The details about one of the most basic and useful stereo mic techniques
By Phil O'Keefe
One of the most commonly used stereo mic techniques is the A-B Stereo pair. A-B Stereo is known by a couple of other names. Spaced Pairs. Time Difference Stereo. All refer to the same basic technique. However, there are two different approaches to A-B Stereo that are often genre dependent. We'll dig in to those differences a bit later - first, let's look at the classic approach to A-B Stereo.
Figure 1: An A-B Stereo pair, as viewed from overhead.
Figure 2: The same Spaced Pair, as viewed from the sound source perspective.
SETTING IT UP
A-B Stereo uses two microphones of the same type. Often matched pairs of omnidirectional condenser microphones are used, although cardioid models are sometimes substituted when a less ambient sounding recording is desired. This is far more common approach when working with multitrack productions as opposed to when making live to stereo recordings. The two microphones are mounted parallel to each other (Fig. 1), spaced apart anywhere from several centimeters to several feet, and aimed directly towards the center of the stage or ensemble to be recorded.
The distance from the sound source to the microphones is, as with most stereo mic techniques, a matter of taste. Omnidirectional microphones tend to capture greater ambience and "room sound" than directional microphones do, and the further back you place them, the more reverb you'll capture. The optimal placement will depend on the nature of the sound source as well as the room it is performing in, as well as the recordist's preferences in terms of the direct sound to ambient sound ratio. For single instruments and small ensembles in a smaller room, a few feet back may be all you need, while you may decide to place the mikes 20 feet or more away when capturing a large ensemble in a performance hall. Experiment until you achieve the results you're after.
CLASSICAL OR ROCK
There are two different approaches and schools of thought when it comes to A-B Stereo. In the classical world, distances between the two microphones are generally kept between 40cm and 60cm (15.75" - 23.62"), and omnidirectional microphones are generally used. The reason behind using omni mikes is due to their ability to remain "flat" in terms of their frequency response, regardless of the mic to sound source distance. Unlike cardioid and other directional microphones, omnidirectional mikes don't suffer from proximity effect. Their bass response stays the same, whether they are very close to, or quite distant from the sound source. This allows you to adjust the source to mic distances without getting a corresponding tonal and frequency balance shift. High frequencies can be attenuated with increased distance from the source too, although this is much less of an issue at most real-world mic placement distances.
The choice of directional microphones can lead to issues with low frequency accuracy, so this is normally not done when the A-B pair will be the sole or primary source for the entire recording, as is commonly the case with classical recordings, or when doing live recordings of small ensembles. In pop recording styles (including rock, hip hop, alternative, etc.), spaced pairs of directional microphones are much more commonly used, and often with wider mic spacing than is typically used for classical A-B stereo pairs. For example, the Glyn Johns approach to drum miking (which I discussed in my article on drum miking) is essentially a spaced pair of cardioid microphones augmented by a kick drum mic, and occasionally, with a snare mic added. In pop music production, the subdued low frequency response is typically either beneficial in helping reduce low-frequency buildup in the mix (in pop music mixing, many tracks are often ran through a high-pass filter to cut out the subsonic "gunk" anyway), and the lessened low frequency response of the A-B Stereo mic pair is typically compensated for by other bass heavy elements of the mix. Again, using the Glyn Johns drum miking technique as an example, the kick drum mic supplements the primary low frequency element of the kit and makes up for the attenuated low frequency response of the distantly placed cardioid microphones.
TIME DIFFERENCE STEREO
So why is A-B Stereo sometimes called "time difference stereo?" A-B Stereo pairs rely on the spacing between the two microphones to generate the stereo image. Our ears are sensitive to very small differences in the arrival time of sound at each of our two ears, and we rely on this as one of the primary tools our brains use to determine directionality of sounds. Due to the spacing of an A-B pair, arrival time differences are captured, as well as phase and amplitude differences. The sound from a source that is closer to one side of the stage will arrive at the mic closest to that side of the stage first, then at the second mic a matter of time later. The further the spacing is between the mikes, the greater the amount of time it takes to arrive at the second microphone.
The wider the spacing is between the microphones, the wider the stereo field and imaging of the recording will be. A-B Stereo can result in very natural recordings that cover a complete 180 degree arc in front of the microphones, giving a very wide sense of stereo imaging with a lot of depth. In fact, if the width of the stereo soundfield is of primary importance to you, then A-B Stereo should be one of the first techniques you consider.
As cool as the resulting stereo image can be, A-B Stereo is not without its faults. Mono compatibility can be an issue, especially with wider mic to mic spacing and with sound sources that are located directly in between the two mikes. If you run with the mikes set too wide and distantly spaced from the sound source, the phantom center stereo image can also disappear. Decca Tree stereo techniques compensate for this by adding a third mic "in the middle" of a widely spaced A-B Stereo pair. Running the mikes widely spaced and placed in close to the sound source can result in an unnaturally wide and large sounding instrument in the mix. If the mikes are located too closely to each other, the stereo image can also be compromised, although distances of as little as 20cm can be sufficient if the mic to sound source distance is kept relatively close too. Because of the possibility of phase cancellation issues when summed to mono, you should always check the mono compatibility by hitting the "mono" button on your interface or mixing console whenever you're using A-B stereo mic configurations. If the sound becomes weak and thin, consider adjusting your mic to source spacing, or the spacing between the two microphones to compensate. A-B Stereo is best reserved for situations where mono compatibility is not a significant concern. When omni microphones are used, it is also an excellent choice whenever greater mic to sound source distances are required, such as when recording large ensembles.
Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Associate Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.