Data representation Flashcards
Character code:
A binary representation of a letter, character or number
ASCII
A standard binary coding system for characters and number.
First was 7 bits (128 permutations), now extended to 8 bits (256 characters).
BUT 256 not enough, doesn’t cover all languages or platforms. Due to the widespread use of the web, increasing need for this. HENCE UNICODE
Unicode:
A standard binary coding system that has superseded ASCII. It includes characters from over 20 countries.
16 bits, constantly updated as aims to cover every language and platform(HW and SW) and every program
Parity bit:
Method of checking binary codes by counting the number of 1s and 0s. Even = even num 1’s. Odd = odd num 1’s
Majority voting:
Produces same data several times and checks the same both time EG 101 - 111000111
Check digits
Digit added onto end to check data is accurate, used in bar code scanners
Worked out by:
- Adding up numbers, add up that answer to get one digit BUT if digits swapped around in transmission, still get same check digit
- Modulo - 11 each num given weighting (scaling factor), times by this, add together, mod 11, remainder is check digit. ( rem = 10 ,X and rem = 11, 0)
Why is hexadecimal preferred?
Useful to represent large numbers with as fewer digits as possible - 2 hex digits represent 1 byte.
USED —> identify memory addresses as less digits than binary representation
Number Systems
Natural N - (0,1,2,3)
Integer Z - (-3,-2,-1,0,2)
Rational Q - (-1/2, 2.5, 3)
Irrational - (Pi, root2, root3)
Real R - EVERYTHING
Ordinal - (1st, 2nd, 3rd) used with size of lists often eg 3rd out of 30
Counting + measurement
- Natural for counting (counter used to keep track how many times looped)
- Real for measurement (microwaves control - both time and temp measured)
Bit
A fundamental unit of info, either a 1 or 0
Why comps use binary?
Processor only handles electricity in a simple way, flowing or not. Hence 2 states 0 (off) and 1 (on)
How many different values can be represented with 5 bits?
2^5 = 32
KMGT
BINARY DECIMAL
Kibi - 2^10 bytes Kila - 10^3
Mebi - 2^20 bytes Mega - 10^6
Gibi - 2^30 bytes Giga - 10^9
Tebi - 2^40 bytes Teba - 10^12
Unsigned binary
Represent positive numbers
Signed binary
Represent positive and negative numbers
Two’s compliment
Max number in binary
2^(n)-1
Most significant bit for 8 bits?
2^(n-1) = 2^7 = 128
Twos compliment - range for 8 bits?
2^(n-1) -1 = 2^(7) - 1 = -128 to 127
128 is 2^(n-1)
Convert from bits to bytes
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Bit - mapped graphic
Image made up of individual pixels, each mapped to 1 bit of memory
Pixels
Individual picture elements each mapped to 1 bit of memory
Resolution
Width * Height (or pixels per inch)
Colour depth
Number of bits/ bytes allocated to representing the colour of a pixel in a bit-mapped graphic
Storage of an image?
Resolution * colour depth
Metadata
Data about data. Bitmaps store this of themselves eg width, colour depth
Analogue signals
Data that is infinitely variable and often represented in the form of a wave
Digital signals
Discrete discontinuous data. either on of off
Analogue to Digital converter
A series of readings are taken at fixed intervals from the wave to create discrete discontinuous values.
The more samples taken, the better accuracy to be like original copy. BUT takes up a greater file size
Synthesized sound
Sound produced digitally
File size (bits) =
Sample rate (hz) * length of recording (secs) * sampling resolution (bits)
ADC Example
EG MIDI uses event messages to control various properties of sound (encoded in binary and provide communication between a midi device and processor
Advans over digital audio
- Small files hence less memory, loads faster (especially when file a web page)
- Completely editable (individual instruments can be selected/ modified)
- Supports wide range of instruments
- Midi files produce v high quality reproduction of instrument
DAC example
- phones when playing music
- modem which is a device embedded within broadband routers that changes (modulates) a digital signal into analogue so that it can be sent down a telephone wire.
When it is received at the other end, another modem demodulates the signal, converting it back into digital form
Data compression
Process of reducing number of bits require to represent data so take up less memory
Lossless compression
No data is lost
Less storage
Uses run-lenght encoding
Uses dictionary based encoding
Run - length encoding
Eliminates repeating data eg blue blue blue = 3B
PICS
Dictionary based encoding
Common strings of data in a text file eg tion rather than rewriting encoded
TEXT
Lossy Compression
Identifies data that can be removed by Run time encoding, higher frequency data is less noticeable hence removed.
AUDIO of VIDEO
