Transcript Audio Technology (Part 2)

SWE 423: Multimedia Systems
Chapter 3: Audio Technology (2)
Audio Signal Representation
• Waveform representation
– Focuses on the exact representation of the
produced audio signal.
• Parametric form representation
– Focuses on the modeling of the signal
generation process.
– Two major forms
• Music synthesis (MIDI Standard)
• Speech synthesis
Waveform Representation
Audio
Source
Human
Ear
Audio
Capture
Playback
(speaker)
Sampling &
Digitization
Storage or
Transmission
Receiver
Digital to
Analog
Audio Generation and Playback
Digitization
• To get audio (or video for that matter) into a
computer, we must digitize it (convert it into
a stream of numbers).
• This is achieved through sampling,
quantization, and coding.
Example Signal
Amplitude
Sampling
• Sampling: The process of converting
continuous time into discrete values.
Sampling Process
1. Time axis divided into fixed intervals
2. Reading of the instantaneous value of the
analog signal is taken at the beginning of
each time interval (interval determined by
a clock pulse)
3. Frequency of clock is called sampling rate
or sampling frequency
• The sampled value is held constant for the
next time interval (sampling and hold circuit)
Sampling Example
Amplitude
Quantization
• The process of converting continuous
sample values into discrete values.
– Size of quantization interval is called
quantization step.
– How many values can a 4-bit quantization
represent? 8-bit? 16-bit?
• The higher the quantization, the resulting sound
quality .............
Quantization Example
Amplitude
Coding
• The process of representing quantized
values digitally
Analog to Digital Conversion
Amplitude
MIDI Interface
• Musical sound can be generated, unlike
other types of sounds.
• Therefore, the Musical Instrument Digital
Interface standard has been developed
– The standard emerged in its final form in
August 1982
– A music description language in binary form
• A given piece of music is represented by a sequence
of numbers that specify how the musical instruments
are to be played at different time instances.
MIDI Components
• An MIDI studio consists of
– Controller: Musical performance device that generates
MIDI signal when played.
• MIDI Signal: A sequence of numbers representing a certain
note.
– Synthesizer: A piano-style keyboard musical instrument
that simulates the sound of real musical instruments
– Sequencer: A device or a computer program that
records a MIDI signal.
– Sound Module: A device that produces pre-recorded
samples when triggered by a MIDI controller or
sequencer
MIDI Components
MIDI Data
MIDI File Organization
Track 1
Header
Chunk
Track
Header
Track 2
Track Track
Chunk Header
Track
Chunk
Actual Music Data
• Describes
–
–
–
–
–
Start/end of a score
Intensity
Instrument
Basis frequency
....
Status
Byte
Data
Bytes
Status
Byte
Data
Bytes
MIDI Data
• MIDI standard specifies 16 channels
– A MIDI device is mapped onto one channel
• E.g. MIDI Guitar controller, MIDI wind machine,
Drum machine.
– 128 instruments are identified by the MIDI
standard
•
•
•
•
•
Electric grand piano (2)
Telephone ring (124)
Helicopter (125)
Applause (126)
Gunshot (127)
MIDI Instruments
• Can play 1 single score (e.g. flute) vs.
multiple scores (e.g. organ)
• Maximum number of scores that can be
played concurrently is an important
property of a synthesizer
– 3..16 scores per channel.
3D Sound Projection
• After experimentation, it was shown that the
use of a two-channel sound (stereo)
produces the best hearing effects for most
people.
Spatial Sound
• Direct sound path: The shortest path between the sound
source and the auditor.
– Carries the first sound waves to the auditors head.
• All other sound paths are reflected
• All sound paths leading to the human ear are influenced by
the auditor’s individual head-related transfer function
(HRTF)
– HRTF is a function of the path’s direction (horizontal and vertical
angles) to the auditor.
Pulse response in a closed room