Two-channel purity? (Skeptimania).
Decades ago, Mercury elaborated on this technique and filled in the space between the two main microphones with a third-allowing for a more stable, mixdown-controllable image in the center. The technique resulted in their vaunted Living Presence recordings, still admired today for their soundstaging qualities and sense of stage depth.
A center fill has been used to good effect for some time in an adapted form for a large number of Decca/London recordings. The highly regarded "Decca Tree," pioneered by Roy Wallace, perfected by Kenneth Wilkinson, and used by such Decca engineering notables as John Dunkerley, Stanley Goodall, James Lock, and Philip Siney, consists of three omni-directional microphones. Each is attached to the end of a T-shaped assembly that is suspended above the conductor's podium. The left/right capsules are about 2 meters apart and the center unit is about 1.5 meters in front of the axis between them. The resulting geometry imparts a quite warm and spacious sound to the recording.
With still wider spacing, the spaced, three-microphone array has in the past been used by engineer Jack Renner of Telarc. Many of the classic Telarc releases were products of this arrangement. When recording large ensembles, Renner often employs omni-directional microphones, with the left and right modules up to nine feet on either side of a centrally positioned one. Keeping the microphones at a wider-than-usual distance apart minimizes the impact of comb-filtered interference effects that some individuals feel color the sound of spaced-array recordings. Telarc's most basic recording technique has long been very popular with a number of knowledgeable listeners and record-review critics and the company's material has become a benchmark for many serious music lovers.
Not every spaced-array advocate likes the wide-track approach, however. Harry Munz, who has recorded some notable material for Gothic records (many of which have been given rave reviews by yours truly), often employed a pair of very high quality omni's spaced only a few feet apart. Actually, the spacing required for best results will often be determined by the size of the ensemble to be recorded and the desired sound-stage spread and focus.
The spaced-microphone arrangement (sometimes supported by ambience-augmenting microphones located out in the audience area), whether utilizing a moderate distance between units or a very wide one, imparts a spacious, open, and sometimes pleasantly diffuse sound to the music when it is played back in typical home listening environments. Many classical-recording enthusiasts in America seem to like this, particularly if they own speaker systems that present a very focused sound-stage image. The technique compliments what truly exemplary soundstaging/imaging speaker systems such as the Dunlavy Cantatas (reviewed in Issue 87), Waveform MC/MC.1 sub/sat systems (Issue 84), and Triad InRoom Silver sub/sat systems (review forthcoming) can deliver.
Not everyone is enthralled by this technique, however. Dissenters, such as researcher and mathematician Dr. Stanley Lipshitz, have in the past indicated their strong preference for the coincident or near-coincident, directional-microphone technique. This is sometimes used by engineers working for Chesky, Opus 3, Nimbus, Hyperion, Teldec, Reference Recordings, and on occasion, Delos and Harmonia Mundi USA, as well as by a number of others.
One configuration, the XY, double-figure-8 approach (sometimes called the Blumlein array), requires the use of directional (front/ back sensitive, side insensitive) microphones located extremely close together and aimed at each flank of the orchestra. By having the direct signals respond only to level differences, as opposed to the spaced-array's response mainly to timing differences, this two-capsule, dipolar-microphone technique insures that the recorded sound is kept comfortably phase coherent.
Apostles of this and several other "intensity stereo" techniques believe that any spaced-array microphone arrangement, be it utilizing two (or three, or more) microphones, will add annoying, phase-dominated, comb-filtering effects to the sound. They note that while the reflected, ambient "hall" sound on any recording should have a somewhat diffuse and phase-dominated quality, the "direct" sound that is coming from the ensemble itself should be coherent and lacking in the time-of-arrival, phase-difference anomalies that a spaced-microphone configuration will by its very nature produce.
They point out that piano recordings are particularly prone to bloating and indistinct focus when recorded with a spaced technique, as evidenced by numerous examples that give the impression that the piano is 20 feet long or that the keyboard is spread out between the speakers. They also claim that coincident-source recordings have the ability to project a valid front-to-back depth with any type of ensemble or solo instrument that spaced-microphone recordings can imperfectly fake--but cannot duplicate. Finally, with small-ensemble performances, they claim that left-to-right sound-stage imaging is going to be more accurate with coincident techniques, particularly when listening from the audiophile-preferred, central "sweet spot."
