A mixing console (or mixing board or simply mixer) is used for a wide variety of purposes in a computer music and recording studio as well as live sound and concert playback situations. Mixers are often classified by their input and output (I/O) capabilities. For example, a 32 x 8 mixer would have 32 individual channel inputs and eight main output channels. Most professional boards have many more input and output capabilities besides the main I/O's. In addition, boards can serve as signal routing devices, moving audio signals from one device to another. Recently, digital mixing consoles, formerly reserved only for very expensive commercial studios, have become much more affordable for smaller studios. These boards handle not only analog signals from devices like microphones, but also mix analog and digital signals with each other from devices such as DAT's, multi-channel tape decks and computer audio interfaces. Such device specifications, in addition to the basic number of channels, include digital I/O terms such as AES/EBU, TDIF, Lightpipe/ADAT, S/PDIF--all different forms of digital connection and transmission protocols. Most digital boards and some analog boards have some form of mixing automation, where predetermined movements of faders, often sync'ed to timecode, can be rehearsed and modified (these were often called 'flying faders'). Digital boards are also highly configurable, where various patches change the function of each input or output channel at the touch of a button. The image below indicates the layout of the various section of a typical mixing console, in this case, a Mackie 8-bus analog board. Input Channel Strips A mixing board will consist of a number of input channels controlled by identical channel strips (vertical arrangement of controls)--if you learn how one strip works, you've learned how they all work. The layout and minor differences in functions vary between manufacturers and models, but they all have the same basic elements. Inputs to a board, patched into the back or top, consist of line level inputs, from devices such as synthesizers and computer audio interfaces, microphones, for which there is often a separate preamp section, and tape inputs, for devices such as CD players (not all boards have separate tape inputs). Below, we will view sections of an input channel and their function one or two at a time. The top module controls aspects of the input lines. In this diagram, there are actually three separate input sources per channel, a microphone input, a line input, and a tape input, along with various means of selecting and routing them through the channel. If a condenser microphone is being used, the phantom power switch or button must be depressed for the microphone to function--this sends a 48 volts current through the mic cable to charge the plate. On this board, the line and mic inputs share a trim pot which allows each individual channel to be balanced with the others regardless of differing input levels or microphone sensitivities (set the trims so the if the channel faders are set to the same value, all channels sound at equal strength). Since the trim pots control the channel preamps, too high a setting may cause the channel to distort--having a meter bridge or LED to warn you of a channel overload is helpful. The MIC/LINE swtich, which choses the mic or line input jack (surprise!)--is located in the channel jack section. When depressed, the flip button switches the normal arrangement of the channel, sending the tape source the main channel fader and the mic or line source to the MIX-B pot (see below). A mixer has a separate set of outputs usually connected to effects devices. Signals can be routed to these devices from the effects sends (sometimes labeled "aux sends" or just "aux") of the input channels. Each channel can have its own relative strength of signal sent to one or more devices by adjusting the effects send pot(s). Look elsewhere on the board for an effects send master, which must be turned up for any signal to be sent. While each channel send pot is designed to set the relative strength of the input signal, the overall strength can be conveniently controlled by the effects send master. Finally, to complete an effects loop and allow the altered signal to be heard, determine how the effects devices are routed back to the board. Some studios route them back through effects returns, while others, such as the CECM studios, route them back through other board input channels. In this way, it is possible to EQ the returning signal differently from the original (a nice compositional idea). Additionally, it is possible to take a returning signal and send it to a second device (NB: do not send a returning effect signal back into the same effects bus if it has a short or no delay--this will produce an undesired feedback loop). The pre/post-fader button determines whether the signal sent from a channel will be altered by the channel fader. In pre-fader position , the signal is sent out at a strength determined only by the effects send pot and not affected by the channel fader. The pre-fader position is useful for techniques where you may wish to fade out the original signal, but still hear the altered sound. In post-fader position, both the effect send pot and the channel fader affect the overall strength of signal sent to the device. The post fader position is useful if you wish both the original and effected sound to fade out completely when the channel fader is pulled down. On this particular console (a Mackie 8-bus), the user can select the source signal for the sends as either the normal channel input signal, or a second set of line inputs called Mix-B. Chapter #3
3. Mixing Console Basics
The EQ (equalization) section is used to adjust the amount of boost or cut (attenuation) for high, midrange and low frequency bands (the exact number of frequency bands varies according to the mixing console model). Certain mixers allow the user to adjust the exact frequency of the bands as well as their boost or cut. For example, on this mixer, notice that the low mid(range) section allows adjustment of the center frequency (lower pot) as well as the amount of boost or cut (upper pot). A common novice mistake is to twirl the lower know while leaving the upper knob set to zero, which will do absolutely nothing to the sound, since you are neither boosting nor cutting the frequencies you are adjusting. The high mid(range) section of this mixer adds a third control (lowest knob) to the boost/cut and center frequency, namely the bandwidth (or Q) or frequency range of this section. Neither the lowest nor highest band is adjustable, just the amount of boost or cut. Both the high mid and low mid are band-pass or band-reject (notch) filters. The Hi and Lo EQ use shelving filters, meaning that they boost or cut most frequencies above 12K (Hi) or below 80 Hz (Lo) equally (picture a horizontal shelf moving up or down without tilting). In addition, mixers may provide a high-pass or rumble button to eliminate unwanted low frequencies caused by vibrations on a mic stand, foot tapping, HVAC, etc. In this case, depressing the low cut button will roll off frequencies beginning at 75 Hz. Finally, be sure to depress the EQ IN button if you wish your EQ settings to have any effect. Toggling this button is a good way to ascertain the effect of your EQ on the sound vs. the original signal. Equalize tracks separately before mixing down a multitrack recording. One caution, however, is that boosting or cutting frequencies will cause the input balances to change. Extreme boosting of frequencies, particularly in the low band, may cause a channel to distort, since it may add up to 15 dB of energy, more than four times the original strength. | |
