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Middle-Side (MS) Recording and Playback Techniques Ian Corbett Sound Engineer and Music Technologist Content •MS Microphone Technique •Matrixing MS to Stereo •Why Use MS Techniques? •MS and the Broadcast Industry •Stereo Loudspeaker Arrays and Phantom Images •Matrixing Left-Right Stereo to MS •Playback Using a Bi-Polar Loudspeaker Array •Using Bi-Polar Playback Techniques in the Theater •Double MS •Suggested Microphones for MS Recording MS Microphone Technique The diagram below shows the typical microphone set up for recording using the MS technique. A cardioid (condenser) microphone is used to pick up the Middle (mono, or front-back) information, and a figure-8 (condenser) microphone is used to pick up the Side (left-right) information. This microphone array may be either two microphones placed as coincidentally as possible (to avoid phase discrepancies between the two diaphragms), or a dedicated dual capsule microphone. The M and S signals need to be ‘matrixed’ or decoded before one can listen to a stereo (left-right) signal. The matrixing is necessary to derive discrete left and right information from the S microphone, which is a single capsule — only one diaphragm. With the S microphone aimed + and - (referring to the front and rear of the capsule) as in the diagram above, the L information is produced by taking the sum of M and S, and the R information by producing the difference between M and S; L = M + S+ (or M + S) R = M + S- (or M - S) where S+ is the regular output of the figure-8 microphone, and S- is the output of the figure-8 microphone, but phase inverted by 180o. Matrixing MS to Stereo Matrixing can either be done ‘manually’ on a mixing board, or by using a dedicated matrix decoder into which the M and S components are fed, and a stereo L-R signal is output (the relative levels of the M and S components being controlled by knobs on the decoder). Three mixer channels are required to matrix the signals manually. Into channel 1, feed the output of the M microphone, pan it center and label it ‘M’. Into channel 2 feed the output of the S microphone, pan in hard L and label it ‘S+’. Channel 3 is fed from the direct out of channel 2 (and therefore is at line level, post fader) and is phase reversed, either by engaging the phase reverse button, or using a phase reversed cable between channel 2’s direct out and channel 3’s line input. Label channel 3 ‘S-’. For the MS matrixing to work properly it is essential that both the S+ and S- channels are at the same level. To balance them: •mute the M channel (1), •pan channel 3 hard left (the same as channel 2), •adjust channel 2’s trim to a healthy level (about 6db less signal than the M channel should be fine) and push the fader up high (so you can really hear what is going on), •set channel 3’s fader at unity. The same signal (but in phase in channel 2, and phase reversed in channel 3) is going to the same monitor speaker. Theoretically, when both + and - are at the same level, complete cancellation will occur, and no sound will be heard. So; •adjust the trim level of channel 3 until the relative levels of both channels are the same, and the sound is attenuated as much as possible, •pan channel 3 hard right. A ‘phasey’ confused sound should result, •reduce the increased level of channel 2, and un-mute and bring in channel 1 (the M component) until a good stereo image is produced. The S component can be adjusted in level by moving only the channel 2 fader. (Channel 3 is being fed post pre-amp, and post fader so will also be brought up and down in level, maintaining the essential relative balance of the two S channels.) An accepted ratio for the signal strengths of the M:S components is 3:1. Why Use MS? When using a typical X-Y coincident pair microphone array, the left and right information is coming from two distinct sources — each microphone, one aimed left, the other aimed right. These signals are then panned left and right to produce a stereo signal. If set up and engineered correctly this X-Y coincident pair technique can produce excellent results, however there are two problems associated with it. The first is that with the microphones set at 90o to each other, a lot of the same signal can be going to both of them, producing a very mono sounding recording. This can be remedied by increasing the angle of separation, however care must be taken not to create a 'hole' in the center of the stereo image as this angle is increased. The pick up pattern of the microphones in use will also dictate what angle of separation is needed to obtain optimum results. The second problem is the occurrence of phase problems due to the distance between the two capsules, particularly when not using a dual capsule microphone. X-Y spaced pair microphone techniques are prone to even more phase problems, and stereo imaging is generally more diffused. Little can be done to correct problems such as the width of the stereo image once an X-Y recording is committed to two-track