Phase problems and the nature of a proper sound-stage presentation are only part of the ongoing debate about microphones and their placement for two-channel recordings. Supporters of the coincident technique note that centrally located images (usually involving soloists) are more sharply focussed. Spaced-array enthusiasts will counter that although the central and near-central images produced by their technique may be more diffuse and less stable when listening from the sweet spot than what is possible with coincident-pickup techniques, the negative effects can be minimized by using the previously noted blended center microphone. Accent microphones on individual instruments can possibly do an even better job of leveling the playing field.
What's more, they point out that the resulting lack of sweet-spot listening tightness with central, half-left, and half-right images that can show up with spaced-microphone techniques is subjectively no worse that what is often encountered in a typical, live classical performances-even ones happening in excellent concert halls. In those live-music situations pinpoint imaging is impossible to experience for anyone but the person conducting the orchestra.
Space-microphone advocates will point out that their favored technique ensures that under home-listening conditions, sound-stage images will be less likely to shift radically toward the nearer speaker if the listener moves away from the central sweet spot, making recordings made that way more suitable for social listening. As they see it, coincident-source-microphone recordings require that "serious" listening be done from a small, central area if full advantage is to be taken of what that technique offers. Only one person gets to experience the recording at its best, with everybody else in the room getting a substandard sound stage.
Most spaced-array advocates feel that even though the direct signals produced by the technique are less phase coherent than those which result from coincident-source practices, the net result is more subjectively realistic when experienced in a typical home-listening environment. Spaced speakers and spaced microphones complement each other. In addition, a few enthusiasts claim that spaced-array techniques, although ideally not in the same purest-oriented class as coincident-microphone practices, allow the recording engineer to do a more customized job of dealing with concert-hall size, shape, and reflectivity deficiencies, particularly with large-ensemble recordings.
Perhaps some kind of compromise is in order, and to this effect a number of other recording engineers and astute listeners swear by variations on the two basic techniques. The technical people at Nimbus records, for instance, made a point of combining the coincident technique with Ambisonic surround processing. Although not very well known in the USA, with proper decoding, this British system supposedly can simulate a reasonably accurate concert-hall environment. The Soundfield, four-capsule microphone used with this kind of recording process was originally designed with Ambisonic recording in mind.
Some experts try to compromise between the spaced- and coincident-array techniques by using the French-pioneered "ORTF," or the Dutch "NOS," near-coincident systems. The former places outward-angled, directional microphones only a few inches apart, while the NOS technique uses somewhat wider spacing and a slightly wider pickup angle between the microphone capsules. (The individual microphones are cardioid models that have progressively weaker sensitivity as the recording angle widens, until there is a near null directly behind the capsule.) The result is quasi-coincident behavior at lower frequencies, due to the long wavelengths involved, with shorter-wavelength frequencies from the midrange on up being given enough of an inter-channel time delay to impart a degree of spaced-array openness to the sound.
With large-ensemble recordings, even engineers dedicated to spaced-array and/or coincident-source techniques usually end up augmenting their main-pickup configuration with additional microphones out in the audience area to pick up ambience. Others may flank the main array with widely spaced microphones, in order to highlight the left and right sides of the orchestra or add "bloom" to the sound. The degree of enhancement can be adjusted later, during the editing sessions.
In addition, many engineers, even those with the purist of motivations, use additional pickups near particularly hard-to-record instruments or vocalists to make them more audible in the final mix. Ironically, this is one way to reduce the dynamic range of a recording, because instruments that are difficult to hear with fully purist techniques (and difficult to hear during live performances, too, for that matter) are increased in level to make them more audible during home-playback situations.
Practices of this kind are almost mandatory if the hall in which the recording is being made has acoustic deficiencies. Recording engineer John Eargle, who has made many superior recordings for both Delos and ProArte (and who is a specialist in the ORTF technique), is particularly well known for his ability to modify orthodox procedures to compensate for hall/orchestra anomalies. He frequently employs level-adjustable, spaced-omni microphones well off to the sides of his main ORTF array to add a controllable amount of spaciousness to his recordings.