To the left is the Mix-B section. Some mixers, such as the Mackie and Tascam line, have a separate bus of inputs for tape or line devices included on the channel inputs. The Mackie board on the left calls these Mix-B. Some boards use direct inserts instead. Both of these types of inputs usually bypass the input sensitivity pots, and so their level is expected to be in a standard -10 to +4 dB range. To balance variations in these levels, many studios such as CECM use level-matching boxes. These inputs effectively double the number of board inputs, since they can be accessed simultaneously with a line or mic signal, so a 32-channel board can actually mix 64 audio inputs. The Pan pot for Mix-B controls the left/right or odd/even balance of the channel's output. A more thorough discussion of panning can be found on the next page. The level knob to the left controls the gain of the Mix B inputs. However, if the source button is depressed, the function of this knob and the regular channel fader are swapped. This knob will then control the normal input gain of the channel, where the channel fader below will control the Mix B gain. If an expected signal is not audible, check the source button. The Mackie also allows the Mix-B signal to go through the Hi/Lo EQ if the split EQ button is depressed. |
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The Pan pot controls the left/right or odd/even balance (see section below) of the channel's output. A stereo instrument coming into two board channels should be panned hard left on one channel and hard right on the other to maintain the maximum stereo effect. On the other hand, when planning the stereo image of a piece (which you should do!), placing mono sounds across the full range of the stereo field should be considered. Having mono sounds panned hard to one speaker or the other tends to make a listener aware of only two locations rather than a 180 degree soundfield and should be reserved for special circumstances The red OL (overload) light indicates that the channels input is potentially distorting because the input level is too high. The solution is to turn down the input sensitivity pot at the top of the channel. The green -20 dB light is a good indicator that signal is getting into the channel. The meter bridge, if set to monitor the channel input, not its output, is a more sophisticated tool for setting input sensitivity levels. Some boards, such as the Mackie make it more difficult to monitor the input on the meter bridge. The mute button will silence the channel when activated, while the solo button will send the channel's signal to a separate "solo" bus, usually controlled by a solo master pot. On most boards, soloing a channel or channels will mute all other unsoloed channels. Solo is designed to isolate sounds for actions such as EQ'ing, checking effect returns, or troubleshooting missing signals. | |
The channel output module controls the level and routing of the signal after it has gone through the input and EQ sections. Selector buttons allow a combination of routing. Buttons labels 1-2, 3-4, etc. route the signal to the board's eight group outputs, where they will most likely be normalled to a multitrack computer audio interface and/or tape deck. The master levels for group outputs are usually found as a separate bank of faders on the right side of the mixing console (see next page). The Stereo or L-R selector routes the signal to the Left/Right or Stereo Mix output, usually controlled by a single master fader (color-coded red on Tascam boards) to the right of the group output faders. The Stereo bus is normally routed to all two-track devices is the studio (which may include DATs, CD recorders, cassettes, etc.). It is also what is normally used for monitoring a stereo mix. Finally, the channel fader (the white vertical sliding tab) controls the level of output of the channel signal being routed into any of the master outputs selected. These are usually calibrated in dB or VU. The U stands for unity gain, the level at which the signal reaches the fader. Above, additional strength is added to the signal. Remember, approximately every 6 dB is a doubling of amplitude. Below should be marked as negative dB, all the way down to -infinity or silence.
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3. Mixing Console Basics | page 5
The aux send master section controls the total amount of signal being sent out to each of six aux devices, combining all the channel aux send amounts mentioned above for each individual bus. It is possible to monitor one or more aux sends by pressing the solo button. Novice users often forget to turn up the aux send masters and consequently no signal is sent to the aux device. This is a good place to mention the gain staging of an effects loop i.e. all the controls where the aux send signal may be boosted or cut. The process begins with the input channel strip aux send pots, which next go through the aux send master pot and into the effect box. The effects box likely has it's own input level pot. These are often very sensitive, so sending the right amount of signal to avoid distortion is imperative. | |
The monitor section controls what signals and levels are sent to the studio speakers, the control room speakers, the talkback circuit (for communicating with artists or thiefs in the studio or conductors on stage), and headphones. Use the source buttons to select the source that you are monitoring. In most cases, this will be set to L/R Mix to monitor what is coming off the main stereo fader. This is one of the first things you should check if you get no sound out of the board. In addition, the master level of the solo bus is set as well as the level of the Mix-B bus. By pressing the "assign to L/R MIX button below the MIX-B LEVEL, you will essentially double the number of input channels asigned to the stereo mix, since all the B-MIX inputs will be added to the regular channel inputs at whatever master level you set in this module. It is important to realize that the level of signal emanating from the control room speakers IS NOT necessarily the level of signal being sent out the board and recorded. That is controlled by the output section (next page). Watch the output meters for levels actually being sent to recording devices...what you hear out the monitors is usually controlled by the output section modified by the CNTRL RM or STUDIO monitor pots (unless your board has a PFL or pre-fader listen option). Since the CECM studio has no separate recording room, the CNTRL RM and STUDIO pots are used to control two different sets of near-field monitor speakers. | |
3. Mixing Console Basics | page 6
The output section has faders for the 8 group outputs (buses) and the Left/Right (L/R) MIx (on the right). Any of the buses can be soloed. The Mackie 8-bus board has the ability to use the 8 group outputs as subgroups for the Left/Right mix. For example, if you were designing a stereo mix but wanted a group fader to control all your synths and another group fader to control all your mics, you would follow this procedure: 1) unassign your mic and synth channels from the L/R Mix |
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