Transcript Basic Acoustics - Loyola University New Orleans

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
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Sound waves are continuous
Infinite number of amplitude points
can be identified between any two
points in time
Digital Audio
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Computers don’t deal with continuous
concepts (infinity)
Digital technology converts analog
audio to computer values
Digital Conversion
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Digitizing a continuous wave = sampling
Amplitude measurements of a sound
signal are regularly sampled
ADC and DAC
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ADC – Analog to Digital Converter
Converts analog signal to digital
samples
DAC – Digital to Analog Converter
Converts digital samples to analog
signal
Characteristics of
Digital Audio
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Sampling Rate
– How often signal is sampled
– Number of samples per second
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Bit Depth
– Size of number used to store samples
– larger number gives more degrees of
value
Sampling Rate
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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.
Humans hear to 20 kHz, requiring
sample rate of at least 40k
Aliasing
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In movies, car wheels appear to move
backwards if between ½ and 1
revolution per frame
In sound, this is not acceptable
Filters are used to remove any
frequencies above Nyquist frequency
Undersampling
Undersampling = Aliases
Critical Sampling
Lowpass Filter
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Reduces or eliminates higher frequencies
Used to remove any frequencies above
Nyquist frequency
Bit Depth (Quantization)
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Amplitude values are stored as binary
numbers
Accuracy depends on how many bits
are available to represent these values
For CD Audio we use 16 bits
Quantization
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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
Quantization
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Quantization error creates a kind of
distortion
Dither adds low-level noise to audio
signal before sampling
Dither turns distortion (bad) into noise
(less bad) – still less noise than analog
Digital Recording Process
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Dither – Low-level noise added (prior
to sampling) to reduce quantization
error distortion
Digital Recording Process
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Lowpass Filter – Removes frequencies
above Nyquist Frequency; cutoff starts
a few thousand hertz lower
Digital Recording Process
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Sample and Hold – Analog voltages
are measured and held long enough to
be read by ADC
Digital Recording Process
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Analog-to-Digital Converter – Converts
analog voltages into binary numbers
Digital Recording Process
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Multiplexer – Combines the parallel
data streams (stereo) into a single
serial bit stream
Digital Recording Process
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Error Correction – Variety of measures
to eliminate, reduce, or compensate
for errors
Digital Recording Process
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Encoding – Encoded for playback
Digital Recording Process
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Storage
Digital Playback Process
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Buffer – To ensure that samples are
processed at a constant rate
Digital Playback Process
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Error Correction – Attempt to
eliminate, reduce, or conceal data
errors
Digital Playback Process
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Demultiplexer – Splits the serial
bitstream into parallel data streams
(stereo)
Digital Playback Process
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DAC – Digital-to-Analog converter
translates binary numbers to voltage
values
Digital Playback Process
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Sample and Hold – Reads the value
from the DAC and holds it until the
DAC’s next stable state
Digital Playback Process
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Lowpass Filter – Smooths the output
from the sample and hold circuit
Digital Playback Process
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Audio – The finished product
Room Acoustics
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Characteristic room sound is
determined by the relationship
between direct and reflected sound
Virtually all sound reaching listeners is
a combination of direct & reflected
At greater distances, most sound is
reflected sound
Room Acoustics
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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
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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
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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
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Specular
– Reflections off smooth and regular
surfaces
– reflection in one direction
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Diffuse
– Reflections off irregular surfaces
– Reflections scattered in many directions
– Contribute to sound of older concert halls
Absorption
Small Room
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Space has potential to act as closed
tube, producing standing wave
Result is amplification of certain
frequencies based on room’s
dimensions
Not a factor in large rooms because air
temperature varies more
Microphones
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Receptor type
– Diaphragm acts as receptor
– Diaphragm vibrates
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Transducer type
– Transducer converts vibrations to electricity
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Directionality
– Determines strength of signal produced by
sounds arriving from different directions
Receptor Types
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Pressure
– Diaphragm responds to sound pressure
changes on only one side of diaphragm
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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
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Dynamic
(Electrodynamic, Electromagnetic, Ribbon, Moving Coil)
– Principle of magnetic induction – wire moves
within a magnetic field, producing a current
– Inexpensive and sturdy
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Condenser (Capacitor)
– Two oppositely-charged metal plates
– Current moves from one to the other
– Sharper transients
– Expensive
Directionality
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Determines the strength of signal
produced by sounds arriving from
different directions
Directionality varies with frequency
Specs often include polar plot with
patterns for different frequencies
Omnidirectional
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Responds equally to sound from all
directions
Pressure mics are omnidirectional
Bidirectional
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Figure-eight response
Responds equally to sounds from front &
back; none from sides
Pressure gradient mics are bidirectional
First-Order Cardioid
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Most common directional microphones
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Cardioid refers to heart-shaped pattern
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Directional patterns are obtained by
combining pressure and pressure
gradient elements in varying proportions
Cardioid Variations
50% Pressure/50% Pres. Gradient
75% Pressure/25% Pres. Gradient
37% Pressure/63% Pres. Gradient
25% Pressure/75% Pres. Gradient
Effects
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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 = Filters
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Effects are created by filter
combinations
Filtering involves combining original
signal with delayed version
Higher internal processing bit rate
means more accurate arithmetic
Simple Delay
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Signal combined with delayed version of
itself.
Multitap Delay
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Series of Simple Delays; output is
combines with a succession of delays.
Feedback Delay
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Combines delayed output with input,
then sends through delay again.
Delay-Based Effects
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Flanging
Chorusing
Phase Shifting
Reverberation
Non-Delay-Based Effects
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Ring Modulation
Amplitude Modulation
Compression/Limiting
Expansion/Noise Gating