# Digital Audio — The Nuts and Bolts A digital audio overview ranging from bit rate, sample rate, and compression types to room acoustics, microphones, and.

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Digital Audio — The Nuts and Bolts A digital audio overview ranging from bit rate, sample rate, and compression types to room acoustics, microphones, and digital effects

Sound Waves/Analog Audio Sound waves are continuous Sound waves are continuous Infinite number of amplitude points can be identified between any two points in time Infinite number of amplitude points can be identified between any two points in time

Digital Audio Computers don’t deal with continuous concepts (infinity) Computers don’t deal with continuous concepts (infinity) Digital technology converts analog audio to computer values Digital technology converts analog audio to computer values

Digital Conversion Digitizing a continuous wave = sampling Digitizing a continuous wave = sampling Amplitude measurements of a sound signal are regularly sampled Amplitude measurements of a sound signal are regularly sampled

ADC and DAC ADC – Analog to Digital Converter ADC – Analog to Digital Converter Converts analog signal to digital samples DAC – Digital to Analog Converter DAC – Digital to Analog Converter Converts digital samples to analog signal

Characteristics of Digital Audio Sampling Rate Sampling Rate –How often signal is sampled –Number of samples per second Bit Depth Bit Depth –Size of number used to store samples –larger number gives more degrees of value

Sampling Rate Harry Nyquist (Bell Labs – 1925) Harry Nyquist (Bell Labs – 1925) Nyquist Theorem: To represent digitally a signal containing frequency components up to X Hz, it is necessary to use a sampling rate of at least 2X. Nyquist Theorem: To represent digitally a signal containing frequency components up to X Hz, it is necessary to use a sampling rate of at least 2X. Humans hear to 20 kHz, requiring sample rate of at least 40k Humans hear to 20 kHz, requiring sample rate of at least 40k

Aliasing In movies, car wheels appear to move backwards if between ½ and 1 revolution per frame In movies, car wheels appear to move backwards if between ½ and 1 revolution per frame In sound, this is not acceptable In sound, this is not acceptable Filters are used to remove any frequencies above Nyquist frequency Filters are used to remove any frequencies above Nyquist frequency

Undersampling

Undersampling = Aliases

Critical Sampling

Lowpass Filter Reduces or eliminates higher frequencies Reduces or eliminates higher frequencies Used to remove any frequencies above Nyquist frequency Used to remove any frequencies above Nyquist frequency

Bit Depth (Quantization) Amplitude values are stored as binary numbers Amplitude values are stored as binary numbers Accuracy depends on how many bits are available to represent these values Accuracy depends on how many bits are available to represent these values For CD Audio we use 16 bits For CD Audio we use 16 bits

Quantization No matter how many bits are used, there is always a margin of error No matter how many bits are used, there is always a margin of error Low-level signals do not use all available bits, so signal-to-error ratio is greater Low-level signals do not use all available bits, so signal-to-error ratio is greater

Quantization Quantization error creates a kind of distortion Quantization error creates a kind of distortion Dither adds low-level noise to audio signal before sampling Dither adds low-level noise to audio signal before sampling Dither turns distortion (bad) into noise (less bad) – still less noise than analog Dither turns distortion (bad) into noise (less bad) – still less noise than analog

Digital Recording Process Dither – Low-level noise added (prior to sampling) to reduce quantization error distortion Dither – Low-level noise added (prior to sampling) to reduce quantization error distortion

Digital Recording Process Lowpass Filter – Removes frequencies above Nyquist Frequency; cutoff starts a few thousand hertz lower Lowpass Filter – Removes frequencies above Nyquist Frequency; cutoff starts a few thousand hertz lower

Digital Recording Process Sample and Hold – Analog voltages are measured and held long enough to be read by ADC Sample and Hold – Analog voltages are measured and held long enough to be read by ADC

Digital Recording Process Analog-to-Digital Converter – Converts analog voltages into binary numbers Analog-to-Digital Converter – Converts analog voltages into binary numbers

Digital Recording Process Multiplexer – Combines the parallel data streams (stereo) into a single serial bit stream Multiplexer – Combines the parallel data streams (stereo) into a single serial bit stream

Digital Recording Process Error Correction – Variety of measures to eliminate, reduce, or compensate for errors Error Correction – Variety of measures to eliminate, reduce, or compensate for errors

Digital Recording Process Encoding – Encoded for playback Encoding – Encoded for playback

Digital Recording Process Storage Storage

Digital Playback Process Buffer – To ensure that samples are processed at a constant rate Buffer – To ensure that samples are processed at a constant rate

Digital Playback Process Error Correction – Attempt to eliminate, reduce, or conceal data errors Error Correction – Attempt to eliminate, reduce, or conceal data errors

Digital Playback Process Demultiplexer – Splits the serial bitstream into parallel data streams (stereo) Demultiplexer – Splits the serial bitstream into parallel data streams (stereo)

