Using stereo microphone recording techniques can be fun and very effective when used in the right context. They can also produce headaches and nightmares when it comes to mixing down the tracks later if setup has not been done properly. Taking time to position microphones appropriately in order to capture the source audio in a balanced and clear way is well worth the effort. Over the course of the next two articles I want to cover the basics of different stereo recording techniques. Which one you should choose in any given situation is based largely on what microphones you have available, the environment you are recording in, and the desired resulting sound. The stereo micing (or “stereo miking” as it is sometimes written)) setups we will cover in this first article (part 1) are spaced pair, coincident pair (X-Y) and near coincident pair (ORTF, NOS, DIN). In part 2 we will cover mid-side(M/S), the Blumlein pair, and the Decca tree methods.
Phase cancellation with Stereo Micing
One problem that can occur in stereo recording is that of phase cancellation. This can often be most problematic when using the spaced-pair technique. The phenomenom known as Phase cancellation occurs when a sound arrives at two different destinations (microphones) at slightly different times, and are mixed together. If one signal is in a positive amplitude cycle, and the other is going through the negative amplitude stage of the cycle when they arrive at the microphones, they will cancel each other out, but if they are both going the same way they will boost each other.
If two signals are partially out of phase with each other there will be a kind of warbling in the resulting mixed sound , but if the two signals are180 degrees out of phase, they will cancel each other entirely and the result will be silence. The phasing issues may not be so noticeable if the two signals are panned hard left and right, but become very evident if they are panned center. It’s always a good idea to check the signals in mono to hear what potential problems there may be.
Ways to minimize the chance of running into phase issues is to use coincident pairs or near coincident pairs (ie: two mics are very close together), or to use the 3:1 rule. The latter technique involves placing the pair of microphones 3 times the distance from each other as they are from the sound source. If you are recording two different sound sources, try to keep their respective microphones at least three times further apart from each other as they are to the sound sources you are attempting to capture.
Coincident Pair (X-Y)
Setting up a coincident pair of microphones is often known as the X-Y technique, and to achieve this method of stereo micing you mount two directional mics with their grilles touching, their diaphragms one over the other, and the mics angled apart. The most commonly seen method is the mounting of two identical cardioid mics on a stereo bar, with one grille above the other, and angled at 120 degrees apart. Narrower or wider angles are also sometimes used. Besides cardioid capsules, other polar patterns of supercardioid, hypercardioid, or bidirectional may also be used. The wider you angle the microphoness, the wider the stereo spread will be.
The advantage of the X-Y system is that it is simple to set up, and will provide a very stable stereo image. The disadvantage of this method is that the stereo spread can tend to sound somewhat narrower than with other techniques. When miking a solo instrument or a small ensemble this can often however, be a good thing. Some engineers like to also mic drum overheads this way, particularly for jazz kit micing, while perhaps prefering the wider stereo spread of ORTF or a spaced pair for rock drums.
Because directional mics are more sensitive to sounds arriving at the front of the mic, and less sensitive to those arriving off axis, sound sources in the center will be captured equally by both of the mics, producing a phantom image midway between the speakers, while sound sources off axis will appear more strongly to the left or right depending on how much is picked up by each mirophone.
The microphones are usually panned hard left and hard right.
Near-Coincident Pair ( ORTF, NOS, DIN)
When micing with a near-coincident pair you place two identical directional mics angled apart, with their grilles spaced a few inches apart on the horizontal plane. The more you angle the mics or the further apart you space them, the greater the stereo spread will be. Even with just a small amount of extra spacing being added it increases the preceived stereo spread of the sound.
The most common method of near-coincident micing is ORTF (Office de Radiodiffusion
Television Française), and it uses two cardioid microphones with a spacing of 17 cm ( 7 inches) between their diaphragms, and angled at 110° between the capsules. Placing of microphones at this spacing is designed to emulate the distance between the human ears, while the angle between the two directional microphones simulates the shadowing effect of a person’s head.
Other variations of this pattern are the Dutch NOS system, where the two cardioid mics are angled at 90° and spaced 30 cm (12 inches) apart, and the German DIN system which uses a 90° angle with a 20 cm (7.9 inch) spacing. The 90° angle between microphones in the NOS and DIN methods is easier to set up by eye than the ORTF 110° angle is, and this smaller angle also means there is less off-axis coloration.
The advantages of the ORTF method is that it is easy to set up, provides a reasonably stable image, and provides a wider stereo spread than X-Y micing does. The spacing of the microphones produces time differences, and this combined with level differences creates the wider stereo effect.
The disadvantage of this method is that when recording some things, the shifts in phase may be a problem, and the sound of instruments (or whatever source you are recording) can appear to wander about in the sound stage even with only small movements of the artist (or other sound source).
The placement of the sound within the stereo image may even appear to wander slightly depending on what note is being played.
As with coincident pairs micing, the microphones are usually panned hard left and hard right.
Coincident Pairs, Near-Coincident Pairs And Frequency Response
A disadvantage of both coincident and near-coincident micing is that most with cardioid mics being most commonly used, this can mean very little of the source is actually on-axis to the microphones. Also the characteristic of most directional mics is to exhibit a proximity effect, which means the further from the source the mic is, the less bass is picked up. In comparison, omni microphones show no proximity effect, and thus may reproduce bass better on sources that are further from the mics. If you are recording something that needs a lot of solid bass response, it may be better to consider using a spaced pair or some other mic technique, and use omni mics.
Spaced Pair (A/B)
The spaced pair method is also known A/B micing, and to achieve this you place two identical mics a few feet (or metres) apart and aimed straight ahead. Mics with any polar pattern may be used, but omni tends to be the most popular choice for spaced pair recording. An exception to this is in the use of spaced pairs for drum overheads, where cadioid mics are used and faced downward towards the drums. The further apart you space the microphones, the greater the stereo spread will be. Positioning is critical when using a spaced pair, and it is a good idea to do some experimenting as to the placement and distance apart to put the microphones. Spaced omni recordings tend not to have as strong a center image as XY recordings, and may suffer from a “hole in the middle” of the stereo image.
Sound sources in the center between the mics produce the same signal from each mic. Therefore a phantom image appears in the center midway between your speakers. If a sound is closer to one mic than the other, the sound will reach the closer microphone before it reaches the one that is further away. While both mics actually produce the same signal, the signal at the mic that is the further away of the two is delayed slightly compared to that of the closer mic. A time delay of just 1.2 milliseconds is enough to move an image completely to one speaker.
When recording with omni mics you will inevitably pick up a fair bit of the sound of the room. That can be a really positive thing if the room is a good sounding one, but if it’s not a great sounding room then you are likely better off using a different technique such as ORTF, M-S or XY. When setting up mics in spaced pairs, take time in positioning them, moving them only by small amounts, because even little changes in placement can make big differences in the sound. You will still always get some phase cancellation, but when you find the right positioning and spacing of the mics, and get a good balance of direct and reflected sound, micing using this technique can sound very natural and lead to recordings that have a spacious feel and great realism. Besides the “hole in the middle” phenomenom, another potential flaw of using spaced mics is that if you play a stereo recording of the two channels back in mono, you may get phase cancellations of various frequencies. It may or may not be noticeable or worth worrying about, but as mentioned previously, it is always a good idea to check things in mono when you are recording just in case there are any problems that could be fixed before you are stuck with them.
Conclusion of Part 1
Well that is a basic introduction to stereo micing and a description of A/B spaced-pairs, X-Y and ORTF techniques. In part 2 we will look at Decca Trees, M-S and Blumlein recording patterns as well as giving a general summary of stereo micing techniques.
By Tony Koretz
© copyright June 2012