Matrixing on a mixing console requires three channels: M, S+, and S−. “Y-cabling” the S signal into two channels is not recommended because all faders, even supposedly identical ones, do not perform exactly the same throughout their travel. Maintaining an identical level of the S+ and S− channels is essential, and simply visually matching fader positions is not accurate enough.
• Label the “M” track, pan it center, and put the fader at unity. Adjust the gain to achieve good input levels. (The level of the M fader can also be below unity if you are balancing the array’s level with other sources in the context of a larger mix.)
• Mute the M channel.
• Label the first S channel “S+,” and connect the S signal to its input. Adjust its gain to achieve good input levels. Pan it hard left.
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• Connect the S+ channel’s post-fader Direct Out (or similar individual post-fader output) to the line input of a third channel, labeled “S−.” Polarity reverse this channel and also pan it hard left. (Yes, left, the same as the other channel. For now.)
• Put both the S+ and S− faders at unity. You will not move the S– fader from unity, so put some board tape over it so it does not get accidentally knocked.
• Adjust the S− gain control until the two S channels cancel out as much as possible.
When maximum cancellation is achieved both the S+ and S− channels are ampli-tude matched.
• Pull down the S+ fader to −∞.
• Pan the S− channel hard right.
• Unmute the M channel.
• Gradually creep up the S+ fader to increase the S component in the signal and widen the stereo image. Reduce its level if the image gets too wide. Do not adjust the S− channel – it has already been calibrated to the S+ channel and its feed is after the S+ fader, so the S+ fader effectively adjusts it as well.
• If you change the level of the M track at any time you will have to adjust the S+
track by exactly the same amount in order to maintain an identical stereo image.
• You could route the M and both S faders through a stereo group fader (and not directly to the L/R output fader) and have a simple way to control the level of the matrixed image in your mix.
The level of the S component in the matrix should not be as high as the M signal. The S faders will not need to be run at unity if the S input levels are trimmed to be similar to the M input levels – they will commonly be at least 6 dB lower than the M fader when producing a good stereo image. If there is too much S component in the image, a strange phasey “hole”
develops in the center – activity is heard on the left and right extremes but not throughout the width of the image.
MS technique has a very different sound to the other stereo techniques discussed in this chapter. It produces a very precise image in which it is easy to pinpoint the location of specific sound sources. It is criticized by some for being too “surgical” and not as impressively immer-sive or enveloping as near-coincident or spaced techniques – which trade MS’s localization accuracy and focus for slightly less accurate “smeared” images that are bigger and more instantly impressive.
MS technique is completely mono compatible – more so than any other technique dis-cussed. If the stereo image is summed to mono, the S+ and S− signals cancel out completely (because they are polarity reversed versions of each other) leaving just the M mic signal as the mono output.
of recordings that are too narrow or wide. If the recorded image of a stereo mic array is too wide or too narrow, the same matrixing technique can also be used during mixing to adjust that image – but this should be a last resort – it is preferable to adjust the mic array to achieve a better image in the first place.
Without matrixing, a left/right stereo source can be made less wide by panning the two channels less extremely – but this may cause phase cancellation artifacts if the signal is not 100 percent mono compatible. Additionally, it is impossible to widen a stereo signal that is too narrow by using the pan pots – the knobs only turn so far!
After matrixing the left-right stereo signal to MS, the relative balances of the M (center) and S (width) components can be adjusted to widen or narrow the image width, and those MS signals matrixed back into stereo using the techniques described earlier.
To matrix LR stereo to MS in a DAW:
• You need to be able to polarity reverse either the L or R channel, so depending on your DAW you may need the stereo track split over separate L and R channels.
• Duplicate these L and R tracks, so there are two sets of them.
• Pan both channels of the first pair of tracks centrally, leaving both faders at unity.
Route them both to an unused internal mono bus in the DAW – “Bus 1” for example.
This is the common “M” signal. Do not route them to the main stereo output.
• Polarity reverse one channel of the second pair of tracks (it doesn’t matter which channel). Pan both channels of this pair centrally, leaving both faders at unity.
Route them both to a second unused internal mono bus in the DAW – “Bus 2” for example. This is the “S” (difference) signal. Do not route them to the main stereo output.
• If you have to use a plug-in to polarity reverse, make sure delay compensation is turned on in your DAW (if it is not automatic) to allow for any delay induced by that plug-in.
• You can now set up three input channels, M, S+, and S–, routed to the main stereo outs, and matrix the MS signal to stereo, changing the image width by manipulating the S level in this second set of channels. The input to the M channel should be
“Bus 1,” and the input to the S+ and S– channels “Bus 2.” Don’t forget to polarity reverse the S– channel, and to pan S+ and S– left and right.
Don’t expect this technique to fix every image problem! If a recorded stereo image is far too wide, has a hole in the middle, or lots of out of phase content between the L and
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R channels, there is little common M signal to be derived. Conversely, if a recorded stereo image is far too narrow and almost mono, there is little S difference signal to be derived. With a shortage of either component the technique is less effective. This technique is best used for subtle changes, not big “rescue” jobs!
MS Mic Arrays @
Example 6.9: An MS array, matrixed to produce a good wide stereo image.
(Compare this example with the stereo image of a spaced pair on an acoustic guitar in Example 11.14.)
