Transcript Multimedia Systems - University of Sussex

Input Transducers
(Microphones)
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Microphones
 Transducers
that convert sound waves
into electrical signals
 There are several different type of
microphones
Dynamic
Condensing
Carbon
granule
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Dynamic
 The
diaphragm is
moved by
changing sound
pressure
 This moves the
coil, which
causes current to
flow as lines of
flux from the
magnet are cut
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Dynamic
 Advantages
Rugged
Inexpensive
to manufacture
Tolerates extremely high sound
pressure levels
Requires no power supply
 Disadvantages
Reduced
high frequency performance
Needs amplification
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Condensing

A pair of parallel metal
plates separated by an
insulator across which is
stored an electrical
charge
 Q=CV
 Moving the plates will
change the voltage
across them causing
current to flow in or out
of the capacitor through
the resistor
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Charging the Condensing Mic
 The
microphone must have some
means of charging the plates
Battery
Phantom
powering
Electret
 The
majority of serious recording work
is carried out with capacitor
microphones
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Condensing
 Advantages
No
coil and the diaphragm can be
made very thin (few microns thick)
thus light weight and less inertia so
better high frequency response.
Capacitor microphones can be made
with virtually any response pattern.
 Disadvantages
More
complex thus more expensive
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Carbon Granule
 When
acoustical
pressure is exerted on
the carbon granules,
the granules are
pressed closer
together which
decreases the
measured resistance.
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Pickup Patterns
 Microphones
can also be classified by
their pick up patterns
Omnidirectional
Unidirectional
Bidirectional
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Omnidirectional
 Pickup
of sound equally well
within a 360 degree radius
around the microphone element
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Omnidirectional
 Advantages
Doesn't
have to held directly in front of
sound source to pick up adequate sound
Gives a sense of the environment
Is not affected by slight changes in micto-source distance
Provides a natural sound in rooms with
good acoustics
Is less susceptible to handling noise and
noise created by wind or popping
Is not subject to the proximity effect
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Omnidirectional
 Disadvantages
Difficult
to use in noisy locations
Susceptible to feedback in
locations where microphone is
feeding a loudspeaker
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Unidirectional (Cardioid)
 Primary
pickup of sound from one
side of the microphone's element
Wide
angle cardioid
Supercardioid
Hypercardioid
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Unidirectional
 Advantages
Discriminates
against
unwanted sound
Reduces danger of
feedback in locations
where microphone is
feeding a loudspeaker
Picks up very little or no
background sounds
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Unidirectional
Disadvantages
May
be subject to the proximity effect
Must be set correctly to avoid sound
source being off-mic
Less natural sound in rooms with good
acoustics
Must take care to avoid covering ports
More susceptible to wind, popping and
handling noises
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Bidirectional

Optimum pickup of sound
from two sides of the
microphone element
 Advantage
- can be used to
reduce number of mics in a
circuit when two voices are
involved in an interview or
performing a voice-over
 Disadvantage - no separate
control over levels of talent,
so they must maintain
proper mic-to-source
distance and remain on-mic
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Proximity Effect
 The
closer the sound source is to
the microphone, the greater the
emphasis on the bass frequencies
in the sound
 Many microphones are equipped
with a bass roll-off switch to
nullify the proximity effect
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Condenser Frequency Response
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Dynamic Frequency Response
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Impedance
 One
important characteristic of a
microphone is its output impedance
 This is a measurement of the AC
resistance looking back into the
microphone
Low
(50-1,000 ohms)
High (5,000+ ohms)
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Low Impedance
 Most
microphones
used in broadcast
applications are
wired for lowimpedance circuits
Low-impedance
circuits
have three wire
connectors: a shield and
two other wires (hot and
cold) to conduct the
current
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Balanced Systems
 Balanced
lines are much less
susceptible to RFI (Radio Frequency
Interference) and the pickup of the other
electrical noise and hum
 In a balanced line, the shield of the
cable is connected to ground, and the
audio signal appears across the two
inner wires which are not connected to
ground
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Balanced Systems
 Output
signal is split into two opposite
phases
Positive
and Negative or Hot and Cold
When returned to the same phase at the
amplifier input noise is cancelled.
23
Phantom Powering
 The
standard power supply for
microphones is 48v
 DC power is supplied along the signal
leads, which means no extra cables
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Connectors
 Two
types of connectors are commonly
used with balanced systems
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High Impedance
 High-impedance
circuits have two
wire connectors: a shield and one
inner wire
Used
in budget systems
High impedance makes the signal more
susceptible to electromagnetic
interference
Signal is affected by cable capacitance in
long runs of cables
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Feedback
 Feedback
occurs when the sound from
the loudspeaker arrives at the
microphone as loud or louder than the
sound arriving directly from the original
sound source (talker, singer, etc.)
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Microphones and Stereo Signals
 Coincident
Right
or XY pair
hand microphone
picks up sound form left
stage and visa a versa
No phase problems as
minimal space between
microphones
Sound source
Cardioid Microphones
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Fin
Fin
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