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Text, Sound and Images | O Level Computer Science 2210 & IGCSE Computer Science 0478 | Detailed Free Notes To Score An A Star (A*)

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Character Sets – ASCII Code and Unicode

  • ASCII code
    • American Standard Code for Information Interchange
      • Set up in 1963
      • Newer version in 1986
    • The standard character set consists of 7-bit codes
      • 0 to 127 in denary.
      • 00 to 7F in hexadecimal
      • These codes represent different numbers, letters and characters that are found on the standard keyboard
      • 32 codes are control codes (0 to 31 in the denary system and 00 to 19 in the hexadecimal system)
      • Between lowercase and uppercase codes for the same character, the sixth bit changes from 1 to 0.
        • 0 represents uppercase
        • 1 represents lowercase
      • Character sets are grouped together in sequences
        • This makes the usability faster
    • Extended ASCII uses 8-bit codes
      • 0 to 255 in denary
      • 0 to FF in hexadecimal
      • It provides another 128 codes that allow characters for the use of non-English alphabets and graphical characters as well.
  • Disadvantages
    • Does not represent non-Western languages
    • Different methods of coding have existed between DOS and Windows over the years.
    • One coding system is called Unicode
      • Represents all languages around the world.
      • Supports many operating systems, search engines and internet browsers that are used globally
      • It overlaps with standard ASCII as the first 128 characters are the same.
      • Unicode can support several thousand characters in total.
      • ASCII uses one byte to represent a character
        • Unicode uses 4 bytes per character
    • The Unicode Consortium was set up in 1991.
      • Version 1.0 published with five goals
        • Create a universal set covering all languages and writing systems
        • Be more efficient than ASCII
        • Unambiguous encoding system where both 16-bit and 32-bit values represent the same characters.
        • Uniform encoding with each character coded as 16-bit or 32-bit code
        • Reserve part of the code for private use.
          • User can assign codes for own characters and symbols

Representation of Sound

  • Soundwaves are air vibrations
  • Ear senses the vibrations and interpret them
  • Each sound wave has a wavelength, amplitude and frequency
  • Sound waves continuously vary, making it analogue data.
    • Computers require digital data.
    • Instead, soundwaves need to be sampled to be stored in the computer
    • Sampling
      • Measuring the amplitude of the sound wave
      • An Analogue to Digital Converter (ADC) is used for this purpose.
        • Converts the analogue data to digital version
    • Sound waves are sampled at regular time intervals
    • Precise measurement of the amplitude is not possible.
    • Approximate values are stored
    • On the graph of sound amplitudes, the x-axis shows the time intervals, and the y-axis shows the sampled amplitude.
    • Given that amplitude range is usually from 0-10 in most samples, a 4 binary bits can be used to represent each amplitude value
    • Increasing the binary value can increase the accuracy of the sampled sound. For example, using 0-127 instead of 0-10 for the sample range will provide more accurate representation.
    • Sampling Resolution
      • Number of bits per sample
      • Also called bit depth
    • Sampling Rate
      • Number of sound samples taken per second
      • Measured in hertz (Hz), 1 hertz means one sample per second
    • How sound clip is recorded?
      • Amplitude is first determined at set sampling rate
      • Approximate representation of sound wave given
      • Each sample is then encoded using series of binary digitals.
    • A higher sampling rate or larger resolution will allow more faithful representation of the original sound.
      • However, increasing sampling rate or resolution can increase file size.
  • Benefits of larger sampling resolution
    • Larger dynamic range
    • Better sound quality
    • Less sound distortion
  • Drawbacks
    • Larger file size
    • Longer to transmit or download
    • Greater processing power required
  • CDs have 16-bit sampling resolution and a 44.1 kHz sample rate, meaning 44 000 samples per second. Therefore, it provide high quality sound reproduction.

Representation of (bitmap) images

  • Bitmap mages are made of pixels
    • Pixels mean picture element
  • Two dimensional matrix of elements
    • Pixels may occur in different shapes
  • Each pixel can be shown as a binary number
    • A bitmap image is stored in the computer as a series of binary numbers
    • Black and white image requires only 1 bit per pixel.
      • Only two colors, so 1 and 0 is enough to represent it.
    • If 2 bits are used to represent each pixel, then each pixel can show one of 4 colors i.e. 00, 01, 10, 11. The rule is, 2 to the power of number of bits so 2^2 =4 colors.
    • Similarly, if 3 bits are used per pixel, it can represent 8 colors as 2^3 = 8.
  • Color Depth
    • Number of bits used to represent each color
    • 8 bit color depth means each pixel can determine one of 256 colors, as 2^8 = 256.
  • Modern computers have 24 bit color depth.
  • Generalization
    • X bits per pixel
    • 2^x colors represented
  • Increasing color depth also increases the size of the file.
  • Image resolution
    • Number of pixels that make up an image
    • An image could contain length x breadth number of pixels
    • For example 4096 x 3072 pixels i.e. 12 582 912 pixels in total
    • Many cameras allow this resolution to be varied before taking the picture
    • Lower resolution means that the picture has less details than a higher resolution picture.
    • Lower resolution can result in the image becoming pixelated or fuzzy.
  • High resolution has a drawback that the file size is very large.
    • Time to download will increase
    • Time to transfer between devices will also increase
    • Lossless reduction / compression is possible to a certain extent before the quality loss becomes noticeable
      • It reduces file size
Hunain Zia

Hunain Zia has previously achieved considerable success in the high-school educational stream, gaining a massive 154 Total A Grades and 7 Major Distinctions, a process in which he broke/ set 11 different world records, including the most A grades ever achieved, most subjects ever appeared in, most distinctions, gaining distinctions across two separate boards (Pearson Edexcel and CAIE) within the same year/ ever, and even 19 hours of constant examination. All records stand intact today. Hunain pursued his Honors Accounting Degree from the prestigious Bentley University, and then completed an LLB (in Honors) from University of London. Currently, he manages multiple social and commercial projects, has founded digital streams and works tirelessly in the education sector.

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