Josephson Engineering OSSDISK stereo microphone baffle
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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 thousands of 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 using 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 (symohony 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 informtion. 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.
B. 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 appromately 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.