Curious as to how to use a Jecklin Disk? Not sure what one is or why you should? Read all about it here straight from the man himself Jürg
Jecklin. Special thanks to Josephson Engineering for the translation and permission to post on our site... enjoy!
The OSS technique (OSS = optimum stereo signal)
The idea of a new microphone arrangement is the result
of the dissatisfaction about the sound of usual music
recordings, which has made itself felt during the course
of approximately 4000 recording sessions. Almost any
sound engineer knows how it feels when the recording
does not sound like he imagines it should, despite the
use of a lot of microphones. In such a case, i.e.,
if the technical expenditure cannot be increased
meaningfully, one should start again and look for a new
recording concept. The present recording technique of
electronic music has grown out of the basic arrangement
of two microphones (stereophonics). Since the recordings
with only two microphones normally are not satisfactory,
additional supporting microphones — often a large
number of them — are used. Therefore, the solution
of the problems has to start with the two insufficient
main microphones. On the one hand, the stereo
main microphone arrangement must guarantee an optimum
sound; and, on the other hand, it must provide the
correct stereo signal for reproduction via two
loudspeakers in the room. The procedure for the
conception of the OSS technique was therefore as
follows:
- I. Selection of suitable microphones
Practical experience has shown that condenser sound
pressure microphones (omnidirectional characteristic)
are superior to all other microphones. Even the sound
balance and the spatial sound distribution of very large
and complex orchestras (symphony orchestras) are
correctly reproduced. This kind of microphone is
therefore used for the OSS arrangement.
- II. Optimum stereo signal
In the usual stereo recording, mainly intensity
differences between the two channel are used as
directional information. This so-called intensity
stereophonics, however, is a mere simulation method with
phantom sound sources between the two reproduction
loudspeakers, which do not actually “stand”
in the room as one would desire. When one listens
directly, there are differences in delay time, frequency
response and intensity between the two ears of the
listener and their combination provides the directional
information. These three parameters change with
frequency and the angel of impact of the sound on the
head. In the case of intensity stererophonics, only one
of these three parameters is taken into consideration
whereas an optimum stereo signal must include all three
in the right combination. In the case of the OSS
arrangement, all three parameters are used for the
directional information and this in a combination which
is ideal for the listener when reproduction comes from
two loudspeakers in the usual arrangement.
Stucture of the OSS arrangement (Jecklin disc)
Two sound pressure microphones are arranged at a
distance of 165 mm. This distance results in the
correct delay time difference between the two
channels. The two microphones are separated by an
acoustically muffled disc of 300 mm diameter. The effect
of this disc is as follows: as the frequency increases,
the two microphones are more and more separated. Below
the value of approximately 200 Hz, the two microphones
record the same. The acoustic muffling of the disc
results in a frequency response difference of the two
channels depending on the angle of impact of the
sound. In addition, there is a sound diffraction around
the disc rim which is dependent on frequency and angle.
- I. Characteristics of the OSS arrangement
The stereo signal is produced by purely acoustic mixing:
on the recording side by the arrangement, and on the
reproducing side by the interplay of the two loudspeaker
and the reproduction room. The two microphones react
only to the sound pressure. The entire acoustic pattern
is recorded in one single spot in the room. The result
of these clear acoustic conditions is a natural sound
and a real spatial acoustic pattern. The simultaneous
reproduction of the spatial sound distribution by the
two loudspeakers cannot be achieved by any other
recording technique. This is audible especially in the
case of organ recordings. In the case of orchestra
recordings, each instrument can be heard in the place
where it is actually played. If the orchestra is
arranged correctly, no supporting microphones are
required.
- II. Working with OSS technique
The technical expenditure is small. Good recordings can
be achieved even with simple technology. The two
microphones together emit one stereo signal. They must
therefore be adjusted to the same output level in the
diffuse sound field. Level differences during recording
must not be balanced out. In the case of one-sided
recording, the OSS arrangement or the arrangement of the
orchestra must be changed.
Sound control at the mixing desk is not possible in
the conventional sense. The sound control is moved into
the studio or into the hall (changing of the orchestra
arrangement). Supporting microphones may be used,
provided they are admixed with a time lag that
corresponds to the delay time of the sound from the
supported instrument to the main microphone. For this
recording technique, not only the intensity relationship
but also the delay time relationship must be correct.
- III. Recording in very reverberant or in acoustically problematic rooms.
In an OSS arrangement, directional microphones cannot be
used. When recording in acoustically unfavorable rooms,
it is better to accept the room as it is than to fight
its acoustics. In practice, natural recording always
sounds better than unnatural, so-called improved
recording. In any room where it is still possible for
musicians to make music, OSS recording is also normally
possible.
- IV. Range of application of the OSS technique
For the recording of orchestras that are harmonious and
are arranged correctly (symphony orchestra, chorus,
brass band, chamber music, individual instruments, etc.).
The OSS technique is not suited for recordings where the
recording technique is designed to co-determine the
sound. The OSS technique is especially well suited for
the recording of concerts aiming at a very natural
sound. The OSS technique may be a step towards a more
natural stereo recording. In comparison with
conventional recordings, OSS recordings were preferred
by all listeners during tests. The professional musicians
are especially happy with the “new” sound.
Recently, many recordings at radio stations have been made
with this technique.
The Jecklin disc in practice
- I. Microphones for recording with the Jecklin disc
For recording with the Jecklin disc, sound pressure
microphones must always be used; i.e.,
“real” sound pressure microphones.
