I would guess that the b1 being the more neutral of the 2 would do best at the bridge end whereas the c1 being more bright might do better at the neck?
As for stereo micing, behold!
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Stereo Microphone Techniques
Firstly, what is stereo? The word ‘stereophonic’ in its original meaning it suggests a ‘solid’ sound image and does not specify how many microphones, channels or loudspeakers are to be used. However, it has come to mean two channels and two loudspeakers using as few or as many microphones that are necessary to get a good result. When it works, you should be able to sit in an equilateral triangle with the speakers, listen to a recording of an orchestra and pinpoint where every instrument is in the sound image. (By the way, some people complain that ‘stereophonic’, as a word, combines both Greek and Latin roots. Just as well perhaps, because if it had been exclusively Latin it would have been ‘crassophonic’!)
When recording a group of instruments or singers, it is possible to use just two or three microphones to pick up the entire ensemble in stereo, and the results can be very satisfying. There are a number of techniques:
* Coincident crossed pair
* Near-coincident crossed pair
* ORTF
* Mercury Living Presence
* Decca Tree
* Spaced omni
* MS
* Binaural
The coincident crossed pair technique traditionally uses two figure-of-eight microphones angled at 90 degrees pointing to the left and right of the sound stage (and, due to the rear pickup of the figure-of-eight mic, to the left and right of the area where the audience would be also). More practically, two cardioid microphones can be used. They would be angled at 120 degrees were it not for the drop off in high frequency response at this angle in most mics. A 110-degree angle of separation is a reasonable compromise. This system was originally proposed in the 1930s and mathematically inclined audio engineers will claim that this gives perfect reproduction of the original sound field from a standard pair of stereo loudspeakers. However perfect the mathematics look on paper, the results do not bear out the theory. The sound can be good, and you can with effort tell where the instruments are supposed to be in the sound image. The problem is that you just don’t feel like you are in the concert hall, or wherever the recording was made. The fact that human beings do not have coincident ears might have something to do with it. Separating the mics by around 10 cm tears the theory into shreds, but it sounds a whole lot better. The ORTF system, named for the Office de Radiodiffusion Television Francaise, uses two cardioid microphones spaced at 17 cm angled outwards at 110 degrees, and is simply an extended near-coincident crossed pair.
The redeeming feature of the coincident crossed pair is that you can mix the left and right signals into mono and it still sounds fine. Mono, but fine. We call this mono compatibility and it is important in many situations – the majority of radio and television listeners still only have one speaker. The further apart the microphones are spaced, the worse the mono compatibility, although near-coincident and ORTF systems are still usable.
Mercury Living Presence was one of the early stereo techniques of the 1950s, used for classical music recordings on the Mercury label. If you imagine trying to figure out how to make a stereo recording when there was no-one around to tell you how to do it, you might work out that one microphone pointing left, another pointing center and a third pointing right might be the way to do it. Record each to its own track on 35mm magnetic film, as used in cinema audio, and there you have it! Nominally omnidirectional microphones were used, but of course the early omni mics did become directional at higher frequencies. Later recordings were made to two-track stereo. These recordings stand up remarkable well today. They may have a little noise and distortion, but the sound is wonderfully clear and alive.
The same can be said of the Decca tree, used by the Decca record company. This is not dissimilar from the Mercury Living Presence system but baffles were used between the microphones in some instances to create separation, and additional microphones might be used where necessary, positioned towards the sides of the orchestra.
Another obvious means of deploying microphones in the early days of stereo was to place three microphones spaced apart at the front of the orchestra, much more distant from each other than in the above systems. If only two microphones are used spaced apart by perhaps as much as two meters or more, what happens on playback is that the sound seems to cluster around the loudspeakers and there is a hole in the middle of the sound image. To prevent this, a centre microphone can be mixed in at a lower level so that the ‘hole’ is filled. There is no theory on earth to explain why this works - being so dissimilar to the human hearing system - but it can work very well. The main drawback is that a recording made in such a way sounds terrible when played in mono.
The MS system, as explained previously, uses a cardioid microphone to pick up an all-round mono signal, and a figure-of-eight mic to pick up the difference between left and right in the sound field. The M and S signals can be combined without too much difficulty to provide conventional left and right signals. This is of practical benefit when it is necessary to record a single performer in stereo. With a coincident crossed pair, one microphone would be pointing to the left of the performer, the other would be pointing to the right. It just seems wrong not to point a microphone directly at the performer, and with the MS system you do, getting the best possible sound quality from the mic. It is sometimes proposed as an advantage of MS than it is possible to control the width of the stereo image by adjusting the level of the S signal. This is exactly the same as adjusting the width by turning the mixing console’s panpots for the left and right signals closer to the centre. Therefore it is in reality no advantage at all.
Binaural stereo attempts to mimic the human hearing system with a dummy head (sometimes face, shoulders and chest too) with two omnidirectional microphones placed in artificial ears just like a real human head. It works well, but only on headphones. A binaural recording played on speakers doesn’t work because the two channels mix on their way to the listener, spoiling the effect. There have been a number of systems attempting to make binaural recordings work on loudspeakers but none has become popular.
In addition to the stereo miking system, it is common to mic up every section of an orchestra, whether it is a classical orchestra, film music, or the backing for a popular music track. Normally the stereo mic system, crossed pair or whatever, is considered the main source of signal, with the other microphones used to compensate for the distance to the rear of the orchestra, and to add just a little presence to instruments where appropriate. Sectional mics shouldn’t be used to compensate for poor balance due to the conductor or arranger. Sometimes however classical composers don’t get the balance quite right and it is not acceptable to change the orchestration. A little technical help is therefore called for.
