Multitrack Mixers (Part 1)
Paul White takes you on a guided tour of the features and facilities of a typical multitrack mixer.
Paul White explains that, though multitrack consoles may look complicated, they're based on exactly the same principles as the simple stereo mixers we looked at last month.
The basic concept of mixing is relatively straightforward, and by the end of this short series, you should no longer be saying "What do all those knobs do?" Indeed, even the largest studio consoles are really just combinations and permutations of the basic concepts covered last month. A studio console doesn't just mix signals, though; it also acts as a central routing system, enabling signals or combinations of signals to be not only mixed to the stereo outputs, but also routed to the various inputs of a multitrack tape recorder. At the same time, it has to function as a 'mixer within a mixer' so that a guide or monitor mix can be set up for the benefit of the performers.
For use with an 8-track tape recorder, an 8-buss mixer is ideal, the term 8-buss meaning that the mixer has eight outputs in addition to the main stereo output. You may also see such a mixer described as an 8-Group console. Any of the input channels may be routed to any of the eight Group outputs, and if two or more channels are routed to the same output, they are automatically mixed together under control of the channel faders. A further master fader is provided at the output to control the overall level of any signals routed to that particular Group. The idea is that, when recording, these Group outputs feed the inputs of the multitrack tape machine, giving the user the ability to route any mixer input to any tape track without having to re-plug any cables.
Routing channels to Groups is generally accomplished by means of routing buttons, located close to the fader of the relevant input channel. On most of the desks you'll come across, a single button handles the routing for a pair of Groups, with the Pan control being used to select between them. For example, if you want to route a channel to output Group 4, you press the routing button marked 3,4 and then turn the Pan control fully clockwise. Turning it fully anticlockwise would send the signal to Group 3, while leaving the Pan in its centre position would send equal amounts of signal to both Groups. On an 8-buss mixer, the layout would be: four routing buttons marked 1,2 3,4 5,6 7,8, with a further button marked L,R for routing the channel directly to the stereo mix. Figure 1 shows how the routing buttons work on a typical console and follows the signal routing to the Group fader and on to the Group output socket.
On a traditional 'split' mixing console, the Group faders and the monitor controls are located on the right hand side of the mixer, with the input channels ranged to the left. Between the two is the master section, which accommodates the master stereo faders, the Aux send and Aux return master controls, and all the other bits and pieces that don't belong anywhere else: test oscillators, talkback mics, monitor switching, headphones and suchlike. A typical 8-buss console needs a minimum of eight monitor channels, but most manufacturers provide at least 16, allowing the mixer to be used with a 16-track recorder, though you have to do a little repatching or creative wiring to get the eight Group signals going onto tape tracks 9 to 16.
On a split console, the monitor controls are invariably located in the section above the Group faders, and the facilities on offer vary depending on the sophistication of the mixer in question. The very least that's needed is a Level and Pan control so that the outputs from the multitrack recorder can be used to produce a rough mix for the performers to work to. Think of the monitor section as an independent stereo mixer, the output of which merges with the stereo output of the main mixer, and you won't go far wrong.
The monitoring arrangements on a modern multitrack tape machine are very well thought out, so it's possible to always provide your monitor mix entirely from the tape machine's outputs, but for the sake of older, less sophisticated machines, there's usually a switch which allows each monitor channel to be fed from either the multitrack output or the Group output (which is the same thing as the multitrack input). Figure 2 shows a multitrack mixer split into its two logical parts, with the monitor mixer shown separately. (In reality, of course, the monitor mixer is housed in the same box as the rest of the mixer.) This is how the mixer behaves during recording — though as well see later, things get switched around a little when it comes to mixing down the multitrack tape to stereo.
In recent years, so-called in-line monitoring has become more popular, for reasons that will become evident shortly. Instead of being limited to eight or 16 monitor channels, an in-line console has one monitor channel for every input channel. And to save space, the monitor controls are not located over on the right, as in a split design, but actually share the channel strips with the input channel controls. In practice, this results in a mixer that is deeper front to back, with the monitor controls taking up a section of channel strip four or five inches long. On the other hand, it means the mixer can be less wide for the same number of channels.
Most monitor channels have at least one Aux send control (enabling the engineer to add a little reverb or whatever to the performer's headphone mix), though on in-line desks, there's usually no room for EQ, not even the simple bass and treble type you get on some split consoles. To make things more flexible — and admittedly a little more complicated — there is often a switching arrangement that allows the monitor channel to 'borrow' additional Aux sends and EQ from the input channel. A common arrangement is for a couple of the Aux sends to be switchable between either the input or monitor channel.
Similarly, it's often possible for the monitor channel to borrow part of the EQ section; if the input channel has a four-band EQ, it may be possible to switch the high and low controls into the monitor path, leaving the input channel with the two sweep mids. But, if the monitor mix is simply a rough guide for the performers, why does it need additional Aux sends and EQ?
