DIGITAL TECHNOLOGY

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Analogue technique: Involves codes and signals, large number of different values (between given limits), hence varies continuously with time

Uses:

- Photocopying: optical and electrostatic processes

- LPs (‘vinyl’): sound variations are stored as variations in a track, dust or scratches will alter output quality

- Cassette tapes: sound variations are stored as variations in the magnetic field orientations in the tape. Each time tape played, quality slightly reduced. Tape can be stretched or damaged

Digital technique: Codes and signals, binary digits/ bits (each bit can only take one of two possible values). Signal repeatedly changes between two available levels.

Bit: Powers of 2, e.g. 1, 2, 4, 8 etc.

Most Significant Bit (MSB): Largest power

Least Significant Bit (LSB): Smallest power

Byte: Eight separate bits of information e.g. 700MB = bits

Uses:

- Floppy discs: different characters stored as series of magnetic variations in disc. External magnetic fields can corrupt the data stored.

- Hard disc: variations stored as series of magnetic variations in the disc. Can store large amounts of microchip memory.

- Computer memory (microchips): variations stored using large numbers of transistors and capacitors within the chip. ROM – fixed information. RAM – flexible information

- DVD: variations of light and sound are stored as series of optical ‘bumps’ and ‘pits’ on DVD track.

Converting between analogue (e.g. music) to digital (e.g. mp3 format):

- Input information sampled at regular intervals

- Each sampled signal converted into one value among a fixed range of possible values/quantum levels

- Each sampled quantum level converted into digital form/ binary form

To improve accuracy of digital information (binary):

- Increasing sampling frequency à capture high frequency detail in original signal

- Increase number of available quantum levels

When n number of bits used, number of different possible values: 2n

American Standard Code for Information Interchange (ASCII): 8-bit code representing 256 or 28 different possible characters and formatting codes.

Analogue e.g. photocopying, vinyl, cassette tapesDigital e.g. floppy disc, hard disc, memory chip, DVDComplexitySimple – direct parallel between pressure variation of sound, and electrical p.d.Complex – complex set of rules for conversion between digital signal and input/outputQualityLiable to damage/corruptionHigh sample frequency and high number of quantum levelsReproducibilityRetrieval can affect quality of future retrievals

Retrieval speed slow, can take significant time

Accurate optical techniques in retrieval, input=output =)

Fast retrieval speed

PortabilityOccupy significantly larger volume, less compact than digitalLarge quantities of data can be stored in a very small device e.g. microchipManipulation of dataIncreased possibility of data corruptionManipulation of data easily achievedAccuracy

Each letter and symbol can be assigned to a specific binary numberKnow advantages and disadvantages!

Implications of increasing data storage:

- Moral/Ethical: Issues of access and ownership of electronic data e.g. the state

- Social: Can be used to record and highlight abuses of human rights, can also be used to control societies’ opinions and views

- Economic: More perfect information and better predictions for the future, and allow quick price comparisons and more competitive markets

- Environmental: Saves resources

Reading data on a CD/DVD: - Track starts in centre and works outwards in a spiral

- Laser beam is focused on track

- Rotational speed of disc controlled so that constant length of track scanned in a given time (CD has a higher speed of revolution near centre compared to outer edges)

- Information read by sensing amplitude of reflected laser beam off bumps and pits along track

From bump or pit, reflected beam gives strong signal – 0

From edge between bump and pit, destructive interference takes place and reflected beam gives weak signal – 1

Bump height =

To find no. of turns = change in radius/ distance between spirals on track

To find length of track = (no. of turns)(2)(average radius)

To find CD playing time = length of track/ scanning velocity

To find average length of track per bit of info = length of track/ no. of bits

Comparing CD and DVD: pits and bumps (that store bits of information) on a DV are much close together than on a DVD, hence can store more data, also DVD allows multilayered data storage.

Capacitors: Devices that can store charge. Charge q stored is proportional to p.d. V applied across capacitor.

use to find change in potential difference across each pixel

Capacitance C: constant of proportionality measured in farads.

1F = 1CV-1

* usually capacitance and p.d. given so that charge stored can be calculated

Charge-coupled device (CCD): A silicon (semiconductor) microchip used to electronically record an image focused onto its surface or in a photovoltaic solar cell

Pixels: A small area on a semiconductor silicon microchip, the smallest part of the microchip that acts as a capacitor and detects photons.

Record digital image using CCD:

During photoexposure, elements within CCD generate a charge proportional to intensity of the incident light due to photoelectric effect. Pixels convert light energy into electrical energy via photoelectric effect. Charge is collected in different pixels. Each pixel acts as a capacitor, amount of charge that builds up and p.d. across each pixel proportional to intensity of incident light (no. of photons)

Charges are ‘coupled’ (transferred) along a line of pixels in sequence. Line by line signal processing ensures charge on each pixel recorded.

Individual charge packets converted to an output voltage at charge amplifier, the value of the pd measured and then digitally encoded in binary code. In this way, light intensity info from each pixel stored.

Another digital signal can be used to represent position of the pixel on the CCD surface

Both types of signals can then be used to reconstruct image, as different light intensities of pixels in different parts of the image has been stored.

Quantum Efficiency (QE): Ratio of number of photoelectrons emitted to number of photons incident on a pixel, measure of level of efficiency of pixel. Greater QE, greater sensitivity of device, able to record images at lower light levels (but uses same amount of energy, same magnification and resolution).

Depends on design of CCD, varies between 0.2 and 0.9

* A perfect CCD would have a ratio of 1 i.e. 1 photoelectron emitted for each incident photon.

Resolution: Greater resolution, greater amount of detail recorded, more memory occupied

* If images of two points are two pixels apart, points on CCD are just resolved

Types of questions to expect: Calculate distance between two points on object and see if their images are resolvable (at least 2 pixels apart)

Magnification: Ratio of length of image on CCD to length of the object (note: not area!) Greater magnification, more pixels used for given section of image, more detailed.

Hence when calculating equivalent separation between object and pixel, and area is given, make sure to take square root of areas first before using magnification in calculations.

Uses of CCDs: (digital!)

- Digital cameras: Convenient usage, image quality must be controlled, image analyzed three times to create colour photograph

- Video cameras: Better quality than analogue images on magnetic videotape, easy to store and transport, re-usable media utilized, useful for security recordings

- Telescopes: Better sensitivity compared to traditional film, allows detailed analysis, reduces exposure time, allows remote operation of telescopes e.g. Hubble space telescope!

- Scanners: Fax machines scan text and transmit image along telephone lines

- X-ray imaging: Better contrast, can be processed, can be easily compared using imaging techniques, easily shared, stored and accessed

Advantages of CCDs:

- Does not use film, cheap

- Quality: Greater quantum efficiency. Traditional film QE=0.1, for CCD QE=0.9 achievable. Very faint objects can be photographed

- Manipulation: Can be enhanced and edited using electronic processing techniques

- Reproducibility: Very fast processing time, can be viewed immediately

- Portability: Easily stored and archived/ much less storage space needed.

(in contrast, film degrades with time, experiences reciprocity failure)

Outline how the variation in potential difference across individual pixels enables a black and white image to be produced by a digital camera:

The charge and pd generated in each pixel is proportional to the incident light as a result of photoelectric effect.

Each pd can be converted to a digital signal, which can then be converted to an image on an LCD screen.

The variation in pd across the collection area of the pixels thus acts as the “map” of the image of the object in the collection area. Each pixels records light intensities in different parts of the image, and thus used to reconstruct the image.

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