Operation in harmony
Jan 1, 1999 12:00 PM, Gordon Moore
In today's modern world of worship, the sound system has become even morecritical in helping convey the message. Going along with this increasedreliance is an increased difficulty in finding trained, skilled techniciansto operate the system, particularly those with a complete understanding ofacoustical gain and the impact of multiple mics on that gain.
There are a few problems associated with an amplified, multiple mic soundsystem. The first is gain control. Each time the number of active mics inan amplified environment increases, the gain of the system increasesaccording to the formula 10logNOM where the subscript NOM means Number ofOpen Mics. Roughly translated, this means the gain increases 3 dB everytime the number of open mics is doubled. In a recording environment, thismeans the background level of the room increases 3 dB. This may be heard inthe recordings of a worship service. In an amplified environment such as achurch, this also means the system will get 3 dB closer to feedback.
In a church, where the sanctuary may be highly reverberant and a lot ofgain is impossible, this means that you have a limit on the number of openmics you can have at full gain. With a manual mixer, the operator must beacutely aware of this relationship and lower the level of one mic beforeraising the level of a second mic. Failure to do so could result infeedback.
When gain is critical, the operator must keep all inactive mics turned downto allow the active mics the most gain from the system. This meanspainstaking attention needs to be paid to the program, and the operatormust anticipate when a lay reader or minister may begin speaking. He willhave to bring up the mic in advance or risk the late-mic syndrome.
Automatic mixers can assist sound systems and their operators in a numberof ways, namely through opening and closing input channels by monitoringthe actual activity of each channel. When the mixer detects enough signalto warrant adding that mic to the mix, the channel is opened. If thechannel has no significant activity, then the mixer keeps that channelattenuated. This reduces noise (ambient and electronic) in the mix.
There are a variety of electronic methods used to control the mics. Somemanufacturers use noise gate type circuitry; others use microprocessors.Each design has its advantages and disadvantages. Regardless of themethodology, it is important to realize that the mics in the system shouldbe set to equivalent levels. The automatic mixer generally needs evenlevels so that its algorithm does not confer a bias to an input that seemssignificantly hotter all the time.
The vast majority of automatic mixers also keep track of how many mics areactive at any time. The mixer controls gain by keeping track of NOM andreducing gain accordingly. This is the NOM attenuation algorithm and variessignificantly from one manufacturer to another. How the NOM algorithmoperates can affect how you would configure a system incorporating theautomatic mixer. Discussion with the manufacturer can assist with designconsiderations. By keeping the number of active mics reduced to theequivalent of one open mic, ambient sound is reduced in the audio signal,reducing noise in both the system and in any recordings from that system.
Automatic mixers can also prevent the late-mic syndrome. Acting withinmilliseconds, today's automatic mixers can keep up with even the mostspontaneous speaker. Clipped first words and lost syllables are artifactsfrom the past and do not appear in the modern units. Some modern designscan even prevent comb filtering and related phase problems when two micspick up the same source.
A tale of two churchesConsider a couple of examples. One church is strictly liturgical; themajority of the service involves the spoken word. The other church'sservices are productions complete with bands, organs, soloists, preachersand lay readers. In both cases, an automatic mic mixer can help.
The liturgical church, a medium-sized congregation of about 250 families,has a music program consisting of a choir, a piano and an electronickeyboard. The service is predominantly spoken word. There are noexperienced audio people in the congregation, and no budget available tohire one. The system needs to be fully automatic with some flexibility.
In this case, an automatic mixer can serve to handle all audio sources. InFigure 1, a block diagram of the typical liturgical system is outlined. Allmics feed into the automatic mixer. If the mixer has enough inputs, theother audio sources can also be mixed here. Be certain that reproducedaudio sources-CDs, tape and organ-are not part of the automatic mix NOMattenuation scheme. If the mixer perceives the CD player as a part of theNOM algorithm, it will affect the NOM and reduce the gain of mics in thesystem. The soloist trying to sing along with a disc may find the leveldropping every time the music comes on. If program signals are allowed toparticipate in the automixer algorithm, then these sources can prevent themics from activating properly. Check with the manufacturer first. Manyautomatic mixers can have inputs set so they do not participate in the NOMprogram. If this will prove to be a problem, then the automixer can feed asmaller program mixer for the complete mix.
