Mixing Console Input Channels

On the right is an example of a typical mixing console input channel.

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. T he line/micbutton selects either the microphone or line input cable. If mic is selected, the phantom power button must be depressed if a condenser mike is being used. Both line and mic inputs have a trim pot which allows each indivi dual channel to be balanced with the others (when all of the channel faders are set to the same value, all channels should sound at equal strength). Since the trim pots control the channel preamps, too high a setting may cause the channel to distort. 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 tape input pot (see below).

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 adju st the exact frequency of the bands as well as their boost or cut. 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 etc.

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 ch annel to distort, since it may add up to 15 dB of energy, more than four times the original strength.

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") of the input channels. Each channel can have its own relative strength of si gnal 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 compos itional idea). Additionally, it is possible to take a returning signal and send it to a second device.

The pre/post-fader button determines whether the signal sent from a channel will be altered by the channel fader. In prefaderposition , the signal is sent out at a strength determined only by the effects send pot and not affected by the channel fader. The prefader position is useful for techniques where you may wish to fade out the original signal, but still hear the altered sound. In postfader position, both the effect send pot and the channel fader affect the overall strength of signal sent to the device. The postfader position is useful if you wish both the original and effected sound to fade out completely when the channel fader is pulled down.

Some mixers, such as the Tascam line, have a separate bus of inputs for tape devices included on the channel inputs. These effectively double the number of input channels, since they can be accessed simultaneously with a line or mic signal. The inputs a re usually routed to the Stereooutput, but there may be selector buttons to route the signals elsewhere. If an expected signal is not audible, check the Flipbutton above. If the channel is "flipped," the tape inputs will be controll ed by the main channel fader, as well as being routed by the main channel output buttons (see below).

The Main Input module controls the line or microphone input if the channel is not "flipped." Selector buttons allow a combination of routing. Buttons labels 1-2, 3-4, etc. route the signal to the board's Group Outputs, where they will most likely be normalledto a multitrack tape recorder. The master level for Group Outputs is usually found as a separate bank of faders on the right side of the mixing console. Tascam usually color-codes these as gray. The Stereo or L-R selector routes the signal to the Stereo output, usually controlled by a single Stereo Master fader (color-coded red on Tascam boards) to the right of the Input Channels. The Stereo bus is normally routed to all two-track devices is the studio (which m ay DATs, stereo reel-to-reel recorders, samplers and computer A-to-D converters for digital editing). The Pan pot controls the left/right or odd/even balance of the channel's output. An stereo instrument coming into two board channels should be p anned 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, placing mono sounds across the full range of the stereo field should be considered. Having mo no 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. The mute button will silence the channel when actived, 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 design to isolate sounds for actions such as EQ'ing, checking effect returns, or trouble shooting missing signals. Finally, each channel has a channel fader that controls to amplitude of signal sent to the selected locations. Some boards may have a channel On/Off button. Some publications recommend turning all unused chan nels off to reduce the level of noise induced by the mixer.

A mixer will usually have a studio monitor section (not pictured here), where an operator can select which signal bus to route to the studio amps and speakers. Common choices are Stereo (L-R), Group Output pairs, Two-Track (a special reser ved input of the board designed to quickly monitor a frequently used tape deck), any of the Effects Send buses. In addition, the monitor section will usually have a pot (often labelled CR to control the volume level of the control room spe akers. This pot does not affect the level of signal sent to the board output devices, just the listening level in the control room. If you are dubbing a long tape, for example, and do not need to monitor it, you may turn the monitor level all the way down without affecting the level of signal sent to the recording device.

If you are having difficulty with the level, quality or routing of signals from the mixer, click here for a troubleshooting checklist.

This document prepared by Prof. Jeffrey Hass, Indiana University School of Music, Center for Electronic and Computer Music, last modified 9/10/01.
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