit electric potential
    • C = Q/V
    • The unit of capacitance is the Farad, F
    • A Farad is one coulomb per unit potential difference.
    • The capacitance of the Earth is 0.1F so we usually deal with pF
  • Charge couple device (CCD)
    • Structure
    • An individual pixel element consists of three sections as shown in the diagram below.
    • The white bars represent electron-collecting zones of low electric potential, on the surface of very thin semiconducting silicon (5 - 8µm). These electron collecting zones are called the potential energy wells and are where photoelectrons are collected.
    • The coloured bars are zones of higher electric potential that act as barriers to keep the electrons in the potential energy wells. The heights of these potentials can be changed by means of three sets of electrodes shown as green lines which run across the surface of the chip and work together to move the electrons in the potential well along the channels.
    • The pixels are laid out in closely spaced columns or channels, which shows a section of three columns each with three pixels, giving 9 pixels in total.
  • CCD
    • A CCD is basically just like any other silicon microchip except it has an array of pixels which are exposed and are sensitive to light. The size of such an array can vary from about 256 x 256 to 2048 x 2048.
    • Each pixel in this array counts the number of photons which fall upon it. The number of photons counted is stored as electric charge in a capacitor by each pixel. Each time a photon hits a pixel, a small amount of electric charge is added to the capacitor.
  • CCD
    • This process continues until the stored number counts in all of the capacitors are read. This process, called 'readout', happens very quickly in such a way that the data is read line by line until the last line has been read.
    • The CCD is then 'flushed' which means that the charges in all of the capacitors are reset to zero. The charge storage process then repeats.
  • Light and pixels
    • Photons have energy that is proportional to the wave frequency:
    • E=hf ,
    • where h is Planck's constant and f is the wave frequency.
    • In a CCD being exposed, photons enter the chip from the rear. When a photon strikes the silicon, it is very likely to interact with an electron in the thin layer of silicon (approximately 8µm) and give sufficient energy to the electron to displace it from the silicon lattice. This creates an electron-hole pair.
    • The displaced electron, or photoelectron, is collected in the nearest potential well while the hole created by the loss of the electron is eventually filled by an electron from the silicon substrate.
    • The more photons which strike the silicon, the more electron-holes produced and so the more electrons stored within each potential well.
  • Photoelectric effect
    • How does the photoelectric effect apply to CCDs?
    • How much charge would a pixel hold if its capacitance was 30pF and the potential changes by 0.2mV?
    • How many photons hit the pixel if the time for the potential change is 10ms?
  • What is a digital image?
      • Essentially, a digital image is just a long string of 1s and 0s that represent all the tiny colored dots -- or pixels -- that collectively make up the image. If you want to get a picture into this form, you have two options:
      • You can take a photograph
      • You can directly sample … you can use a digital camera
  • What is a digital image?
      • The first line shows the initial state with the coloured bucket three from the left hand side.
      • The second line shows position of the coloured bucket after the first swap. The free bucket at the right hand side contains the charge which is then digitised. The potential formed by the charge is amplified and converted into a digital signal by an Analogue to Digital Converter (ADC).
      • After each swap, the buckets move one place further to the right.
    • Consider a CCD where the maximum output voltage from a pixel, after amplification, is 0.8V. If this is to be converted into a 3-bit binary number, then there will be 8 different quantisation levels
    • e.g. The voltage at the output is 0.42 V. This falls in the quantisation level represented by 4 and will give a digital signal of 100.
  • Capturing image
    • The image sensor employed by most digital cameras is a charge coupled device (CCD). Some cameras use complementary metal oxide semiconductor (CMOS) technology instead. Both CCD and CMOS image sensors convert light into electrons.
    • A simplified way to think about these sensors is to think of a 2-D array of thousands or millions of tiny solar cells.
    A CMOS sensor
  • Digitisation of the light
    • http://www.olympusmicro.com/primer/digitalimaging/concepts/concepts.html
    • Electrode measures the potential difference developed across the pixel and this is then converted into a digital signal
    • Pixel position is also stored
  • Capturing Color
    • Unfortunately, each photosite is colorblind. It only keeps track of the total intensity of the light that strikes its surface. In order to get a full color image, most sensors use filtering to look at the light in its three primary colors. Once the camera records all three colors, it combines them to create the full spectrum
    For illustrations and explanations visit: http://electronics.howstuffworks.com/digital-camera3.htm
  • Definitions
    • You need to know
    • Quantum efficiency
    • Magnification
    • Resolution
  • Quantum Efficiency
    • This is the ratio of the number of photoelectrons emitted to the number of photons incident on the pixel
    • This will never be 100% due to scattering or non-interaction with substrate
    • Quantum efficiency is a measure of the sensitivity of a light detector. Many photoelectric materials typically emit an electron for every 5 to 10 incident photons and so therefore have a quantum efficiency of between 10 and 20%.
    • Typical values are 70-80%
  • Magnification
    • The ratio of the length of the image on the CCD to the length of the real object
  • Magnification
    • Resolution
    • Two points on an object will be resolved if the images of the two points are at least two pixels apart
  • Digital Camera Resolution
    • The more pixels a camera has, the more detail it can capture and the larger pictures can be without becoming blurry or "grainy."
    Photo courtesy Morguefile The size of an image taken at different resolutions
  • Image quality
    • Larger magnification = more pixels activated = more detailed
    • Greater resolution = more pixels /unit length = more detailed
  • Use of CCDs
    • Digital cameras
    • Video cameras
    • Telescopes
    • Medical imaging
    • Photocopiers
    • Barcode readers
  • Advantages over emulsion
    • Reusable - once an image has been captured, the CCD can then be reset ready for the next image to be captured. Photographic emulsion is a 'one off' process and cannot be reused.
    • Greater sensitivity - modern CCDs are over 1000 million times more sensitive than the human eye
    • Greater colour response - modern CCDs will respond to electromagnetic radiation over a wider range of wavelengths than either the human eye or photographic emulsion
    • Linear response - the output voltage from a CCD is proportional to the charge collected by each pixel, which in turn is proportional to the number of photons incident on the CCD.
  • Image retrieval
    • Light focussed on CCD
    • Photoelectric effect
    • Number of electrons released from each pixel will vary
    • Potential change occurs
    • Pixel location sorted along with pd change as a digital signal
    • Digital signal converted to image