Download Audio Players 3 Observations about Audio Players

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Audio Players
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Introductory Question
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Audio players record sound in digital form but
play it in analog form. The transformation
from digital to analog form takes place
A.
before the final amplifier inside the player.
at the outlet into which you plug the earpieces.
in the earpieces themselves.
B.
C.
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Observations about Audio Players
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They are part computer, part sound system.
They require electric power, typically batteries.
They reproduce sound nearly perfectly.
They are sensitive to static charge.
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4 Questions about Audio Players
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How does an audio player “store” sound?
How does it move sound information around?
How does the audio player’s computer work?
How does the audio player’s amplifier work?
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Question 1
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How does an audio player “store” sound?
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Representing Sound
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There is no sound inside an audio player
It uses representations of sound information,
sequences of air pressure measurements
 that contain everything needed to recreate the
sound.
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It stores and retrieves them in digital form,
prepares them for playback in analog form,
 and finally uses them to reproduce sound itself.
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Digital Representation
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Several discrete “symbols” represent a number
A symbol can be any discrete physical quantity:
a positive or negative charge on a capacitor
 an integer value of voltage on a wire
 a north or south magnetic pole on a magnet
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The symbols collectively represent one number,
so this symbol approach is insensitive to noise
 and digital representations can be “perfect.”
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Analog Representation
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One physical quantity represents one number
Any continuous physical quantity can be used:
the voltage on a wire,
 the current in a circuit,
 the strength of a permanent magnet.
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Each physical quantity represents a number,
so this analog approach is sensitive to noise
 and analog representations are “imperfect.”
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The Player’s Representations
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An audio player uses
digital representation for storage and retrieval,
 but analog representation for the actual playback.
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Storage and retrieval involves a digital computer
Playback involves an analog amplifier
Between them is a digital-to-analog converter
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Question 2
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How does it move sound information around?
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MOSFET Transistors
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The electronic components we’ve encountered
so far are all relatively passive:
Wires: carry current from place to place
 Capacitors: store charge
 Resistors: imperfect conductors that drop voltage
 Diodes: block reverse current flow
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To manipulate charge, we need active switches
MOSFET transistors are electronic switches that
allow tiny charges to control large current flows
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MOSFET Transistor Off
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Two back-to-back pn-Junctions, with gate above
Normally does not conduct electricity at all
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MOSFET Transistor On
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Charge on gate can change the channel type
so entire device becomes one type and conducts.
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MOSFET Summary
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An electrically controllable resistor
A tiny amount of charge alters its resistance
can manipulate digital information by switching
symbols on or off.
can manipulate analog information by
controlling charge, current flow, or voltage.
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Question 3
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How does the audio player’s computer work?
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Audio Player’s Digital Computer
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Computers perform logical operations with bits
A bit is a base-two digit
 so it can hold one of only two symbols: 0 or 1.
 Bit-wise representation of numbers is called binary
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Two of the simplest bit logical operations are
inversion (NOT)
 not-and (NAND)
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Any function can be built from these two
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Inverter (NOT)
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Takes one input bit, provides one output bit
Output symbol is inverse of input symbol
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Not-AND (NAND)
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Takes two input bits, provides one output bit
Output is inverse “and” of input symbols
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CMOS Logic
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Bits are represented by charge
The symbol “1” is represented by positive charge
 The symbol “0” is represented by negative or no
charge
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Logic is built from n-channel and p-channel
MOSFETS in complementary pairs
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CMOS Inverter
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Input charge delivered to two complementary
MOSFETs
Positive charge on input
delivers negative charge
to output
Negative charge on input
delivers positive charge
to output
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CMOS NAND
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Positive on both inputs delivers negative charge
to output
Negative on either input
delivers positive charge
to output
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Question 4
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How does the audio player’s amplifier work?
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Audio Player’s Audio Amplifier
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Three circuits:
Input circuit: current/voltage represents sound
 Output circuit: amplified “sound” current/voltage
 Power circuit: provides power for amplification
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Amplifier produces “enlarged” copy of input
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Amplifier Components
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Diodes – one-way devices for current
Capacitors – store charge, shift voltages
Resistors – provide voltage drops, limit current
Transistors – control current flow
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Resistors
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Simple ohmic devices
Voltage drop is proportional to current
 Resistance is the proportionality constant
 Many values of resistance are available
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Reduce a current’s voltage
Produce a current proportional to voltage
Limit current based on voltage drop
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Amplifier (Part 1)
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As resistance of MOSFET drops:
Current from +9V to 0V increases
 Voltage drop of 50 resistor increases
 Voltage at “A” goes down
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Amplifier (Part 2)
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100K resistor ensures that MOSFET is
partially conducting
If it is off, gate becomes more +
 If it is on, gate becomes less +
 Balance is struck at partly +
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Voltage at “A” is about 4.5V
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Amplifier (Part 3)
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Any charge flowing through input circuit is
placed on the Gate
Voltage “A” changes
Input capacitor shifts
charge voltage so that
it matches gate voltage
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Amplifier (Part 4)
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Changes in voltage “A” cause changes in output
current
Output capacitor shifts
charge voltage so that it
matches speaker voltage
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Earpieces
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Sound is reproduced by a moving surface
Surface is pushed and pulled electromagnetically
Surface’s wire coil is surrounded by a magnetic field,
 so current in that coil experiences the Lorentz force
 and the coil accelerates.
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Varying currents causes varying accelerations,
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and the surface reproduces sound
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Introductory Question (revisited)
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Audio players record sound in digital form but
play it in analog form. The transformation
from digital to analog form takes place
A.
before the final amplifier inside the player.
at the outlet into which you plug the earpieces.
in the earpieces themselves.
B.
C.
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Summary about Audio Players
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Represent sound in digital and analog forms
Use MOSFETs to work with sound information
Digital computer comprised of CMOS logic
Analog amplifier based on MOSFETs.