Advanced Lecture

 Equalization, or EQ is the process of using passive or active electronic elements or digital algorithms for the purpose of altering (originally flattening) the frequency response characteristics of a system.

 Parametric equalizer (or parametric "EQ") is an electronic multi-band variable equalizer used in sound recording and live sound reinforcement.  THREE primary parameters of an internal band-pass filter which are amplitude, center frequency and bandwidth.  Bandwidth is typically labeled "Q" on the unit, which stands for Quality. The amplitude of each band can be controlled, and the center frequency can be shifted, and widened or narrowed.  Most channel EQ sections on Mixing Consoles have semi- or fully parametric EQ

Quality Factor  The quality factor ("Q") in equalization defines the sharpness of the band of frequencies that are affected by the EQ.  How "pure" this reactance was (it's "quality factor") determined how sharply the circuit "tunes" to just one frequency, thus a higher "Q" meant the equalizer affected less frequencies.

The Q and bandwidth Q=0.7=BW of 2 octaves Q=1.0=BW of 1.33 octaves Q=1.4=BW of 1 octave Q=2.0=BW of 0.67 octaves

Tonal enhancement  Parametric equalization is most often used to enhance frequencies which are not "speaking" well on a specific instrument.  You can specifically target a frequency range  A subtle, precise boost of the frequency which does not respond well may help to resolve this problem. REASONS TO USE PARAMETRIC EQ

Resonance reduction  Parametric equalizers can be used to precisely remove (or enhance) excessive resonance, which can create a "boomy" or "ringy" sound.  In cases where instruments such as acoustic guitars or double basses have excessive "boominess" in some frequencies, a parametric equalizer can be used to select the frequency band that is overly resonant and reduce its volume.

Reducing extraneous noises  Parametric equalization can also be used to reduce extraneous noises such as the sound of a guitar pick or the left-hand fingers for string instruments, or the creaking of a damper pedal of a piano.  These sounds often cannot be fully removed without changing the tone quality

Feedback prevention  Certain frequencies looping through an audio system though a microphone in close proximity to the speaker it is being sent to.  Parametric equalization for "feedback cancellation" typically uses a notch filter, which cuts the frequency where feedback is occurring.  A NOTCH FILTER, narrow bandwidth, -6dB  Often used in LIVE SOUND

Correction  All speakers have peaks and dips in their output at certain frequencies.  Parametric equalization can be used to boost or cut these peaks and dips to flatten the frequency response.

 One common type of equalizer is the graphic equalizer which consists of a bank of sliders for boosting and cutting different bands (or frequency ranges) of sound.  The number and width of filters depends on application. A simple car audio equalizer might have one bank of filters controlling two channels for easy adjustment of stereo sound, and contain five to ten filter bands.

More on EQ  A typical equalizer for professional live sound reinforcement has 25 to 31 bands, necessary for quick control of feedback tones and room modes. Such an equalizer is called a 1/3-octave equalizer (spoken informally as "third-octave EQ") because the center frequency of each filter is spaced one third of an octave away from its neighbors, three filters to an octave.  Equalizers with half as many filters per octave are common where less precise general tone-shaping is desired—this design is called a 2/3-octave equalizer.

 Equalizers are deliberately designed to create fairly minor changes in the signal.  For more drastic effects, such as removing some region of the signal entirely, a FILTER is required.  A filter is a circuit that sharply reduces the amplitude of signals of frequency outside of specified limits.

More on Filters  The unaffected region is called the PASSBAND, and the filter type is named after the passband as low-pass, high-pass, or band-pass.  The point where the signal attenuation just becomes noticeable (a reduction of 3 dB) is termed the CUTOFF FREQUENCY. A low-pass filter with a cutoff of 500 hz will attenuate any signal of frequency above 500hz.

Dynamics Processing and Envelope Compressor Recap  A compressor is a device that regulates the gain or level of an audio signal as a function of (usually) the changes in amplitude of that signal, according to a fairly complex set of rules.  Sometimes we use it to prevent overly loud signal peaks from distorting, and sometimes to smooth out the level variations to make a signal (particularly a voice signal) more continuously and easily audible.  Sometimes we use it to reduce the overall dynamic range of an audio signal.

 All compressors work by sending the incoming audio signal through an ACTIVE GAIN STAGE (usually a voltage-controlled amplifier or its digital equivalent).  At the same time, the signal is also sent, in parallel, to a so-called level detector, which studies the signal and converts it into a control voltage (or digital equivalent).

 VCA Voltage Controlled Amplifier compressors are the most versatile of all and so are the greatest in number.  The VCA can quickly change gain in response to many different detectors looking at the same signal.  VCA compressors are for the really tough cases where you want strict control over level and dynamics.  However, they can be as gentle as any other compressor or anywhere in between. Types of Compressors

More on Compressors  Control voltage is manipulated (here's where each compressor gets "its own sound") and then regulates the gain of the active gain stage.  As the amplitude of the audio signal gets greater the control voltage is reduced, then the net result will be that as the audio signal gets louder, the gain stage will make it softer, hence "compressing" its dynamic range

 A type of audio compressor that uses an electro optical attenuator to control the dynamics of the processed signal.  An electro optical attenuator basically consists of a light source whose intensity is proportional to the level of the input signal, and a photoconductive cell whose resistance decreases as the light intensity presented to it increases.  This photoconductive cell controls the volume of the amplifier that attenuates the volume of the output signal.  Thus when a louder signal is input, the light shines brighter, the photocell's resistance goes down, and the amplifier reduces its gain, producing the effect of audio compression. Optical

 Teletronix LA-2 and the transistorized UREI LA-3 leveling amps are examples of this type of compressor.  Compressors using this method are used a lot for bass guitar, vocals, program mix and drums.  These compressors offer simple, natural sounding control (unless pressed hard).

 Computer based/digital compressors Computer-based or digital compressors are now more prevalent, offering the ability to process audio in the digital domain under precise computer control.  Digital audio stored in a computer has the advantage of being "predicatively" processed.  You can have zero attack time gain reduction as well as nearly infinite control of all parameters of compression on a moment-to-moment basis.

 FET Field Effect Transistor compressors use a special transistor to vary gain. FETs were the first transistor to emulate tubes in the way they worked internally.  Inherently a high-impedance device, the FET compressor sounds like no other box, and not many examples exist because of the expense of the extra attendant circuitry required.  FET compressors are extremely fast, clean and reliable.  UREI's 1176LN peak limiter and LA Audio's Classic II stereo compressor/limiter are examples of FET-based compressors.