Capturing and Mixing the Audio Signal

from Rock Hardware: The Instruments, Equipment, and Technology of Rock by Tony Bacon

First of the many links in the PA ("Public Address" system) chain is the microphone. The job of a microphone - to convert sound waves into equivalent electrical vibrations - seems quite simple; after all, the ordinary telephone has used microphones successfully for over 80 years. However, with live sound microphones, the stress is on the word equivalent. The carbon microphones used in telephones, although tough and reliable, are horrendously noisy and only reproduce a fraction of the range of audible frequencies. On the other hand, a live sound microphone must not generate any audible noise, it must be equally sensitive to all the frequencies it will be asked to reproduce and it must not produce spurious harmonics (or distortion) regardless of the intensity of the sound it is asked to handle. Such microphones exist; indeed, the best microphones convert sound into electricity much more accurately than their opposite numbers, loudspeakers. Regrettably, subtlety and finesse in converting sound into electricity tends to go hand in hand with fragility and rock performers are not renowned for fastidious relationships with microphones. Phil Lewis, for instance, vocalist of Girl, smashed a $200 (£100) microphone against the stage each night as part of his act.

Somehow, fidelity has to be consolidated with robustness, and only two of the many methods of converting sound into electricity are practical for the stage. The dynamic or moving coil microphone is the heavy duty, high fidelity equivalent of the toy microphones which accompany oriental cassette recorders. This microphone works rather like a speaker in reverse. Because the moving coil mike is inherently tough and was already highly developed when rock and roll began, it became the standard. However, its reproductive qualities are not as accurate as the alternative: the capacitor (or condensor) microphone. This requires a DC voltage, which can be a nuisance, but otherwise, modern versions are probably superior to moving coil microphones in all respects excepting ruggedness. Ironically, although the capacitor microphone was developed nearly 60 years ago, the only practical head amplifiers available until the late 1960s used tube and these made the microphone impossibly heavy, bulky and delicate for stage use. As a result, only in the latter half of the 1970s did the capacitor microphone find a place on the stage, when its new sturdiness made it acceptable to the habitual users of moving coil microphones.

Capacitor microphones exhibit a very flat frequency response, while moving coil mikes are often peaky in the upper midrange. This peakiness adds character, particularly to vocal sounds, and microphones are often chosen on the basis of such character or coloration.

The simplest way of miking up a band is to place a single microphone (mono) or a crossed pair of microphones (stereo) in front of the stage. This technique is admirably suited for purist recording, but it does not work at all for PA. The microphone has to be a fair distance from the musicians for it to "hear" in perspective. This separation between the sound source and the mike lowers the sound level heard by the mike, which in turn means the signal has to be am-amplified to a greater degree than is strictly necessary. To achieve this separation the microphone must be positioned out front, dangerously close to or even in line with the sound from the PA speakers. The inevitable result of this arrangement (aggravated by the need for extra microphone gain) is that acoustic feedback occurs long before any useful amplification of the original sound can be attained. In any case, the balance in the sound level of the individual instruments is totally out of the sound engineer's control, and the musicians themselves cannot hear what cacophonous sound is being suffered by the audience. Finally, this simple miking method produces a very reverberant sound, full of ambience, which, although realistic, includes audience noise and lacks both "tightness" and the all-important feeling of spontaneity.

For these reasons, the best and most popular way to mike up is to give each instrument its own close-seated microphone, a technique called "close miking". Because rock instruments are quite noisy, stage microphones are almost exclusively unidirectional, that is, they are especially sensitive in one direction only. Careful positioning of the myriad microphones on the drumkit is essential to limit the degree of sound spill from, say, adjacent toms and snares.

These two diagrams contrast the miking up arrangement for a five-piece band in a small venue (1) with the miking arrangement for a major venue (2). The symbols indicate mike placement. Despite the fact that amps and drumkits are usually loud enough on their own in small clubs, most groups mike up bass and lead amps and the bass drum.

Close miking means that the microphones are subjected to very high sound levels. Therefore the microphone gain required is very small and the likelihood of acoustic feedback is neatly deflated. Because each instrument has its own microphone, the sound engineer can control the overall balance of the band's sound, as heard by the audience, by mixing the microphone signals out front. The emphasis in PA microphone techniques, then, is to capture each instrument as a discrete entity.

Direct Injection, Stage Box and Multicore

Apart from the purely acoustic instruments, guitars and keyboards have to be fed into the PA. Since these instruments are electric, microphones can be avoided by tapping off the electrical signal somewhere between the guitar or keyboard and the instrument amplifier's loudspeaker.

Left Joni Mitchell surrounded by microphones. Subtle acoustic sounds are difficult to capture: the solution is to place mikes as close to the sound source as possible. The sound engineer can then control the balance by mixing the signals from the various mikes to achieve the desired effect.

