PART 4: Paul White looks at the multitrack recording console and its role in your multitrack recording system. This is the fourth article in a five‑part series.
If you're working with a cassette multitracker, the chances are that it has a relatively simple mixer section, and because everything is in one box, you don't have any wiring to worry about. All that changes when you move up to a larger multitrack system based around a separate mixer and tape machine. Separate multitrack mixers are much more flexible and powerful than the ones you find in most cassette multitrackers, but by the same token, they are potentially more confusing and you also have to deal with the wiring. A studio console doesn't just mix signals — it also acts as a central routing system, sending signals to the different tape machine tracks, adding effects from external processors, and mixing the outputs from the tape machine to produce a final, stereo mix. At the same time, it has to function as a 'mixer within a mixer' so that a separate control room monitor mix can be set up while the performers are recording or overdubbing.
Channel Flannel
If you're new to mixers, you might find the terminology a little confusing, and one mistake that even experienced users make is to refer to a mixer as having so many 'tracks'. In fact mixers don't have tracks, they have channels and groups — it's tape recorders that have tracks.
A mixer channel is simply another name for an input and its associated controls; a typical mixer channel will include features such as gain control, EQ, an insert point, Aux sends and a fader, though on a multitrack mixer there are two different kinds of channel. The main input channel generally has the most comprehensive facilities and is used to feed microphones and line‑level sources such as keyboards and samplers into the mixer while recording. When you come to mix, this channel is switched to handle the output from the multitrack tape machine. The other type of channel is the monitor channel, so‑called because it is used to set up a guide monitor mix based on the multitrack outputs while the performers are recording their original parts or overdubbing new parts. Without this facility to monitor the multitrack outputs, there'd be no way for the engineer to hear what was going on, and no way for the performers to hear what was already on tape.
Monitor channels tend to have fewer facilities than the main input channels because their main job is to provide a rough mix during the session. They may have little or no EQ and fewer Aux sends than the main channel, though on an in‑line mixer (so‑called because the main input and monitor controls are located in the same channel strip), there's often provision to switch all or part of the EQ between the main and monitor signal paths, and the same is often true of the Aux sends. More on this later.
Because there is no need for a monitor mix when the recording is completed, most consoles allow the monitor channels to be used as extra line inputs at the mixing stage. These may be used as additional effects returns or to add sequenced MIDI instruments into the mix.
Group Therapy
The next piece of terminology to explore is the group. Whereas a stereo mixer just has a left and a right output, a multitrack mixer has several additional outputs, each with its own fader, the idea being that a different combination of signals can be routed to each of several tape tracks. These separate outputs are known as groups, and for use with an 8‑track tape recorder, an 8‑group mixer is ideal. The term 8‑group simply means that the mixer has eight mix outputs in addition to the main stereo output. (You might also see such a mixer described as an 8‑buss console, because the eight groups are fed from eight mix busses.) Any of the input channels may be routed to any of the eight group outputs (or to the stereo output), and if two or more channels are routed to the same output, they are automatically mixed together; the channel faders set the relative levels of the various elements in the mix and the group fader controls the overall level. During recording, these group outputs feed directly into the inputs of the multitrack tape machine, enabling any mixer input to be routed to any tape track without the need to re‑plug any cables.
Routing Around
On a cassette multitracker, routing the desired input signals to tape is relatively straightforward, but using a multitrack mixer the procedure may be slightly unfamiliar. Next to the channel faders, you'll find a set of routing buttons which are used to send the channel signals either to the various group outputs (which are connected to the multitrack inputs) or to the stereo mix which, at mixdown, is used to feed the stereo mastering recorder. Invariably, a single button handles the routing for a pair of groups, with the Pan control being used to change the balance of what is sent to the odd‑ and even‑numbered groups. If you want to route a channel only to output group 2, for example, you'd press the routing button marked 1,2 and turn the Pan control fully clockwise so that all the signal went to group 2 and none to group 1. Leaving the Pan control in its centre position would send equal amounts of signal to groups 1 and 2. To record something in stereo (for example, the different drum mics over a drum kit), the relevant channels would be routed to a pair of groups and the Pan control used to position the various sounds between them. The outputs from these two groups would then be recorded to two tracks of tape. When mixing, these two tape tracks would be panned hard left and right to maintain the stereo image you created while recording.
