SLR 13 - coding sound and music

Question 1

How does digital to analogue conversion work?

Answer 1

• you use a digital to analogue converter
  the device reads a bit pattern representing an analogue signal
• it outputs an alternating, analogue electrical current

Question 2

What’s the difference between analogue and digital data?

Answer 2

analogue is continuous (smooth curves) whereas whereas digital is discrete (sharp edges and vertical drops)

Question 3

What can analogue sound waves do?

Answer 3

can take ay values and can change as frequently as required

Question 4

What can digital sound waves do?

Answer 4

must always take one of a specified range of values and can only change value at specified intervals

Question 5

How does analogue to digital conversion work?

Answer 5

• sensors (temp sensors and microphone) output an analogue signal
• Computers use ADCs convert them
• the device samples the analogue signal at regular intervals

Question 6

What is frequency?

Answer 6

• given in Hertz
• it determines the number samples taken per second

Question 7

What does a larger number of frequencies mean?

Answer 7

Better reproduction of an analogue signal

Question 8

How do computers represent sound?

Answer 8

as a sequence of samples, each taking a discrete digital value

Question 8

What is sampling rate and what is it measured in?

Answer 8

the number of samples per second (hertz)

Question 9

What is sample resolution?

Answer 9

the number of bits allocated to each sample

Question 10

A higher sample resolution means…

Answer 10

greater audio quality and increased file size

Question 11

What is Nyquist’s theorem?

Answer 11

the sampling rate of a digital audio file must be at least twice the frequency of the sound so that the sound may be accurately represented

Question 12

What does MIDI stand for?

Answer 12

Musical Instrument Digital Interface

Question 13

What is MIDI used with?

Answer 13

electronic musical instruments which can be connected to computers

Question 14

What does MIDI store instead of sound samples?

Answer 14

a series of event messages, each representing an event in a piece of music

Question 15

What information could event messages contain?

Answer 15

• the duration of a note
• the instrument with which the note is played
• the volume of the note
• if a note should be sustained

Question 16

Ads of MIDI?

Answer 16

• allows for easy manipulation of music without a loss of quality
• the duration of notes can be changed and they can be transposed
• MIDI have smaller file sizes and lossless (no info lost)

Question 17

Disads of MIDI?

Answer 17

MIDI can’t be used for storing speech and sometimes results in a less realistic sound

Question 18

What are the two types of compression?

Answer 18

lossy and lossless

Question 19

What is lossy compression?

Answer 19

some information is lost to reduce the file size
could be by reducing the resolution or lowering sample resolution in an audio file

Question 20

What is lossless compression?

Answer 20

there is no loss of information when using lossless compression
reduces file size without reducing quality

Question 21

What are two types of lossless compression?

Answer 21

• run length encoding
• dictionary-based methods

Question 22

What is RLE (run length encoding)

Answer 22

reduces the size of the file by removing repeated information and replacing it with one occurrence of the repeated info followed by the number of times it is to be repeated

Question 23

What is dictionary based lossless compression?

Answer 23

a dictionary containing repeated data is appended to a file

Question 24

How does dictionary based lossless compression effect file size?

Answer 24

- should result in a significant reduction to file size - but the dictionary used to compress the data has to be present in the file, which will increase the file size

Question 25

What is encryption?

Answer 25

the process of scrambling data so that it cannot be understood if intercepted to keep it secure during transmission

Question 26

What is unencrypted information known as ?

Answer 26

plaintext

Question 27

What is encrypted information known as ?

Answer 27

ciphertext

Question 28

What is a cipher?

Answer 28

A type of encryption method

Question 29

How would you decrypt ciphertext?

Answer 29

- the encryption method - the key

Question 30

How do Caesar ciphers encrypt information?

Answer 30

- by replacing characters - one character is always replaced by the same character

Question 31

What are the two types of cipher?

Answer 31

- shift cipher - substitution cipher

Question 32

How do shift ciphers work?

Answer 32

All the letters in the alphabet are shifted by the same amount

Question 33

How do substitution ciphers work?

Answer 33

the letters are randomly replaced

Question 34

Cons of a Caesar cipher:

Answer 34

- Caesar ciphers are easily cracked - once you discover one character the shift cipher can be completely cracked and a key found

Question 35

What is a vernam cipher?

Answer 35

An example of a one-time pad cipher - each key should only ever be used once

Question 36

What are two requirements of the Vernam Cipher?

Answer 36

- random -at least as long as the plaintext that is to be encrypted

Question 37

How does a Vernam cipher work?

Answer 37

1. aligning the characters o the plaintext and then the key 2.Converting each character to binary (using an information coding system) 3. Apply a logical OR when Converting the rest back to a character

Question 38

When do Vernam ciphers offer perfect security?

Answer 38

1. the encryption key is qual to or longer than the plain text message 2. the key is truly random 3. the key is used only once and then destroyed

Question 39

What makes a scheme computationally secure?

Answer 39

if it cannot be cracked in a reasonable time