ORTF stereo
Coincident or Near-Coincident
Mic Placement Techniques
From DPA Microphones online publications
With history and comments by Tom Bates
ORTF stereo
Two first order cardioid
microphones spaced 17 cm
and angled 110° creating the
stereo image.
The ORTF stereo technique uses two first
order cardioid microphones with a spacing of 17 cm between the microphone diaphragms,
and with an 110° angle between the capsules. This technique is well suited for reproducing
stereo cues that are similar to those that are used by the human ear to perceive directional
information in the horizontal plane. The spacing of the microphones emulates the distance
between the human ears, and the angle between the two directional microphones emulates
the shadow effect of the human head.
The ORTF stereo technique provides the recording with a wider stereo image than XY
stereo and still preserves a reasonable amount of mono-information. Care must be taken
when using this technique at larger distances, as the directional microphones exhibit
proximity effect and will result in low frequency loss. You could add low frequency with an
Equalizer to desired taste.
The French Radio Organization developed this technique and created a standard that others have
modified and followed. The distance from the ensemble will determine the amount of room
reverberation - further away from the ensemble will have more reverberant room sound and closer
placement will have less room sound.
ORTF stereo
Two first order cardioid
microphones spaced 17 cm
and angled 110° creating the
stereo image.
The ORTF stereo technique uses two
first order cardioid microphones with a spacing of 17 cm between the microphone
diaphragms, and with an 110° angle between the capsules. This technique is well suited
for reproducing stereo cues that are similar to those that are used by the human ear to
perceive directional information in the horizontal plane. The spacing of the microphones
emulates the distance between the human ears, and the angle between the two
directional microphones emulates the shadow effect of the human head.
The ORTF stereo technique provides the
recording with a wider stereo image than XY
stereo and still preserves a reasonable
amount of mono-information. Care must be
taken when using this technique at larger
distances, as the directional microphones
exhibit proximity effect and will result in low
frequency loss. You could add low frequency
with an Equalizer to desired taste.
The French Radio Organization developed
this technique and created a standard that
others have modified and followed. The
distance from the ensemble will determine
the amount of room reverberation - further
away from the ensemble will have more reverberant room sound and closer placement
will have less room sound.
NOS stereo
Two first order cardioid
microphones spaced 30 cm
and angled 90° creating the
stereo image.
The NOS Stereo Technique uses two
cardioid microphones spaced 30 cm
apart and angled at 90° to create a stereo
image, which means a combination of difference-in-level stereo and difference-in-time
stereo. If used at larger distances to the sound source the NOS stereo technique will
loose the low frequencies due to the use of pressure gradient microphones and the
influence of the proximity on these type of microphones. The NOS stereo technique is
more useful at shorter distances, for example on piano, small ensembles or used for
creating stereo on a instrument section in a classical orchestra.
This technique was developed by Dutch Broadcasting (Nederlandsche Omroep
Stichting). This angle is normal, but 30 cm of separation is wider than any human head.
RAI stereo
(Italian Broadcasting Corporation) The RAI technique is similar to ORTF;
however, the cardioid microphones are 21cm apart and at an angle of 100
degrees.
DIN stereo
Two first order cardioid microphones spaced 20 cm and
angled 90° creating the stereo image.
DIN stereo uses two cardioid microphones spaced 20 cm apart and angled at 90° to
create a stereo image. The DIN stereo produces a blend of intensity stereo signals and
time delay stereo signals, due to the off-axis attenuation of the cardioid microphones
together with the 20 cm spacing. If used at larger distances to the sound source the DIN
stereo technique will loose the low frequencies due to the use of pressure gradient
microphones and the influence of the proximity on these type of microphones. The DIN
stereo technique is more useful at shorter distances, for example on piano, small
ensembles or used for creating stereo on a instrument section in a classical orchestra.
(Proposed standard in Germany)
Olson stereo
The Olson technique is also similar to ORTF; however, the cardioid microphones
are 20cm apart and at an angle of 135 degrees. This is the widest angle of
acceptance of any of these techniques. Often the audio center is indistinct
because each mic receives it at 67.5 degrees off access.
XY stereo
Two first order cardioid
microphones in the same
point (coincident) and angled
to create the stereo image.
