Calrec Soundfield (Mark 4)
The Calrec Soundfield Microphone (SFM) is surely due to be seen as a major contribution to the recording of excellent stereo now that Compact Discs (CDs) are showing up all manner of original 'mastering' faults. It deserves to be used more and more in 'crossed pair' microphone situations where its attributes over other crossed pairs are many.
Made in Yorkshire by Calrec Audio Ltd, the SFM has been around for a number of years. It is now in its Mark 4 guise and up to now has had both its admirers and its detractors. I believe here, I can offer a couple of good reasons to change the views of the majority of the latter - I'll come to these points later. In the meantime, let's cover the basis of the system and then its attributes.
It is certainly not simplifying the SFM too much to suggest that it is merely an extension of the Mid & Side (M-S) system which has been known about since the 1930s. HSR gave an insight into M-S in its fourth issue (Dec 83). Briefly, with M-S the use of two mics, one of which must have a 'figure of eight' (Side facing) pickup response and where, for example, the other is an 'omni' (Mid), one gets by suitable mixing (or matrixing) of the in-phase and out-of-phase signals, a couple of crossed cardioids at 180° (Figure 1). By varying the contribution (in the mixture) of the side facing 'figure of eight', the stereo width of the pair is controllable. The SFM simply takes this operational principle and goes much further with it.
To grasp the basics of the SFM system, simply see it as three 'figure of eights' and one omni mic! The 'figure of eights' point forward and back (called X), side to side (called Y), up and down (called Z) while the omni points nowhere in particular (and is called W). The various signals from the various 'separate' mics are called the B Format signals, of which more later (Figure 2).
By the same matrixing techniques employed for M-S, one can derive a superb stereo pair with all manner of facilities and textbook (yet very practical) attributes.
1. The polar patterns of the pair can be varied remotely from Figure 8s, through Hyper-Cardioid, Cardioid and Sub-Cardioid to Omni. Mind you, many capacitor mic polar patterns can also be varied remotely, but with the SFM there is much more!
2. The angle of their crossing can be similarly altered from 0° to 180°.
3. The pair and, in effect, the whole mic can be turned through 360° in four switched quadrants. This is called Azimuth.
4. The 'mic' can be tilted, remotely 45° upwards and downwards.
5. There is a remote Dominance facility, which in effect allows the mic to be moved forwards or backwards from where it actually is!
The use of these controls allows all sorts of 'balance' aspects to be catered for. To be specific, the stereo width produced at a given mic position, is a factor of the polar pattern selected (fig 8 widest, cardioid narrowest) and capsule angle. The ratio of direct sound (from in front of the mic) to indirect sound (from back and sides) is a factor of the polar pattern (fig 8 - more from back and sides, but there is the phase change to consider at 90° and 270°: cardioid - less from the back and more from the sides) and the Dominance control (controlling front and back sensitivity). Of course, these controls are overlapping and in a sense interdependent in their effect - they are all using the same basic material, the B Format signals.
When out 'on location', the remoted rotation of the mic is a boon when setting up, particularly if it is being slung from a boom stand, say. There is a red LED to show the front from a distance, but slung mics have been known to move subsequently! The tilt control, especially when in 'figure of eight' mode, very effectively allows balancing, of say, the front sound of an orchestra with respect to the back, or an orchestra against a soloist in front or a choir behind. So, figure of eights, coupled with front dominance can, with the addition of tilt and rotation, give an amazing amount of sound control assisting the balance of instruments and performers and the balance against the room acoustic (ambience).
A special note about the Dominance control. It is not magic, so for instance when used 'forward' you don't get the widening of the sound stage that a physical move of the mic would produce, but just more direct and less indirect pickup of sound.
|POLAR PATTERNS||ANGLE 0-180°||AZIMUTH 0-360°||ELEVATION +/- 45°||DOMINANCE FRONT/BACK|
|X Figure 8 front to back||*||*||*||*||*|
|Y Figure 8 side to side||*||*||*|
|Z Figure 8 up and down||*|
Within the control unit the facilities discussed make use of particular combinations of the B Format signals as follows:
Imagine that one recorded the B Format signals! Then on playback, all the options discussed would be available in better monitoring conditions to allow a better choice to be made. The trouble is, in this digital era there isn't a low cost 4 channel digital recorder to encode all four signals, but there are, apparently, the possibilities of using the fine PCM F1 and a Beta HiFi video recorder so that four channels could be available! Stay tuned to these pages for news of any successful outcome of tests along these lines.
