Stereo Mic Techniques
All you ever wanted to know about stereo mic techniques, explained in plain English.
Pop music tends to be associated with close micing only - but a well-rounded engineer or producer should have an understanding of traditional stereo miking, used in recording acoustic instruments, choirs and orchestras or film sound tracks. Paul White explains the techniques you need.
A successful stereo microphone technique must emulate the way in which we utilise our own two ears to experience sound in a three-dimensional space, and logic might suggest that using two microphones in place of our own ears would be the nearest we could get. Research work has actually been undertaken utilising tiny microphone capsules fitted within the ear, but a far less uncomfortable approach is to build a 'dummy head' and incorporate the microphones into that.
If the sound is picked up from within the ear of a dummy head, then it must be reproduced by loudspeakers in the same physical position in order to maintain any degree of accuracy — in other words, the recording must be replayed via headphones. Encouraging results can be obtained in this way, and several impressive recordings have been made, though front and rear sounds still appear to be confused. Unfortunately, the technique doesn't work nearly so well when the sound is replayed via loudspeakers.
When we use stereo loudspeakers, some of the sound from the left speaker reaches the right ear and vice versa. This introduces a new set of conditions, and different microphone techniques are required. In real life, sound arrives from a wide variety of directions — left/right, up/down, front/back and all angles in between. But with speakers, all the sound comes from just two locations. Sophisticated though the human hearing system is, it appears to be quite forgiving when it comes to stereophonic sound, and though no manmade recording equals the experience of the live event, even a relatively crude stereo recording can provide a satisfying and pleasurable listening experience.
Possibly the most popular method of stereo miking is the coincident or XY pair. This comprises two high-quality cardioid or figure-of-eight mics of similar characteristics, mounted at approximately 90 degrees to each other and with their capsules as close to each other as is physically possible. As directional microphones are used, one will tend to pick up sound mainly from one side of the soundstage and the other from the opposite side. Figure-of-eight mics will also capture the left and right sound from the rear of the room, including audience noise and room reverberation, though as far as the recording is concerned, these rear sounds will also appear to emanate from the front.
XY provides the necessary change in level from left to right but makes no attempt to account for the masking effect of the human head, or the phase effects caused by the distance between one ear and the other. However, because cardioids often have a less than perfect high-frequency response when presented with off-axis sounds, the high-frequency attenuation caused by the presence of the human head may, to a degree, be simulated quite by accident!
Because the two microphone capsules are so close to each other that they can be considered coincident (occupying the same space at the same time), all sounds arrive at both capsules at the same time, regardless of their direction; consequently, there are no phase problems that might destroy mono compatibility. However, there are also negative aspects to this arrangement.
The lack of phase information means that some of the auditory clues we rely on in nature are missing, which results in a 'dilution' of the stereo image. Also, because cardioids and figure-of-eight mics tend to have their most accurate frequency response on or near their axes, the important central area of the soundstage may be less accurately reproduced than the edges. The degree to which this occurs depends on the off-axis characteristics of the particular microphones used. When the two microphones are summed together to provide a mono signal, the lack of phase errors ensures that no significant compatibility problems exist other than those caused by the small physical separation between the capsules.
An alternative coincident microphone technique, which produces a more stable centre image than the XY pair, is the so-called M&S pair, which is also mono compatible. Figure 2 shows a figure-of-eight and an omni microphone arranged so as to be physically coincident, the figure-of-eight microphone having its axis pointing across the soundstage rather than at it. In situations where ambient sound should be minimised, the omni may be exchanged for a forward-facing cardioid.
The 'middle' mic picks up the centre stage sound in mono, so if a mono feed is required, it is only necessary to leave out the feed from the figure-of-eight 'side' mic. During normal stereo operation, the centre mic provides a mono picture, and the side mic provides the information that describes how the left and right signals differ from the centre signal. To extract discrete left and right signals, it is necessary to use some form of 'sum and difference' matrixing circuitry, though in practice, most mixing desks can accomplish this. Summing the outputs from the two mics gives one side signal, while subtracting the side signal from the middle gives the other side. This may be done using a specially constructed sum and difference box, by using a mixing console that has phase reversal buttons, or by using a special split mic lead with the hot and cold connections reversed on one of the connectors. Figure 3 shows how this is accomplished in practice.
