Week 8 Sound
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卡片预览 (11 张)
Analogue data
Data that varies in a continuous manner e.g. sound
Digital data
Data that takes the form of discrete values
Principles of operation of an ADC [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.
Principles of operation of a DAC [2]
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.
Sampling Rate
The number of samples that are taken per second
Sampling Resolution
The number of bits used per sample
Frequency
Number of waves (passing through a point) per second
Nyquist’s Theorem
The sampling rate must be at least twice the rate of the highest frequency in the original sound; This theorem gives us the minimum sampling rate.
Describe how music is represented using MIDI. [2]
• Music represented as a sequence of MIDI events / instructions; • Playback is a combination of event messages in a specified order; • Examples of data: Channel, Note on/off, Pitch/frequency/note number, Volume/loudness, Velocity, Key pressure/aftertouch, Duration, Instrument; • MIDI messages are 2–3 bytes long; • First byte is a status byte (others are data bytes); MSB=1 indicates status byte, 0 indicates data bytes;
Explain two advantages of using MIDI instead of sampled sound to represent music. [2]
• File sizes are smaller / more compact representation; • Easy to modify or edit at note level; • Easy to change instruments; • Simple to compose algorithmically; • Musical score can be generated directly; • Records musician’s inputs, not sound; • No data lost through sampling; • Better quality (no noise / no sampling error).
Equation for calculating sound file size in bits
Sampling resolution x Sampling rate x Length (seconds)