Craig Dory, of Dorian, is also well known for his innovative microphone techniques, as are Keith Johnson of Reference Recordings, Ralph Couzens of Chandos, Bob Katz of Chesky, Paul Goodman when he was working for RCA, and John McClure when he was working for Columbia. Notable freelance classical engineers such as Tony Faulkner, Ben Connellan, Mike Clements, Mike Hatch, Don Hartridge, Christopher Greenleaf, and Jonathan Stokes are also masters of purist innovation, as were renowned Decca engineers John Dunkerley, John Pellow, and Simon Eadon.
Needless to say, not every recording engineer is enamored of simple, or semi-simple, microphone techniques, be they spaced-array or coincident. Many recent classical releases by Decca/London have been masterpieces of advanced multi-microphone techniques, and Jac Holzman, founder of Elektra and Nonesuch, pioneered the intelligent use of multi-microphone techniques for classical material in the 1950s and 1960s.
Deutsche Grammophon has used as many as 32 channels to record a symphonic work, mixing their inputs down to 2 channels by deftly blending the total. Columbia and Sony have also made a large number of multi-microphone classical recordings. While the results of heavy-handed multi-microphone use by these and a few other companies have sometimes been disappointing, a fair number of such rigorously controlled recordings, even classical ones, sound quite good. And of course, some pop transcriptions have sounded downright spectacular.
Indeed, in the pop-music realm, multiple microphones and elaborate mixing techniques are the norm. Recording engineers such as Ed Cherney, Frank Filipetti, Chip Davis, George Massenburg, Roger Nichols, Hugh Padgham, Bill Schnee, Alan Sides, Bruce Swedien, and Al Schmitt are past masters of multi-microphone usage. And of course, during the 1950s, 60s, and 70s, Bill Putnam and his Universal Recording Company set the standard for refined pop-recording techniques for a generation. Ironically, the use of multiple microphones and complex mixers has allowed some pop-music engineers to move ahead of their classical counterparts in some respects, because those technologies lend themselves so well to the creation of discrete-channel surround-sound recordings.
With classical music, steely, harsh-sounding violins are the most conspicuous indication that the multiple-microphone technique has been executed incorrectly, because the procedure nearly always puts the directional microphones close to the direct field of the violins. When this is done, the mellow, mostly off-axis blend of the sound that reaches an audience during a live concert may not be properly reproduced. Correctly locating a microphone for the best blend can be a time-consuming job, because the unit must be placed in that part of the direct field that most closely simulates the reverberant sound encountered at a live concert.
Many multi-capsule classical recordings also come across as one-dimensional sounding because proper timing clues are not reproduced. Sounds picked up by microphones placed very close to instruments at the rear of the orchestra are added to the mix simultaneously with close-miked sounds picked up from instruments at the front. However, at a live concert the rear-most sounds arrive later than those from up front, which is one of several ways that we sense the depth of the orchestra. In addition, at a live concert the sounds coming from the rear of an orchestra tend to generate a more reverberant soundfield and blend than what is produced up front. This also aids the listener in sensing depth. It is difficult to simulate these effects with multi-miking, although individual-track, digital-delay systems can help.
Nevertheless, given that time-delay phase artifacts and levels can be precisely controlled during the mixing process, when placed under skillful control, multi-microphone techniques can do a remarkable job of simulating an "intensity-stereo" recording. Indeed, the technique can greatly reduce many of the phase problems disliked by coincident-source advocates-while still allowing the kind of final control that recording engineers and producers (and sometimes even conductors) can feel comfortable with.
Whether the use of scads of microphones and extremely complex mixing consoles will be the norm as the world of hi-fi sound reproduction further advances into the realm of surround sound is a question that nobody can answer right now. It is likely that at the very least a center-channel microphone, in combination with hall-ambiance microphones will become mandatory, as will careful post-production mixing.
For better or worse, it appears certain that the era of absolutely pure two-channel recording techniques is all but over.
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|Date:||Sep 1, 2002|
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