Representation in computer systems

REPRESENTING CHARACTERS:

KEY WORD:

Characters - A symbol which represents a letter, number or symbol

ASCII - A universal character set which uses 7 bits to represent the characters of the English language only.

Unicode - A character set which uses 16 bits to represent the characters of every language of the world and emoticons

• Each time you hit a key on the keyboard, the computer generates a code for each letter which is processed by the CPU.

• ASCII = American Standard Code for Information Interchange

ASCII character set:

• The ASCII system required 127 different codes.

• In binary, 127 is 1111111, so the ASCII system uses 7 bits.

  • Lowercase = 26

  • Uppercase = 26

  • Numeric symbols = 10

  • Punctuation, symbols and ‘space‘ = 33

  • Non-printable control codes = 32

• As 8 bit machines became standard, the ASCII character set made use of the extra bit(providing a further 128 characters)

• This new character set using all 8 bits is known as ‘extended ASCII‘

Unicode:

• A new character set was developed to accommodate the various other languages of the world.

• This is called Unicode.

• The original 127 ASCII characters still have the same code values, others have just been added on.

REPRESENTING IMAGES:

KEY WORD:

Pixels - a single picture elements(dot of colour/shade) which combines with others to form a digital image

Colour Depth - The amount of bits used to define pixels in a digital image.

Resolution - The amount of pixels in a given area of an image. Images with high resolution will have more details as they have a greater number of pixels in a given area of the image.

BITMAP IMAGES:

• Bitmap images are made up of rows of ‘dots’ called ‘pixels‘(picture elements).

• Each pixel is represented by a binary number.

• Behind the scenes, this 1 bit(with each colour represented by a bit) is in fact a series of numbers.

• 8bit colour - Each screen pixel is represented by a single byte(8bits). The byte represents one of the 256 colours.

• 24bit colour(true colour) - Each screen pixel is represented by 3bytes. Each byte represents either a shade of red, green or blue which combine to provide the pixel colour.

EFFECTS OF RESOLUTION ON QUALITY AND FILE SIZE OF AN IMAGE:

DOTS PER INCH(DPI)/ RESOLUTION:

• Higher DPI means higher resolution which in turn means a more defined image but also also more data is needed and therefore a bigger file size.

EFFECTS OF COLOUR DEPTH ON THE QUALITY AND FILE SIZE OF AN IMAGE:

COLOUR DEPTH:

• Higher number of colours that a pixel could be means a higher colour depth which in turn means a more real looking image but also more data is needed therefore increasing the file size.

• A low colour depth means that each pixel is represented by a short binary number. A high colour depth means each pixel is represented by a long binary number.

• A shorter number means few possible colours can be represented by the number whilst a higher number means more colours can be represented.

• Each pixel may be represented by a 48bit long number so each pixel can be one of 281 trillion colours in a high colour depth but each pixel may be represented by a nibble so each pixel can be only one of 16 colours in a low colour depth.

METADATA:

• ‘Data about Data‘ is also known as Metadata.

• This provides information about the file e.g. file size, height, width etc

REPRESENTING SOUNDS:

KEY WORDS:

Sampling - The process of measuring the frequency of sound, at regular intervals, in order to digitally capture and record audio.

Sample rate - The rate at which samples are taken. Faster sample rates will increase the quality of the recorded sound.

Bit depth - The amount of bits used to represent each sample. Higher bit depths will increase the quality of the recorded sound.

ANALOGUE VS DIGITAL:

• Analogue sound(basically sound waves that continuously vary) are pure and of perfect quality.

• Any computer recorded sound is not pure, not real and not of perfect quality.

• It is because sound has been digitised - it has been sampled at set intervals.

WHAT IS SAMPLING?

• Sampling is recording snippet of sound at set intevals.

• Analogue sound is continuous over time, digitised is made up of lots of ‘sound bites‘ over time.

SAMPLE RATE:

• The sampling process happens very fast to make sure we do not hear gaps between sound recordings.

• In a typical audio CD, music has been sampled 44,100 times per second or 44.1kHz.

• This can be carried out across 1 or 2 ‘channels‘

  • 1=mono 2=stereo

SAMPLE RESOLUTION AND BIT DEPTH

• Sample resolution describes the number of bits used in each sample.

• A higher sample resolution will mean more data is recorded per sample, therefore sound quality is improved.

• Bit depth of 16bits and a rate of 44.1kHz.

• For a 3 minutes song:

  • 44,100 samples per second, over 180 seconds is 7,938,000 samples.

• To work out file size, multiply the number of samples per second by the number of seconds in total.

• Then multiply the sample resolution by the result from the first calculation.

• To convert to bytes, divide by 8.

• To get to Megabytes, divide by 1,000,000

• Metadata tells the computer what type of file it is so that the computer can use the file correctly.

COMPRESSION:

KEY WORDS:

Compression - The process of reducing the size of a file.

Lossy Compression - A compression method which reduces the size of a file by permanently removing some of the file’s data.

Lossless Compression - A compression method which reduces the size of a file by temporarily removing some of the file’s data.

COMPRESSION:

• Compression software reduces the size of files.

  • Less storage is required

  • Faster download times - improving online experience

  • Faster streaming speeds of video/audio files

Lossy Compression:

• Unrequired data is removed from a file

Lossless Compression:

• Data is temporarily removed from a file but is added back(rebuilt) when the file is to be used again.

• An algorithm is used to later reconstruct the file by reproducing the sequence data.