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Audio PlayersAudio Players

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Introductory QuestionIntroductory Question

Audio players record sound in digital Audio players record sound in digital form but play it in analog form. The form but play it in analog form. The transformation from digital to analog transformation from digital to analog form takes placeform takes place

A.A. before the final amplifier inside the before the final amplifier inside the player.player.

B.B. at the outlet into which you plug the at the outlet into which you plug the earpieces.earpieces.

C.C. in the earpieces themselves.in the earpieces themselves.

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Observations about Observations about Audio PlayersAudio Players

They are part computer, part sound They are part computer, part sound system.system.

They require electric power, They require electric power, typically batteries.typically batteries.

They reproduce sound nearly They reproduce sound nearly perfectly.perfectly.

They are sensitive to static charge.They are sensitive to static charge.

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4 Questions about Audio 4 Questions about Audio PlayersPlayers

How does an audio player “store” How does an audio player “store” sound?sound?

How does it move sound information How does it move sound information around?around?

How does the audio player’s How does the audio player’s computer work?computer work?

How does the audio player’s How does the audio player’s amplifier work?amplifier work?

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Question 1Question 1

How does an audio player “store” How does an audio player “store” sound?sound?

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Representing SoundRepresenting Sound

There is no sound inside an audio playerThere is no sound inside an audio player It uses representations of sound information,It uses representations of sound information,

sequences of air pressure measurementssequences of air pressure measurements that contain everything needed to recreate the that contain everything needed to recreate the

sound.sound. It stores and retrieves them in digital form,It stores and retrieves them in digital form,

prepares them for playback in analog form,prepares them for playback in analog form, and finally uses them to reproduce sound itself.and finally uses them to reproduce sound itself.

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Digital RepresentationDigital Representation

Several discrete “symbols” represent a Several discrete “symbols” represent a numbernumber

A symbol can be any discrete physical A symbol can be any discrete physical quantity:quantity: a positive or negative charge on a capacitora positive or negative charge on a capacitor an integer value of voltage on a wirean integer value of voltage on a wire a north or south magnetic pole on a magneta north or south magnetic pole on a magnet

The symbols collectively represent one The symbols collectively represent one number,number, so this symbol approach is insensitive to noiseso this symbol approach is insensitive to noise and digital representations can be “perfect.”and digital representations can be “perfect.”

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Analog RepresentationAnalog Representation

One physical quantity represents one One physical quantity represents one numbernumber

Any continuous physical quantity can be Any continuous physical quantity can be used:used: the voltage on a wire,the voltage on a wire, the current in a circuit,the current in a circuit, the strength of a permanent magnet.the strength of a permanent magnet.

Each physical quantity represents a number,Each physical quantity represents a number, so this analog approach is sensitive to noiseso this analog approach is sensitive to noise and analog representations are “imperfect.”and analog representations are “imperfect.”

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The Player’s The Player’s RepresentationsRepresentations

An audio player usesAn audio player uses digital representation for storage and digital representation for storage and

retrieval,retrieval, but analog representation for the actual but analog representation for the actual

playback.playback. Storage and retrieval involves a digital Storage and retrieval involves a digital

computercomputer Playback involves an analog amplifierPlayback involves an analog amplifier Between them is a digital-to-analog Between them is a digital-to-analog

converterconverter

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Question 2Question 2

How does it move sound information How does it move sound information around?around?

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MOSFET TransistorsMOSFET Transistors The electronic components we’ve The electronic components we’ve

encountered so far are all relatively passive:encountered so far are all relatively passive: Wires: carry current from place to placeWires: carry current from place to place Capacitors: store chargeCapacitors: store charge Resistors: imperfect conductors that drop voltageResistors: imperfect conductors that drop voltage Diodes: block reverse current flowDiodes: block reverse current flow

To manipulate charge, we need active To manipulate charge, we need active switchesswitches

MOSFET transistors are electronic switches MOSFET transistors are electronic switches that allow tiny charges to control large that allow tiny charges to control large current flowscurrent flows

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MOSFET Transistor OffMOSFET Transistor Off

Two back-to-back pn-Junctions, with Two back-to-back pn-Junctions, with gate abovegate above

Normally does not conduct Normally does not conduct electricity at allelectricity at all

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MOSFET Transistor OnMOSFET Transistor On

Charge on gate can change the Charge on gate can change the channel typechannel type

so entire device becomes one type so entire device becomes one type and conducts.and conducts.

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MOSFET SummaryMOSFET Summary

An electrically controllable resistorAn electrically controllable resistor A tiny amount of charge alters its A tiny amount of charge alters its

resistanceresistance can manipulate digital information can manipulate digital information

by switching symbols on or off.by switching symbols on or off. can manipulate analog information can manipulate analog information

by controlling charge, current flow, by controlling charge, current flow, or voltage.or voltage.

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Question 3Question 3

How does the audio player’s How does the audio player’s computer work?computer work?

