RESERVAS TÉCNICAS

Example 6.4: A ORTF near-coincident pair of overheads, centered over the drum set.

Example 6.5: A NOS near-coincident pair of overheads, centered over the drum set.

Example 6.6: A DIN near-coincident pair of overheads, centered over the drum set.

6.5 SPACED PAIR (AB) TECHNIQUES

In spaced pair or AB techniques, two microphones which face directly forwards are positioned with their capsules 40 to 60 cm apart (15 to 24 in), as shown in Figure 6.7. Omnidirectional microphones are generally used for spaced pair techniques – but there are many applications, such as drum overheads, when spaced directional mics can be used.

Sound source “1,” located centrally, travels the same distance to each microphone – the sound arrives at each mic simultaneously at the same amplitude. Both loudspeakers then reproduce this sound at the same time and at the same amplitude creating a phantom image centrally located between the loudspeakers.

Sound source “2,” located to the extreme left, travels a significantly shorter distance to the left mic than to the right mic, so the left mic picks it up a millisecond or two (and even several milliseconds with more extreme mic spacing) before the right mic. This sound source is also slightly quieter when it arrives at the right mic because the right mic is further from it than the left mic, but in most cases this amplitude difference is trivial. On reproduction, the left loudspeaker propagates the sound the same number of milliseconds before, and possibly just slightly louder than the right loudspeaker.

Fi g u r e 6 .6 A DIN array.

L R

20 cm

<7.8 in)

Sound source “3” is picked up by the right mic slightly before the left mic, and is fraction-ally louder in the right mic. Therefore the right loudspeaker reproduces the sound slightly before, and slightly louder than the left loudspeaker.

Time arrival information is the primary indicator of directionality in spaced mic techniques.

Mic spacing of approximately 50 cm (20 in) creates a maximum time arrival difference of approximately 1.5 ms between the mics, which produces a natural listening experience for a listener in the sweet-spot of a stereo pair of loudspeakers.

A suitably spaced AB mic array can capture an image that very closely matches the natural acoustic image, or pleasingly expands it.

General characteristics of AB spaced pair techniques include:

• A wide, expansive stereo image created solely by time arrival information. The image is not necessarily the most focused or precise, but it is the most enveloping.

• If the mic spacing is too wide, sources in the center can lack definition and focus.

• As the spacing between the mics is widened, longer than natural time arrival differences are generated, and the image becomes weaker and made up of “ghostly”

separated left/right components with a “hole” in the center where nothing can really be localized.

• The relatively large time arrival differences between the mics make this the least mono compatible stereo technique. It is vitally important to check for phasing and comb filtering before recording, and to adjust the array spacing subtly to minimize any issues – although it will probably be impossible to remove all mono compatibility artifacts.

• The perceived image width can be increased or narrowed by increasing or decreasing the spacing between the mics – but don’t go too wide on a close sound source (the image will have a hole in the middle), or too narrow when the mics are further away (the image will become too mono).

Fi g u r e 6 .7 A spaced pair of microphones. Sound sources are shown at various positions in the array’s pickup, and their perceived playback locations are shown between the loudspeakers.

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rear of the mic array.

• If a wide sound source such as a symphony orchestra, choir, or large drum set is being recorded, it is possible to augment a spaced pair with a third, center mic, and position the left and right mics equidistantly wider than if they were just a pair. This extra center panned information can either aid mono compatibility by adding common sound to both loudspeakers, or further decrease it by adding a third time displaced signal to the mix – it is entirely situational, depending on the spacing and relative balance between the mics.

Spaced Mic Arrays @

Example 6.7: A spaced pair of omnidirectional overheads, 50 cm (20 in) apart, centered over the drum set.

Example 6.8: A widely spaced pair of cardioid overheads, each mic positioned over the edge of the drum set’s width.

6.6 MS (MIDDLE-SIDE) TECHNIQUES

In all of the stereo techniques previously discussed, left and right microphones were present – and each were routed discretely to their respective loudspeaker (for a full width stereo image).

An MS array, as shown in Figure 6.8, is significantly different. It doesn’t pick up left/right information. Instead, MS techniques use a forward facing microphone (most commonly a car-dioid mic, but it could be any polar pattern) to pick up the center, or middle (M) information, and a bidirectional microphone turned sideways at 90° to pick up side (S) information (which is a combination of the sounds coming from the left and right sides of the array).

The S mic is a single mic capsule with a single combined output of the sum of the sound waves hitting its front and rear – there is not a separate left and right capsule or output.

Sound source “1,” in the center of the array’s pick up, is only picked up by the M mic – it is in the null point of the S microphone.

Sound source “2,” to the left, is picked up by a combination of both the M mic and the S mic. Its polarity in the S mic is normal because it is “in front” (+) of that mic.

Sound source “3,” to the right, is also picked up by both mics, but its polarity in the S mic is reversed because it is “to the rear” (−) of that mic. It is also louder in the M mic and quieter in the S mic than sound source “1” because of its position relative to each mic’s axis.

CONVERTING MS TO LR STEREO

In order to be reproduced on stereo loudspeakers (or headphones) an MS array needs to be matrixed (or decoded) into stereo using a sum and difference matrix.

Raw MS tracks can be recorded and then matrixed to stereo at a later time, or matrixed prior to the recording device to record the derived L/R stereo tracks. If you record raw MS tracks you should monitor a matrixed L/R stereo form of the signal in order to check the array is working properly and that the matrixed image is desirable. To record L/R stereo tracks a hardware matrix is necessary after the mic pre-amps and before the recording device.

Hardware matrixes usually have few controls, and possibly only one – an S level control which varies the level of the S+ and S− signals that are combined with the M signal – to adjust the image width. Matrixing is also possible on a mixing console or in a DAW.

THE SUM AND DIFFERENCE MATRIX

The left side of an MS mic array’s pick-up, as shown in Figure 6.8, is a combination of the M mic and normal polarity sound picked up by the front of the S mic (S+).

LEFT = M + S+

The right side of the mic array’s pick-up is a combination of the M mic and polarity reversed sound from the rear of the S mic, (S−).

RIGHT = M + S−

Those equations simplify to:

LEFT = M + S RIGHT = M − S

LEFT is the sum of the M and S mics, and RIGHT is the difference between the M and S mics.

Fi g u r e 6 . 8 An MS array.

• Label the original track “S+” and pan it hard left.

• Label the duplicate track “S−,” polarity reverse it, and pan it hard right.

• Put both the S+ and S− faders at −∞, then group them together so that they are always at exactly the same level when you move them.

• With the S+ and S– faders at –∞ the image will be mono because all you are hearing is the center panned M mic. Increase the width of the image by increasing the level of the S+ and S− faders. Decrease the stereo width by decreasing the level of the S+ and S− tracks.

• Once the desired stereo image has been achieved, group together the M and both S faders so that their relative balance (and the stereo image produced) stays identi-cal if you adjust their overall level.

• If you adjust any pre-amp, plug-in, or gain controls related to these three channels you will have to re-balance the M and S channel levels to maintain the stereo image.

• Any insert effects (EQ or compression, etc.) should be applied to the matrixed L/R stereo signal by routing the outputs of the M, S+, and S− tracks to a group or aux track (instead of the main stereo outs) and applying them on that track.