If the sound is a clean one (electric bass or keyboards) the signal is tapped off as close to the instrument as possible for the sake of clarity. If the sound is a dirty one, which includes the sound of an overdriven amplifier and speaker, a microphone can be placed in front of the instrument amplifier's speaker to capture the raunchy sound in its entirety. Nonetheless, direct injection can be added to supplement this sound to taste, and with both types of signal sources to blend, the mixing engineer can reproduce both clean and dirty styles of playing without cornpromise. The direct injection (DI) box is simply a means of connecting amplifiers on stage to the PA without creating unhealthy hums and buzzes. The DI box also frequently incorporates an attenuator which cuts the signal and an amplifier to allow minute voltages from guitar pickups and the large voltages driven into instrument amplifier speakers to be fed into the PA at a fairly standard level and without overload, noise or loss of clarity.

BELOW: The moving coil microphone is the mike most often used for rock vocals. It works in a similar way to a generator or dynamo. Sound waves vibrate the diaphragm, which is curved for rigidity rigidity and connected to a voice coil. The voice coil moves through a strong magnetic field and generates a tiny electrical current (although at the high sound levels encountered on stage, the output voltage can reach one volt, l0,000 times greater than the output from mikes in a broadcasting studio). Air vents a the rear of the diaphragm make the microphone most sensitive to sound arriving from the front(directional). The windshield prevents "popping" noises when the microphone is held close to the singer's mouth The transformer adjusts the signal to match the mixing desk input, preventing unwanted hums and ensuring the treble response is not limited.

Below: The direct injection Hall Equalization (DI) box allows signals to be taken from the instrument itself and routed directly to the mixer, eliminating problems of sound "spill" between mikes. DI boxes are often used for capturing the "clean" sounds of bass and keyboards, if the amplifier's tone is not part of the overall sound.

Above: Roger Daltrey of The Who is noted for his energetic stage presence, expressed, typically, by swinging microphones. Stage microphones have to be sturdy to accommodate this type of treatment.

Cables lead from each microphone and DI box and are collated at the stage box. Then a very thick cable, affectionately termed "multicore"or "snake", containing 80 wires or more leads the signals to the mixing desk, which is sited somewhere in the center of the auditorium, usually toward the rear.

Although the mixing desks seen at major concerts look dauntingly like mission control, once the functions of one of the channels are grasped, operating the desk becomes mainly repetitious. The signal from each microphone and DI box is allocated a channel. The desk is designed to route and control the level and tonality of a large number of signals, both singly and in groups, without spurious interaction, or the addition of distortion, hiss or other noises. Big rock concert desks are broadly similar to their counterparts in film and recording studios, but the emphasisis on ruggedness, the subgrouping accords with the number of instruments typically encountered in bands and the inputs are designed to handle the high level signals that originate from wild rock performers and close miking techniques.

Right A typical channel from a 12-channel mixer Each instrument or microphone is assigned a channel; for example lead vocals may be assigned the first channel, bass the second, and so on. At the rear of each channel is an input socket and a slide switch which selects the appropriate input sensitivity "line" or mike signals. Input gain adjusts the level of the input signal Some amplification is necessary to maintain a large ratio between hums and desirable sounds, but too much level will cause overload. The equalization controls (bass.midrange and treble) are an extension of the bass and treble controls found on domestic hi fi equipment. These are used for cutting and boosting the frequencies in the different frequency bands High midrange can be used to accentuate vocal "presence" or to harden the drum sound.

Boosting the Iow midrange gives the sound warmth. The echo send level control determines the level sent to an auxiliary submixer used to feed effects units;it is also sometimes used to provide an additional mix for the monitors The foldback level control allows the engineer to set up a separate mix for the stage monitors The panpot deploys the signal between the left and right speaker stacks(similar to a domestic stereo's balance control). Turning the control knob creates the impression of a sound image moving across the stage, particularly effective for drums Otherwise. the panpots are set up to position each sound according to the relevant instruments position - drums center bass stage right, and so on. The channel fader is used as a volume control, unlike the input gain which is usually disregarded once set up. The normal position for the fader is "3/4 up". After the channel fader and panpot, the signals from each channels are mixed down and pass to the left and right output sockets via a pair of master faders. These allow the overall level fed to each side of the stage to be adjusted easily Sometimes a master equalization section is also provided in the output section. Two meters show whether the output levels are overloading any part of the system. The outputs for echo send and foldback are in mono.

In the smaller PA setups, fewer channels are required and the desk shrinks into a mixer, with perhaps four or six channels, and the facilities reduced to the bare essentials - volume,bass, midrange and treble controls for each microphone and a master volume control.Although cosmetically different, such mixers are essentially similar to their forerunner, theWEM Audiomaster. Size or complexity regardless, the desk is the control center of the PA. The sound engineers or musicians can aim to reproduce the music per se, or can add their own interpretation. Mixing a band is much more than merely summing the signals from all the microphones or twiddling knobs. Despite all its advantages, close miking does not reproduce the natural sound of many instruments; skillful use of the equalization (or tone) controls is necessary to counteract this deficiency. A good engineer must also have rapport with the audience and know how to make the musicians come through to the best of their ability: success in PA is largely a matter of the person on the desk being in accord with the musicians on stage. A complex desk provides a great versatility in control but without unique human attributes such as creativity, wit and skill, the desk is merely an expensive showpiece.