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Audio Player’s Digital Audio Player’s Digital ComputerComputer

Computers perform logical operations with Computers perform logical operations with bitsbits A bit is a base-two digitA bit is a base-two digit so it can hold one of only two symbols: 0 or 1.so it can hold one of only two symbols: 0 or 1. Bit-wise representation of numbers is called Bit-wise representation of numbers is called binarybinary

Two of the simplest bit logical operations areTwo of the simplest bit logical operations are inversion (NOT)inversion (NOT) not-and (NAND)not-and (NAND)

Any function can be built from these twoAny function can be built from these two

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Inverter (NOT)Inverter (NOT)

Takes one input bit, provides one Takes one input bit, provides one output bitoutput bit

Output symbol is inverse of input Output symbol is inverse of input symbolsymbol

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Not-AND (NAND)Not-AND (NAND)

Takes two input bits, provides one Takes two input bits, provides one output bitoutput bit

Output is inverse “and” of input Output is inverse “and” of input symbolssymbols

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CMOS LogicCMOS Logic

Bits are represented by chargeBits are represented by charge The symbol “1” is represented by The symbol “1” is represented by

positive chargepositive charge The symbol “0” is represented by The symbol “0” is represented by

negative or no chargenegative or no charge Logic is built from n-channel and p-Logic is built from n-channel and p-

channel MOSFETS in channel MOSFETS in complementary pairscomplementary pairs

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CMOS InverterCMOS Inverter

Input charge delivered to two Input charge delivered to two complementary MOSFETscomplementary MOSFETs

Positive charge on inputPositive charge on inputdelivers negative chargedelivers negative chargeto outputto output

Negative charge on inputNegative charge on inputdelivers positive chargedelivers positive chargeto outputto output

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CMOS NANDCMOS NAND

Positive on both inputs delivers Positive on both inputs delivers negative charge to outputnegative charge to output

Negative on either inputNegative on either inputdelivers positive chargedelivers positive chargeto outputto output

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Question 4Question 4

How does the audio player’s How does the audio player’s amplifier work?amplifier work?

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Audio Player’s Audio Audio Player’s Audio AmplifierAmplifier

Three circuits:Three circuits: Input circuit: current/voltage Input circuit: current/voltage

represents soundrepresents sound Output circuit: amplified “sound” Output circuit: amplified “sound”

current/voltagecurrent/voltage Power circuit: provides power for Power circuit: provides power for

amplificationamplification Amplifier produces “enlarged” copy Amplifier produces “enlarged” copy

of inputof input

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Amplifier ComponentsAmplifier Components

Diodes – one-way devices for currentDiodes – one-way devices for current Capacitors – store charge, shift Capacitors – store charge, shift

voltagesvoltages Resistors – provide voltage drops, Resistors – provide voltage drops,

limit currentlimit current Transistors – control current flowTransistors – control current flow

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ResistorsResistors

Simple ohmic devicesSimple ohmic devices Voltage drop is proportional to currentVoltage drop is proportional to current Resistance is the proportionality constantResistance is the proportionality constant Many values of resistance are availableMany values of resistance are available

Reduce a current’s voltageReduce a current’s voltage Produce a current proportional to Produce a current proportional to

voltagevoltage Limit current based on voltage dropLimit current based on voltage drop

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Amplifier Amplifier (Part 1)(Part 1)

As resistance of MOSFET drops:As resistance of MOSFET drops: Current from +9V to 0V increasesCurrent from +9V to 0V increases Voltage drop of 50Voltage drop of 50 resistor increases resistor increases Voltage at “A” goes downVoltage at “A” goes down

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Amplifier Amplifier (Part 2)(Part 2)

100K100K resistor ensures that resistor ensures that MOSFET is partially conductingMOSFET is partially conducting If it is off, gate becomes more +If it is off, gate becomes more + If it is on, gate becomes less +If it is on, gate becomes less + Balance is struck at partly +Balance is struck at partly +

Voltage at “A” is about 4.5VVoltage at “A” is about 4.5V

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Amplifier Amplifier (Part 3)(Part 3)

Any charge flowing through input Any charge flowing through input circuit is placed on the Gatecircuit is placed on the Gate

Voltage “A” changesVoltage “A” changes Input capacitor shiftsInput capacitor shifts

charge voltage so thatcharge voltage so thatit matches gate voltageit matches gate voltage

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Amplifier Amplifier (Part 4)(Part 4)

Changes in voltage “A” cause Changes in voltage “A” cause changes in output currentchanges in output current

Output capacitor shiftsOutput capacitor shiftscharge voltage so that itcharge voltage so that itmatches speaker voltagematches speaker voltage

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EarpiecesEarpieces

Sound is reproduced by a moving surfaceSound is reproduced by a moving surface Surface is pushed and pulled Surface is pushed and pulled

electromagneticallyelectromagnetically Surface’s wire coil is surrounded by a magnetic Surface’s wire coil is surrounded by a magnetic

field,field, so current in that coil experiences the Lorentz so current in that coil experiences the Lorentz

forceforce and the coil accelerates.and the coil accelerates.

Varying currents causes varying Varying currents causes varying accelerations,accelerations, and the surface reproduces soundand the surface reproduces sound

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Introductory Question Introductory Question (revisited)(revisited)

Audio players record sound in digital Audio players record sound in digital form but play it in analog form. The form but play it in analog form. The transformation from digital to analog transformation from digital to analog form takes placeform takes place

A.A. before the final amplifier inside the before the final amplifier inside the player.player.

B.B. at the outlet into which you plug the at the outlet into which you plug the earpieces.earpieces.

C.C. in the earpieces themselves.in the earpieces themselves.

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Summary about Audio Summary about Audio PlayersPlayers

Represent sound in digital and Represent sound in digital and analog formsanalog forms

Use MOSFETs to work with sound Use MOSFETs to work with sound informationinformation

Digital computer comprised of Digital computer comprised of CMOS logicCMOS logic

Analog amplifier based on Analog amplifier based on MOSFETs.MOSFETs.