Data Storage and File Compression | O Level Computer Science 2210 & IGCSE Computer Science 0478 | Detailed Free Notes To Score An A Star (A*)

Measurement of Data Storage

• Bit
  • Basic unit of computer memory storage
  • It either has a value of 0 or 1.
  • Word comes from Binary digIT.
• Byte
  • Smallest unit of memory in computer
  • 1 byte has 8 bits.
• Nibble
  • Half the byte
  • Contains 4 bits
• One byte doesn’t allow to store much information so memory size is in the following multiples
  • 1 KB or 1 kilobyte is 1000 bytes
  • 1 MB or 1 megabyte is 1000000 bytes
  • 1 GB or 1 gigabyte is 1 000 000 000 bytes
  • 1 TB or 1 terabyte is 1 000 000 000 000 bytes
  • 1 PB or 1 petabyte is 1 000 000 000 000 000 bytes
  • 1 EB or 1 exabyte is 1 000 000 000 000 000 000 bytes
• It is only focused on some storage devices
  • Technically inaccurate
  • It uses SI base 10 system where 1 kilo is 1000.
• Memory size is actually measured in terms of powers of 2.
• Therefore, the system adopted by the International Electrotechnical Commission (IEC) is based on the binary system
  • 1 KiB or 1 kibibyte is 2^10  or 1024 bytes
  • 1 MiB or 1 mebibyte is 2^20 or 1048576 bytes
  • 1 GiB or 1 gibibyte is 2^30 or 1073741824 bytes
  • 1 TiB or 1 tebibyte is 2^40 or 1099511627776 bytes
  • 1 PiB or 1 pebibyte is 2^50 or 1125899906842624 bytes
  • 1 EiB or 1 exbibyte is 2^60 or 1152921504606846976 bytes
• This system is more accurate
  • Internal memories such as RAM or ROM are measured through this system.

Calculation of File Size

• File size of an image is as follows
  • Image resolutions (in pixels) x Color Depth (in bits)
• Mono Sound File
  • Sample Rate (in Hz) x Sample Resolution (in bits) x Length of Sample (in seconds)
• Stereo Sound File
  • The result of the mono sound file calculation will be multiplied with 2

Data Compression

• Necessary to reduce or compress the file size
  • Save storage space
  • Reduce time take for streaming
  • Reduce time taken for download, transfer or upload
  • Bandwidth
    • The maximum rate of transfer of data across a network, measured in bits per second.
    • It is used up when we upload or download something
    • Fewer bits in compressed files will use less bandwidth, ensuring faster transfer
  • Reduces costs
    • Cloud storage costs are based on the size of data stored
    • ISP will charge less with less amount of data transferred

Lossy and Lossless file Compression

• Lossy File Compression
  • Algorithm eliminates unnecessary data from the file.
  • Original file can not be reconstructed once compressed
  • Some loss of detail occurs
  • The algorithm decides which parts of the file can be discarded.
  • Lossy compression
    • Will reduce the resolution/ bit or color depth of images
    • Sound file may see a fall in sampling rate or resolution
  • Final file is smaller than lossless files
    • Benefits in storage issues
    • Benefits in data transfer rate requirements
    • Common lossy file compression
      • MPEG-3 (MP3)
        • MP3 files are used for playing music
        • It is a compression technology that reduces the size of normal music files by about 90%.
        • MP3 files are never of the same quality of a CD or DVD
          • It is still satisfactory in most cases
        • The algorithm removes the sounds that human ear can’t hear
          • Sounds outside the human ear listening range
          • Perceptual Music Shaping
            • If two sounds are playing at the same time, the louder sound can only be heard by our ear
            • Therefore, the softer sound is eliminated
      • MPEG-4 (MP4)
        • Allows storage of multimedia files instead of just sound.
        • It retains acceptable quality of sounds and videos
        • No real loss in discernable quality
        • Usually we can stream videos and music online in this format
      • JPEG
        • A raw bitmap file is very large
        • Such files are temporary
        • JPEG is a lossy compression used for bitmap images
        • A new file is formed and the original file can no longer be reconstructed
        • JPEG compression occurs on two key concepts
          • Human eye can not detect the differences in color shades as well as it determines the image brightness differences.
            • Our eyes are more sensitive to brightness variations compared to color variations
          • By separating pixel color from brightness, images can be split into 8 x 8 pixel blocks.
            • Certain information can then be discarded
            • It will not cause any real of noticeable deterioration in quality.
• Lossless File Compression
  • All data from the original uncompressed file can be reconstructed
    • Important where any data loss will be disastrous
    • For example, very large and complex spreadsheets being transferred
    • Or a very large computer applications being transferred
  • Lossless file compression does not lose any data form the original file
  • One method is run-length encoding (RLE)
    • Reversible file compression
    • Reduces the size of a string of adjacent, identical data
      • For example repeated alphabets or repeated colors
    • A repeating string is encoded in two values
      • First value tells the number of identical items in the run
      • The second value represents the code of the data item. For example, the keyboard character that was being repeated.
  • Only effective where there is long run of repeated units/ bits.
  • How RLE is used on text based data?
    • If there is a string called bbbbdddddaaaaacc.
    • Each character requires one byte, then the string requires 16 bytes.
    • Then we can use the following method using ASCII code.
      • bbbb becomes 04 (showing that 4 times it is repeated) and 98 (which is the ASCII code of b), similarly, for the d, it will be 05 100, for the a it will be 05 97 and c will be 02 99.
    • Here, we can see that 8 bytes of memory will now be required if 1 byte of memory is required for each data here. It reduces the original size b half.
    • If the data is something like cdcdcdcdcd, then we need to use a flag
      • A flag preceding the data indicates that the upcoming data has a number of repeating units.
      • If no flag used, the following data is taken at face value, and a run of 1.
    • For example
      • If the string is aaaaaaaa bbbbbbbb c d c d c d eeeeeeee
      • Then, without a flag, it will code as 0897 0898 01 99 01 100 01 99 01 100 01 99 01 100 08 101
      • Here, the original data will use 32 bytes of data, the compressed one use 18 bytes
      • One the other hand, we can use a flag, at 255, which will reduce the over all bytes used.
        • 255 08 97 255 10 98 99 100 99 100 99 100 255 08 101
        • 15 values used so 15 bytes required only.
  • RLE can be used with images
    • It can be used for colored and uncolored images
  • Real life reductions are not very large when using lossless compression
    • Other data such as file header etc. have to be stored as well.