Equalisation
That was the idea back in the
days when they were first
conceived as a tool used to
get flat response in telephone
lines and to make up for the
deficiencies in audio
equipment and acoustic
spaces.
Nowadays it could more
aptly be named an
"unequaliser" since they
are more often used
creatively to alter the
relative balance of
frequencies to produce
desired tonal
characteristics in sounds.
An equaliser has the ability to boost and/or cut the
energy (amplitude) in specified frequency ranges
by employing one or more filter circuits.
There are many different types of EQ's in use
today in many widely varying applications, but they
fundamentally all do the same thing.
There are three types of
eq we will be covering
here:
basic filters,
parametric eq,
and graphic eq.
Filters
A filter is an electronic device designed to reduce a
signal's energy at a specific frequency in a given
bandwidth.
A true filter acts as a subtractive device, and does not
add anything to the signal.
In some filters however, an amplifier is incorporated into
the circuit, which allows the frequency to be boosted or
cut. This is called an active filter.
Filters of different frequencies are often combined to
create equalisers.
Filters are found on most consoles in the form of
High Pass and Low Pass filters.
High Pass filters "pass" the highs, or allow the
highs to pass through the signal chain, while
filtering out the low frequencies.
Low pass filters do the opposite.
Sometimes the frequency is sweepable,
meaning that the point at which the filter starts
acting, is variable.
Parametric Eq
Parametric eq is found on each input channel
of most mixing consoles.
This is a type of equaliser having several
"parameters" for control of various filters that
can be applied to audio signals. Parametric
equalisers are most widely used in situations
where very fine control over the audio signal
is needed.
the Allen and Heath
GL2400 EQ, shown at left,
is quasi-parametric.
This channel uses another kind of
equalisation called fixed band eq, which
also happens to be shelving eq. 12k and
above can be cut or boost up to 15db, and
80hz and below can be cut or boost by up
to 15db as well.
Then there are the two sweepable mid eq
pots, sweeping from 35Hz to 1 k, and from
500 Hz all the way up to 15k.
the Allen and Heath
GL2400 EQ, shown at left,
is quasi-parametric.
In order for an equaliser to be parametric it
must at least have control over gain, Q,
and frequency.
In most cases each of these controls are on
rotary pots, but there are a few "graphic style"
parametric equalisers on the market. Some
equalisers have selectable frequencies that can
be adjusted, but no Q control. These are known
as quasi-parametric or sweepable equalisers. The
Allen & Heath consoles we have at VUT are this
kind, with no Q control.
In order for an equaliser to be parametric it
must at least have control over gain, Q,
and frequency.
"Q" refers to the width of the frequency
band being adjusted. Fully parametric eqs
have variable Q.
What does Q really
mean?
The resonance of an electronic circuit. "Q"
actually refers to quality factor. Q is a measure
of the sharpness of a resonant peak. The term
Q is often used interchangeably with
"bandwidth". This is not entirely correct. It is
more accurate to say that Q determines
bandwidth (a subtle but distinct difference).
Q is most often used in reference to
synthesizer filters (sometimes referred to as
resonance) and equalisers, but it also applies
to capacitors (a measure of efficiency, the ratio
of capacitive reactance to resistance at a high
frequency) and speakers (a measure of
directivity).
Parametric Equalisers:
Some Examples
These are standalone
Eq units, performing
the same function to
the eq component in
a console.
Channel eq from
the Solid State
Logic (SSL)
Console.
But the worst is still to come; an electronic filter is not phaselinear. Phase shifts below and above the centre frequency of
every EQ filter. The higher the Q (31 band EQ) the more phase
shift will occur.
Every band that boost or cuts adds its phase distortion to the end
result. Just as you think it can't get worse; it can! Steep filters
have another flaw; the ringing effect. A high Q bandfilter (an EQ
stage) uses input energy to slowly get into resonance, and when
the input signal vanishes, the bandfilter will use the stored energy
to resonate a little further.
To illustrate this just take an empty beer
bottle; blow over the top to get the whooo
resonant sound of the bottle. You will notice
that the tone will slowly rise in volume and
when you stop blowing it will slowly die out. It
is impossible to start and stop this resonant
tone abruptly. Well, the electronic equivalent the 31 band EQ- shows the same problem 31
times...
• When doing your sound check, try to always put the
signal you are eq'ing back into context. Changing
things in isolation may result in a badly eq'd general
mix.
• Use restraint with live eq- and if you see patterns
developing across your channels, consider changing
the graphic across FOH instead.
• If you are doing monitors from front of house,
everything you eq on a channel will be also be
changing in whatever monitor send it is in as well.
• When tuning with a graphic, it can be a good learning
tool to switch the graphic in and out of bypass to see
how much gain you are losing and to make sure you
are making the sound better and not worse!