PHY2464 -The Physical Basis of Music
PHY
-2464
PHY-2464
Physical Basis of Music
Presentation
Presentation 24
24
Sound
Sound Reproduction
Reproduction and
and Synthesis:
Synthesis:
Mostly
Mostly Analog
Analog Systems
Systems
Adapted
Adapted in
in part
part from
from Sam
Sam Matteson’s
Matteson’s Unit
Unit 44 Session
Session 38
38
Sam
Trickey
Sam Trickey
April
April 11,
11, 2005
2005
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Pres. 24 Sound Reproduction & Synthesis - Analog
Training Wheels of Electricity
Two kinds of charges: “positive” and
“negative”; chemically important examples are
protons and electrons
Conductor - a material in which charge
can move freely (most commonly, metals)
“Voltage” or “potential difference”
is the electrical “push” available to
move charge.
“Current” is the amount of charge
moved per unit time.
PHY2464 -The Physical Basis of Music
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Pres. 24 Sound Reproduction & Synthesis - Analog
Training Wheels of Electricity – cont’d
Waterfall analogy
Height ↔ voltage (potential difference)
Flow (gallons per minute) ↔ current
Units: volts, amperes
R
Ohm’s law: V = IR
I→
V
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Training Wheels of Magnetism
Magnetic field – fundamental physical interaction
between two current-carrying wires
Natural magnets exist because of atomic-level
currents (compass needles) or planetary
properties (Earth’s field)
Maxwell: A time-varying electrical
field inevitably has a time-varying
magnetic field associated with it and
conversely: E(t) ↔ B(t)
PHY2464 -The Physical Basis of Music
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Pres. 24 Sound Reproduction & Synthesis - Analog
Transducer: a device for converting signal
from one form of energy to another
More generally, but abstractly, a system for
mapping information from one representation
to another.
Reciprocity: A transducer that
converts a signal from mode A to
mode B can, in principle, be operated
to convert mode B into mode A.
(Always possible, not always
practical.)
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Pres. 24 Sound Reproduction & Synthesis - Analog
Sound reproduction – map a source sound
(transient!) into a non-transient form for
subsequent playback
Sound amplification – map a source sound into a
faithful replica of itself for simultaneous
(essentially) delivery to hearer(s).
Sound synthesis – map a source signal or signals
into a set of desired sounds.
PHY2464 -The Physical Basis of Music
PHYPHY-2464
Pres. 24 Sound Reproduction & Synthesis - Analog
Analog reproduction - Lay of the land:
•
•
•
•
Requirements: input transducer(s), such as a microphone,
to produce an electrical signal, amplification, recording
medium, recording transducer system, playback transducer
and amplifier system.
Input transducers are “microphones”
microphones” - Common types of
microphones are (1) condenser; (2) magnetic; (3)
dynamic; (4) ribbon; (5) piezoelectric; (6) carbon.
Analog recording media include vinyl record, magnetic
tape, magnetic disk, and optical film.
Playback uses physical principles in which mechanical
oscillations, magnetic domains, and optical density
variations produce electrical signals that are analogous to
the recorded sound.
PHYPHY-2464
Pres. 24 Sound Reproduction & Synthesis - Analog
Aside: An electric guitar transducer is a magnetic pickup
that produces an electrical signal from the timetimevariation of the magnetic field caused by the strings.
Steel Guitar String
N
S
Pickup
PHY2464 -The Physical Basis of Music
PHYPHY-2464
Pres. 24 Sound Reproduction & Synthesis - Analog
The Faraday Effect : a time-varying magnetic
field produces a voltage and current in a
coil.
Current
N
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Pres. 24 Sound Reproduction & Synthesis - Analog
A Condenser microphone uses electric induction to
produce an electrical signal. (Condenser is old name
for capacitor; a device for holding charge)
Charge = V (ε
(εo S/x)
Voltage V
+
+
+
+
Current I
+
+
PHY2464 -The Physical Basis of Music
PHYPHY-2464
Pres. 24 Sound Reproduction & Synthesis - Analog
Magnetic Microphone: a small magnet, attached
to a moving diaphragm induces a voltage
and current in a coil.
Current I
Voltage V
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Pres. 24 Sound Reproduction & Synthesis - Analog
Dynamic Microphone: a diaphragm moves a
small coil in a magnetic field inducing a
voltage and current.