Microphones with a switchable directional
characteristic, insofar as they are designed as double
diaphragm microphones, may not be used.
- II. Operating instructions
- 1. General
The disc has two microphone mounts (21mm) and one stand
connecting piece. By putting the stand connecting piece
into the individual borings, the setting angle of the
disc can be adjusted to any situation. The measuring
cord located in the center of the disc serves for the
adjustment of the microphones on the disc. The measuring
cord can be adjusted on both sides.
- 2. Mounting of the microphones onto the Jecklin disc
The microphones are pushed from the rear through the
holding clamps of the Jecklin disc. Take care
that the microphone capsules on both sides are located
at a distance of approx 8 cm directly above the disc
center. This can be checked by means of the pulled-in
measuring cord. In this case, the angles of the
microphones are drawn slightly towards the outside.
- 3. Mounting of the connecting piece
The Jecklin disc with the microphones is at first held
by hand in the position that is required for the recording.
By loosening the fastening screws at the stand
intermediate piece, the intermediate piece can be
removed and set into any of the borings at the disc rim;
i.e., the disc is attached at the preselected angle.
Thus the balance of the entire arrangement is guaranteed
and the adjustment to any situation is made possible.
Note from David Josephson: It isn't mentioned by
Jecklin, but “the position that is required by the
recording” is determined by the desired tone color
balance. Most microphones are more sensitive toward the
front than toward the sides and rear for high frequency
sounds. Rotating the disc maintains the capsules at the
correct distance from the disc but allows their angle to
the sound to be adjusted for more or less high frequency
information.
- III. Recording with the Jecklin disc
Since the two microphones together emit a uniform stereo
signal, they have to be adjusted prior to the recording
in the diffuse sound field to the same output level. This
can be checked best by means of a headphone: the spatial
impression must be uniform. The room must not
“hang” on one side. During the microphone
test or the recording, level differences between the two
channels must not be balanced out. In the case of
one-sidedness, the disc must be turned or rearranged
accordingly. A sound control in the conventional sense,
balancing at the mixing desk, is not possible; i.e.,
the sound control is moved from the control room into the
studio or concert hall.
Placing of the Jecklin disc before the sound source is
less critical than microphone placement in the case of
other recording techniques. Depending on the distance
of the disc from the sound source, the recording sounds
nearer or more remote without being automatically too
close or of too large an auditory perspective. When the
disc is being used, the optimum distance from the sound
source is larger than it is the case of any other
recording technique. The disc must, however, be located
within the diffuse-field distance of the room (the
diffuse-field distance is the distance at which the
direct sound from the source and the diffuse sound
portion of the room are equal). The exact postion and
the correct distance from the sound source, however,
must always be determined by ear. Here, one can usually
proceed as follows:
- Too near: Locate the disc at a larger distance, or higher.
- Too far: Locate the disc nearer or lower.
- Wrong spatial sound distribution: if the instruments in
the rear of the orchestra are recorded too loud, locate
the disc lower. If they are too soft, the disc must be
located higher.
- Level variations: if the level of one channel of a
correctly adjusted disc (adjusted in the diffuse sound
field) is higher than the other, the disc must be turned
accordingly.
- Distance of the disc from the sound source: as already
mentioned, the disc must be located within the
diffuse field. This distance can be calculated. In the
following table, the diffuse-field distance depending
on the room volume and the reverberation time for
virtually all possible cases is indicated.
Room volume, | R e v e r b e r a t i o n t i m e, s e c o n d s
cubic meters | 1.0 1.5 1.75 2.0 2.25 2.5 3.0 3.5 4.0
500 1.27 1.04 0.96 0.9 0.85 0.8 0.74 0.68 0.64
1000 1.8 1.47 1.35 1.41 1.2 1.14 1.04 0.96 0.9
2000 2.55 2.08 1.93 1.8 1.7 1.61 1.47 1.36 1.27
5000 4.03 3.29 3.04 2.85 2.69 2.55 2.33 2.16 2.02
10000 4.65 4.3 4.03 3.8 3.6 3.3 3.05 2.85
15000 5.28 4.93 4.65 4.42 4.03 3.73 3.49
20000 5.7 5.37 5.1 4.65 4.31 4.03
The table shows diffuse field distance, in meters, from the center of a sound
source, as a function of room volume and reverberation time. The disc should
be placed closer to the sound source than this distance.
- IV. Checking the technical equipment
Successful recording depends on the perfect functioning
of all of the technical equipment used. In practice,
this can be achieved most easily be means of the
following check list.
- 1. Initial Setup
- Check the correct lateral position of the loudspeakers
(headphone): monitor switch at the tape recorder to
“tape” (check with a known recording).
- Adjust the reproduction volume (with a known recording).
- Check the correct lateral position and the functioning
of the microphones (monitor switch to “input”).
- Check the tape run (visually). remove buckled tapes (the
tapes are often deformed only at the beginning of the
tape reel. Prewind accordingly).
- Check the recording function (monitor switch to
“tape”, recording key pushed).
- During the microphone test
- Adjust the channel controller levels for the correct
balance and the correct control of a loud passage.
- Recording test: compare “input” and
“tape”.
- During the recording
- Do not touch the controllers (short sound overshooting
is usually not that bad and is readjusted too late anyway).
- Always monitor behind the tape (monitor switch to
“tape”); do not change the reproduction
volume.
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