PHY2464 - The Physical Basis of Music
4/18/2005
PHY
-2464
PHY-2464
Physical Basis of Music
Presentation
Presentation 25
25
Sound
Sound Reproduction
Reproduction and
and Synthesis:
Synthesis:
Distortion
Distortion and
and Its
Its Correction
Correction
Adapted
Adapted in
in substantial
substantial part
part from
from
Sam
Matteson’s
Unit
4
Session
Sam Matteson’s Unit 4 Session 39
39
Sam
Sam Trickey
Trickey
April
April 15,
15, 2005
2005
Pres. 25 Sound Reproduction & Synthesis – Distortion
Microphone directional sensitivity :
Omni ((-directional)
microphone;
Overhead plot
PHY2464 - The Physical Basis of Music
Pres. 25 Sound Reproduction & Synthesis – Distortion
Microphone directional sensitivity :
Cardioid
microphone
Pres. 25 Sound Reproduction & Synthesis – Distortion
Microphone directional sensitivity :
• “Omni” microphones are sensitive equally in all
directions.
• “Cardioid” microphones have a “heart-shaped”
sensitivity plot ⇒ most sensitive in one direction
• Directional sensitivity can be good (pick up the
sound of a golf ball being driven) or bad (lower
the level of a sound that was supposed to be
detected.)
4/18/2005
PHY2464 - The Physical Basis of Music
4/18/2005
Pres. 25 Sound Reproduction & Synthesis – Distortion
Amplification:
Gain G = Vout / Vin
Vout = G Vin
Vout
Vin
device
Pres. 25 Sound Reproduction & Synthesis – Distortion
Distortion in amplification:
Gain depends on amplitude
Vout = G(Vin ) Vin
Vout
Vin
device
Non-linearity
PHY2464 - The Physical Basis of Music
4/18/2005
Pres. 25 Sound Reproduction & Synthesis – Distortion
Distortion in amplification:
Gain saturates
Vout = G Vin.,
if Vout< Vmax
Vmax
Vout
Vin
device
Clipping
Pres. 25 Sound Reproduction & Synthesis – Distortion
Distortion in amplification:
Gain frequency filtering
Vout = G(f) Vin
Vout
device
Low f gain
High f gain
Vin
PHY2464 - The Physical Basis of Music
4/18/2005
Pres. 25 Sound Reproduction & Synthesis – Distortion
Distortion in amplification:
Gain frequency filtering
Vout = G(f) Vin
“Roll off”
Log(Gain)
Log(Frequency)
Pres. 25 Sound Reproduction & Synthesis – Distortion
Distortion in reproduction & amplification:
▫ Non-
causes distortion of waveform by
linearity:
gain that depends on the amplitude
of the signal.
▫ Clipping: extreme form of non-linearity - output
reaches the maximum current or voltage
the system can deliver. Adds spurious
high frequency components to spectrum.
▫ Roll-off: gain drop-off with increasing
frequency. Distorts upper
frequency regime of spectrum.
PHY2464 - The Physical Basis of Music
4/18/2005
Pres. 25 Sound Reproduction & Synthesis – Distortion
Equalization “EQ”:
Vout = G(f) Vin
EQ Channels
Vout = G(f) Vin* EQ(f)
Log(Gain * EQ)
Log(Gain)
Log(Frequency)
Pres. 25 Sound Reproduction & Synthesis – Distortion
Equalization EQ in words:
•
Equalization is a gain adjustment in multiple
frequency bands that compensates – at least
approximately - for distortion due to gain and
response variations with frequency in other parts
of the system.
PHY2464 - The Physical Basis of Music
4/18/2005
Pres. 25 Sound Reproduction & Synthesis – Distortion
VU:
•
VU = abbreviation for “volume unit,” the
electronic equivalent for Gain of dB.
• Thus, 10 VU is 10 dB of gain or 10x; 20 VU is 20 dB of
gain or 100x, etc.