XY stereo set-up is a coincidence stereo
technique using two cardioid
microphones in the same point and
angled typically 90° to produce a stereo image. Opening angles of 120° to 135° or even
up to 180° between the capsules are also seen, which will change the recording angle
and stereo spread. Theoretically, the two microphone capsules need to be at exactly the
same point to avoid any phase problems due to the distance between the capsules. As
this is not possible, the best approximation to placing two microphones at the same point
is to put one microphone on top of the other with the diaphragms vertically aligned. In
this way, sound sources in the horizontal plane will be picked up as if the two
microphones are placed at the same point.
The stereo image is produced by the offaxis attenuation of the cardioid
microphones. While A-B stereo is a
difference-in-time-stereo, the XY stereo is
a difference-in-level-stereo. But as the
off-axis attenuation of a first-order
cardioid microphone is only 6 dB in 90°,
the channel separation is limited, and
wide stereo images are not possible with
this recording method. Therefore, XY
stereo is often used where high monocompatibility is needed - for example, in
broadcasting situations where many
listeners still receive the audio on mono
equipment.
Since the sound-sources are mainly picked up off-axis when using the XY stereo setup,
high demands are placed on the off-axis response of the microphones used. And as
described earlier, the use of directional microphones at large distances will reduce the
amount of low frequency information in the recording, due to the proximity effect
exhibited by the directional microphones. The XY configuration is therefore often a close
miking application choice.
Baffled stereo
Spaced microphone stereo
techniques using an acoustic
absorbent baffle.
Baffled stereo is a generic term for a lot
of different stereo techniques using an
acoustic baffle to enhance the channel
separation of the stereo signals. When
placed between the two microphones in a
spaced stereo set-up like A-B stereo,
ORTF stereo, DIN stereo or NOS stereo,
the shadow effect from the baffle will have a positive influence on the attenuation of offaxis sound sources and thereby enhancing the channel separation. Baffles should be
made from an acoustic absorbent and non-reflective material to prevent any reflections
on the surface of the baffle to cause coloring of the audio.
One of the more well known baffled stereo principles is the so called Jecklin Disc
developed by the Swiss sound engineer Jürg Jecklin. This technique uses two Type
4003 or 4006 omnidirectional microphones spaced 36 cm and a special acoustic treated
disc with a diameter of Ø35 cm placed between the microphones.
This addition to near coincident mic technique places an acoustically absorbent baffle
between the two microphones to lessen the immediate travel of sound from the sides
across from one microphone to the other. This resembles, in some approximate way,
the blocking of sound by the human head between the two ears.
Binaural stereo
Two omnidirectional microphones placed in the ears of a
dummy head creating the stereo image.
The Binaural recording technique uses two omnidirectional microphones placed in the
ears of a dummy head and torso. This two-channel system emulates the human
perception of sound, and will provide the recording with important aural information
about the distance and the direction of the sound-sources. When replayed on
headphones, the listener will experience a spherical sound image, where all the soundsources are reproduced with correct spherical direction.
Binaural recordings are often used as ambience sound or in virtual reality applications.
This can enhance the clues for localization, but those enhancements are usually easier
to perceive on headphones than on speakers.
Blumlein stereo
Two bi-directional microphones placed in the same point and
angled 90° creating the stereo image.
The Blumlein stereo set-up is a coincidence stereo technique, which uses two bidirectional microphones in the same point and angled at 90° to each other. This stereo
technique will normally give the best results when used at shorter distances to the sound
source, as bi-directional microphones are using the pressure gradient transducer
technology and therefore is under influence of the proximity effect. At larger distances
these microphones therefore will loose the low frequencies. The Blumlein stereo is
purely producing intensity related stereo information. It has a higher channel separation
than the XY stereo, but has the disadvantage, that sound sources located behind the
stereo pair also will be picked up and even be reproduced with inverted phase.
Because the microphones pick up in front and back, both the ensemble and the room
sound are well represented.
MS-stereo
One first order cardioid microphone and one bi-directional
microphone in the same point and angled 90° creating a
stereo image through a so called MS-matrix.
MS-Stereo uses a cardioid microphone capsule as center channel and a bi-directional
microphone (figure-of-eight-microphone) at the same point, angled at 90° as the socalled surround channel. The MS-signal can not be monitored directly on a left-right
system. The MS-matrix uses the phase cues between the center and the surround
microphone to produce a left-right signal suitable for a normal stereo system. Due to the
presence of the center microphone, this technique is well suited for stereo recordings
where a good mono-compatibility is needed, and is extremely popular in broadcasting.