tape (without using MS techniques described later in this document). MS signals can either be matrixed to L-R stereo information and recorded onto two track tape, or without matrixing, the M and S components can be recorded onto tape, and the matrixing done at a later date, maybe in better mixing/matrixing facility. MS and the Broadcast Industry If a matrixed MS recorded signal is collapsed into mono, what happens? The S+ is summed with the S-. A 'plus' and 'minus' cancel out completely, so all that is left is the M component which carries all the important sound information, just not the ‘frosting’ supplied by the S component. An acceptable mono signal is produced, with no phase anomalies audibly present. The same cannot be said of collapsing an X-Y recorded signal into mono, a process which is prone to phase problems. AM radio is a mono broadcast medium, so the original recording engineer has to have produced a mono compatible mix. FM radio broadcasts L and R signals, and when listening on a mono radio the mono signal is produced by summing the L and R — since people still listen on mono radios the issue of mono compatibility is still a concern. Television, now a stereo medium, broadcasts in MS, the decoding occurring inside the stereo TV. Before the days of stereo TV, sound was just a mono (or 'M') signal. By adding an S component to this existing mono signal, stereo images are produced (matrixed) by suitably equipped TV sets — mono sets simply ignore the new S component information. Since most viewers have mono TV sets, mono compatability is (currently) of great concern to TV broadcasters. Stereo Loudspeaker Arrays and Phantom Images The problem with listening using a conventional two loudspeaker stereo array is that most of the 'bread and butter' information is panned centrally. This means that there are equal amounts of the same signal going to both the left and right speakers, producing a 'phantom' image in the middle of the stereo sound-stage. The problem with this phantom image is that one has to be situated in the correct location to hear and place the image correctly in the middle of the speakers. An experiment to try: mic up a piano (mono), or take a mono recording of a piano, and listen to it panned center (listening from the 'sweet-spot') — this is a 'phantom' image, positioned in the space between the two loudspeakers. Next, pan the signal hard left or right, and listen to it from directly in front of the speaker. The single speaker, non-phantom image should sound crisper, and less fluffy. The reason for this is that when signals are traveling from the left and right speakers, sound from the left speaker is going to one's left ear, and sound from the right speaker is going to one's right ear. This is fine! What also happens is that sound from the left speaker goes to one's right ear, and sound from the right speaker goes to one's left ear, slightly time delayed due to the extra distance it has to travel around one's head. This causes phase cancellation and anomalies which distort the stereo image. Matrixing Left-Right Stereo to MS It is possible to take any Left-Right stereo signal, and matrix it into it's M and S components. Why do this? Mastering engineers have used this technique to widen (or narrow) the stereo image of recordings that are too 'mono'. A stereo L-R signal can be narrowed by panning the L and R channels more centrally, but may cause phase cancellation artifacts if the signal is not fully mono compatible. It is impossible to widen the stereo image by using pan pots. Matrixing a L-R stereo signal into it's M and S components allows one to widen the stereo image by increasing the S component in relation to the M component, and also to listen to playbacks using a technique that eliminates the center phantom image problems described previously. One way of matrixing a L-R stereo signal to MS requires a mixing board with a mono output, an augmented speaker array (and a soldering iron to make the necessary cables): •bring the Left and Right channels of the source material in on two mixer channels, •pan them hard left and right respectively, and assign them to a pair of output busses (for example, Group outs 1 and 2), •derive the M component by assigning both of these channels to the mono output bus on the board. The S component requires a special cable to go from the two Group outs (as above) to a single BALANCED line input: •connect the + wire of the LEFT/GROUP output (as assigned above) to the TIP (or the + XLR pin) of the balanced line input connector. •connect the + wire of the RIGHT/GROUP output (as assigned above) to the RING (or the - XLR pin) of the balanced line input connector. •connect both of the shields to the sleeve (or XLR pin 1) of the input connector. This new plug is the S component. These M and S signals can either be brought into other channels of the same mixer and fed to the L-R outputs, matrixed to form L-R again in exactly the same way as described in the 'Matrixing MS to Stereo' portion of this