Sound Representation

Question 1

What is the difference between digital and analogue data?

Answer 1

Analogue:
- constantly changing amplitude and frequency
- no defined ranges
- infinite precision
- cannot be represented by computers
Digital:
- discrete and finite values
- limited precision
- expressed as binary numbers

Question 2

How do ADC’s work?

Answer 2

• Regular samples are taken (of the analogue signal);
• Samples are quantised // the height of each sample is approximated to an integer value // height of samples measured // amplitude/volume measured;
• Each integer value is encoded as a binary value // measurements are coded in a fixed number of bits; output the binary numbers as digital signals/voltage levels;

Question 3

How do DAC’s work?

Answer 3

• Digital samples are converted to electrical voltages to reproduce an approximation of an analogue signal

Question 4

How can you determine bit depth or sample resolution? How does this affect the sound?

Answer 4

• The number of binary digits used to represent amplitude
• Higher resolution increases sound quality
• However, also increases file size

Question 5

How to find file size?

Answer 5

• File size = bit rate x time
• File size = bit depth (resolution) x sampling frequency x time

Question 6

What is the Nyquist theorem to determine the correct sampling frequency to use?

Answer 6

• sampling rate (frequency) should be at least 2 x the greatest frequency
• a higher rate than 44,000 Hz is UNNECESSARY because it increases file size AND humans cannot hear

Question 7

How does MIDI work?

Answer 7

• rather than storing samples of sounds, it stores series of event messages
• each event message is a series of instructions to recreate music
• used with electronic musical instruments connected to computer

Question 8

What information is stored in the event messages?

Answer 8

• which note to play
• duration of a note
• which instrument on which to play
• how loud a note is (volume)
• if it should be sustained

Question 9

What are the advantages of MIDI?

Answer 9

• can easily edit the music WITHOUT LOSS OF QUALITY
• notes can be transposed and duration can be altered
• MIDI files are smaller in size
• lossless (no information lost ABOUT NOTES)

Question 10

What are the disadvantages of MIDI?

Answer 10

• sound outputted is dependent on each setup’s instruments
• cannot be used for storing speech
• less realistic sound