Unit 1 Computer Science

Unit 1 Computer Science

1. Section 5: Data Representation

   • Character Coding Schemes
     • Unicode: Uses 16 bits to represent characters (2 bytes)
     • ASCII: Uses 8 bits (1 byte)
     • Parity bits are extra bits added to a bit pattern for error checking
       • Odd Parity: odd number of 1's in bit pattern
       • Even Parity: even number of 1's in but patter.
       • Hamming Code
         • Hamming code is still used today as it provides a good balance between error detection & error correction
         • It can self correct single errors by using a few parity bits in a bit pattern
       • Majority Vote: To determine the correct answer, therefore send it through more than once
     • Gray Code: Is designed so thatonly one output bit changes when a counter is built
       • Prevents Data error that may occur with pure binary, during state changes.
         • Helps data be more reliable
       • How to workout Gray Code
         • Keep first binary bit the same
         • Add the first pairs of Binary bits
         • Add the next pair
         • 1+1=10 [drop the carry & write the last digit]
       • Gray code counter toggles only one output at a time, where as a pure binary counter toggles multiple parts
       • Used in telecommunications in order to minimize the impact of analog noise on a digital communication
         • Where analogue information is being converted to digital informaition
           • visa versa
   • Bin & Hex Numbers
     • Definitions
       • Two's Complement: Representing negative numbers
       • Hex Number: Uses digits 0-9 and letters A-F
       • Word: Groups of bytes
     • Converting negative Denary number to two's complement binary
       • 1) Find binary value of the positive number. 2) Change all 1's to 0's and 0'2 to 1's. 3) Add 1 to the result.
     • Converting negative two's complement number to denary
       • Write the binary digits below the place values(1,2,4,8,16 etc). The left most bit is negative. Then add up values which have a 1 under them.
     • Binary Subtraction
       • Use two's complement on the number being subtracted and add the two binary numbers
   • Bitmapped and Vector Graphics
     • Definitions
       • Pixel: Smallest block of colour in an image
       • Bitmap: Created when pixels of an image mapped to positions in memory that store binary codes representing the colour of each pixel
       • Resolution of VDU screen: Number of pixels per row by the Number of pixels per column
       • Resolution of an Image: A quantity expressed as the number of pixels per inch
       • Colour Depth: The number of bits used to represent ta colour of a single pixel in a bitmapped image
       • Vector Graphic Object: Graphic component such as a line, rectangle or a circle
       • Vector Graphics: records information about the objects that make up an image
       • Drawing List: List of drawing components that create the vector graphics
       • Property: describes things such as the size, direction, thickness, shading, font, size or typeface
       • Data Compression: squeezes data into a smaller number of bytes than the data would occupy if uncompressed
     • Bitmap: Samples the colour in each grid cell of the original then assigns and stores a binary code to represent the average colour in each grid
     • Stored binary codes are used to reproduce a copy of the original
       • Size of grid cell is too big to allow accurate copy of the original to be reproduced. reducing size of each cell improved the resulting image
         • resizing= loss of quality
       • Binary code for each pixel is stored in memory (Before written in file)
       • File contains bitmapped representation of image
     • Colours selected for each pixel is coded in a fixed number of bits
       • 1 Bit Per pixel gives 2 possible colours (B or W)
       • 2 Bits Per Pixel gives 4 possible colours (2^2)
       • 8 Bit Per Pixel gives 256 possible colours (2^8)
       • The size of the pixels determine the sharpness of an image. The amount of pixels don't
         • The smaller the pixels the better
     • The higher the Resolution the greater number of pixels
     • The size of a bitmap is the size in pixels multiplied by its colour depth in bytes
     • Comparing Bitmapped and Vector Graphics
       • Bitmap
         • When enlarged the pixels are more visible- creating a staircase effect
         • Geometric Images require more bytes
         • Images that have continuous areas of colours require less bytes
       • Vector
         • Avoid distortion when magnified as the scaling is applied to the line's endpoint
           • Takes into account the change in lie thickness
           • Scaling is called geometric as vector graphics deal with objects and not pixels
           • The coordinates are changed therefore the drawing increasing in size and you don't loose resolution
         • Geometric Images require less bytes
         • Images that have continuous areas of colours require more bytes
         • Geometric images load faster from secondary storage and download faster over the internet
     • Run length Encoding
       • Compression technique
       • If 3+ consecutive pixels contain the same binary code for the colour of the pixel then the binary code is split into 2 bytes
         • First byte stores the count of the number of identical cells
         • Second byte Stores the binary code of the colour of the pixel
     • Lossy Compression: Reduces resolution of background; uses fewer bits for colours to which the human eye is less sensitive & fewer bits for low intensity colours
   • Sound in a Computer
     • Definitions
       • Analogue Data: varies in a continuous manner
       • Digital Data: takes form of discrete values
       • Analogue Signal: Electrical signal that varies in a continuous manner
       • Digital Signals: electrical signals with voltage changes that are in discrete steps
       • PCM (Pulse Code Modulation): process for coding sampled analogue signal by recording the height of each sample in a binary electrical equivalent.
       • PAM (Pulse Amplitude Modulation): process samples analogue signals at regular time intervals & produces electrical pulses of height proportional to the original signal's amplitude.
       • Quantisation noise: difference between original amplitude and its sampled value
     • Transducer converts energy from one form to another
       • A loud speaker: electrical to sound energy
     • To process analogue data it must be converted into analogue signals
     • How to Record Sound:
       • ADC is used to sample and electrical equivalent of the sound and convert the samples into digital signals using two voltage levels
         • Play back the sound: DAC is used to convert the digital signal representing the recorded sound into analogue electrical signal.
           • Heavily relies on the PCM technique
     • PCM form of an analogue by an ADC:
       • 1) Samples taken at fixed regular intervals. Sampling Frequency/ Rate must be atleast 2x the highest frequency in the Analogue signal- PAM
         • 2) To produce PCM: PAM samples are quantised. Height of each PAM pulse is approximated by an n-bit integer
           • 3) Height of each PCM pulse is encoded in n-bits to produce the digital output in binary form
       • Process is reversed using a DCA: Produces an analogue signal that is an approximation of the original.. Staircase effect is the result of approximation during PCM quantisation
     • Sampling Frequency/Rate: No. of samples taken per second (Hertz [Hz])
     • Nyquist Theorem
       • The more bits per sample the more accurate representation of the data being sampled
       • Sample at a frequency at least twice the rate of the highest frequency in the sampled signal
       • More frequent and accurate sampling means accurate data
       • Bandwidth is information carrying capacity
         • More capacity for preserve high low frequencies than needed. Takes less bandwidth if only some frequencies need to be transmitted- telephone speech
     • Storing Sound
       • WAV -1 minute=2.5MB [When storing on CD] (Windows)
       • MPEG
         • MP3 compressed format- used for music available online
         • Removes frequency tat the brain or ears can sense- reduces the file size before other compression techniques are applied
         • Is a compression algorithm
     • Synthesising sounds
       • MIDI: (Musical Information Digital Interface): Representing sounds made by an instrument
       • use digital means to generate audio signals resembling instrument sounds or human voice
       • Very compact and flexible for many purposes. Easy to transpose then into a different key, play on different instruments
       • Streaming Sounds
         • Streaming clients (RealPlayer) receives audio data and put it in buffer to be stored until used. As long as the player is not trying to access data that hasn't been received yet it will be successful
     • Editing sounds
       • Goldwave or Audacity can take separate digital recordings or sound waves and merge and mix them into one file

Imagens e arquivos do mapa