The basic idea is that the Video record facility would be used for the PCM F1 digital storage of, say Y and W signals, with the full bandwidth Beta HiFi analogue channels used for X and Z. Possibly some matrixing would need to be resorted to, but either way the availability of coherent B Format signals on playback is envisaged.
What has a tetrahedron got to do with B Format? Well, basically it's a means to an end. The four 'mics', which effectively provide the B Format signals, couldn't be physically constructed very close together, and still remain symmetrical. So preceding the B Format stage in the SFM Control Unit is the A Format derived from the mic itself. The photograph shows the inside of an SFM with the cover off the capsule end. Below the head (in the body proper) are the A Format amplifiers. There are four Sub-Cardioid capacitor capsules arranged in a regular tetrahedron, which are relatively close together and totally symmetrical. The B Format signals are derived in what is known as the A/B Matrix, part of the SFM Control Unit. The A Format signals are carried down 100 metres of balanced multicore cable, which also carries the head amplifier powering current and capsule polarising voltage.
Apart from the unique control over the mic pickup already discussed, there are positive virtues in the nature of the stereo produced and the quality of sound reproduction available. If you accept that crossed pair stereo has advantages over spaced omnis (heavens forbid!) or pan-potted multimic set-ups, then an SFM takes the attainment of such stereo further.
Other stereo mics or crossed ordinary pairs are asymmetrical and only effectively coincident up to lower mid frequencies, due to the self-imposed physical constraints. An SFM is symmetrical and the B Format itself ends up being coincident to above 10kHz. The end result is pin point stereo imaging without the fudging inherent in other pairs. Interestingly it takes a digital system, like the PCM F1 to reveal this properly, for there is a fair amount of fudging of stereo going on in good old analogue recording for various reasons!
Also while ordinary crossed pairs will give a reasonably coherent acoustic portrayal, an SFM resolves it more accurately for the reasons discussed above and due to its frequency performance.
Conventional capacitor mics have their directivity created by various acoustic labyrinths, mechanical resonance schemes, or back to back dual diaphragm arrangements. This particularly applies to cardioid systems. The result is often a compromise between on an off-axis frequency response together with the accuracy of the polar patterns at various frequencies.
An SFM starts off with relatively little dependance on this. After the capsules, one has the B Format stage, deduced by electronic means, leading to the electronic derivement again of the actual crossed pair. The result is low colouration and a flatter, smoother and more extended frequency response, with uniform polar patterns across the audio range. No need to resort to spaced omnis to get the advantage of extended LF?
Many early capacitor mics have upper high frequency peaks and early low frequency roll-offs (hence the resurgence of omnis?) apart from their questionable off-axis HF performance, which dulls the centre sound stage pickup. Irregular polar patterns also smudge the stereo imaging and coherence of acoustic portrayal.
It's all a matter of degree, of course. There are certainly some fine 'stereo mic' recordings made without the Soundfield mic, but when it comes to low colouration, sharp pin point accuracy, SFM stereo accuracy scores! Even more so now with digital recording and the improved playback conditions.
To reinforce this aspect, the Compact Disc medium's criticism seems to come down to deficiencies in the original recordings, particularly it seems with choice of mics and the number in use. Stan Curtis in HiFi For Pleasure magazine, said of some Calrec Soundfield mic and PCM F1 recordings "This is what Compact Discs should sound like."
Price complete with Control Unit, Flight Case & 100m Multicore on drum - £2600.
SFM Hire Service
The complete system may be hired from: (Contact Details)
Review by Mike Skeet
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