Output from the mixer feeds the EQ-amp-loudspeaker chain. This simplesystem is turned on with a single power switch at the beginning of eachservice. It is highly recommended to use a sequential power-up product sothe system does not thump at power up and power down. No operator isrequired. A single mic type is used everywhere with all floor jacks andwireless mics preset to correct levels for the typical user. Many automaticmixers have provision for external pots to exert level control throughinternal VCAs in the mixer. A simple 10K linear pot can be mounted on apanel for the head usher or music director to adjust system level. Whenattendance is low, system level can be lowered. Adding a small resistor inparallel to the pot allows you to set up the system with a minimum/maximumrange. It cannot be turned all the way off, nor can it be turned up intoear-bleeding feedback.
This system is simple, requires no operator and can be locked up forsecurity. Although somewhat more expensive than a similar systemincorporating a manual mixer, this design offers maximum gain beforefeedback without requiring a trained operator. The advantage to the houseof worship is obvious. The system is always available on demand forweddings, funerals and other special functions. Because it can be literallylocked up in a secure area, the equipment can be better protected fromtheft.
The second example, the progressive church, is the big church. Themembership is measured in the thousands. The services are enthusiastic,multimedia events with a broadcast feed. Here, the budget has a soundprofessional on the payroll whose job depends on delivering reliable audioevery service. He has to run a large console, and the minister is bothenthusiastic and unpredictable. Keeping up with the music program keepsthese sound system operators busy. Minding the multiple mic locations addsto the need for them to do the four-handed two-step. Late mics are aregular problem and seem even more glaring in light of the broadcast feed.
In this case, as with any church with a large music program, the mics wherethere is typically a speaking voice should be the least of their worries.An automatic mixer incorporated here can help stabilize the system withoutjeopardizing the gain structure of the remainder of the system. Theminister, associate minister, lay reader, pulpits, altar, baptistery andother similar locations will all have mics that can be controlled by anautomatic mixer.
There are three approaches for using the automixer here. The first approach(Figure 2) is to take the combined mix of the automixer and bring it intothe larger console as a single channel. The operator is concerned with onlycontrolling the single voice channel. The advantage is that the soundoperator needs to worry about only one channel. The disadvantage is that hehas little individual voice mic control. This may or may not be a problem.Depending on the design of the automixer, adjusting the levels of the micsat the automixer may be counterproductive because these levels may affectthe automix algorithm and cause inconsistent performance. If the mics arerarely changed (such as fixed pulpit and altar mics), and the program isfairly consistent, then this approach greatly simplifies the consoleoperators' job. He can concentrate on properly mixing the instruments andchoir without worry about other mics.
The second approach (Figure 3) is to use an automixer with direct channeloutputs that are brought into the console as individual channels. Eachdirect output carries only the individual mic, but the signal ispost-attenuator. This means the automixer is still opening and closing thelevel of the mic and adjusting levels according to the NOM algorithm. Usethis approach if the system is prone to major changes, such as changing theministers' mic or moving various mics to different inputs. The consoleoperator still retains adjustability on each channel without adverselyaffecting the automixer algorithm. If a section of the service is reachedwhere some or all of the voice mics should be brought down, the operatorcan do so. Later in the service, the operator can bring all or some of themback up at the main console. There is no worrying about feedback becauseindividual channel NOM control is still controlled by the automaticmixer-no feedback, no excruciating gaps in the audio while someonescrambles to open the active mic.
