digital audio — the nuts and bolts a digital audio overview ranging from bit rate, sample rate,...

Digital Audio —Digital Audio —The Nuts and The Nuts and BoltsBolts

A digital audio overview ranging from A digital audio overview ranging from bit rate, sample rate, and compression bit rate, sample rate, and compression types to room acoustics, microphones, types to room acoustics, microphones, and digital effectsand digital effects

Sound Waves/Analog Sound Waves/Analog AudioAudio Sound waves are continuousSound waves are continuous

Infinite number of amplitude Infinite number of amplitude points can be identified between points can be identified between any two points in timeany two points in time

Digital AudioDigital Audio

Computers don’t deal with Computers don’t deal with continuous concepts (infinity)continuous concepts (infinity)

Digital technology converts Digital technology converts analog audio to computer valuesanalog audio to computer values

Digital ConversionDigital Conversion

Digitizing a continuous wave = Digitizing a continuous wave = samplingsampling

Amplitude measurements of a Amplitude measurements of a sound signal are regularly sampledsound signal are regularly sampled

ADC and DACADC and DAC

ADC – Analog to Digital ConverterADC – Analog to Digital Converter

Converts analog signal to Converts analog signal to digital samplesdigital samples

DAC – Digital to Analog ConverterDAC – Digital to Analog Converter

Converts digital samples to Converts digital samples to analog signalanalog signal

Characteristics ofCharacteristics ofDigital AudioDigital Audio Sampling RateSampling Rate

– How often signal is sampledHow often signal is sampled– Number of samples per secondNumber of samples per second

Bit DepthBit Depth– Size of number used to store Size of number used to store

samplessamples– larger number gives more degrees larger number gives more degrees

of valueof value

Sampling RateSampling Rate

Harry Nyquist (Bell Labs – 1925)Harry Nyquist (Bell Labs – 1925)

Nyquist Theorem: To represent Nyquist Theorem: To represent digitally a signal containing digitally a signal containing frequency components up to X Hz, it frequency components up to X Hz, it is necessary to use a sampling rate is necessary to use a sampling rate of at least 2X.of at least 2X.

Humans hear to 20 kHz, requiring Humans hear to 20 kHz, requiring sample rate of at least 40ksample rate of at least 40k

AliasingAliasing

In movies, car wheels appear to In movies, car wheels appear to move backwards if between ½ and move backwards if between ½ and 1 revolution per frame1 revolution per frame

In sound, this is not acceptableIn sound, this is not acceptable

Filters are used to remove any Filters are used to remove any frequencies above Nyquist frequencies above Nyquist frequencyfrequency

UndersamplingUndersampling

Undersampling = Undersampling = AliasesAliases

Critical SamplingCritical Sampling

Lowpass FilterLowpass Filter

Reduces or eliminates higher Reduces or eliminates higher frequenciesfrequencies

Used to remove any frequencies Used to remove any frequencies above Nyquist frequencyabove Nyquist frequency

Bit Depth Bit Depth (Quantization)(Quantization) Amplitude values are stored as Amplitude values are stored as

binary numbersbinary numbers

Accuracy depends on how many Accuracy depends on how many bits are available to represent bits are available to represent these valuesthese values

For CD Audio we use 16 bitsFor CD Audio we use 16 bits

QuantizationQuantization

No matter how many bits are No matter how many bits are used, there is always a margin of used, there is always a margin of errorerror

Low-level signals do not use all Low-level signals do not use all available bits, so signal-to-error available bits, so signal-to-error ratio is greaterratio is greater

QuantizationQuantization

Quantization error creates a kind Quantization error creates a kind of distortionof distortion

Dither adds low-level noise to Dither adds low-level noise to audio signal before samplingaudio signal before sampling

Dither turns distortion (bad) into Dither turns distortion (bad) into noise (less bad) – still less noise noise (less bad) – still less noise than analogthan analog

Digital Recording Digital Recording ProcessProcess Dither – Low-level noise added Dither – Low-level noise added

(prior to sampling) to reduce (prior to sampling) to reduce quantization error distortionquantization error distortion

Digital Recording Digital Recording ProcessProcess Lowpass Filter – Removes frequencies Lowpass Filter – Removes frequencies

above Nyquist Frequency; cutoff above Nyquist Frequency; cutoff startsstarts a few thousand hertz lowera few thousand hertz lower