document. The stereo width of the originally L-R source material can now be changed by varying the amount of S component relative to the M component. If the board you are using does not have a MONO output buss, you can assign the L and R inputs to two odd (or even) numbered Group Outs, and use a Y cable to sum these outputs into one input connector. Playback Using a Bi-Polar Loudspeaker Array Using a bi-polar loudspeaker array to listen to MS material requires three loudspeakers, arranged as in figure 4 — center, left and right, but with the left and right speakers to either side of the listener's head. It is essential that the distance between the listener and each speaker is the same (making this technique somewhat unsuitable for the 'real world', but still a unique listening experience). To apply this technique to a non-matrixed MS recording: •connect the center loudspeaker to the M component source, •split the S component source into two lines, •feed the L speaker with the non-phase reversed signal (S+), •feed the R speaker with the phase reversed signal (S-). If one has matrixed a L-R stereo recording into it's M and S components (as in the section 'Matrixing MS to Stereo', above): •assign each of the three input channels to a different Group output buss (or assign the M to a Group out, and the S+ and S- to the L-R outs respectively), •feed the center speaker from the Group output buss that the first input channel (M) is assigned to, •feed the left speaker from the Group output buss that the second input channel (S+) is assigned to, •feed the right speaker from the Group output buss that the third input channel (S-) is assigned to. Using this technique for listening, one has full control over the relative M:S levels, and experiences a sonic image that has more clarity and integrity than the equivalent L-R stereo image. The distance between the listener's head and the speakers will affect the size of the 'sweet-spot' proportionally. However this 'sweet-spot' is small, so there is little 'real world' application for this speaker technique. Using Bi-Polar Playback Techniques in the Theater A large sonic space can be created by using bi-polar speaker arrays in the theater. Distinct L-R panning will not be achievable due to the few number of people sitting in a spot equidistant to all speakers, however ambiances work very well. This technique is particularly useful if the production is 'in the round' or with seats on either side of a long central stage. It can also be applied to more conventional spaces as well. The M component is sent to a speaker array above the central stage area. S+ and S- speakers are placed in each corner of the room, alternating S+, S- around the room. In a more conventional space, M speakers could be hung center stage, and S+ speakers in one rear corner of the hall, S- speakers in the other. Double MS Double MS microphone technique was developed during the 1980s. It involves using two MS microphone arrays, one for direct sound, and a second array to pick up room ambience and reverberation — usually positioned facing the rear of the room at or just beyond the critical distance of the room. (Critical distance being the point at which the direct sound and reflected sound are at approximately equal intensity). The result of keeping this distance between the two microphone arrays is that any leakage of the direct sounds into the ambient microphone do not dominate the desired ambient sounds going to that microphone array. This makes it possible to control the mix of direct and ambient elements, keeping phase and time delay effects to a minimum. Care must be taken not to position the ambient microphone array too far back, however, which may cause 'slap' effects when it is mixed with the direct sounds. Double MS recordings typically feature a spacious sound with good stereo imaging and definition. The ambience is more controllable than when using some other microphone techniques, and most importantly, the stereo image produced will collapse to mono without problems, the reasons for which have already been discussed. Another use for double MS techniques is in surround recording. Versions of 5.1 stereo surround are currently being developed for music recording, based on double MS techniques, with an added sixth channel overhead. If you are able to make a double MS recording onto multi-track tape and play it back using a front and rear pair of monitor speakers, a very believable quad image will be produced, exhibiting quality and believability that is unobtainable using X-Y microphone techniques. Suggested Microphones for MS Recording •A pair of AKG C-414's (switchable polar patterns). •A pair of Neumann U-87's (switchable polar patterns). •Shure SM69, dual capsule microphone (with switchable polar patterns for each capsule). •AKG C-422, dual capsule microphone (with switchable polar patterns for each capsule). •Soundfield ST-250, multi capsule microphone (with switchable polar patterns).
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