Data Representation (P1)

Types of Primary Storage - Virtual Memory (Definition, How it works)

An extension of RAM that enables data that is not currently being used to be transferred to the hard disk to free up space in the RAM.

1. RAM is full

2. New data needed to be stored in RAM, so data not currently in use is moved to hard drive

3. Space created; new data transferred into RAM

   • When original data is required again, it is moved back from virtual memory into RAM.

Virtual Memory Advantages

• Allows more programs to be run at the same time.

• More efficient utilisation of memory

• Prevent requirements of more RAM

Virtual Memory Disadvantages

• Slower access times compared to RAM

• Thrashing - more time is spent swapping and transferring than actually working: performance decrease

Binary

Binary is a base 2 number system that only has 2 possible values - 0 or 1.

Data storage hierarchy

1. Bit - (A 0 or 1)

2. Nibble - (4 bits, like 0101)

3. Byte - (Single character, like A)

4. Kilobyte - (Short email)

5. Megabyte - (Minute of music)

6. Gigabyte - (500 photos)

7. Terabyte - (500 hours of films)

8. Petabyte - (1.5M CDs)

Denary

Denary (decimal) is the number system that has 10 possible values - 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9.

Why do computers use binary?

Computer systems consist of switches that only have two values - on (1) or off (0).

Storage Characteristics - CRAD PC

• Capacity

• Reliability

• Access Speed

• Durability

• Portability

• Cost

Access Speed

How quickly data on the device can be read or edited.

Capacity

The maximum amount of data that can be stored on the device.

Cost

The price to purchase a storage device.

Durability

The strength of the device to last without breaking.

Portability

How easy it is to carry/move the device.

Reliability

The likelihood of the device continuing to perform well over time.

Hexidecimal

A base 16 number system: a shorthand for binary.

• Has 16 possible values: 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F

Hexadecimal Advantages

• Less prone to errors when reading/writing

• Simpler - uses fewer characters to write the the same value in binary

Binary Addition - Overflow error

An overflow error occurs when a binary value is too large to be stored in the bits available.

• With a byte, (8 bits), largest number that can be held is 255. A sum of two binary numbers that equal > 255 will result in an overflow error as its too large to be held in 8 bits.

Binary Shift (Definition, Shift types)

A method used to multiply and divide binary numbers.

• Shift to left = multiply

• Shift to right = divide

Shift by 1 → Effect of 2

Shift by 2 → Effect of 4

Shift by 3 → Effect of 8

Character Set: Why it’s needed and Two Main Types

A table that matches a character to a binary value. Necessary for computers to exchange data and humans to input characters.

• Two types: ASCII and Unicode

ASCII + Possible characters

A 7-bit code able to represent the English alphabet, numbers, some symbols and control characters.

• → 128 possible characters

Extended ASCII + Bits + Possible characters

Uses 8 bits instead of 7, to include extra symbols, mathematical symbols and some non-English characters.

• Uses 1 byte (8 bits)

  → 256 possible characters

Unicode + Bits + Possible characters

Character set used for symbols outside the English language, requiring more storage than ASCII.

• Uses 2 bytes (16 bits)

  → 65,536 possible characters

• Allows different languages, punctuation, symbols and emojis.

File Size of Text File Equation

bits per character * number of characters

e.g. A small text file uses ASCII, (8 bits per character) there are 300 characters in the file.

→ 300 × 8 = 2,400 bits

→ 300 bytes / 0.3 kilobytes

Pixel

Individual dots of colour stored as binary codes.

Resolution - Impact on image quality and file size

Total number of pixels in an image.

• A higher resolution means more pixels are used to represent the image, increasing image quality.

  → Also increases file size as more data is needing to be stored.

Bit (colour) depth - Impact on image quality and file size

Number of bits used to represent the colour of each pixel, determining the number of colours displayed.

• Higher bit depth means more colours can be represented, increasing image quality.

  → Also increases file size as more data is needing to be stored.

Metadata and examples

Additional data about a file.

• Height and width in pixels

• Colour depth

• Resolution

• Date created

• File type

• Author details

Bitmap Images and Characteristics

Image made of pixels where each pixel is assigned a binary value and represents a single dot of colour.

• Increased number of pixels = Increased resolution = Increased quality

• Larger file size - storing information of every pixel

• Suitable for photos, images capturing real-world objects.

• Re-sizing issues - stretching or shrinking pixels leads to quality loss.

Vector Images and Characteristics

Images created using mathematical equations and points, rather than individual pixels.

• Usually smaller in file size - only store mathematical information

• Ideal for illustrations, Logos and Web design - can be rescaled without quality loss

• Resize-ability - can be resized without quality loss, not pixel based

File size of Bitmap Equation

file size = resolution * colour depth

→ resolution = width of pixels * height in pixels

→ colour depth == bit depth

Converting Analogue Sound to Binary

Analogue sound waves must be digitally recorded and stored as binary.

• To record the sound, the amplitude (height) of the analogue sound waves is measured and recorded in binary at specific intervals.

Sampling an Analogue Sound Wave

Digital sampling is discrete (separate) and not continuous like analogue waves.

• To get the highest quality sound, sample rates must increase (samples per given time) to recreate the analogue wave as closely as possible.

Sample Rate

Number of samples taken per second, measured in Hertz.

• Higher sample rate means better audio quality - digital wave resembles the analogue wave more closely.

  → Larger files sizes - more data is stored for each individual sample.

Bit Depth - Impact on sound quality and file size

Number of bits available to represent each sample.

→ Common bit depth is 16 bits.

• Higher bit depth means more bits available to be used for each sample - quality increase as digital wave more closely resembles the analogue wave.

• Increase file size - each sample stores additional bits.

Sound File Size Equation

sound file size = sample rate * bit depth * duration

E.g. A short audio sample has a bit depth of 4 and a sample rate of 10 samples per second. The clip is 15 seconds long.

→ 4 × 10 × 15 = 600 bits

→ 600 / 8 =75

→ 75 bytes

Compression and benefits

Process of making a file’s size smaller.

• Files take up less storage space.

• Files can be transferred quicker.

• Files can be read from or written to quicker.

Lossy Compression

Method of data compression that reduces file size by permanently removing some data.

• Algorithms remove data that is least likely to be noticed

• Original file cannot be restored from compressed version

• Suitable for images and videos

Lossless Compression

Method of data compression where file size is reduced without permanently removing any data, meaning no quality is lost.

• Algorithms looks for patterns in the data - repeated data is only stored once, including information about restoration

• Original file can be restored

• Suitable for text files and audio