Digital Recording Digital Recording ProcessProcess Sample and Hold – Analog Sample and Hold – Analog

voltages are measured and held voltages are measured and held long enough to be read by ADClong enough to be read by ADC

Digital Recording Digital Recording ProcessProcess Analog-to-Digital Converter – Analog-to-Digital Converter –

Converts analog voltages into Converts analog voltages into binary numbersbinary numbers

Digital Recording Digital Recording ProcessProcess Multiplexer – Combines the Multiplexer – Combines the

parallel data streams (stereo) into parallel data streams (stereo) into a single serial bit streama single serial bit stream

Digital Recording Digital Recording ProcessProcess Error Correction – Variety of Error Correction – Variety of

measures to eliminate, reduce, or measures to eliminate, reduce, or compensate for errorscompensate for errors

Digital Recording Digital Recording ProcessProcess Encoding – Encoded for playbackEncoding – Encoded for playback

Digital Recording Digital Recording ProcessProcess StorageStorage

Digital Playback Digital Playback ProcessProcess Buffer – To ensure that samples Buffer – To ensure that samples

are processed at a constant rateare processed at a constant rate

Digital Playback Digital Playback ProcessProcess Error Correction – Attempt to Error Correction – Attempt to

eliminate, reduce, or conceal data eliminate, reduce, or conceal data errorserrors

Digital Playback Digital Playback ProcessProcess Demultiplexer – Splits the serial Demultiplexer – Splits the serial

bitstream into parallel data bitstream into parallel data streams (stereo)streams (stereo)

Digital Playback Digital Playback ProcessProcess DAC – Digital-to-Analog converter DAC – Digital-to-Analog converter

translates binary numbers to translates binary numbers to voltage valuesvoltage values

Digital Playback Digital Playback ProcessProcess Sample and Hold – Reads the Sample and Hold – Reads the

value from the DAC and holds it value from the DAC and holds it until the DAC’s next stable stateuntil the DAC’s next stable state

Digital Playback Digital Playback ProcessProcess Lowpass Filter – Smooths the Lowpass Filter – Smooths the

output from the sample and hold output from the sample and hold circuitcircuit

Digital Playback Digital Playback ProcessProcess Audio – The finished productAudio – The finished product

Room AcousticsRoom Acoustics

Characteristic room sound is Characteristic room sound is determined by the relationship determined by the relationship between direct and reflected soundbetween direct and reflected sound

Virtually all sound reaching listeners Virtually all sound reaching listeners is a combination of direct & is a combination of direct & reflectedreflected

At greater distances, most sound is At greater distances, most sound is reflected soundreflected sound


Direct SoundDirect Sound– Directly from the source to the Directly from the source to the

listenerlistener– Direct sound arrives Direct sound arrives beforebefore reflected reflected

sound; even if reflected sound is sound; even if reflected sound is louder, we hear direct sound first louder, we hear direct sound first and determine direction of the and determine direction of the sourcesource


Early ReflectionsEarly Reflections– First-order reflections that reach the First-order reflections that reach the

listener after reflecting listener after reflecting onceonce from from the floor, ceiling, or wallsthe floor, ceiling, or walls

– If arriving in the first 35ms after the If arriving in the first 35ms after the direct sound, reinforces with clarity direct sound, reinforces with clarity & intelligibility& intelligibility

– ““Intimate” halls have first-order Intimate” halls have first-order reflections of less than 20msreflections of less than 20ms


Diffuse ReverberationsDiffuse Reverberations– Second- (and higher) order reflectionsSecond- (and higher) order reflections– Reverberation time is the time required Reverberation time is the time required

for the SPL to drop 60dBfor the SPL to drop 60dB– Larger room is likely to have longer Larger room is likely to have longer

reverberation time than a smaller roomreverberation time than a smaller room– Reverberation time is frequency Reverberation time is frequency

dependent; lower frequencies dependent; lower frequencies reverberate longerreverberate longer

Types of ReflectionsTypes of Reflections

SpecularSpecular– Reflections off smooth and regular Reflections off smooth and regular

surfacessurfaces– reflection in one directionreflection in one direction

DiffuseDiffuse– Reflections off irregular surfacesReflections off irregular surfaces– Reflections scattered in many Reflections scattered in many

directionsdirections– Contribute to sound of older concert Contribute to sound of older concert

hallshalls

AbsorptionAbsorption

Small RoomSmall Room

Space has potential to act as closed Space has potential to act as closed tube, producing standing wavetube, producing standing wave

