Data Representation

Number Systems

Binary System

• Base 2 number system
• Has two possible values only (0 and 1)
• 0 represents OFF and 1 represents ON
• A point to be noted is that the most left bit is called the MSB (Most Significant Bit)

Denary System

• Base 10 number system
• Has values from 0 to 9

Hexadecimal (aka Hex)

• Base 16 number system
• Has values from 0 to 9 followed by A to F
• A represents 10, B represents 11 and so on until 15 which is F

Number Conversions

Converting Binary to Denary

• Take the binary value and place it in columns of 2 raised to the power of the number of values from the right starting from 0.e.g. For binary value 11101110, place it in a table like this:

• As it can be seen that it starts from 1 and then goes till 128 from left to right
• Now values with 1 are to be added together giving the final answer, as for the example, it is 128 + 64 + 32 + 8 + 4 + 2 = 238

Converting Denary to Binary

• Take the value and successively divide it by 2 creating a table like follows:

• Note that when the value itself is not divisible by 2, it is divided by the previous value of the current number and 1 is added to the remainder column for that specific number
• When you reach 0, the remainder has to be read from bottom to top giving us the binary value ( as in this case, it is 1 0 0 0 1 1 1 0 )

Converting Hexadecimal to Binary

• Separate each value from each other and convert them to denary

• Each separate denary value to be converted to binary

• All the binary values to be merged together

  e.g.

Hexadecimal : 2    1    F    D 
Denary      : 2    1    15   13
Binary      : 0010 0001 1111 1101

Final Answer: 0010000111111101

Converting Binary To Hexadecimal

• Divide the binary value into groups of 4 starting from the right. If at the end, the last division is less than 4, add 0s until it reaches 4
• For each group, find the denary value as shown above, and then convert each denary value to its corresponding hexadecimal value (if less than 10, then itself, else, 10 is A, 11 is B, 12 is C, 13 is D, 14 is E and 15 is F).
• After conversion, just put all the hexadecimal values in order to get the final answer

Given Value : 1 0 0 0 0 1 1 1 1 1 1 1 0 1
When grouped: 10 0001 1111 1101

After 2 values added to left: 0010 0001 1111 1101

After Conversion to Denary: 2 1 15 13

Denary to Hexadecimal: 21FD

Converting Hexadecimal to Denary

• Convert the value to binary as shown above, and then convert the final answer to denary

Converting Denary to Hexadecimal

• Convert the value to binary, and then convert it to hexadecimal as explained above

Addition of Binary

• Binary values are not added the way denary values are added, as when adding 1 and 1, we cannot write two because it doesn’t exist in binary.

Points to note:

• 0 + 0 = 0
• 1 + 0 / 0 + 1 = 1
• 1 + 1 = 0 (1 carry)
• 1 + 1 + 1 = 1 (1 carry)

Overflow:

• When adding two values, if the solution exceeds the limit of given values, e.g., the solution has 9 bits, but the question had 8 bits per value, the 9th bit (most left bit) is called overflow.
• This indicates that the memory doesn’t have enough space to store the answer to the addition done in the previous part.

Steps to add two values (with example):

• The values we will add are 1 1 0 1 1 1 0 and 1 1 0 1 1 1 1 0

  1. Convert both the bytes into 8 bits (add zero to the left-hand side to match them).

     e.g., 1 1 0 1 1 1 0 would become 0 1 1 0 1 1 1 0

  2. Add the values as follows with the points given above

Note: We move from RHS to LHS, and when adding values, we use the rules given above. If the bit crosses the limit (overflows), we put the value in brackets, denoting it is overflow.

iii. The solution would now be  (1) 0 1 0 0 1 1 0 0

Logical Shifts

• The logical shift means moving a binary value to the left or the right
• When doing a logical shift, keep in mind that the bit being emptied is going to become 0

Explaining with an example:

• Shifting 10101010 - 1 place left:

  1. The furthest bit in the direction to be logically shifted is removed ( in this case, one at the LHS is removed) - ==(if it were 2 places, 2 bits would have been removed)==
  2. Every bit is moved in given places to the given direction ( every bit is moved one place to the left in this case, and the leftover bit in the right is marked 0, so 10101010 would become 01010100)

Two’s complement (Binary Numbers):

• Two’s complement is a method used to represent negative values in binary. Here, the MSB ( Most Significant Bit) is replaced from 128 to -128; thus, the range of values in a two’s complement byte is -128 to 127

Converting binary values to two’s complement

• Firstly, write the binary value and locate the first one from the right; e.g., 1101100 would have the first one at the third position from the right.
• Now, switch every value to the left of the first one located above (not switching the one), e.g., the value in our example becomes 0010100, which is the two’s complement of itself.

Converting negative values to two complement

• Find the binary equivalent of the value ignoring the - sign
• Convert the binary value to two’s complement
• Make the MSB 1, if not already

Converting two’s complement value to denary:

• We do it the same way as a normal value is converted from binary to denary; we only just replace 128 with -128 e.g., for 10111010 we do the:

-128 + 32 + 16 + 8 + 2 = -70

Use of the Hexadecimal System

Examples:

• Defining colours in Hypertext Markup Language (HTML)
• Media Access Control (MAC) addresses (a number that uniquely identifies a device on a network)
• Assembly languages and machine code
• Memory Dumps
• Debugging (method to find errors in a program)
• Display error codes (numbers refer to the memory location of the error)
• IP (Internet Protocol) addresses

Memory Dumps

• Hexadecimal is used when developing new software or when trying to trace errors.
• Memory dump is when the memory contents are output to a printer or monitor.

Assembly code and machine code (low-level languages)

• Computer memory is machine code/ assembly code
• Using hexadecimal makes writing code easier, faster, and less error-prone than binary.
• Using machine code (binary) takes a long time to key in values and is prone to errors.

Text

ASCII

• The standard ASCII code character set consists of 7-bit code that represent the letters, numbers and characters found on a standard keyboard, together with 32 control codes
• Uppercase and lowercase characters have different ASCII values
• Every subsequent value in ASCII is the previous value + 1. e.g. “a” is 97 in ASCII , “b” will be 98 (which is 97 + 1)
• Important ASCII values (in denary) to remember are as follows:

• 0 is at 48
• A is at 65
• a is at 97

• ASCII uses one byte to store the value

• When ASCII value of a character is converted to binary, it can be seen that the sixth bit changes from 1 to 0 when going from lowercase to uppercase of a character and the rest remains the same. e.g.

  

Unicode

• ASCII does not contain all of the international languages, thus Unicode is used to solve this problem

• For the first 128 values, it is the same to ASCII

• Unicode supports up to four bytes per character allowing multiple languages and more data to be stored

  
  
  To represent text in binary, a computer uses a character set, which is a collection of characters and the corresponding binary codes that represent them

Sound

• Sound is analogue, and for it to be converted to digital form, it is sampled
• The sound waves are sampled at regular time intervals where the amplitude is measured, however, it cannot be measured precisely, so approximate values are stored

How is sound recorded

• The amplitude of the sound wave is first determined at set time intervals
• The value is converted to digital form
• Each sample of the sound wave is then encoded as a series of binary digits
• A series of readings gives an approximate representation of the sound wave

Sampling Resolution:

• The number of bits per sample is known as the sampling resolution (aka bit depth)
• Increasing the sampling resolution increases the accuracy of the sampled sound as more detail is stored about the amplitude of the sound
• Increasing the sampling resolution also increases the memory usage of the file as more bits are being used to store the data

Sampling rate

• Sampling rate is the number of sound samples taken per second which is measured in Hertz (Hz)
• Using a higher sampling rate would allow more accurate sound as less estimations will be done between samples

Images

Bitmap images

• Bitmap images are made up of pixels
• A bitmap image is stored in a computer as a series of binary numbers

