Transcript Signal Processing

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.
http://en.wikipedia.org/wiki/Equalization#cite_note-0
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Parametric equalizer (or parametric "EQ") is an electronic
multi-band variable equalizer used in sound recording and
live sound reinforcement.
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THREE primary parameters of an internal band-pass filter
which are amplitude, center frequency and bandwidth.
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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 semior fully parametric EQ
Quality Factor
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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
REASONS TO USE PARAMETRIC EQ
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.
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.
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A NOTCH FILTER, narrow bandwidth, -6dB
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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
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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
http://www.tvtechnology.com/article/88542
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.
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Sometimes we use it to reduce the overall dynamic range
of an audio signal.
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All compressors work by sending the
incoming audio signal through an ACTIVE
GAIN STAGE (usually a voltage-controlled
amplifier or its digital equivalent).
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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).
Types of Compressors
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VCA •
Voltage Controlled Amplifier compressors are the
most versatile of all and so are the greatest in number.
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The VCA can quickly change gain in response to many
different detectors looking at the same signal.
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VCA compressors are for the really tough cases where
you want strict control over level and dynamics.
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However, they can be as gentle as any other
compressor or anywhere in between.
More on Compressors
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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
Optical

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.
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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.
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Teletronix LA-2 and the transistorized UREI LA-3
leveling amps are examples of this type of
compressor.
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Compressors using this method are used a lot for
bass guitar, vocals, program mix and drums.
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These compressors offer simple, natural
sounding control (unless pressed hard).
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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.
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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.
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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.
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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.
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UREI's 1176LN peak limiter and LA Audio's
Classic II stereo compressor/limiter are
examples of FET-based compressors.