SLR 13

Sound Key Phrase

Measure the amplitude of the wave at regular intervals per second and store as a binary value

Analogue Sound

• Naturally occurs

• Continuous form

• Smooth curves

Digital Sound

• Converted from analogue to a discrete format

• Looks boxy

• Still continuous

• Stored by sound samples – taken at regular intervals

Sound Graphs

Y = bit depth – number of binary values

X = Sample rate/ frequency = Hz – 1 Hz = 1 sample/second

Nyquist’s Theorem

• Bit depth = double the frequency we can hear

• Higher bit depth = more accuracy

File Size

• Frequency x Bit Depth x Seconds

MIDI

• Computer generated sound – ex. EDM

• Musical Instrument Digital Interface

• Need to know what it can control:

  • Event messages

  • Pitch

  • Duration

  • Timbre

  • Vibrato

  • Volume

  • Tempo

Positives of MIDI

• Smaller file size than analogue

• Easy to learn – drag and drop interface

Negatives of MIDI

• Doesn't sound as authentic

Compression

• Doesn't sound as authentic

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Compression

• Reduce file size – create free space

• Not needed as much now – more built in storage

• Needed to send/ upload files – can be too large upload

• Needed on websites – otherwise it will load slowly

• Mobile phone networks and ISPs

• Streaming sites – music and videos

Lossy Compression

• Permanently loses data

• If too much detail is lost – it becomes unusable – e.g. on a text file the loss of letters will cause confusion

• Saves more space than lossless but reduces quality

• Group together pixels with a similar colour – loss of quality is not very noticeable

• Doesn’t work on vector images – drawing list

• Sound – removes sounds outside the frequency we can hear - MP3

Lossless Compression

• Data isn't removed permanently so an algorithm can be used to restore the original

• Uses repeated data – doesn't compress much so file size is larger than lossy

• File quality is exactly the same

• Run Length Encoding – lossless technique - 2 white, 6 black, 8 white...

• Records the value and how many times it repeats

• Compresses less with more colours – could have a row which cannot be compressed – each pixel is a different colour

• Can't be used with vector images

• Dictionary based – each repeated pattern is given a binary value and these are stored in a “dictionary”

Encryption

• Transformation of data from plaintext to ciphertext – prevents unauthorised access

• Use a key to undo the cipher – key is generated from the type of encryption algorithm

• Cracking – if you do not have the key but try to decrypt the cipher

Caeser Cipher

• invented by Julius Caeser – shift the alphabet n positions – n is the key – needs a maximum of 25 attempts in a brute force attack

• Good cipher but resources(examples, key) , time, can a human work out the cipher

Cryptoanalysis

the process of cracking a cipher using time and resources

Non-random Key

• the process of cracking a cipher using time and resources

• a cipher with a non-random key is susceptible to cryptanalysis attacks if given enough time and resources

• even random keys are not random – mathematically generated and so can be solved

Truly Random Key

• collected from something physical - could be called a phenomenon

  • White noise

  • Radioactive decay

  • Timing of a hard disk read/write head

Vernam Cipher

• Created one-time pad ciphers – offer perfect security if correctly used

• Three conditions must be met for the one-time pad ciphers to run:

  • Key is equal to or longer than the plaintext message

  • The key is truly random

  • The key is used once, then destroyed – must be shared securely in person