Data Representation

• Compression

  A method of reducing file sizes, particularly in digital media such as photos, audio and video
• Lossless compression

  Reduces the size of a data file. There is no loss of data quality. The file can be decompressed and will be in its original form.
• Lossy compression

  Makes a file smaller by removing bits and therefore lowering the quality. In images for example, this can be done by removing pixels that are similar and are next to eachother.
• Bitmap image

  An image made up of pixels. This type of image loses quality if its width and/or height are increased.
• Pixel

  Basic unit of programmable color on a computer display or in a computer image.
• ASCII

  Is a character encoding standard for electronic communication. It is representedin text in computers, telecommunications equipment, and other devices.
• Unicode


  Inside the computer text characters are coded I digital form. It is an international code used to represent text characters.
• File extension

  A dot followed by the letters/

  file extension is the ending of a file that helps identify the computer to know with which program it needs to open the file

  Examples: .docx , .exe ...
• Binary

  The binary number system uses two digits, 1 and 0
• Denary


  Number System which we use in everyday life, using ten digits to represent numbers.
• Hexadecimal


  Its a number sytem based on 16. It has 16 digits. Place value goes up in powers of 16.
• 8-bit binary


  A sucession of eight binary digits.
• Bit

  Binary numbers are made up of the digits 1 and 0. We abbreviate the term from "binary digit".
• Byte

  Eight bits, able to represent 256 distinct values. Abbreviated "b".
• Nibble

  4 bits (half a byte)
• Kilobyte

  1000 bytes = 1k or kb
• Bit depth

  Is the number of bits available for each sample - the higher it is the more accurately the wave is measured.
• Sample rate

  In digitising sound, the sample rate is the number of samples of a sound wave taken per second. Also called frequency, measured in Hertz.
• Colour depth

  The number of bits used for the color of a single pixel. The number of colours available is 2 to the power n where n is colour depth.
• WAV

  Uncompressed digital sound format which stores music accurately and with a high sample rate. WAV files are very large and wouldn't fit on a phone
• MP3


  A lossy compressed audio format widely used for websites, and digital audio players. This digital format uses much less storage than WAV.
• MP4


  Lossy compressed file format most commonly used to store video.
• Pixelation

  When you zoom too close to a bitmap image, the individual pixels become visible and the quality of the image decreases
• Two's complement

  A method of representing negative numbers in the binary system. In an 8-bit two's complement integer, the leftmost bit indicates the sign. 1 means it is a negative numbers.
• Rules of binary addition

  1) 0 + 0 = 02) 0 + 1 = 13) 1 + 0 = 14) 1 + 1 = 0 Carry 15) 1 + 1 + 1 = 1 Carry 1you must show all carry bits to get full marks.
• Converting to and from two's complement

  flip each binary digit and then add 1.
• Logical shifts


  Where all the bits move either right or left, causing some bits to drop, and a 0 move in to the new space.
• Logical left shift

  A single shift multiplies a binary number by 2
• logical right shift


  A single logical shift divides a number by 2
• File size of an image (in bytes)


  (height x width x colour depth) divided by 8
• File size of a sound file (in bytes)

  (resolution x sample rate x length in seconds) divided by 8
• Overflow error


  When data is lost after binary addition or a logical shift. It is either the most significant bit (MSB) or the least significant bit (LSB) that is lost.
• Convert from bytes to kilobytes (when calculating file size)

  divide by 1000
• Run Length Encoding (RLE)

  An example of lossless compression. The data AAABBBBCCCCC would be compressed into 3A4B5C.
• Lossless compression - e.g. she sells sea shells on the sea shore

  A compression algorithm which doesn't remove any data. Repeated data or patterns in the data are identified and indexed, and a key created. The position of the data is stored, so the file can be reconstructed from the key.