The stereo micing (stereo miking) setups we covered in the first article (part 1), were the spaced pair (A/B), coincident pair (X-Y) and near coincident pair (ORTF, NOS, DIN). In part 2 we will cover the mid-side (M/S), the Blumlein pair, and the Decca tree methods.
Mid-Side (M/S) Recording
The mid-side method was developed by Alan Blumlein, who worked as an engineer for EMI. He patented this technique in 1933, and it was used on some of the first stereophonic recordings.This method utilises a coincident technique where a bidirectional microphone (Figure of 8 ) faces sideways, and a cardioid microphone at an angle of 90° to it, faces forward toward the sound source. Blumlein’s original patent used an an omni mic instead of a cardioid mic, and this is still sometimes used today. The two mics are placed one above the other, and as close together as possible without actually touching.
In this configuration the null in the figure 8 pattern is facing toward the sound source and the mic picks up reverberant sound from the left and right side of the room. This is known as the side mic. The mid mic which is aimed at the source picks up sound dirctly from the source. If you play back audio from these two mic tracks you will not get any sense of stereo at all, as the signals need to be matrixed and decoded in order to create a stereo image.
Because the side mic is utilising a figure-8 pattern, the two sides of the mic will be 180 degrees out of phase. There is a positive charge on one side of the diaphragm, and an equal negative charge on the other side. The front of the mic, which has the positive (+) side, is normally aimed towards the left of the sound stage, and the rear facing negative (-) side, is pointed towards the right.
The sound from each microphone is recorded to it’s own separate track. The sound from the side mic then needs to be split in two.
You assign the cardioid mic to a single track. You put this track as the first track, or track to the left in your DAW, because decoders are usually designed to see it arranged this way. To record the side bi-directional mic you can use any of the following methods:
a) Split the signal from the bi-directional side mic and record it to two separate tracks and invert the polarity of one of these tracks later.
b) Some microphone preamps are available that actually have m/s decoders built in.Record two tracks this way.
c) Record the side mic to only one track and then use a decoder plug-in.
d) Record the side mic to only one track and then clone it later and invert the clone track’s polarity.
Cloning Method With No M/S Decoder
If you are not using a decoder a good idea is to have the two bi-directional tracks labeled as Side+ (left track) and Side- (right track), and invert the Side- track’s phase by using a trim plug or channel phase invert switch. Arrange the tracks in your DAW with the “Mid track” to the left, next you have the “Side+ track”, and thirdly you have the “Side- track”
If possible link the two “Side” tracks and pan Side+ hard left and Side- hard right. You can then vary the amount of stereo information in the mix depending on how much level you give the side tracks. If you listen to just the mid mic you have a mono recording.When you raise the level of the side mic tracks, the stereo width will increase, and when you lower them it will decreases. The fact that you can adjust the amount of stereo information in the recording after it has been done is an advantage that Mid-Side recording has over other methods.
Using M/S Decoder Plugins
A disadvantage of recording only one mid mic and one side mic and splitting the side mic into two channels later, is that while you are recording you can’t hear the stereo effect of the M/S configuration till after you have recorded and split the side mic. To hear the stereo effect while you are actually recording you can plit the signal that is being input from the side mic and decode as you are recording, or you can use as special decoder plugin. There are some good plugin decoders available. The Voxengo MSED decoder is a good free plugin that does this job well. It is available foor download at http://www.voxengo.com/product/msed/ .
When using a decoder there is no need to split the side mic to two tracks. You send two stereo tracks to the decoder, a mid mic track and a side mic track. The decoder plugin has controls that enable you to vary the levels of the mid and side mics (width control), as well as often having mutes for each channel, channel swapping and other features. Some even have asymmetry and rotation controls that give you the ability to vary where the stereo image is sitting. This can be a handy feature if you find your stereo image appears to be off-center ( more on that later).
Potential Imaging Issues With M/S Recording
Besides being a good method of recording a small ensmble or group, the M/S technique can be a great one for recording solo instruments such acoustic guitar. However, when close micing an instrument such as a guitar it can happen that the image of the mid and side mics combined may appear off center. Why is that? Well, Mid-Side recording and decoding is based on using polarity differences to accurately reproduce the location of a sound source in stereo field.
