Representing Sound Flashcards
(18 cards)
What is an analogue signal?
A continuous signal that varies with time that can take an infinite set of values
What is a digital signal?
A signal that must always take one of a specified range of values and can only change at specified intervals
What is analogue data?
Data that has no limits to the values that it can take. Represents information in the real world
What is digital data?
Data that can only take particular values
What is the sample resolution of a recording?
Number of bits used to represent each sample
What is the sample frequency/rate of a recording?
Number of samples taken per second
What is quantisation?
- The process of assigning an integer value to the amplitude of a signal at a specific point in time
OR - mapping of continuous infinite values to a smaller set of discrete finite values
What is a quantisation error?
The difference between the analogue signal and the closest available digital value at each sampling instant
What is Nyquist’s theorem?
The sampling rate must be at least twice the highest frequency of the signal being sampled such that it can be accurately reproduced.
What frequency are digital recordings stored in?
44.1KHz
What is a MIDI file?
A set of stored instructions (event messages) that can be used to tell musical instruments and synthesisers how to produce a piece of music.
What does a MIDI event message contain?
- when the note should be played
- pitch, volume and duration
- The instrument that should play that note
- Clock signals to coordinate multiple instruments
- Other control data (tempo, special effects)
Advantages of MIDI
- Musical content can be easily edited (notes, volume or duration of note) without LOSS OF QUALITY
- notes can be transposed and duration can be altered
- More compact representation of audio, MIDI files much smaller than mp4/mp3
- No data is lost through sampling
Disadvantages of MIDI?
What the file actually sounded like depends entirely on the instrument/synthesiser being used so the same file can sound totally different in different scenarios
Cant represent speech
Digital to analogue converters DACs
Digital to analogue converters convert digital samples into electrical voltages to reproduce an approximation of an analogue signal
bit rate of audio file
bit rate = sampling frequency x sample size
Sampling Sound
sound travels as analogue waves.
The amplitude of the wave determines the volume of the sound
the frequency of the wave determines the pitch of the sound.
amplitude of sound wave is sampled at regular intervals by ADC
value of each sample’s amplitude is quantised and stored as a binary value
resulting file contains binary representations of the amplitudes of the original sound wave captured at every sampling interval
How do ADC’s work?
- Regular samples are taken (of the analogue signal);
- Samples are quantised
- Each integer value is encoded as a binary value