Result is amplification of certain Result is amplification of certain frequencies based on room’s frequencies based on room’s dimensionsdimensions

Not a factor in large rooms because Not a factor in large rooms because air temperature varies moreair temperature varies more

MicrophonesMicrophones

Receptor typeReceptor type– Diaphragm acts as receptorDiaphragm acts as receptor– Diaphragm vibratesDiaphragm vibrates

Transducer typeTransducer type– Transducer converts vibrations to Transducer converts vibrations to

electricityelectricity DirectionalityDirectionality

– Determines strength of signal produced Determines strength of signal produced by sounds arriving from different by sounds arriving from different directionsdirections

Receptor TypesReceptor Types

PressurePressure– Diaphragm responds to sound pressure Diaphragm responds to sound pressure

changes on only one side of diaphragmchanges on only one side of diaphragm Pressure GradientPressure Gradient

– Diaphragm responds to sound pressure Diaphragm responds to sound pressure changes from the front or rearchanges from the front or rear

– Signal is determined by difference Signal is determined by difference (gradient) of pressures from either side(gradient) of pressures from either side

Transducer TypesTransducer Types

Dynamic Dynamic (Electrodynamic, Electromagnetic, Ribbon, Moving (Electrodynamic, Electromagnetic, Ribbon, Moving Coil)Coil)

– Principle of Principle of magnetic induction – magnetic induction – wire wire moves within a magnetic field, producing a moves within a magnetic field, producing a currentcurrent

– Inexpensive and sturdyInexpensive and sturdy Condenser (Capacitor)Condenser (Capacitor)

– Two oppositely-charged metal platesTwo oppositely-charged metal plates– Current moves from one to the otherCurrent moves from one to the other– Sharper transientsSharper transients– ExpensiveExpensive

DirectionalityDirectionality

Determines the strength of signal Determines the strength of signal produced by sounds arriving from produced by sounds arriving from different directionsdifferent directions

Directionality varies with frequencyDirectionality varies with frequency

Specs often include polar plot with Specs often include polar plot with patterns for different frequenciespatterns for different frequencies

Responds equally to sound from all Responds equally to sound from all directionsdirections

Pressure mics are omnidirectionalPressure mics are omnidirectional

OmnidirectionalOmnidirectional

BidirectionalBidirectional

Figure-eight responseFigure-eight response Responds equally to sounds from Responds equally to sounds from

front & back; none from sidesfront & back; none from sides Pressure gradient mics are Pressure gradient mics are

bidirectionalbidirectional

First-Order CardioidFirst-Order Cardioid

Most common directional microphonesMost common directional microphones

Cardioid refers to heart-shaped patternCardioid refers to heart-shaped pattern

Directional patterns are obtained by Directional patterns are obtained by combining pressure and pressure combining pressure and pressure gradient elements in varying gradient elements in varying proportionsproportions

Cardioid VariationsCardioid Variations

50% Pressure/50% Pres. Gradient




EffectsEffects

All music that is recorded or All music that is recorded or amplified relies on effects to amplified relies on effects to enhance the sound.enhance the sound.

Effects are necessary to make Effects are necessary to make electronic audio signals sound like electronic audio signals sound like natural sound.natural sound.

Effects = FiltersEffects = Filters

Effects are created by filter Effects are created by filter combinationscombinations

Filtering involves combining Filtering involves combining original signal with delayed original signal with delayed versionversion

Higher internal processing bit rate Higher internal processing bit rate means more accurate arithmeticmeans more accurate arithmetic

Simple DelaySimple Delay

Signal combined with delayed version Signal combined with delayed version of itself.of itself.

Multitap DelayMultitap Delay

Series of Simple Delays; output is Series of Simple Delays; output is combines with a succession of combines with a succession of delays.delays.

Feedback DelayFeedback Delay

Combines delayed output with Combines delayed output with input, then sends through delay input, then sends through delay again.again.

Delay-Based EffectsDelay-Based Effects

FlangingFlanging ChorusingChorusing Phase ShiftingPhase Shifting ReverberationReverberation

Non-Delay-Based Non-Delay-Based EffectsEffects Ring ModulationRing Modulation Amplitude ModulationAmplitude Modulation Compression/LimitingCompression/Limiting Expansion/Noise GatingExpansion/Noise Gating

digital audio — the nuts and bolts a digital audio overview ranging from bit rate, sample rate,...

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