Sounds that enter the on left side of the M/S array will enter the positive side of the bi-directional microphone’s capsule, and so will have the same-polarity as the mid microphone (which is always positive polarity), while sounds that enter on the right will enter the negative lobe of the bi-directional mic and so are the opposite polarity of the mid mic. Mid-side decoding sends the positive polarity information (Mid and Side) to the left speaker, and the negative polarity information to the right speaker. This therefore recreates the stereo spread as it appeared in front of the microphone array. If you close mic an instrument such as a guitar and angle the instrument in such a way that more sound is hitting one lobe than the other, the sound can appear off-center. If the instrument is positioned where one lobe of the mic is closer to a wall than the other, the sound of the room may reflect louder in one side of the mic than the other too. To get a good center balance it is important to have the instrument straight-on to the array. If imaging is a problem, moving a little further away from the mic can benefit this too, and choosing a position in the room that is not too close to a wall on one side can also help. If you still find your image is not properly centered (for whatever reason) the assymmetry and rotation controls found in some decoders, and in stereo imaging plugins can be used to help correct the issue.
M/S Recording Applications
The Mid-Side microphone recording technique is frequently used in broadcast, because the recordings are 100% mono compatible. When played back in mono the opposite polarities of each side of the bi-directional mic cancel each other out, meaning you only hear the sound from the mid mic. M/S micing is also a popular method for studio use and in concert recording,. Because of it’s flexibility it is often employed in live recording also.
Generally it’s a good idea that the mics used be fairly similar in sound. On a large sound source this is specially important. The channels share panning information which means the tone quality will change as you listen across the stereo field if mics with different sound qualities are used. It is less important to match the mics on sources such as an acoustic guitar where the stereo field is narrower. Sometimes the use of a brighter sounding mic on the side ( bi-directional) can give the recording a more spacious sound.
Blumlein Pair Recording
The Blumlein Pair is another recording technique that was devised by Alan Blumlein in the 1930s. Blumlein was looking for ways to get good accurate stereo image in sound for film. He discovered that if you set up two bi-directional mics as a coincident pair at right angles to each other it captures a very realistic stereo image. The figure of 8 pattern mics capture the sound of the room to the rear of the mic capsules, along with the sound from at the front lobes, and this produces a stereo image not unlike an X-Y coincident pair arrangement, but with extra room ambience from behind the mics. In the Blumlein array the four-lobed polar pattern gets mixed to two channels. The two mics are panned left and right. The sound in the left-panned channel consists of signals from the front-left and rear-right, while the right channel has the front-right and rear-left signals in it. This method can produce a very natural sounding recording. The mics in the Blumlein Pair array are oriented in such a way that the line that bisects the angle between the two microphones is pointing towards the sound source.
Blumlein Pair Pros And Cons
Because bi-directional microphones pick up equally from the front and the rear lobes you need to place them relatively close to the sound source in order to avoid picking up excessive amounts of reverberent room sound. Because you need to place them closer, the sound from the center can tend to gain more emphasis than sound to the sides. If you have a great sounding room using the Blumlein method can work well, but if the room sound is not particularly flattering, then it is likely better to use an X-Y coincident pair or another micing method. One advantage of using bi-directional mics however, is that you don’t have to contend with proximity effects, because they don’t enhance bass response as you get closer to the source. This means you can get in closer and still keep the frequency balance intact.
Spending some time finding the best place in the room to put the mics can make the difference between a reasonable recorded sound, and a really good one.
The Decca Tree Recording Method
During the 1950′s the Decca Tree recording tecnique was developed by a team of engineers that worked at Decca Records. It involves the use of a spaced pair of omni mics with an added center fill mic. It is most commonly used to record orchestras. Two microphones are placed about two meters (six feet) apart, and a third microphone is placed halfway between the two mics and about one or one and a half meters (3-5 feet) closer to the sound source (usually above the conductor). In other words, the microphones are placed in a kind of “T” arrangement. The signals from the side mics are panned hard left and hard right, and the signal from the centre mic is then routed to both left and right channels. The signal from the centre mic is used to fill in the centre “hole” that would otherwise be found in the use of a widely spaced pair of mics.
There are many factors to consider when deciding which stereo micing technique to use. Things to consider such as how important mono compatibility is, how much room sound do you want in the recording, how wide do you want the stereo image to be, and what microphones do you have available, can all influence the choice of micing technique. Each of the methods we have covered has it’s own advantages and disadvantages. Depending on the room, the type of recording and the desired result, you can hopefully use this basic guide to help you make a decision which technique to use.
By Tony Koretz
© copyright July 2012