Audio Players 1 Audio Players

Audio Players 2 Introductory Question Audio players record sound in digital form but play it in analog form. The transformation from digital to analog form takes place 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. B. at the outlet into which you plug the earpieces. C. in the earpieces themselves.

Audio Players 3 Observations about Audio Players They are part computer, part sound system. They are part computer, part sound system. They require electric power, typically batteries. They require electric power, typically batteries. They reproduce sound nearly perfectly. They reproduce sound nearly perfectly. They are sensitive to static charge. They are sensitive to static charge.

Audio Players 4 4 Questions about Audio Players How does an audio player “store” sound? How does an audio player “store” sound? How does it move sound information around? How does it move sound information around? How does the audio player’s computer work? How does the audio player’s computer work? How does the audio player’s amplifier work? How does the audio player’s amplifier work?

Audio Players 5 Question 1 How does an audio player “store” sound? How does an audio player “store” sound?

Audio Players 6 Representing Sound There is no sound inside an audio player There is no sound inside an audio player It uses representations of sound information, It uses representations of sound information, sequences of air pressure measurements sequences of air pressure measurements that contain everything needed to recreate the sound. that contain everything needed to recreate the 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.

Audio Players 7 Digital Representation Several discrete “symbols” represent a number Several discrete “symbols” represent a number A symbol can be any discrete physical quantity: A symbol can be any discrete physical quantity: a positive or negative charge on a capacitor a positive or negative charge on a capacitor an integer value of voltage on a wire an integer value of voltage on a wire a north or south magnetic pole on a magnet a north or south magnetic pole on a magnet The symbols collectively represent one number, The symbols collectively represent one number, so this symbol approach is insensitive to noise so this symbol approach is insensitive to noise and digital representations can be “perfect.” and digital representations can be “perfect.”

Audio Players 8 Analog Representation One physical quantity represents one number One physical quantity represents one number Any continuous physical quantity can be used: Any continuous physical quantity can be 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 noise so this analog approach is sensitive to noise and analog representations are “imperfect.” and analog representations are “imperfect.”

Audio Players 9 The Player’s Representations An audio player uses An audio player uses digital representation for storage and retrieval, digital representation for storage and retrieval, but analog representation for the actual playback. but analog representation for the actual playback. Storage and retrieval involves a digital computer Storage and retrieval involves a digital computer Playback involves an analog amplifier Playback involves an analog amplifier Between them is a digital-to-analog converter Between them is a digital-to-analog converter

Audio Players 10 Question 2 How does it move sound information around? How does it move sound information around?

Audio Players 11 MOSFET Transistors The electronic components we’ve encountered so far are all relatively passive: The electronic components we’ve encountered so far are all relatively passive: Wires: carry current from place to place Wires: carry current from place to place Capacitors: store charge Capacitors: store charge Resistors: imperfect conductors that drop voltage Resistors: imperfect conductors that drop voltage Diodes: block reverse current flow Diodes: block reverse current flow To manipulate charge, we need active switches To manipulate charge, we need active switches MOSFET transistors are electronic switches that allow tiny charges to control large current flows MOSFET transistors are electronic switches that allow tiny charges to control large current flows

Audio Players 12 MOSFET Transistor Off Two back-to-back pn-Junctions, with gate above Two back-to-back pn-Junctions, with gate above Normally does not conduct electricity at all Normally does not conduct electricity at all

Audio Players 13 MOSFET Transistor On Charge on gate can change the channel type Charge on gate can change the channel type so entire device becomes one type and conducts. so entire device becomes one type and conducts.

Audio Players 14 MOSFET Summary An electrically controllable resistor An electrically controllable resistor A tiny amount of charge alters its resistance A tiny amount of charge alters its resistance can manipulate digital information by switching symbols on or off. can manipulate digital information by switching symbols on or off. can manipulate analog information by controlling charge, current flow, or voltage. can manipulate analog information by controlling charge, current flow, or voltage.

Audio Players 15 Question 3 How does the audio player’s computer work? How does the audio player’s computer work?