For proper operation, the side and mid signals should be set up with the same gain, (assuming the two mics are of equal sensitivity), but by changing the level of the 'side' signal channels, the stereo width can be modified. With the side signals turned right down, we are left with mono, and if the side signals are turned up higher than the centre signal, then the result is an artificially widened stereo image. If the side signal is significantly higher than the mid signal, the stereo image will be diminished and the sound will take on a distinctly out-of-phase character.
Care must be taken to set the two 'side' channels to the same level, when using a mixer to extract the left/right information. This may be done by panning both the in-phase and out-of-phase signals from the figure-of-eight mic to the centre and adjusting their levels so that they cancel each other out. The channels may be panned back to their original left and right positions.
The two coincident systems discussed so far create a level difference between left and right sounds, but no account has been taken of the time delay caused by the distance between the ears. We could space the microphones of a coincident pair by a few inches to create an additional sense of space; however, the phase cancellations that occur when the signal is summed to mono cause a degree of colouration, due to the inevitable comb filtering effects brought about by the different times of arrival of off-axis sounds. This technique is the basis of the so-called ORTFE system favoured by some European broadcast institutions.
A very common approach in professional circles is to use spaced omni microphones, and though the spacing causes mono compatibility problems, the comb filtering effects seem to be less pronounced. Such a mic arrangement is known as an 'AB Pair' and is extensively used for recordings destined to be replayed over loudspeakers. Where the sound-stage is wide, as in the case of an orchestra, then a third omni mic can be added centre stage to avoid any weakness in the centre of the image. If phasing problems are evident, then all you can do is to change the mic positions and spacings so that the effect is minimised, trying to keep the inter-mic spacing as large as possible relative to the distance between microphone and sound source. Figure 4 shows a spaced omni setup.
The distance of the microphones from the performers must be carefully adjusted, so that the desired balance of direct and reverberant sound is being picked up. If the sound is too ambient, then the mics need to be brought closer to the performers. Conversely, if the sound is too dry, the microphones may be moved further away. Don't move the microphones too far in one go, because a relatively small change in distance can significantly change the sound. If you can arrange to monitor a rehearsal using good headphones while you 'fine tune' the mic positions, you'll save a lot of trial and error. Where the room ambience is too obtrusive, cardioid pattern microphones may be used in place of the omnis, though models with good off-axis performance are essential. Excellent budget recordings may also be made using the inexpensive Tandy PZM microphones in place of conventional omni pattern microphones.
Large ensembles such as orchestras make balancing difficult, as the only way to change the mix is to physically move the performers forward or backward. Some recording engineers do just that, while others prefer to fine-tune the balance with spot microphones. These are omni or cardioid pattern mics positioned close to the performers who need more reinforcement, and their contribution is added to the main stereo mix, where it is panned to match the stereo image from the main stereo mic array. The spot mic signals should be added to the stereo mix as sparingly as possible, so that the main stereo mics still provide the main signal.
A potential problem when employing spot mics is that the sound arrives at the spot mics before it reaches the main stereo mic array — sound travels at around one foot per millisecond. This can degrade the stereo imaging and cause phase cancellation effects that rob transient sounds of their clarity. Professionals use a high-quality digital delay line to delay the spot mics, in order to bring them back into line with the stereo mics. Separate delays would be used for each spot mic to ensure accuracy, but acceptable results can be achieved by feeding all the spot mics to a subgroup and then delaying these so that the most distant spot mic is brought into line with the main stereo array. Allow 0.91 ms of delay for every foot of distance between the main and spot mics.
Feature by Paul White
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