M-Audio Microphone manual Blumlein, Mid-Side

separation and captures more room reflections. In general, the X-Y technique using cardioids yields an accurate stereo image exhibiting minimal acoustic reflections, although the separation is not as significant as some other stereo miking techniques.

Blumlein

NaBlumleind after British stereo pioneer Alan Blumlein, the technique takes advantage of the polar patterns

inherent in figure 8 (bidirectional) mics. Recall that figure 8 patterns pick up equally well on two sides while exhibiting strong rejection at 90 degrees off axis to those sides. In the Blumlein technique, two figure 8 patterns are oriented 90 degrees from each other with the positive sides facing the left and right sides of the sound source. Due to the inherent side rejection, the area of greatest sensitivity of one mic is the area of least sensitivity of the companion mic. While the patterns overlap in the center, the signal from each is 3dB down and, when combined, pick up a uniform center signal.

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The Blumlein arrangement relies on a matched pair

of coincident figure-8 patterns

The Blumlein arrangement yields very good stereo separation. Due to the fact that figure 8￿s are equally sensitive on the back lobes, this configuration also picks up significant room reflections. There are drawbacks to this technique, however.The fact that the back of the left mic is also picking up reflections from the right rear of the room makes for poor mono compatibility. Further, reverberant sounds coming from the sides of the acoustic space can enter the positive lobe of one mic and the negative lobe of the other, thus causing the impression of poor localization and/or hollow effects that can be disturbing. As a result, Blumlein is best used in situations where the sound source, acoustic space and mic placement are optimal. Since this is a rarity, other stereo techniques offering superior control are more frequently used.

ORTF Sound Source

Developed by the French national broadcastingORTF agency,

Office de Radio T￿l￿vision Fran￿aise, the technique is intended to emulate the placement of ears in the average adult human head. Two cardioid capsules are placed 17cm (about 6 - 3/4 inches) apart at a 110 degree angle to one another. ORTF can produce the wide imagery and depth common to the Blumlein technique, however the use of cardioids means that the configuration captures much less reverberant reflection.

17 cm

110 º

The ORTF technique positions a matched pair of mics in a configuration similar to that of human ears

The specified distance for ORTF makes wavelengths below about 500 Hz effectively phase coherent. The time delays or phase incoherence above that frequency typically contribute to a sense of stereo separation, along with the perception of a pleasing open or airy quality. ORTF also exhibits adequate monophonic compatibility. Similar experiments by the Dutch broadcasting counterpart￿Nederlandsche Omroep Stichting￿yielded the NOS technique where a pair of

cardioids are placed 30cm apart at a 90 degree angle.

Mid-Side

The Mid-Sidetechnique utilizes special processing to capture very precise stereo imagery with excellent mono applicability. A ￿mid￿ microphone (typically a cardioid) faces the center of the sound source and captures the primary sound. A figure 8 (the ￿side￿) is placed along the same vertical axis with its lobes facing right and left, thereby picking up the extreme left and right information due to the side rejection inherent in the figure 8 pattern.

This configuration does not constitute stereo until the signals are processed through an M-S encoder matrix such as the M-Audio Octane Preamp. The encoder adds the mid and side signals

Choosing & Using Microphones 22