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MB Electronics Jecklin Disc.



The term 'home', as used in home recording, implies that, unlike a professional installation, a set up has been 'superimposed' over a domestic scenario, and that consequently there have probably been a few compromises made.

One of the purely practical limitations for most of us is that of space. Not many private houses have rooms which can accommodate a full band lineup with complete drum kit and brass section etc, and even if it were possible to cram the band in, the acoustics of the average living room would probably be a little boxey and resonant, resulting in undesirable colouring of the sound.

Acoustic treatment of a room as a whole tends to be prohibitively expensive or cosmetically unacceptable. Consequently, the home studio is usually the domain of the overdub technique, wherein each instrument is recorded separately, so that any acoustic treatment can be kept simple, and can be limited to a small booth of screens, the settee and blankets draped over boom arms etc, or improvised variations there on.

However, having said that, there may be those who like the natural sound of their living room or who have gone to the trouble of acoustic treatment, to the point where it can be used quite effectively as a whole. If that's you, then this review might open up a new realm of possibility.

MB Electronic's OSS System is based on the Jecklin Disc, which is a device for use in making live stereo recordings, and can be used more or less wherever a stereo pair of mics might be deployed. As the name suggests, it is basically a plastic disc used to acoustically separate a pair of microphones, but before going into the details of the thing, let's take a look at the reasons and scientific basis for its design.

Binaural



The phenomenon of stereo sound is based on the fact that we have two ears, and that these two ears are physically separated by the mass of the head, we have a binaural hearing system. As sound travels at a finite speed (approximately 1,000 feet per second through the air) this means that acoustic information arriving from the left hand side will reach the left ear a fraction of a second before it reaches the right ear, thereby introducing a relative phase shift, and further, that the head will act on sound above a frequency of around 1kHz to cause a shadow, such that the sound will arrive at the right ear somewhat attenuated. To sum up then: sound arriving from the left will reach the right ear after it reaches the left and at a lower level.

The brain interprets this phase and intensity information to discern the direction of the sound source.

Dummy Head



A well established technique used to try and capture this information on tape is 'dummy head' binaural recording. A model of a human head has a microphone placed in each ear, the outputs of which are recorded on to separate tracks of a tape recorder, just as with a normal stereo recording. When listened to on headphones, such recordings can impart an extraordinary degree of spatial awareness to the listener.

There are a number of very definite inherent limitations within this technique: there is no strong vertical or back/front information apparent, the sound image still tends to be inside the listener's head, and of particular interest here, when listened back to on loudspeakers (as opposed to headphones) there is an acoustic 'hole' in the centre of the image. This is where the disc in question claims to have the upper hand: it provides detailed and accurate spatial information, and works well on both speakers and headphones. It also costs less than a good quality dummy head.

The Disc



Aesthetically, the disc is far less impressive than a head, and for what it is, the price of £66.20 is hardly inexpensive. It is a plastic disc, 12" in diameter and about ⅓" thick, covered on both sides with about the same thickness of foam rubber. A standard adaptor, included, allows it to be attached, edge-on, to a suitable mic stand.

The idea is that a microphone is positioned either side of the disc, using the mic adaptors attached to it, and that sound will be absorbed, reflected delayed and generally shadowed by the disc, in various degrees depending upon its angle of incidence (where it's come from), thus providing the desired phase and intensity information required for the binaural effect.

The symmetrical positioning of the mics with relation to the disc is fairly critical for accurate imaging. With this in mind, a captive cord is threaded through a hole in the centre of the disc, and by extending the cord perpendicularly to the disc, on either side of it, the correct position for the head of each microphone can be established.

Needless to say, there are some marked differences between the form of a plastic disc and that of a human head. The distance between the mics is about 7", which is similar to the spacing of a pair of human ears, but the 'sight lines', that is to say the angles of access and obstruction caused by the disc, are going to vary considerably. It's rather difficult to state any basic rules concerning these differences as they will change with the angle of incidence.

A diameter of 12" is somewhat larger than the average head and consequently, for certain oblique angles, it will cause shadowing down to a lower frequency. However, this is obviously only effective in one plane; the disc's lack of depth will leave sound coming in from a more acute front or rear angle relatively unaltered. Furthermore, unlike with the flush-mounted position of the ear, the mics will be receiving both direct sound together with that reflected from the disc. According to the manufacturer, it is the combination of these differences which solves the hole in the middle problem.

The idea of using a disc in this way is by no means original, the likes of Radio London have been using a slightly less sophisticated one for some time in the recording of classical concerts, and have found it to be satisfactory.

If home recording, for you, means Vivaldi evenings with the local string quartet, then the disc has very obvious and worthwhile applications. However, if overdubbing individual instruments, as previously mentioned, does constitute the bulk of your work, it may be necessary to actually look for ways in which to use it.

If you can afford it, the system can be bought as a whole in a robust, executive style flight case. It comes complete with a wide selection of connection adaptors, mic stand adaptors and a pair of PMB 640 capacitor mics, but suffice it to say that this is a "price on application" deal, and considering that each mic on its own will cost almost £190.00, it won't be cheap!

For the purposes of this review, we borrowed just the disc, together with a pair of the relatively new PMB E350 electret mics, which retail at a more reasonable £72.22, including an omnidirectional capsule. Any good quality mic will suffice as long as it has an omni polar response pattern, and fits into the fairly small mic adaptors mounted on the disc.

This range of less expensive devices is based on a battery-powered amplifier unit which forms the body of the microphone. In addition to a -10dB pad, there is a switch to modify the frequency response of the unit to suit 'music', ie. full range, or 'speech' for which there was no response curve to be studied, although a bass rolloff was apparent.

Regardless of whether or not you have a use for the disc-based system, the microphones are certainly worth a look. They have the flexibility of a modular system, and also include a 'shot gun', hypercardiod capsule. They don't need a power supply, beyond the readily available internal battery, and considering their price tag, they exhibit a bright, smooth response, which could be of use in many general applications.

Recording



I made a few recordings in my living room, which has no acoustic treatment beyond the usual sparse collection of soft furnishings, a carpet and other assorted pieces of furniture. It is, in fact, a fairly live room, though probably reasonably representative of an average domestic acoustic.

Using piano, acoustic guitar and clarinet, plus the services of a couple of friends, I experimented using different disc positions, and with the forgivable exception of the piano, moving around whilst actually playing. The results were quite impressive, although inevitably, the less-than-ideal acoustic of the room was apparent. Allowing for this, imaging was clear and indeed, there was no hole apparent in the centre of the image. However, with such a limited ensemble, a good deal of space was naturally to be expected.

The most impressive spatial effects were to be had in the 'close field', that is to say, close in to the microphones. We all wandered around the disc, talking, whispering closely into them, and making various percussive sounds. Had anyone seen us, they might have found reasonable grounds for committal, but in fact the experiment proved useful in that it produced an extraordinarily realistic spatial field, when listened to on headphones. The effect was not so impressive in the 'far field', and headphones were undoubtedly the preferred means of monitoring.

This preference for headphones and near-field sounds is quite in keeping with all other dummy head and binaural recordings, and this is one of the reasons that it never became as commercially successful as people once thought it might.

Summary



The Jecklin Disc is an interesting device, and it certainly does what it claims to do produce useful, binaural recordings. However, as far as the average home recordist is concerned, there are probably better things to spend your money on... the PMB mics, for instance.



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Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - Apr 1984

Donated & scanned by: Mike Gorman

Review

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