The third approach is a variation on the second method. If the console haspatch points after the console pre-amps, the automatic mixer's individualchannels can be patched in here. The advantage lies in using the boardpre-amp for setting the mic trim levels. The automatic mixer gets evenlytrimmed line level signals that the algorithm can reference for a smoothaction. The operators still retain full board control, but the automixergoverns the NOM attenuation scheme, thereby providing feedback protectionand reduced system noise.
With either approach, there is an additional benefit. The system can beconfigured for a hands-off operation. At the end of a service, theoperators can set up the main board for the voice mics and power down thesystem. Later that week, at other special functions, the voice mics are alloperational, and the sound system operators do not have to be present. Thisreduces reliance and burden on system operators. Their absence will notimpact the church nearly as much.
Real-world experienceThe following system is an actual installation in Rio Rancho, New Mexico,where I was drafted to design and install a new system. The old systemconsisted of a consumer-level stereo system with badly placed loudspeakersof dubious response, one borrowed dynamic mic, two borrowed wireless micsand a boombox for music. A large tube organ with separate amp/loudspeakersand a piano rounded out the system. When program audio was needed, someonehad to sit at the back of the church, turn on a cassette by hand, or achoir member turned on the boom box. No EQ was used. The room has highreverberance. Intelligibility also suffered from a heavy low-frequency echoand poor high-frequency response. The old system was gain structured forthe badly placed loudspeakers and four open mics. The open mics accountedfor a 6 dB loss. The old loudspeaker placement also accounted for about 6dB of lost gain.
The new system consists of a Lectrosonics AV62 multi-media mixer, an eightchannel Lectrosonics AM8 automatic mixer, a Sony CD player, a Sharpcassette deck and four Lectrosonics VHF wireless mics. There are threefloor jacks for hardwired mics, a line level drop in the back for visitingyouth groups to plug in their manual mixers and an electronic keyboard. Newhardwired mics include the AKG D8080 and the Lectrosonics Vari-mic.
The wireless mics, mic drops and line-level drop feed into the automaticmixer. All electronic sources and the automatic mixer feed into themultimedia mixer. The electronic sources are not part of the automaticmixer algorithm. This prevents the CD player or keyboard from dominating amic signal. The combined output is sent to an EQ and from there into theQSC amp. The amp then drives an overhead loudspeaker cluster. Future plansinclude auditory assistance and monitor loudspeakers for the choir. Thereis also potential for MIDI control through the mixer's software.
Because of the acoustics of the sanctuary, the choir does not need anyamplification. Soloists use the mics. The automatic mixer is good atdealing with these performances. Softer instruments, such as guitar, arealso run through the automixer with smooth results. The piano does not needa mic. The organ was donated to another church and replaced by theelectronic keyboard that is connected to the multimedia mixer.
The system is a vast improvement. By replacing the old mixer with anautomatic mixer, the mic gains could be set as if each mic was at fullgain. Proper equalization also helped gain and intelligibility. Now, thesermon is heard at a much higher level with an improved frequency response.Best of all, I do not have to be at every service to keep the system up andrunning. The music director simply plugs in the mics needed for theservice. Even the big Christmas cantata took a minimum of effort to preparefor-largely a matter of mic placement to follow the traffic flow ofsoloists and readers. A single wall switch activates the system and a wallmounted control panel allows some level adjustment on the system. The rackis locked up at all times, and only two people have the key. Some of theacoustic issues still need work, but the electronics package has improvedintelligibility and overall gain levels. Gain is about 15 db better than before.
Automatic mixers can have a place in all modern worship environmentswhether a simple liturgy or a complex program service. Any system can bedesigned for simplified operation, including the ability to make even themost complex installation hands-free for simple events. Improved gaincontrol, lowered system noise and elimination of late mics benefit anylevel of church service. As other demands are placed on the church soundsystem by emerging media, simplification of the mix and control of the mostbasic component, the mic, can only ease the job of the designer, theinstaller and the operator.