Data Representation

Data Representation

1. Text

   • ASCII Code
     • American Standard for Information Interchanage
     • Character set
     • 7-bit set of codes that allows 128 different characters.
     • When a key is pressed on the keyboard a number that represents the key is sent to the computer.
     • Letters are stored sequentially.
     • 1 character = 1 byte
     • Extended ASCII
       • 8-bit character set that represents 256 different characters
       • Useful for European languages.
       • Extra bit ( parity bit) used as an error checking bit.
     • Programming
       • ord()
         • Inbuilt ord function is used to find out the ASCII Code.
       • chr()
         • Inbuilt chr function is used to lookup the character.
   • 5 bits is not enough to encode all the characters of the keyboard.

2. Sound

   • Hearing Sound
     • Sound is created by vibrations causing changes in air pressure.
     • Ears can detect changes as the air pressure makes the ear drums vibrate.
     • Our brain processes the vibration interpreting it as sound.
   • Sound Wave
     • Amplitude: measure of the air pressure which is continuously changing.
       • But in an electrical signal representation the Amplitude measures the change in voltage.
     • Wavelength: measure of the pitch of sound.
     • Sound is analogue as it is continuously changing.
   • Recording Sound
     • Microphone
       • Converts sound to electricity.
       • Transducer: converts acoustical energy to electrical energy.
     • Sound Card
       • Converts the electrical signal from the microphone into a binary signal.
       • The voltage level at regular intervals is checked and rounded to a specific level.
       • The binary representation is stored.
       • It contains an analogue to digital converter (ADC) which is used for the conversion of real world analogue signals into digital form.
     • Sample Rate
       • The computer samples the sound wave at regular intervals.
       • Each time the wave is sampled, the computer records its amplitude and codes this as a number in binary.
       • The more the sampling higher the accuracy of the sound recording.
       • At each sampling, the value of the electrical signal is rounded to its nearest fixed point.
       • Definition: The number of samples taken per second (measured in Hz, or KHz).
   • Sample Resolution
     • Higher the number of bits is used to store the sound the better the sound reproduction.
     • 16 bits typical sound's sample resolution.
     • Definition: The number of bits used to store the amplitude per sample (measured in bits).
     • Sample: measure of the amplitude of a wave at a given point.
   • Sound File Size
     • File size (in bits) = Rate * Res * Secs
     • File size (bytes) = File size (bits) / 8
     • Mono sound is when sound comes out of one speaker only.
     • Stereo sound is when sound comes out of two speakers (two channels)
     • When sound is recorded in stereo, the file size is doubled because all sound has to be recorded twice – once for each speaker.

3. Images

   • Photographs taken by cameras are displayed by the camera recording the changing colours continuously.
   • This is by breaking the picture down into very small elements called pixels (picture elements).
   • The greater the number of pixels means the higher resolution (quality) of the image.
   • A picture with resolution 1024 x 768 has 768 lines of pixels and 1024 pixels in each line therefore 787,200 pixels in total.
   • Colour Depth
     • The computer stores the colour of each pixel in order to store a picture.
     • The number of colours you can use is determined by the number of bits you allow for each pixel.
     • 8 bits allow 256 different colours for each pixel.
     • The number of bits used for each pixel is called the colour depth.
   • Difference in File Size
     • Vector Images
       • Not all pictures are stored as bitmaps.
       • Some pictures are stored as vector images.
       • The computer stores specific details in binary including the size, colour and coordinates of the lines/shapes.
     • Bitmap Images
       • All photos are stored as bitmap style images.
       • They are often compressed to take up less space.
       • Bitmap files are always slightly larger than the calculated size because of other information including the resolution.
       • Some bitmap files store a code for all the colours used in the image which is called meta data.

4. Data

   • Computers can only store data in binary form.
   • Data inside a computer is transmitted as a series of electrical signals that are either on or off.
   • To be able to process any kind of data they must be converted into binary form.