When it comes to mixing down the multitrack tape, it's a case of all-change. Switching the console's input channels to 'Remix' feeds the tape machine outputs directly into the input channels and connects the apparently redundant monitor channel inputs to the console's line inputs. This allows additional line signals to be added to the stereo mix using the monitor channel controls, which is immensely useful if you have a rack of MIDI instruments sync'ed to tape which you want to add into the mix 'live'. Furthermore, if you have a lot of effects with stereo outputs, these spare monitor channels make excellent extra effects returns.
You might also think that the Group routing facilities would be redundant when you come to mix, but they can still be put to good use, courtesy of a button which routes the Group output back into the main stereo mix. This usefulness is best illustrated by example. Imagine you have a drum kit recorded over six tracks of your multitrack tape — not an uncommon occurrence. To change the overall level of the drum kit in the mix, you have to change the level faders on all six channels, which isn't particularly convenient. A neater way of working is to form what is known as a Subgroup of the drums by routing those channels, not to the L,R stereo mix, but instead, to a pair of Groups, for example, 1,2. The whole stereo drum mix can now be controlled by the two Group faders, 1 for the left signal and 2 for the right. Selecting the 'Groups to Stereo' option normally routes all odd-numbered Group faders to the left of the mix and all even numbered ones to the right, though some of the more sophisticated consoles actually provide Group Pan controls.
Creating Subgroups in this way greatly simplifies mixing, because logical groups of instruments can be allocated their own subgroups, thus reducing the number of faders which need to be moved during the mix. Common examples are Subgroups of drums, backing vocals and keyboards. If effects are to be added to these Subgroups using the Aux sends on the input channels, ensure that the effects returns are routed to the same Subgroup. Figure 3 shows a block diagram of a mixer at mixdown, illustrating how the monitor channels are pressed into use as extra line inputs and how Subgroups are created.
Most mixers have Mute buttons on their input channels, and these do just as you'd imagine — they turn the channel off. Hitting Mute also silences any post-fade Aux (effects send) sends, though the pre-fade (foldback) sends are generally unaffected. MIDI-controlled muting systems are becoming increasingly popular, as they allow the mixer channels to be turned on and off automatically under the control of an external MIDI sequencer sync'ed to a time code track on the multitrack recorder. This has an obvious advantage in that it offers a convenient means to mute tape tracks during pauses or breaks in the recorded material, so that the bare minimum of unnecessary noise is added into the final mix.
A prime example of this might be the lead guitar solo, which can be unmuted a fraction of a second before the first note is played and muted again after the last note fades away. This eliminates the buzz, hum and finger noise that invariably precedes rock guitar solos.
The PFL button is usually found close to the Mute button on most consoles, though on some mixers, there may be a Solo button instead. These two functions are similar, but not identical. PFL is short for Pre Fade Listen, a system that allows you to select any channel to be heard in isolation over the studio monitors; it is heard at a fixed level regardless of whether the channel fader is up, half up or completely off. That's how the term Pre fade listen comes about — you're listening to the signal before it gets to the fader. When the PFL button is pressed, all the channels on which the PFL has not been pressed are excluded from the monitor mix and, at the same time, the signal level of the channel you are checking is displayed on one of the console's meters. By adjusting the input gain control (sometimes called Gain Trim) until the signal is just peaking at 0VU, the input signal is optimised for minimum noise — a good habit to get into.
Solo also isolates the channel in the monitor mix but this time, the fader setting does affect what you hear. Most solo systems also retain the Pan position of the signal being checked so that, in effect, when you press a solo button, all that really happens is that all unsolo'd channels disappear from the monitor mix. Because the stereo placement and level of the original signal is retained, the term 'Solo In Place' or SIP is also commonly used. The ability to isolate sounds in the monitoring system (and usually the headphones) at the touch of a button is very valuable in checking individual sounds for excessive noise, distortion and so on. On a studio console, the main stereo output, (which normally feeds directly to the master stereo recorder), is not affected when PFL or SIP are used.
Most consoles have at least a couple of dedicated Auxiliary returns, and it has been known for these to confuse users, who often assume that Aux return 1 must be used with Aux send 1 and so on. This may well be appropriate, but it certainly isn't compulsory. An Aux return is simply an additional line input channel but with fewer facilities than the main input channels. On smaller desks they will be permanently routed to the stereo mix buss while larger desks will provide the same routing arrangement as on the main input channels. There's nothing to stop you from feeding the outputs from your effects units into spare input channels, and equally, there's nothing to stop you from using your spare Aux returns as additional line inputs for sequenced MIDI sound sources if you so wish. The numbering is purely a matter of convenience to help you keep track of things.
Next month, we'll take a closer look at the facilities offered by a typical multitrack mixer.
Feature by Paul White
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