Audio Players 16 Audio Player’s Digital Computer Computers perform logical operations with bits Computers perform logical operations with bits A bit is a base-two digit A 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 binary Bit-wise representation of numbers is called binary Two of the simplest bit logical operations are Two of the simplest bit logical operations are inversion (NOT) inversion (NOT) not-and (NAND) not-and (NAND) Any function can be built from these two Any function can be built from these two

Audio Players 17 Inverter (NOT) Takes one input bit, provides one output bit Takes one input bit, provides one output bit Output symbol is inverse of input symbol Output symbol is inverse of input symbol

Audio Players 18 Not-AND (NAND) Takes two input bits, provides one output bit Takes two input bits, provides one output bit Output is inverse “and” of input symbols Output is inverse “and” of input symbols

Audio Players 19 CMOS Logic Bits are represented by charge Bits are represented by charge The symbol “1” is represented by positive charge The symbol “1” is represented by positive charge The symbol “0” is represented by negative or no charge The symbol “0” is represented by negative or no charge Logic is built from n-channel and p-channel MOSFETS in complementary pairs Logic is built from n-channel and p-channel MOSFETS in complementary pairs

Audio Players 20 CMOS Inverter Input charge delivered to two complementary MOSFETs Input charge delivered to two complementary MOSFETs Positive charge on input delivers negative charge to output Positive charge on input delivers negative charge to output Negative charge on input delivers positive charge to output Negative charge on input delivers positive charge to output

Audio Players 21 CMOS NAND Positive on both inputs delivers negative charge to output Positive on both inputs delivers negative charge to output Negative on either input delivers positive charge to output Negative on either input delivers positive charge to output

Audio Players 22 Question 4 How does the audio player’s amplifier work? How does the audio player’s amplifier work?

Audio Players 23 Audio Player’s Audio Amplifier Three circuits: Three circuits: Input circuit: current/voltage represents sound Input circuit: current/voltage represents sound Output circuit: amplified “sound” current/voltage Output circuit: amplified “sound” current/voltage Power circuit: provides power for amplification Power circuit: provides power for amplification Amplifier produces “enlarged” copy of input Amplifier produces “enlarged” copy of input

Audio Players 24 Amplifier Components Diodes – one-way devices for current Diodes – one-way devices for current Capacitors – store charge, shift voltages Capacitors – store charge, shift voltages Resistors – provide voltage drops, limit current Resistors – provide voltage drops, limit current Transistors – control current flow Transistors – control current flow

Audio Players 25 Resistors Simple ohmic devices Simple ohmic devices Voltage drop is proportional to current Voltage drop is proportional to current Resistance is the proportionality constant Resistance is the proportionality constant Many values of resistance are available Many values of resistance are available Reduce a current’s voltage Reduce a current’s voltage Produce a current proportional to voltage Produce a current proportional to voltage Limit current based on voltage drop Limit current based on voltage drop

Audio Players 26 Amplifier (Part 1) As resistance of MOSFET drops: As resistance of MOSFET drops: Current from +9V to 0V increases Current from +9V to 0V increases Voltage drop of 50  resistor increases Voltage drop of 50  resistor increases Voltage at “A” goes down Voltage at “A” goes down

Audio Players 27 Amplifier (Part 2) 100K  resistor ensures that MOSFET is partially conducting 100K  resistor ensures that MOSFET 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 4.5V Voltage at “A” is about 4.5V

Audio Players 28 Amplifier (Part 3) Any charge flowing through input circuit is placed on the Gate Any charge flowing through input circuit is placed on the Gate Voltage “A” changes Voltage “A” changes Input capacitor shifts charge voltage so that it matches gate voltage Input capacitor shifts charge voltage so that it matches gate voltage

Audio Players 29 Amplifier (Part 4) Changes in voltage “A” cause changes in output current Changes in voltage “A” cause changes in output current Output capacitor shifts charge voltage so that it matches speaker voltage Output capacitor shifts charge voltage so that it matches speaker voltage

Audio Players 30 Earpieces Sound is reproduced by a moving surface Sound is reproduced by a moving surface Surface is pushed and pulled electromagnetically Surface is pushed and pulled electromagnetically Surface’s wire coil is surrounded by a magnetic field, Surface’s wire coil is surrounded by a magnetic field, so current in that coil experiences the Lorentz force so current in that coil experiences the Lorentz force and the coil accelerates. and the coil accelerates. Varying currents causes varying accelerations, Varying currents causes varying accelerations, and the surface reproduces sound and the surface reproduces sound

Audio Players 31 Introductory Question (revisited) Audio players record sound in digital form but play it in analog form. The transformation from digital to analog form takes place 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. B. at the outlet into which you plug the earpieces. C. in the earpieces themselves.

Audio Players 32 Summary about Audio Players Represent sound in digital and analog forms Represent sound in digital and analog forms Use MOSFETs to work with sound information Use MOSFETs to work with sound information Digital computer comprised of CMOS logic Digital computer comprised of CMOS logic Analog amplifier based on MOSFETs. Analog amplifier based on MOSFETs.