Distortion probable
Clipping probable
VU
Pres. 25 Sound Reproduction & Synthesis – Distortion
Feedback:
•
•
Positive Feedback is the (generally) undesirable phenomenon in
which the output “feeds back”
back” into the input of a system.
If the system gain is greater than one (1x), an uncontrollable
oscillation results. In a sound reinforcement system (e.g., PA
system) this is the source of the awful “squeal”
squeal” or “shriek”
shriek”
Speaker
Feedback
Microphone
PHY2464 - The Physical Basis of Music
4/18/2005
Pres. 25 Sound Reproduction & Synthesis – Distortion
Positive Feedback:
•
To control positive feedback ⇒ break the feedback loop:
• Reduce gain
• Adjust EQ to reduce gain in offending band
• Move microphone or use hypercardioid
Speaker
Feedback
Microphone
Pres. 25 Sound Reproduction & Synthesis – Distortion
Microphone directional sensitivity: Cardioid vs.
Hypercardioid
Cardioid
microphone
Hypercardioid
PHY2464 - The Physical Basis of Music
4/18/2005
Pres. 25 Sound Reproduction & Synthesis – Distortion
Speakers:
•
“(Loud)speaker”:
• An electro-acoustic transducer from electrical
signals to sound.
Input
Current
Pres. 25 Sound Reproduction & Synthesis – Distortion
Speakers:
•
“CrossCross-Over Network”
Network”:
• An electrical network that shifts the electrical signal to
the speaker in an array that is appropriate for a
particular frequency range.
Input
CrossCrossOver
High
Tweeter
Frequency
Network Low
Frequency
Woofer
PHY2464 - The Physical Basis of Music
4/18/2005
Pres. 25 Sound Reproduction & Synthesis – Distortion
Speakers:
•
“Column Speaker”
Speaker”:
• An arrangement of speakers in which interference
effects produce a “beam”
beam” of sound.
Pres. 25 Sound Reproduction & Synthesis – Distortion
Speakers:
•
“Column Speaker”
Speaker”:
• An arrangement of speakers in which interference
effects produce a “beam”
beam” of sound.
“Beam”
PHY2464 - The Physical Basis of Music
4/18/2005
Pres. 25 Sound Reproduction & Synthesis – Distortion
Speakers:
•
“Column Speaker”
Speaker”:
• A column speaker can be used to give more uniform
sound reinforcement than from other speakers.
¾ of room
1/r2
Pres. 25 Sound Reproduction & Synthesis – Distortion
Speakers:
•
“Distributed Sound”
Sound”:
• A sound reinforcement system in which the electrical
signal is fed to a large number of distributed speakers.
• Each speaker is fed with a calibrated delay
relative to the preceding one
Δt
Δt
Δt
Δt
PHY2464 - The Physical Basis of Music
Pres. 25 Sound Reproduction & Synthesis – Distortion
Summary:
Distortion can be spatial, temporal, or frequency
•
dependent
•
“Cardioid”
Cardioid” and “omni”
omni” refer to the directional
sensitivity of microphones.
•
Sound reinforcement requires amplification,
characterized by gain.
•
Distortion results when the output waveform is a
distorted version of the input.
•
EQ equalizes the response of a room and system by
adjusting the gain of the amplifier for various frequency
bands to offset distortions in the rest of the system.
•
VU are “volume units”
units” = electronic gain equivalent to
deciBels.
.
deciBels
Pres. 25 Sound Reproduction & Synthesis – Distortion
Summary (concluded):
•
•
•
•
•
Positive feedback is an uncontrolled rebroadcast (recycling) of the output sound.
Loudspeakers are electro-acoustic devices that
convert electrical signals into sound.
A cross-over network shifts an electrical signal
to the speaker appropriate for a specific freq.
band.
Column speakers use interference to produce a
“beam” of sound.
Distributed sound reinforcement is an array of
individual speakers with calibrated time delays.
4/18/2005