Representing Sound

Question 1

What is analogue data?

Answer 1

Data that varies in a continuous manner. E.g. sound

Question 2

What is digital data?

Answer 2

Data that takes the form of discrete values.

Question 3

How does an analogue to digital converter (ADC) work?

Answer 3

Samples of the analogue signal are taken at regular intervals.

The amplitudes of the samples are quantised. An integer number is given to approximate the amplitude of the wave.

Each integer value is encoded as a binary value.

Question 4

What factors affect the quality of a recording?

Answer 4

Sampling rate and sampling resolution.

Question 5

What is the definition of sampling rate?

Answer 5

The number of samples that are taken per second.

Question 6

What is the definition of sampling resolution?

Answer 6

The number of bits used per sample.

Question 7

What is Nyquist’s Theorem?

Answer 7

The sampling rate must be at least twice the rate of the highest frequency in the original sound.

This means theorem gives us the minimum sampling rate.

Question 8

What is the definition of frequency?

Answer 8

The number of waves per second.

Question 9

When are digital to analogue converters (DAC) used?

Answer 9

When playing digital recordings e.g. through a speaker.

Question 10

What is the principle of operation of DAC?

Answer 10

An analogue voltage is created equivalent to the bit value of the samples.

The voltages are passed through a filter to smooth the transition between voltage levels.

Question 11

What is MIDI?

Answer 11

A way of representing the sounds made by an instrument.

MIDI does not store itself (it’s not a recording). It stores information that describes the sound (using event messages).

Question 12

How does MIDI represent music?

Answer 12

Music is represented as a sequence of MIDI event messages.
Data present in these messages include volume / velocity / duration / instrument etc.

Question 13

What are the advantages of using MIDI?

Answer 13

• more compact representation
• easy to modify/edit notes
• a simple method to compose algorithmically