Digital Playback Process DAC – Digital-to-Analog converter translates binary numbers to voltage values DAC – Digital-to-Analog converter translates binary numbers to voltage values

Digital Playback Process Sample and Hold – Reads the value from the DAC and holds it until the DAC’s next stable state Sample and Hold – Reads the value from the DAC and holds it until the DAC’s next stable state

Digital Playback Process Lowpass Filter – Smooths the output from the sample and hold circuit Lowpass Filter – Smooths the output from the sample and hold circuit

Digital Playback Process Audio – The finished product Audio – The finished product

Room Acoustics Characteristic room sound is determined by the relationship between direct and reflected sound Characteristic room sound is determined by the relationship between direct and reflected sound Virtually all sound reaching listeners is a combination of direct & reflected Virtually all sound reaching listeners is a combination of direct & reflected At greater distances, most sound is reflected sound At greater distances, most sound is reflected sound

Room Acoustics Direct Sound Direct Sound –Directly from the source to the listener –Direct sound arrives before reflected sound; even if reflected sound is louder, we hear direct sound first and determine direction of the source

Room Acoustics Early Reflections Early Reflections –First-order reflections that reach the listener after reflecting once from the floor, ceiling, or walls –If arriving in the first 35ms after the direct sound, reinforces with clarity & intelligibility –“Intimate” halls have first-order reflections of less than 20ms

Room Acoustics Diffuse Reverberations Diffuse Reverberations –Second- (and higher) order reflections –Reverberation time is the time required for the SPL to drop 60dB –Larger room is likely to have longer reverberation time than a smaller room –Reverberation time is frequency dependent; lower frequencies reverberate longer

Types of Reflections Specular Specular –Reflections off smooth and regular surfaces –reflection in one direction Diffuse Diffuse –Reflections off irregular surfaces –Reflections scattered in many directions –Contribute to sound of older concert halls

Absorption

Small Room Space has potential to act as closed tube, producing standing wave Space has potential to act as closed tube, producing standing wave Result is amplification of certain frequencies based on room’s dimensions Result is amplification of certain frequencies based on room’s dimensions Not a factor in large rooms because air temperature varies more Not a factor in large rooms because air temperature varies more

Microphones Receptor type Receptor type –Diaphragm acts as receptor –Diaphragm vibrates Transducer type Transducer type –Transducer converts vibrations to electricity Directionality Directionality –Determines strength of signal produced by sounds arriving from different directions

Receptor Types Pressure Pressure –Diaphragm responds to sound pressure changes on only one side of diaphragm Pressure Gradient Pressure Gradient –Diaphragm responds to sound pressure changes from the front or rear –Signal is determined by difference (gradient) of pressures from either side

Transducer Types Dynamic (Electrodynamic, Electromagnetic, Ribbon, Moving Coil) Dynamic (Electrodynamic, Electromagnetic, Ribbon, Moving Coil) –Principle of magnetic induction – wire moves within a magnetic field, producing a current –Inexpensive and sturdy Condenser (Capacitor) Condenser (Capacitor) –Two oppositely-charged metal plates –Current moves from one to the other –Sharper transients –Expensive

Directionality Determines the strength of signal produced by sounds arriving from different directions Determines the strength of signal produced by sounds arriving from different directions Directionality varies with frequency Directionality varies with frequency Specs often include polar plot with patterns for different frequencies Specs often include polar plot with patterns for different frequencies

Responds equally to sound from all directions Responds equally to sound from all directions Pressure mics are omnidirectional Pressure mics are omnidirectional Omnidirectional

Bidirectional Figure-eight response Figure-eight response Responds equally to sounds from front & back; none from sides Responds equally to sounds from front & back; none from sides Pressure gradient mics are bidirectional Pressure gradient mics are bidirectional

First-Order Cardioid Most common directional microphones Most common directional microphones Cardioid refers to heart-shaped pattern Cardioid refers to heart-shaped pattern Directional patterns are obtained by combining pressure and pressure gradient elements in varying proportions Directional patterns are obtained by combining pressure and pressure gradient elements in varying proportions

Effects All music that is recorded or amplified relies on effects to enhance the sound. All music that is recorded or amplified relies on effects to enhance the sound. Effects are necessary to make electronic audio signals sound like natural sound. Effects are necessary to make electronic audio signals sound like natural sound.

Effects = Filters Effects are created by filter combinations Effects are created by filter combinations Filtering involves combining original signal with delayed version Filtering involves combining original signal with delayed version Higher internal processing bit rate means more accurate arithmetic Higher internal processing bit rate means more accurate arithmetic

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

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

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

Delay-Based Effects Flanging Flanging Chorusing Chorusing Phase Shifting Phase Shifting Reverberation Reverberation

Non-Delay-Based Effects Ring Modulation Ring Modulation Amplitude Modulation Amplitude Modulation Compression/Limiting Compression/Limiting Expansion/Noise Gating Expansion/Noise Gating

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