Current I
N
S
N
Voltage V
PHY2464 -The Physical Basis of Music
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Pres. 24 Sound Reproduction & Synthesis - Analog
Ribbon Microphone uses a light weight strip of
metal moving in a magnetic field to produce
electrical signals.
Current I
N
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Voltage V
Pres. 24 Sound Reproduction & Synthesis - Analog
Piezoelectric Microphone: diaphragm bends a
thin crystal which produces a small voltage
when deformed.
+
+
Voltage V
+
PHY2464 -The Physical Basis of Music
PHYPHY-2464
Pres. 24 Sound Reproduction & Synthesis - Analog
Carbon Microphone: a diaphragm compresses a
sealed container of carbon granules,
changing its resistance.
Current I
I = V/R
Voltage V
PHYPHY-2464
Pres. 24 Sound Reproduction & Synthesis - Analog
Microphones:
•
Electrostatic — Induction
• Condenser
•
Magnetic — Faraday Effect
•
•
•
•
•
Magnetic
Dynamic
Ribbon
Piezoelectric — Piezoelectric Effect
Carbon — Ohm’s Law
Note! In every case the sound drives a mechanical diaphragm
whose motion is tranduced into an electrical signal.
PHY2464 -The Physical Basis of Music
PHYPHY-2464
Pres. 24 Sound Reproduction & Synthesis - Analog
Amplifiers (≈1920 →1970 technology)
Vacuum Tube is an electronic device that amplifies
electrical signals by permitting a small voltage to
modulate a much larger current.
Cathode
Vout
Vin
Grid
Plate
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Gain = Vout /Vin
Pres. 24 Sound Reproduction & Synthesis - Analog
Amplification:
Gain G = Vout / Vin
Vout = G Vin
Vout
Vin
PHY2464 -The Physical Basis of Music
PHYPHY-2464
Pres. 24 Sound Reproduction & Synthesis - Analog
Amplifiers (Solid state technology) A Field Effect
Transistor (FET) is a solid state electronic device
that permits amplification of electrical signals.
Gate
Source
Drain
Silicon
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Silicon
Dioxide
Pres. 24 Sound Reproduction & Synthesis - Analog
Analog Recording Media:
•
Mechanical
• Punched paper rolls (player pianos and organs),
punched metal disks (music boxes) – not analog in the
same sense as the rest.
• Edison’
Edison’s Phonograph
• Vinyl LP Records
•
Magnetic
• Magnetic Tape (before that, wire, briefly)
• Magnetic Disk
•
Optical
• Audio Track on motion picture film (“
(“talkie”
talkie”)
PHY2464 -The Physical Basis of Music
PHYPHY-2464
Pres. 24 Sound Reproduction & Synthesis - Analog
Mechanical:
•
Edison’s Phonograph
PHYPHY-2464
Pres. 24 Sound Reproduction & Synthesis - Analog
Mechanical analogs:
• Edison’s Phonograph
• Vinyl LP Records
PHY2464 -The Physical Basis of Music
PHYPHY-2464
Pres. 24 Sound Reproduction & Synthesis - Analog
Analog Recording Media:
• Magnetic
• Magnetic Tape
• Magnetic Disk
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Analog Recording Media:
•
Magnetic
• Magnetic Tape
• Magnetic Disk
FerroFerro-metal
Oxide
Tracks
PHY2464 -The Physical Basis of Music
Pres. 24 Sound Reproduction & Synthesis - Analog
Analog Recording Media:
•
Optical
• Audio Track on motion picture film
Stereo Audio Tracks
Pres. 24 Sound Reproduction & Synthesis - Analog
Analog Recording Media:
•
Optical
• Audio Track on motion picture film
Stereo Audio Tracks
PHY2464 -The Physical Basis of Music
Pres. 24 Sound Reproduction & Synthesis - Analog
Summary (partial):
•
•
•
•
Common types of microphones are (1) condenser;
(2) magnetic; (3) dynamic; (4) ribbon;
(5) piezoelectric; (6) carbon.
Vacuum tubes and transistors provide amplification by
permitting the input to modulate a larger output current
and voltage.
Analog recording media have included mechanical,
magnetic, & optical forms. Magnetic dominates today.
Playback uses physical principles in which mechanical
oscillations, magnetic domains, and optical density
variations produce electrical signals that are analogous
to the recorded sound.