The cardioid microphone is panned to center. The figure 8 microphone is split into two
channels and panned hard left and right. The phase of the left remains normal while the
right is phase reversed. By increasing the level of the figure 8 microphone channels, the
room acoustics and angle of acceptance will increase. The MS stereo technique has
excellent mono compatibility.
Spread Stereo Mic Placement Technique
A-B stereo
Two spaced microphones
creating a stereo image.
The A-B Stereo Technique uses two
spaced (omnidirectional or cardioid)
microphones to record audio signals. The
microphone spacing introduces small
differences in the time or phase
information contained in the audio signals
(according to the relative directions of the
sound sources). As the human ear can sense time and phase differences in the audio
signals and use them for localization, time and phase differences will act as stereo cues
to enable the listener to "capture the space" in the recording, and experience a vivid
stereo image of the complete sound-field, including the positioning of each separate
sound-source and the spatial boundaries of the room itself.
Microphone spacing
An important consideration when setting up for A-B stereo recordings is the distance
between the two microphones. Since the acoustic character of the stereo recording is
very much a question of taste, it is impossible to give fast rules for stereo microphone
spacing, although it is a good idea to keep some important acoustic factors in mind.
Since the stereo width of a recording is frequency-dependent, the deeper the tonal
qualities you wish to reproduce in stereo, the wider your microphone spacing should be.
Using a recommended microphone spacing of a quarter of the wavelength of the
deepest tone, and taking into account the human ear's reduced ability to localize
frequencies below 150Hz, leads to an optimal microphone spacing of between 40 and
60 cm. Smaller microphone spacings are often used close to sound-sources to prevent
the sound image of a particular musical instrument from becoming "too wide" and
unnatural. Spacings down to 17 to 20 cm are detectable by the human ear, as this
distance is equivalent to the distance between the two ears themselves.
It should also be noted, that an increase in microphone spacing will decrease the
system's ability to reproduce the signals from sound-sources positioned directly between
the microphones. This will also lead to a reduction in the quality of the stereo recording
when it is played in mono.
Distance between microphones and sound-source
The ideal distance from the microphone pair to the sound-source not only depends on
the type and size of the sound-source and on the surroundings in which the recording is
to be made, but also on individual taste. The position from which the listener
experiences the event - and hence the position from which the micro-phones record the
event - should be chosen with care and feeling.
Critical musical recordings, such as a full orchestra in a concert hall, are good examples
of the importance of correct stereo microphone positioning. Here the microphones would
typically be placed above or behind the conductor. And although most instruments
project their sound in an upwards direction, the microphones should be placed high
enough so that the individual musicians do not shadow each other.
The mix of direct and diffuse sound in a recording is also of crucial importance, so much
time can often be used establishing the optimum positioning of the microphones. It is
here that the versatility of our A-B Stereo Kits comes into play. Using the different
acoustical attachments for the microphones, the amount of ambience and the tonal color
of the recording can be adjusted without adding any noise. The choice of floor and
ceiling mounting of the boom can give you added flexibility when positioning the
microphones.
Omnidirectional microphones and A-B Stereo are often the preferred choice when the
distance between microphone and the sound source is large. The reason is that
omnidirectional microphones are able to capture the true low frequencies of the soundsource regardless of the distance, while directional microphones are influenced by the
proximity effect. Directional microphones will therefore exhibit loss of low frequencies at
larger distances. The DPA Wide Cardioid microphones are however designed with a
richer bass response, making the bass loss at distances much less critical. Therefore
these mics are a good alternative to omnis when a little directionality is preferred or
needed. The sound color is very similar to our omnis.
Spaced Cardioids
Unlike the techniques discussed so far, spacing cardiod microphones apart and aiming
them in toward the musicians is not a technique born of attempting to model the human
listening experience. However, it can often prove more effective if a desire to include
more of the room sound, or to achieve a flattering capture of the instruments, has led to
a mic placement that is farther apart. If these events coincide with a narrow ensemble
width to be recorded (such as a string quartet on a large stage, or a solo piano), then a
more satisfying spread of the perceived stereo width can be created for the listener
Spaced Omnis
This technique uses two (or three for large ensembles) omni-directional microphones
evenly placed in front of an ensemble. This technique is often blamed for producing
phasing problems, but I have not often experienced that problem. Some cite a 3:1 rule
which they say should be utilized. The 3 to 1 rule states that for every 1 unit of distance
from the sound to the microphone should be 3 units of distance between microphones.
