Representing Images, Sound and other data

Question 1

What are analogue sounds?

Answer 1

Analogue data and signals vary in a continuous way.
For example, the output from a microphone into a tape recorder is an analogue signal that varies as a function of the pressure of the sound.

Question 1

What is a bit pattern?

Answer 1

A bit pattern is an arrangement of binary digits arranged in a sequence; they can be used to represent text (ASCII and Unicode) as well as images (bitmaps and vector graphics), video and sound.

Question 2

What are Digital sounds?

Answer 2

Digital data and signals consist of discrete discontinuous binary digits or a sequence of 0s and 1s.
For example, a sound wave is sampled at certain times and stored as a series of numbers in a digital device

Question 3

Why is it important to convert ADC to digital?

Answer 3

It is necessary to convert analogue signals using an analogue-to-digital converter (ADC) into a digital format so that they can be input and processed in a computer. The output from a computer may need to be converted into an analogue signal using a
digital-to-analogue converter (DAC) to interface with an analogue device

Question 4

How do you convert analogue to digital? - (ADC)

Answer 4

Analogue to digital converters (ADCs) receive signals input via an analogue sensor in the form of voltage waveforms which are converted into a digital format so they can be read into a computer.

Question 5

How do you convert digital to analogue? - (DAC)

Answer 5

Digital to analogue converters convert a discrete digital signal into a continuously varying analogue signal; most commonly, this is to an analogue sound signal that is used to drive audio speakers.

Question 6

What are Digital images composed of?

Answer 6

Digital images are composed of a series of pixels or dots, so a pixel is the smallest element in a digital image

Question 7

What is colour depth?

Answer 7

Colour depth is the number of bits used for each pixel, where 1 bit is black and white and 8 bits will give 256 colours.

Question 8

How is Image Resolution calculated?

Answer 8

Image resolution is calculated using the pixel dimensions (height x width).

Question 9

What is display resolution?

Answer 9

Display Resolution is the number of dots per inch (DPI).
More dots per inch give a better image resolution.

Question 10

What is Metadata?

Answer 10

Metadata is ‘data about data’; metadata in an image file provides details of properties such as the image dimensions and colour depth.

Question 11

Why is metadata used by a computer?

Answer 11

Metadata is used by the computer to help it interpret an image file, since all data is basically stored in binary format

Question 12

How do you calculate image storage?

Answer 12

Storage = width (pixels) x height (pixels) x colour depth (bits per pixel)

Question 13

What are vector graphics?

Answer 13

Vector graphics are produced in graphics packages and are created using a series of objects and their coordinates.

Question 14

What are Objects?

Answer 14

Objects are made up of a series of geometric shapes and a set of coordinates for the object.

Question 15

How do you edit objects with the images?

Answer 15

• Shape of the object
• Colour of lines or fill in an object
• Size of object and thickness of lines
• Reference position, by dragging the object to a new position in the editor

Question 16

What are bitmapped graphics?

Answer 16

-Bitmap is created from rows of different colored pixels that together form an image.
- Bitmap images store individual pixels.
- Where bitmapped images are resized, the image quality is degraded and can appear fuzzy.

Question 17

What are some features of vector graphics?

Answer 17

• Vector graphics are used in engineering to create accurate and precise technical drawings. They are also used to create clip art and logos.
• Vector graphics are made up of objects and their coordinates, taking up less storage space and memory than bitmapped images.
• Vector graphics can be rescaled or resized with no loss of quality.
• Vector graphics use more processing power when editing.

Question 18

What is sampling rate?

Answer 18

Sampling rate (measured in Hz) is the number of samples taken per second from the analogue input to create a digital signal

Question 19

What is Nyquist theorem?

Answer 19

Nyquist’s theorem indicates that the sound must be sampled at twice the highest analogue input frequency to create an accurate representation of the original input waveform.

Question 20

What is sampling resolution?

Answer 20

Sampling resolution (or audio bit depth) is the number of bits used to store each sample.

Question 21

What is MIDI?

Answer 21

MIDI (Musical Instrument Digital Interface) is a protocol that is used to synthesise musical instruments; it is used for editing, recording and playing back music input from keyboard, voice and other musical instruments, such as guitars or drums.

Question 22

What are the advantages of MIDI?