Example 6.10: An MS array. This example starts with the S component turned all the way down in the matrix, leaving just the mono center. As the S component is turned up you can hear the image widen. By about 20 seconds there is too much S component and the image gets “phasey” with a “hole” in the middle.
6.7 THE DECCA TREE
The Decca Tree was originally developed by the Decca record label for orchestral recording and can be heard on countless records and film soundtracks. It is formed from three omnidirec-tional microphones, tradiomnidirec-tionally large diaphragm condensers, which are set up in a triangle as shown in Figure 6.9. The L mic is panned left, the R mic panned right, and the C mic panned to the center. The gains of the three mics should be set equally, although adjusting the C mic up or down can narrow or widen the perceived image.
Many variations of the Decca Tree are possible:
• Cardioid or wide-cardioid mics can be used instead of omnidirectional mics.
Fi g u r e 6 . 9 A Decca Tree.
c
( 4 - 6 f t ) 140 - 200 cm
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70 -1 0 0 cm ( 2 - 3 f t )
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This array is famed for its expansive and immersive sound. It is not necessarily the most precise in terms of imaging accuracy – but its image characteristics are definitely pleasing.
Neither is it the most mono compatible – with three spaced mics and the associated time arrival differences between them. Mono compatibility should definitely be checked prior to recording.
6.8 BINAURAL AND BAFFLE TECHNIQUES
Stereo loudspeaker systems create expansive and wide stereo images because of the inter-aural crosstalk and associated HRTF effects of the sound from each loudspeaker travelling to both of the listener’s ears. The stereo microphone techniques discussed so far are designed primar-ily for loudspeaker system reproduction – and while they work fairly well on headphones, the perceived image is not the same as it would be on a loudspeaker system. Headphones, being binaural and not technically “stereo,” produce a different image – lacking the inter-aural crosstalk and HRTF effects of a true stereo system.
Binaural microphone techniques capture characteristics similar to natural HRTF time delay and frequency filtering effects, at the time of recording. These pre-recorded HRTF effects (that would otherwise be missing when listening on headphones) are then reproduced by the headphones and interpreted as inter-aural crosstalk and HRTF effects by our brain.
For this reason binaural mic techniques reproduce and image well on headphones, but not on stereo loudspeaker systems where an additional dose of HRTF effects are naturally and unavoidably imposed by the playback system and the listener’s head.
Several manufacturers produce head-shaped or spherical baffles which have mic capsules permanently mounted in positions similar to our ears. Other flat baffle systems can also be purchased or easily home built to separate two standard omnidirectional mics. The presence of the baffle creates some acoustical separation between the two omnidirectional mics. The spacing between these baffled mics (and the size of the head-shaped or spherical baffle) approximates the separation of the ears on the average human head.
The Jecklin Disc is a baffle technique that can be easily constructed and is shown in Figure 6.10. A circular baffle, 25 to 35 cm (10 to 14 in) in diameter, with both sides covered with acoustically absorbent material is positioned between two omnidirectional mics that are 36 cm (14 in) apart. The mics are angled outwards at +20° and −20° from center, although this angle is variable.
Baffles can be added to any of the near-coincident techniques previously discussed to increase channel separation and image width, but accuracy when reproduced on stereo loud-speaker systems will be sacrificed.
PRACTICAL EXERCISE
Record a piano, acoustic guitar, drum set (or any other sound source that has width and takes up horizontal space) using a variety of the stereo mic techniques discussed in this chapter. Pan each channel hard left and hard right respectively, and listen carefully to the stereo images produced by each. Answer the following questions:
• When summed to mono how much does the image collapse and narrow? When put back into stereo how much does the image expand and widen?
• When summed to mono do you hear tonal changes or does the amplitude get signifi-cantly quieter? (These would indicate mono compatibility issues.)
• How does the overall sense of width and space compare between the techniques?
Do some sound wider and more spacious than others? Do some sound smaller and more compact?
• How clear and accurate is the image between the speakers? Can you really pinpoint the precise location of a specific sound? A good test for this is to record some of the mic techniques over the hammers of a grand piano and listen to the transition of the notes from left to right. How much do you hear the notes moving across the sound-stage? Do they move slowly and smoothly across and through the center of the soundstage, or do they hang in a certain loudspeaker before suddenly jumping to the other side?
• Tonally or timbrally, how does each technique compare? Are some darker, muddier, or more muffled sounding than others? Are some brighter, more open, and more transparent than others?
Figu r e 6 .10 A Jecklin Disc separates two spaced omnidirectional mics, creating a binaural mic array.
36 cm (14 in)
DISC FO A M COVERED
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2 0 ° f 2 cm
(3/4 in)
3 fra n (13 3/4 in) 2 0°
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IN THIS CHAPTER:
7.1 Art and Science 144
7.2 Distance and Tonal Qualities 144
7.3 “Zoom Factor” 145
7.4 Off-Axis Response 147
7.5 Direct vs Reflected Sound 149
7.6 Floor Reflections – the Good, the Bad, and Boundary Mics 150
7.7 Distance and Stereo Arrays 151
7.8 Spill – Enemy or Creative Tool? 153
7.9 Why Minor Changes in Mic Position Change the Sound So Much 155
7.10 Experimentation and Exploration 155
7.11 Practical Tips to Help Set Mic Position 157