(Example: microphones are 5 feet in front of the ensemble - the distance between the
two microphones cannot be less than 15 feet - 7.5 feet each from the center line.) You
must adapt to the situation, however, so such rules are only rough guidelines. Spaced
omnis look like spaced cardioids, but the sound is quite different. Omnis must usually be
placed much closer to the ensemble or there may be too much room sound. They have
better balance of instruments in larger ensembles, but less of a distinct stereo separation
than their cardioid counterparts.
Omni Out-Riggers
For large ensembles, all of the techniques mentioned above can be enhanced by adding
a pair of omni out-riggers. These are placed some distance left and right of center,
usually following the 3:1 rule cited above.
.
Decca Tree
3 Omnis in a triangle
Originally introduced by the record label Decca, the "tree"
consists of a three-point pick-up by omnidirectional
microphones in a (often equilateral) triangle pointing at the
sound source. The two outside microphones are so far apart
that a "hole in the middle" will occur without adding the center
mic. The center mic should be mixed in to fill up this hole,
taking care not to make the sound perspective too mono.
Outer distances are seen from 60cm to 120cm. The third, in
the center, can be slightly lower than and in front of the
outside pair. Depending on the acoustic variables of the hall where the ensemble or
orchestra is being recorded, the tree can be raised or lowered to achieve the best result.
This is a very successful placement style as it ensures a natural, seamless sound stage
for the listeners, and allows them to experience the full dynamic performance in context.
Often, the Tree is placed right behind or over the conductor which results in a sound
balance very close to his intention. The 3 microphones furthermore come closer to the
orchestra sections than AB setups, providing even more clarity and sharper imaging - a
more intense and detailed reproduction.
With the DPA Acoustic Pressure Equalisers you
are able to modify the frequency response of the
4003 or 4006. This is done without adding any
noise or distortion from EQs because the lift is
done acoustically at the diaphragm. The L40B
sphere will introduce a presence boost and more
directionality at higher frequencies, which in
sound color is identical to old legendary
microphone types originally used by Decca, only
with the much higher degree of naturalness
offered by a DPA microphone.
Double MS
A time coincident, compact and adjustable surround configuration
The principle of the double MS technique is using a frontand a backwards pointing MS set, sharing the same Side
microphone. Therefore, just 3 microphones are needed,
but quite a processing/mixing task is necessary. As
always with MS set-ups, 2 different transducer
technologies are used for the cardioid Mid mic and the bidirectional Side mic resulting in risk of different frequency
and phase responses of sound reproduction from the
sides or the front.
Therefore, this double MS technique can be attained by
using 4 identical - even matched - DPA 4021 cardioid
microphones angled 90° to each other.
Acknowledgement: Some of this material was drawn from DPA online publications:
http://www.dpamicrophones.com/
Overview of Stereophonic Recording Techniques
Stereo recording
principle
Name
coincident
microphone placement
X/Y
M/S
level differences +
minor arrival-time
differences
microphones separated by an acoustically opaque object
major arrivaltime differences
ORTF (for example)
Jecklin disk (for ex.)
5 cm – 30 cm
depends on the
object between
them
40 cm – 80 cm
or greater (up to
several meters)
typically 20°
0° – 90°
A/B
Geometry
Distance (d)
between microphones
Angle between
the main axes of
the microphones
Acoustic operating principle of
the microphone
0 cm
usually vertically aligned
distance and angle
are interdependent
70° – 180°
90°
0° – 180°
pressure-gradient transducer
(e.g. SCHOEPS cardioid MK 4
or CCM 4)
Sonic impression
clean, clear, often bright
usually pressure transducers*
(e.g. SCHOEPS MK 2S or CCM 2S)
depending on the microphones used
big, spacious; especially good low-frequency reproduction when omnidirectional condenser microphones are used
Spaciousness
often rather limited
satisfactory
good
very good
Localization
potentially very good,
except that the center of
the stereo image can be
over-emphasized (not a
problem with figure 8s)
good
adequate
indistinct
(potentially
unstable)
* These recording methods can also employ
pressure gradient microphones, though this
is not often done.
**The appropriate angle between microphones
depends on their directional pattern and the
recording angle (the range within which the
sound sources should be placed, as “seen“
by the microphone.)
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