Memory and Storage: Character Sets and Sound

• 1.2 Memory and Storage

Character Sets

• Allow computers to understand letters, numbers, and other characters.
• Logically ordered with the the value for A is lower than B.
• ASCII (American Standard Code for Information Interchange) is a character set.
• In ASCII, each character is given a unique binary code.
• The code is 8 bits/1 byte long offering 256 possible characters.
• Unicode uses 2 bytes making more characters possible, which accommodates languages with thousands of characters.

Storing Sound

• Computers only understand binary, therefore sound has be encoded into binary.
• Sound is broken down into thousands of samples per second, each of which is stored as binary data.
• Sample rate: Measured in Hz, refers to the number of samples per second.
  • More samples leads to more detail, clearer sound requiring more space.
• Bit Depth: Number of bits available for each sample.
  • Higher bit depth equals higher quality but also more space needed.
• Duration: Higher duration leads to longer audio, and therefore more space needed.

Storing Images

• Images are stored as a series of pixels in binary.
• Each pixel has a specific color, represented by a color code.
• Images also contain metadata.
  • Structure of the file.
  • Size of the grid.
  • Other info such as date.
• Resolution: number of pixels in the image.
  • Higher Resolution equals to more pixels, a clearer image, and more space needed.
• Colour Depth: number of bits used to store the colour for each pixel.
  • 1 bit allows 2 values, 2 bits allow 4 values etc.
  • Higher colour depth equals more realistic colours, hence more space needed.

Compression

• Encoding of data to needs fewer bits/bytes to represent it.
• Reduces space needed for storage.
• Data must be decompressed to be used.
• Lossless Compression
  • Compresses data files without losing any of the information.
  • Reversible, the original data can be reconstructed.
  • Not all files can be compressed with this method.
• Lossy Compression
  • Some of the information is lost.
  • It is used when this is acceptable for files like audio.
  • Produces smaller files with decreased accuracy.

Units of Data Storage

• Bit
• Nibble: contains 4 bits
• Byte: contains 8 bits
• Kilobyte (KB): about 1,000 bytes
• Megabyte (MB): about1,000 KB
• Gigabyte (GB): about 1,000 MB
• Terabyte (TB): about 1,000 GB
• Petabyte (PB): about 1,000 TB

Converting Binary to Hex

• Example: converting the binary number 0110 to hex
• Breakdown:
  • 8 4 2 1
  • 0 1 1 0
  • 4 + 2 = 6
• Therefore the value is 6

Converting Between Denary and Binary

• Example: convert denary number 11 to binary
• Breakdown:
  • 8 4 2 1
  • 1 0 1 1
• Therefore the value is 1011

Converting Denary to Hexadecimal

• Example: converting the denary number 62 to a hex number.
• Breakdown:
  • 62 ÷ 16 = 3 R 14
  • 3 ÷ 16 = 0 R 3
• Result: 3E, where E = 14

Binary

• Is a number system made up only of 1s and 0s.
• Only two possibilities (base two number system).
• Computers use binary because the CPU contains transistors, which are either on or off.

Hexadecimal

• Uses the number system 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F.
• There are 16 possibilities so is a base sixteen number system.
• Binary strings are long and difficult to work with, this makes Hexadecimal more useful.
• Hex is easily converted to binary, as there is 1 hex digit per nibble.
• Hex is less prone to error.

Choosing Storage Media

• When choosing a storage media, consider the following factors:
  • Capacity: How much data the storage media can hold.
    • Larger files requires larger amounts of storage.
  • Speed: How quickly the data can be written and read back.
    • Live websites requires faster data accessibility
  • Portability: How easy to move storage media.
    • Consider size and compatibility.
  • Durability: Length of time the storage media is expected to last and how easily damaged.
  • Reliability: How likely the storage media is to fail and how likely errors are to occur.
  • Cost: How expensive the storage media is and any required hardware.

Primary Storage

• Faster than the main storage of the computer.
• Holds data and instructions needed by the CPU.
• RAM (Random Access Memory): Loses its data when the computer is switched off (volatile).
  • It is responsible for saving data about programs currently open.
  • Much faster than a HDD or SSD, so the CPU has to spend less time during the "fetch" part of the process.
  • It is expensive compared to HDD or SSDs.
• ROM (Read Only Memory): Values stored in ROM remain when the computer is switched off (non volatile).
  • Virus attacks are nonexistent.
  • Values stored cannot be accidentally changed.
  • Data is written permanently when constructed.
  • Contains the instructions needed for booting computer.

Virtual Memory

• Simulated memory that is written to a file on the hard drive.
• Allows more memory to be used than there is in the system.
• Needed to run more applications than RAM can support.

Virtual Memory Implementation

• Operating system sets up virtual memory using the Virtual Memory Manager (VMM).
• The VMM creates a file on the hard disk, large enough for the extra memory needed.
• The Operating System can then processes the virtual memory as if it were real memory stored in RAM.
• Swapping or paging is the process operating system employs to move data between RAM and virtual memory.
• Data which processes do not currently need is moved out of the RAM to virtual memory.

Secondary Storage

• Storage devices that are not constantly connected to the computer.
• Storage devices not directly accessible by the system's CPU.
• Used to back up data stored in primary storage.
• When there is a need for larger storage capacity.
• Types of Secondary Storage:
  • Optical Storage: Uses a laser beam to write and read data.
    • CDs and DVDs are examples.
    • Inexpensive, reliable, robust, relatively large capacity.
  • Magnetic Storage: Uses different magnetic patterns to store data.
    • Examples include tape cartridge and hard drive.
    • Large capacity, to store operating systems with other files and programs, reliable and cost-effective.
  • Solid State Storage
    • Data is stored within flash chips.
    • Examples include USB drives and SSDs.
    • Flexible, faster access, for portable devices, smaller in size, robust, easy to use.

Examples of Choosing Storage Media

• A portable barcode scanner uses solid state flash media.
  • Capacity: barcodes consume limited amount of data, therefore high capacity is not of high importance.
  • Speed: flash media is quick that will not hinder the device's operation.
  • Portability: Flash media is small and light, and will easily fit within the scanner
  • Durability: Flash media has no moving parts so damage is reduced.
  • Reliability: Flash media is highly reliable.
  • Cost: Flash media is more expensive than other storage alternatives, but their advantages are significant.
• Films are sold on DVD and BluRay disks
  • Capacity: allows for longer, higher quality movies
  • Speed: slower speeds than flash memory but sufficient for the task.
  • Portability: lightweight, small and commonly used
  • Durability: durable if stored correctly.
  • Reliability: reliable if stored correctly.
  • Cost: very cheap to produce in high volume.

Memory Data Flow

• CPU to Main Memory
• Main Memory to Secondary Storage

Calculating File Sizes

• Sound file size = sample rate x duration (s) x bit depth
• Image file size = colour depth x image height (px) x image width (px)
• Text file size = bits per character x number of characters

Explanation of character sets like ASCII and Unicode and how computers store sound. Sound is encoded into binary by breaking it down into samples per second. Sample rate and bit depth affect sound quality and storage space.