Answer 22

• Music data that has been loaded onto the computer can be arranged and manipulated in many different ways.
• A musician or music producer can make use of overlays to create a whole band sound.
• It is straightforward to create a musical score from the edited music

Question 23

What is data compression?

Answer 23

Data compression reduces file sizes to allow download times and file storage requirements to also be reduced.

Question 24

What is loseless compression?

Answer 24

Lossless compression techniques allow the original data to be perfectly reconstructed, resulting in no loss of data.

Question 25

What are the advantages and uses of loseless compression?

Answer 25

• An image can be compressed to low resolution for display on a website, but may also be needed in a higher resolution in a publication.
• A program may be compressed to be downloaded over the Internet, but it will need to be expanded to an exact copy of the original file before it runs.
• A text document might be compressed into a zip file for attachment and transmission using email. Again the document will need to be expanded to an exact copy of the original to ensure that it can be understood.

Question 26

What is lossy compression?

Answer 26

Lossy compression techniques result in a loss of data, so the original data cannot be perfectly reconstructed.

Question 27

What are the advantages and uses of lossy compression?

Answer 27

• A copy of an image might be compressed to a small file size for display on a website, but there is no need to expand back to an exact copy of the original file.
• In sound files unnecessary data can be reduced to compress the files, but overall output quality is acceptable for the user.

Question 28

What is run length encoding?

Answer 28

Run-length encoding (RLE) is a form of lossless compression where a sequence that contains a repeated value of data is replaced by a single value of that data with a count of the number of times it occurs

Question 29

What is dictionary compression?

Answer 29

The dictionary-based compression method (sometimes known as a substitution technique). The dictionary or look-up table contains encoded bit strings that contain fewer bits than the original code

Question 30

Why does the encoded bits of Dictionary compression contain a fewer bits than the original code?

Answer 30

• Variable-length strings of symbols are encoded as single symbols or tokens.
• The token is used as the index for the look-up table or dictionary.
• Compression takes place where the tokens are smaller than the original variable-length strings

Question 31

What type of method is dictionary compression?

Answer 31

• loseless compression

Question 32

What is Encryption?

Answer 32

Encryption is used to make stored data more secure from hackers, by making it unreadable to people who do not have the key to decrypt or decode it. This method is commonly used to protect data transmitted over the Internet.

Question 33

How does encryption operate?

Answer 33

Encryption operates by modifying plain text using an encryption algorithm; this takes place by using an encryption key which stipulates how the message will be encoded. An authorised user is able to decode this message by using a secret decryption key based on a decryption algorithm

Question 34

What is caesar cipher?

Answer 34

The Caesar cipher is a commonly used and simple substitution cipher; using this technique each of the plaintext letters in a message is replaced by a different letter of the alphabet a fixed position from the original letter.

Question 35

What is a ciper algorithm?

Answer 35

A cipher algorithm or cryptography is the process of creating ciphertext by scrambling plaintext (encryption) and turning it back to the original plaintext (decryption).

Question 36

What is a vernam cipher?

Answer 36

The Vernam cipher or ’one-time pad cipher’, is a digital data stream cipher that is combined with a same length random stream of date to create the ciphertext, based on the Boolean exclusive OR function

Question 37

Why is a vernam cipher known as a one time pad?

Answer 37

• It must only be used once.

Question 38

What happens if the key is chosen and used only once?

Answer 38

If the key is randomly chosen and used only once, unlike other ciphers, the resultant ciphertext will be impossible to break no matter how much time is spent trying to crack the code, so the Vernam cipher is a perfect form of security.

Question 39

What is computational security?

Answer 39

Computational security is a term used to indicate the effectiveness of a cipher.

Question 40

What is vernam cipher considered as?

Answer 40

As stated, the Vernam cipher is considered to be 100% mathematically secure

Question 41

Where are bitmap images used?

Answer 41

Bitmapped images are used in digital photography and website applications, where they provide realistic images.

Question 42

What is the advantage of bitmap images over vector graphics?

Answer 42

Bitmap images use less processing power than vector graphics.

Question 43

What is the disadvantage of bitmap images over vector graphics?

Answer 43

• Bitmap images store individual pixels and, therefore, take more storage space and memory than vector images.