CH 06: Memory and Data Storage

File Formats

Musical Instrument Digital Interface (MIDI)

• associated with the storage of music files
  • communications protocol that allows interaction between electronic musical instruments
  • does not contain any sounds
  • uses 8-bit serial transmission
  • operates on 16 different channels numbered 0 to 15
  • Examples of MIDI commands:
    • note on/off: indicates a key has been pressed/released
    • Key pressure: indicates how hard the keys have been pressed
• Pitch Byte: tells MIDI device which note to play
• Velocity Byte: tells device how loud to play the note
• MIDI files saved in the computer has file extension .mid
• Sequencer Software: interprets raw .mid files
• MIDI files are considerably smaller than MP3 files

MPEG-3 (MP3) and MPEG-4 (MP4)

MPEG-3 (MP3)

• uses audio compression to convert music and other sounds to an MP3 file format
  • reduces size by about 90%
• used in devices such as MP3 players, computers, and mobile phones
• File Compression: software used to covert CD files
  • less but satisfactory quality than CD files
• Perceptual Music Shaping: removes sounds that the human ear can’t hear properly
• MP3 files are known as Lossy Format: part of the original file is lost after compression
• Bit Rate: number of bits per second used when creating files

MPEG-4 (MP4)

• allows the storage of multimedia files rather than just sound
  • music, videos, photos, and animations can be stored in MP4 format

Joint Photographic Experts Group (JPEG)

• another form of lossy format due to compression
• reduces photo file size by reducing Picture Resolution: number of pixels per centimeter
  • reduces raw bitmap image by a factor of 5 to 15
• Raw Bitmap can be a TIFF or BMP image (.TIF or .BMP)
• TIFF and BMP files have the highest quality

Text and Numbers

• usually stored in an ASCII format. In spreadsheets, numbers can be stored in different formats:
  • real (1,2,3,4)
  • integer (3)
  • date (12/08/2016)
  • time (19:34:54)
  • currency ($56.65)
• number files tends to be lossless under file compression
• When using ASCII format to store text on file then the table below is used to store each of the characters:

• The word “computer” can be stored as:

    “67 79 77 80 85 84 69 82” or

    “%43 %4F %4D %50 %55 %54 %45 %52”

• To compress text files, repeated sections of words are put into a data dictionary to be substituted into a single bit of data
  • For example the sentence:

    “THIS SECTION SHOWS YOU HOW THIS WOULD WORK” * “THIS” will be replaced by “1” * “HOW” will be replaced by “2” * “OU” will be replaced by “3”

• The compressed text will now be:

    “1 SECTION S2S Y3 2 1 W3LD WORK”

• once decompressed, the replaced sections of words will be converted back to its original form hence lossless compression

Lossless and Lossy File Compression

Lossless File Compression

• all the data bits from the original file are reconstructed when file is uncompressed
  • important for files where loss of data will be disastrous like a spreadsheet file

Lossy File Compression

• file compression algorithm eliminates unnecessary bits of data as seen in MP3 and JPEG formats
  • applicable for files wherein loss of data does not degrade quality

Memory and Storage

• has three distinct groups
  • Primary Memory
  • Secondary Storage
  • Off-line storage

Primary Memory

Random Access Memory (RAM)

• volatile/temporary memory
• used to store
  • data
  • files
  • part of operating system that are currently in use
• can be written to or read from and the contents of the memory can be changed
• the larger the size of RAM the faster the computer will operate

RAM is much faster to write and read from but it is very volatile

• Dynamic RAM (DRAM): consists of chips that are made up of a number of transistors and capacitors
  • Capacitors: holds the bits of information (0 or 1)
  • Transistors: acts like a switch; allows chip control circuitry to read or change capacitor value
• Dynamic Ram needs to be constantly refreshed (capacitors must be recharged every 15 microseconds)
• Advantages of DRAMs over SRAMs:
  • less expensive to manufacture
  • consume less power
  • higher storage capacity
• Static Ram (SRAM)
  • does not need to be constantly refreshed
  • makes use of “flip flops” which hold each bit of memory
  • much faster than DRAM in data accessing
  • SRAM is used in the processor’s Memory Cache where most program instructions are stored

Read Only Memory (ROM)

• non-volatile/permanent memories
• often used to store start-up instructions
• can only be read; cannot be rewritten
  • Example application and comparison of ROM and RAM in remote-controlled toy car:

Secondary Storage

Hard Disk Drives (HDD)

• most common method to store data on a computer
  • digital data is stored on the magnetic surfaces of the disks or platters
  • spins at about 7000 times a second
  • each platter has two surfaces which can be used to store data
• Data is stored on the surface in sectors and tracks
  • a sector on a given track will contain a fixed number of bytes

• HDDs have slower data access compared to RAM. Constant read-write movement will cause an increase of latency
• Latency: time it takes for a specific block of data on a data track to rotate around to the read-write head
  • extreme effects of latency can have messages pop up such as “please wait” or “not responding”

Solid-State Drives (SSD)

• has no moving parts; does not rely on magnetic properties
  • data is received at the same rate
  • works by controlling the movement of electrons within NAND chips producing non-volatile rewritable memory
• Electronically Erasable Programmable Read-Only Memory (EEPROM)
  • more expensive than NAND technology
  • allows data to be read or erased
  • more useful in certain applications where data needs to be accessed or erased in byte-sized chunks
• Advantages of SSD over HDD:
  • more reliable
  • considerably lighter
  • don’t have initial “warm up” stage
  • lower power consumption
  • very thin form factor
  • faster data access
• Main drawback of SSDs is the longevity of the technology (Most rated for only 3 years of read-write)

Off-line Storage

CD/DVD Disks

• both CDs and DVDs are described as Optical Storage Devices which use laser light to read and write data on the surface of disks
• both use a thin layer of metal alloy or light-sensitive organic dye to store data
• both use a single, spiral track that runs from the center of the disk to the edge
• DVD technology uses dual-layering: increases the storage capacity by joining together two layers of standard DVD with a transparent spacer (polycarbonate), and a very thin reflector
• DVDs use lasers with a wavelength of 650 nanometers; CDs use lasers with a wavelength of 780 nanometers
• The shorter the wavelength, the greater the storage capacity

DVD-RAM

• uses a number of concentric tracks
• concentric tracks allow simultaneous read and write operations to take place

• have greater longevity (30 years); ideal for archiving

Blu-ray Disks

• uses blue laser instead of red; blue laser has a wavelength of 405 nanometers
• can store up to five times more than normal DVD
• uses a single 1.1 mm polycarbonate disk
• Birefringence: light is refracted into two separate beams causing reading errors
  • since Blu-rays only uses one layer, it does not suffer from Birefringence
• automatically come with a secure encryption system which helps prevent piracy and copyright infringement

Differences between CDs, DVDs, and Blu-ray

• DVDs:
  • mostly used in movies and games
  • used in supplying software
  • back-up systems for photos, music, and multimedia files)
• CDs:
  • used in supplying software
  • back-up systems for photos, music, and multimedia files)
• Blu-rays:
  • mostly used in movies and games

Future of Optical Media

USB Flash Memories

• uses solid-state technology
• usually connects to a computer through USB port
• small, lightweight devices; suitable for file transfers between computers
• can be used as a small back up device for multimedia files

eXtreme Digital (XD) or Secure Digital (SD) Cards

• allow photos to be transferred from the camera to computer via USB port
• made up of NAND chips

Removable Hard Disk Drives

• essentially HDD but can be connected externally to a computer using one of the USB ports
• can be used as a back up device with huge storage capacity