Color Depth

• The number of bits used to represent each colour is called the colour depth.
• An 8 bit colour depth means that each pixel can be one of 256 colours (because 2 to the power of 8 = 256)
• A 1 bit color depth means each pixel can store 1 color (because 2 to the power of 1 is 2) - ( This is done as the bit can either be 0 or 1, with 0 being white and 1 being black)
• Increasing colour depth increases the size of the file when storing an image

Image Resolution

• Image resolution refers to the number of pixels that make up an image; for example, an image could contain 4096 × 3072 pixels
• Photographs with a lower resolution have less detail than those with a higher resolution
• When a bitmap image is ‘ blurry ‘ or ‘ fizzy ’ due to having low amount of pixels in it or when zoomed, it is known as being pixelated.
• High resolution images use high amounts of memory as compared to low resolution ones

Measurement of the Size of Computer Memories

• A binary digit is referred to as a BIT
• 8 bits is a byte
• 4 bits is a nibble
• Byte is used to measure memory size

IECB System (more commonly used):

Conventional System:

Calculation of file size

• The file size of an image is calculated as: image resolution (in pixels) × colour depth (in bits)
• The size of a mono sound file is calculated as: sample rate (in Hz) × sample resolution (in bits) × length of sample (in seconds). (For a stereo sound file, you would then multiply the result by two.)

File types

Musical Instrument Digital Format (MIDI)

• Storage of music files

• Communications protocol that allows electronic musical instruments to interact with each other

• Stored as a series of demands but no actual music notes

• Uses 8-bit serial transmission (asynchronous)

• Each MIDI command has a sequence of bytes:

  • First byte is the status byte – informs the MIDI device what function to preform

  • Encoded in the status byte is the MIDI channel (operates on 16 different channels)

• Examples of MIDI commands:

  • Note on/off: indicates that a key has been pressed

  • Key pressure: indicates how hard it has been pressed (loudness of music)

• Needs a lot of memory storage

MP3

• Uses technology known as Audio Compression to convert music and other sounds into an MP3 file format

• This compression reduces the normal file size by 90%

  • Done using file compression algorithms which use Perceptual Music Shaping

  • Removes sounds that human ear cannot hear properly

  • Certain sounds are removed without affecting the quality too much

• CD files are converted using File Compression Software

• Use lossy format as the original file is lost following the compression algorithm

MP4

• This format allows the storage of multimedia files rather than just sound

• Music, videos, photos and animations can be stored

• Videos, could be streamed without losing any real discernible quality

Joint Photographic Experts Group (JPEG)

• JPEG is a file formats used to reduce photographic file sizes

• Reducing picture resolution is changing the number of pixels per centimetre

• When photographic file undergoes compression, file size is reduced

• JPEG will reduce the raw bitmap image by a factor between 5 and 15

Lossless and Lossy File Compression

Lossless File Compression

• All the data bits from the original file are reconstructed when the file again is uncompressed
• Important for files where loss of data would be disastrous (spreadsheet)
• An algorithm is used to compress data
• No data is lost
• Repeated patterns/text are grouped together in indexes

Run-Length Encoding

• It reduces the size of a string of adjacent, identical data (e.g. repeated colours in an image)
• A repeating string is encoded into two values: the first value represents the number of identical data items (e.g. characters) and the second value represents the code of the data item (such as ASCII code if it is a keyboard character) e.g. ‘aaaaabbbbccddddd’ becomes “05 97 04 98 02 99 05 100”
• RLE is only effective where there is a long run of repeated units/bits
• One difficulty is that RLE compression isn't very good for strings like "cdcdcdcdcd". We use a flag to solve this, e.g. 255 can be made the flag. Now 255 will be put before every repeating value, e.g. our previous example becomes 255 05 97 255 04 98 255 02 99 255 05 100 where 255 now indicated that now the next character/set of characters is approaching

Lossy File Compression

• The file compression algorithm eliminates unnecessary bits of data like MP3 and JPEG formats
• Impossible to get original file back once compressed
• Reduces file quality
• In this, resolution of the image is reduced, and colour depth is reduced