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 Spectral Processing Devices
• Equalizers
• Filters
• Filtering Effects:
      Wah Wah Pedal, Envelope Filters
• Changing the amplitudes of specific
  frequencies across the audible
  frequency spectrum.

• Audible Frequency Spectrum:
    Humans: 20 Hz to 20 kHz
                   Just for Fun
•     Species          Approximate Range (Hz)
•     human                     20-23,000
•     dog                       67-45,000
•     cat                       45-64,000
•     cow                       23-35,000
•     sheep                     100-30,000
•     rabbit           360-42,000
•     rat                       200-76,000
•     mouse                     1,000-91,000
•     bat                       2,000-110,000
•     beluga whale              1,000-123,000
•     porpoise                  75-150,000
•     goldfish                  20-3,000
•     chicken                   125-2,000

    Source: www.lsu.edu/deafness/HearingRange.html

Affects audio wave shape components:

 1. Fundamental - The perceived note
 2. Overtones - Define the timbre of the
        Types of EQ curves
• Bell Curve or Peak/Dip: Boost/Cuts around a
  center frequency.
• Low Shelving and High Shelving: boost/cuts
  from a set frequency equally onward.
• High Pass Filter: Cuts lows from a set
  frequency onward by a slope rate
• Low Pass Filter: Cuts highs from a set
  frequency onward by a slope rate.
          Bell or Peak/Dip EQ Curve
    • Boost/Cuts around a center frequency

Source: www.harmony-central.com
  Bell or Peak/Dip EQ Curve

1. Gain: Boost/Cut (Decibels)
2. Center Frequency: (Hertz):
    selectable: incremental steps
    sweepable: continuous control
3. Bandwidth: (Octaves) or Q (No Units)
                          Bandwidth and Q
   • Measurement across points -3 dB from
     center frequency.

Source: Modern Recording Techniques, Huber
           Bandwidth and Q

• Q is another way we can express bandwidth.
• Q=Center Frequency (Hz) / Bandwidth (Hz)
• Inverse relationship. BW increases, Q decreases.

• 1 octave bandwidth = 1.41 Q
• 2 octave bandwidth = .67 Q
                      Shelving EQ Curves
      • Boosts/Cuts from a cutoff (turnover)
        frequency equally onward

  Low Shelving Curve              High Shelving Curve
Source: www.harmony-central.com
      Shelving EQ Controls
1. Gain: Boost/Cut (dB)
2. Turnover Frequency: (Hz)

  Usually no BW or slope control
                     Turnover (or Cutoff )

        * Note that the turnover frequency is 3 dB from the shelf.
Source: Understanding Audio, Dan Thompson
          Filter EQ Curves

• High Pass Filter (HPF): Cuts Lows
    AKA Low Frequency Roll off

• Low Pass Filter (HPF): Cuts Highs

• Band Pass Filter (BPF): Combination of HPF
  and LPF, Cuts Highs and Lows.
                             Low Pass Filter

                                         • Cuts High
                                         • Cutoff Freq.
                                          -3 dB from
                                         • Slope:Rate of

Source: www.harmony-central.com
                                              HPF has a turnover frequency (at -3 dB) of
                                              700 Hz. Slope = 6 dB / octave

                                                        LPF has a turnover frequency of 700 Hz.
                                                        Slope = 12 dB / octave

Source: Modern Recording Techniques, Huber & Runstein
             Filter Controls
• HPF & LPF: Frequency Control
  – filters only cut, no gain control
  – Slope: usually a preset rate.
  – Common Slope Rates: 6 dB/octave,
    9 dB / octave, 12 dB / octave, 18 dB / octave

• BPF: Bandwidth control and Center
  Frequency control, no gain control
                    EQ Bands
• Band = A range of frequencies to be affected
• Band = A set of controls

• How many bands? Count the gain controls.

    Note: HPF, LPF, and BPF usually not classified as bands on
    analog EQ devices.
    A single curve (either peak/ dip or shelving ) is a single band.
           Equalizer Types
1. Graphic - Multiple Bands (usually 10,15,or
   30) broken down by octave increments.
   Fixed frequency, fixed bandwidth. Variable
   +/- gain.

2. Parametric - “More versatile EQ”
    Up to 5 bands (analog device), with variable
    frequency and gain control. Also may have
    bandwidth or Q adjustment.
                      Graphic Eq
                      •   10-band graphic EQ
                      •   mono channel
                      •   Freq: 1 octave ranges
                      •   Gain: +/- 12 dB
                      •   BW = 1 octave

Source: www.api.com
                      Semi-Parametric EQ
                      •   4 Bands
                      •   Hi Band: Bell or Hi Shelving
                      •   Low Band: Bell or Low Shelving
                      •   Mid Bands (2): Bell Curve
                      •   Gain: +/- 12 dB
                      •   Frequency Adjust (ranges)
                      •   BW: Preset => Semi

Source: www.api.com
   Semi vs. Fully Parametric
• Semi: preset or limited (switched)
  bandwidth adjustment.

• Fully: sweepable (continuous)
  bandwidth adjustment.
   Selectable vs. Sweepable
      Frequency Control

• Selectable: Switched in increments.
  Non-continuous. Limited Options.
• Sweepable: Continuous adjustment.
  – Ranges (ex. 20 Hz - 1kHz)
  – Full Spectrum (ex. 20 Hz - 20 kHz)
                        Fully Parametric Eq

   •    Dual Mono Design (2 mono channels)
   •    4 bands per channel
   •    High/Low Bands: Shelving or Bell Curve
   •    High/Low Bands:No BW control=>Semi
                        Fully Parametric Eq

    • Mid Bands(2): Bell Curve
    • Mid Bands have BW control=>fully
    • Overall Classification: Fully Parametric
Source: www.manleylabs.com

                  Parametric Pultec EQ

  •    Dual Mono Design (or Stereo), Tube Design
  •    3 Bands per Channel
  •    Parametric => Adjustable Frequency Control
  •    Selectable Frequency(Non continuous control)
  •    Low Band: Low Shelving. +/- Gain
  •    Mid Band: Bell Curve, Gain: Boost only
  •    Mid Band: BW control
  •    Hi Band: Hi Shelving, Gain: Cut Only
                          Wah Wah Pedal

                             • Pedal Controls a Center
                             • Bell Curve with Gain
                               Boost or Band pass
                               Filter design

Source: www.zsounds.com
                 Envelope Filter

     Wah Wah effect triggered by an input voltage
     Sensitivity: processes once a threshold level is passed
     Range: Sweep Frequency Range
     Blend: Direct / Processed mix

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