Input Transducers

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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
• QCV
• 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
  mic-to-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 low-impedance 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.

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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 or XY pair
• Right 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