|
1970's MXR
Phase 90
Bass guitar
with direct box and mic
Two microphones
positioned apart from each other will capture
sound at different times. When brought together
they will be out of phase. The Phazer helps
solve the problem.
Comb
filtering is an effect that is caused when
multiple ‘out of phase’ sound
sources are combined. Not only is the fundamental
affected, so are the harmonics.
|
|
 |
Messing
around with ‘time travel’ is not new…
In the dark ages of the 1970's, MXR™'s ground
breaking 'Phase 90' effect pedal ignited interest
in phasing by way of modulating the phase effect
for electric guitar. This inspired Radial’s
chief engineer Denis Rozon to start messing with
phase circuits in the early 1980s. Fast forward
20 years MXR is re-launched by Dunlop and these
‘vintage’ remakes are once again available
to the masses. Radial has taken a different route,
targeting not guitar pedals, but audio signal paths.
What few realize is that beyond the sweeping phase
effect, minute phase adjustments can lead to actually
creating more natural sounds. It has to do with
physics or more precisely, the physical location
of the desired sound source and the various devices
used to capture the sound.
Sound travels at roughly 1130 feet per second (340
meters per second) while electricity is estimated
to travel at 650,000 feet per second (200,000 meters
per second) or roughly 600 times faster. This means
that if you were to combine the direct feed from
a guitar amplifier with a microphone, by the time
the amplifier pushes the speaker outwards and the
sound travels through air before it enters the mic,
it will be minutely delayed with respect to the
direct (hard wired) signal.
This ‘phase mismatch’ is even more pronounced
when two microphones are used on the same instrument.
Imagine, for instance, an acoustic guitar with one
microphone positioned directly adjacent to the sound
hole and a second mic elevated and 10 feet (3 meters)
away from the instrument in an effort to capture
the natural space and ambiance. Some sound engineers
often spend hours moving microphones around the
room in an effort to find the sweet spot or the
position where phase anomalies will be less pronounced.
These phase anomalies create an effect known as
comb filtering. This occurs when a sound is combined
with a delayed version of itself in a given acoustic
space. When the fundamentals and harmonics mix together
the frequencies will either combine to amplify each
other when in phase or cancel each other out when
out of phase, depending on where you place the two
microphones relative to each other. Visually, the
resulting frequency response curve looks somewhat
like a comb, hence the name comb-filter. This usually
produces a hollow or unnatural tone.
Reality check #1: Comb filtering is an integral
part of all sounds. Our ears and brain use phase
along with frequency and loudness to localize
sound. In fact it is impossible to be in perfect
phase at all frequencies as each of the infinite
number of frequencies has a different wave length.
In other words, if you perfectly time-align 500Hz
(wavelength is 2.2 feet long or about 1.5 meter),
then 510 Hz will be out of phase because the wavelength
is slightly longer.
Reality check #2: Since you cannot ever be in
perfect phase, don’t stress over it. Use
your ears and listen. This is exactly what those
finicky engineers do when trying to get a great
sound. They listen to the combined sound of the
two mics using their ears. It is no different
when combining two sounds electronically by adjusting
the phase. This is in fact what the Phazer is
all about. It is a device that lets you precisely
adjust the phase relationship between two sources
by delaying one of the sources. Phase adjusting
is most often applied to the nearest source so
that the fundamental frequencies line up and sound
best.
A common application is electric guitar. In live
touring, the goal during sound check is to get the
sound up fast so that the band and crew can go have
dinner before the show starts. Taking a direct feed
from an amp, using a device like the Radial
JDX amplifier DI for example, provides
a consistent sound because inconsistencies such
as room acoustics or mic placement are eliminated.
But since the time when Keith Richards started playing
guitar, the venerable Shure SM57 has always enjoyed
front row positioning an inch or two away from the
speaker. This familiar look and sound is important
on many levels. For the guitarist, it provides a
safety net whereby he knows his amp sound will be
projected through the PA system. For the FOH engineer,
the familiar ground makes for a good starting point
in capturing the sound for the evening’s event...
but potential problems abound. If the microphone
moves ever so slightly, either to the side or further
from the speaker, the sound will be completely different.
So, as a means to circumvent this problem, a direct
feed using a device like the Radial JDX is an excellent
solution and has become a standard practice for
the smart engineer.
This solves one problem, but unfortunately creates
another. The direct feed from the microphone traveling
at two thirds the speed of light will arrive at
the mixing console slightly ahead of the slower
mic signal. The Phazer solves this problem by allowing
the engineer to slow down the direct feed so that
both signals sound good together. Within seconds,
you can combine two sounds and get better, more
consistent results.
Designed to be positioned at the mixing board,
the Phazer is usually inserted into the signal
path using the insert ports on a console. This
allows the Phazer to be used on all types of sources:
microphones, direct feeds, transducers and so
on.
It is important to emphasize that the Phazer is
a tool designed to spur on the creative process.
In this day and age of virtually unlimited tracks,
try using the Phazer in creative ways that can
produce some fun and unexpected results. If you
do not like the effect, mute the track –
the point is to have fun, and experiment. Who
knows what results you can achieve!
|
|
