Chapter1_InformationRepresentation

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DataRepresentation

Data Representation

Prefixes

Binary prefix name symbol value Decimal prefix name symbol value
Kibi Ki $2^{10}$ Kilo k $10^3$
Mebi Mi $2^{20}$ Mega M $10^6$
Gibi Gi $2^{30}$ Giga G $10^9$
Tebi Ti $2^{40}$ Tera T $10^{12}$

Overflow: the result of carrying out a calculation which produces a value too large for the computer’s allocated word size

Applications

Application of hexadecimal system

  • Memory dumps
    • Memory contents are output to printer or monitor
  • MAC address
  • Error message
  • IP address
  • Unicode
  • Colour in HTML

Application of Binary Coded Decimal (BCD)

  • Calculator
  • Clock

Character sets

Character set:

  • The symbols that computer uses
  • A list of characters recognized by computer software and hardware
  • Each character has a character code
  • The binary code for each character in the string is stored in sequence

Disadvantage of ASCII:

  • Only 256 characters can be represented
  • Uses values 0 to 127/256
  • Many characters in other languages cannot be represented
  • The extended ASCII the characters from 128 to 255 may be coded differently in different system.

UNICODE:

  • UNICODE has greater range of characters than ASCII
  • UNICODE represents most written language in the world, while ASCII does not; used for English only
  • ASCII uses 7-8 bits per character, whereas UNICODE uses up to 4 bytes per character
  • UNICODE is standardized while ASCII is not

Multimedia

Multimedia

Graphics

Bitmap Vector graphic
Definitions · Made up of pixels (picture elements)
· Stored in a two-dimensional matrix of pixels
· Each pixel has a colour
· Stored as binary number
· The number of bits used to represent a pixel is called colour depth		 · A series of geometric shapes
· Drawing object
· Exact dimension is not stored Stored coordinates
· Contains a drawing list
· Commands/formulae for creating each individual object
· Property for that object Eg: colour, thickness
Properties · Takes up more memory
· Enlarging the bitmap can means that the image is pixelated
· Can be compressed with significant reduction in file size
· Suitable for photographs/scanned image
· Uses less processing power
· Individual elements of a bitmap cannot be grouped
· It is possible to change/edit each pixel to change the design		 · Made up of geometric shapes which require definition/attributes
· Stores a set of instructions about how to draw the shape
· Takes up less memory
· Vector graphic image can be enlarged without being pixelated
· Do not compress well
· Suitable for geometric shape
· Individual elements of a vector graphic can be grouped
· Vector graphics need to be ‘rasterised’ in order to display or print.
· Image is redrawn with small adjustment
· It is necessary to change each of the geometric shape to alter the design
Available formats .jpeg, .bmp, .png .svg , .cgm, .odg
Defined in XML text files which, therefore, allows them to be compressed.

Pixel: smallest picture element which can be drawn

Screen resolution: the number of pixels which can be view horizontally and vertically

Image resolution: the number of pixels that make up an image

Resolution: the number of pixels per column and per row

Pixel density: number of pixels per square centimetre.

Sound

Sampling analogue sound:

  • Amplitude measured
  • At regular time interval
  • The value of sample is recorded as binary number.

Increasing sampling resolution:

  • More bits used to represent one sample
  • Larger file size
    • Takes longer to transmit/download the file
    • Requires greater processing power
  • More accurate representation of sound
    • Less sound distortion
    • Larger dynamic range
    • Better sound quality

Decrease sample rate:

  • Fewer samples per unit time
  • File size will reduce
  • Larger gaps/ space between samples // greater quantization errors
  • Sound accuracy will reduce.

Sampling:

  • amplitude of sound wave taken at different points in time.
  • Measurement the value of the analogue signal at regular time interval.

Sampling rate:

  • Number of time that the amplitude of (analogue) sound wave is taken
  • Per unit time
  • Higher sampling rate results in more accurate digital representation.

Sampling resolution:

  • Resolution is the number of distinct value able to encode/represent each sample
  • Specified the number of bits used to store each sample
  • Also called bit depth
  • The higher the sampling resolution, the lower the quantization error
  • The higher the sampling, the less sound distortion.
  • Usually 8 bits, 16 bits, 24 bits or 32 bits.
  • Benefits:
    • Allows for larger dynamic ranges
    • More accurate representation/ sound quality
  • Drawbacks:
    • Bigger files / larger memory
    • Takes longer to transmit/download
    • Greater processing power needed

Sound editing software:

  • Edit start time, stop time and duration of any sound/time
  • Extract/delete/save part of a clip
  • Frequency, amplitude, pitch alteration
  • Conversion between different audio file formats
  • Use of filters
  • Mix/merge multiple sound sources

Sound edit:

  • Fading
    • Change a volume of a section of sound for it to get louder.
  • Removing sound element
    • Delete sections of the sound wave.
  • Copy
    • Repeat elements of the sound wave.

Sound:

  • Analogue value
  • Use ADC (analogue digital converter)
  • To convert to digital value

Compression

Compression

Lossy data compression:

  • Lossy may result in lost of detail compared to the original file
  • Lossy compression make decision about what parts of the file are important and not important.
  • Certain parts of the music can be eliminated without significantly degrading the listener’s experience.
  • Discards softer sounds if two sounds played together.
  • Perceptual music shaping: only keeps sound that human ear can hear
  • Reduce to about 10%
  • E.g: mp3, jpeg

Method1: Reduce the colour depth

  • Reduce the number of bits per colour
  • Each pixel has fewer bits

Method2: Reduce the resolution

  • Fewer pixels per unit measurement
  • Fewer pixels are stored.

Lossless data compression:

  • Lose none of the original detail
  • Based on some form of replacement: Run line Encoding
  • Maximum compression to about 50%.

RLE(Run line encoding):

  • Lossless method of compression
  • Reduces the size of a string of adjacent, identical characters
  • The repeating string is encoded into two values
  • One value represent the number of characters in the run
  • The other value is the code of the character in the run
  • The run value and run count combination may be preceded by a control character

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