On an 8‑group mixer (the most popular format for project studio use), the routing buttons would be marked 1,2 3,4 5,6 7,8, with a further L,R button for routing the channel directly to the stereo mix. Figure 1 shows the signal flow through the routing buttons of a typical console, including the signal path to the group fader and group output socket. Note that you don't have to use an 8‑group mixer to work with an 8‑track tape machine — you could use a 4‑group mixer, as long as you don't need to record more than four groups at a time. You can still record more than four tape tracks at a time by using channel direct outputs, insert sends, or even spare aux sends, to feed additional tape tracks. By connecting group output 1 to tape inputs 1 and 5, group output 2 to tape inputs 2 and 6 and so on, you can still record on all eight tracks without having to re‑plug any cables. The tape machine record status buttons determine which of the two possible tracks each group will record onto. Figure 2 shows this arrangement, and in practice, many 16 and 24‑track tape machines are used this way.
Split And In‑Line Monitoring
On a conventional 'split' mixing console, the group faders and the monitor channel controls are situated on the right‑hand side of the mixer. Between the main input channels and the monitor section is the master section, which includes features such as the master stereo faders, and the Aux send and Aux return master controls. It's here that you'll also find things like the test oscillator, the talkback mic, Mix/
2‑Track switching, headphone level control, and so on. An 8‑group console needs a minimum of eight monitor channels to provide an off‑tape monitor mix, but many models have 16, allowing the mixer to be used with a 16‑track recorder using the split wiring arrangement illustrated in Figure 2. The basic requirements for a monitor channel are Level and Pan controls so that a stereo cue mix can be provided from the outputs of the multitrack recorder. In practice, though, most monitor channels have some form of basic EQ and also Aux controls that feed the same Aux busses as the main mixer input channels. Figure 3 (overleaf) shows a multitrack mixer, with the monitor section shown separately. In reality, the monitor section is housed in the same box as the rest of the mixer, but it can help to visualise it as a separate mixer within a mixer. The diagram shows the signal flow during recording.
Why is it that the monitor section always monitors the tape machine outputs? What happens if you want to hear what you're playing at the same time as you're recording? Fortunately, modern multitrack tape machines are far simpler than they used to be when there was a time delay between the record and playback head, and when there were separate switches on the tape machine to switch each tape track between input, sync and playback monitor mode.
These days, the tape machine takes care of monitor switching without you even having to think about it, and the relevant signal is always present at the tape output socket. If a track is being recorded, you hear the tape input, but if the track is in playback mode, you hear what's on tape. On older, less sophisticated tape 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).
Your first encounter with a multitrack mixer may be daunting, but you can take comfort from the fact that one channel is exactly the same as the next, and once you've got your head around the basic routing system, you're almost home and dry.
Currently, nearly all budget recording consoles follow the in‑line format, which differs from the split concept in that the monitor controls reside in the same channel strip as the main input channel controls. This may seem confusing at first, but the advantage is that instead of being limited to just eight or 16 monitor channels, there's one monitor channel for every input channel. This layout usually results in a mixer that is deeper front to back than an equivalent split design, but it also means the mixer can be made less wide for the same number of channels. This can be an important consideration in the project studio where space is limited.
Most monitor channels have at least one Aux send control, but there may be little or no room for EQ controls. Depending on the model and make of in‑line mixer, you may get just a simple bass and treble EQ, or no EQ at all. To mitigate the evident restriction of having no EQ, there is often a switching arrangement that allows the monitor channel to share part of the main input channel's EQ. For example, if the main EQ is a 4‑band affair with two sweep mids, the monitor channel may be able to commandeer the Hi and Lo EQ, leaving the main Input channel with two sweeps. In such cases, the sweep range is usually wide enough to allow the two mids to cover the entire audio spectrum. Another common arrangement is for a couple of the Aux sends to be switchable between either the input or monitor channel.
Mixing Down
If you're using a split console, when it comes to the final stereo mix, you have to route all your tape tracks to the main input channels and then set up the mix from scratch. With an in‑line console, however, as each track is recorded, you can operate a flip switch at the top of the channel strip to route the tape signal through the main input channel path, leaving the monitor input free for later use. The benefit of working this way is that you can be working on your mix and fine‑tuning the balance and EQ as you go along. By the time you've finished recording, you should have the basis of a good mix set up.
Having lots of free monitor channels (now connected to the channel line inputs, courtesy of the Flip switch) more line signals can be added to the stereo mix using the monitor channel Level and Pan controls. In these days of large MIDI systems, many of which are used sync'ed to tape, these additional inputs are a necessity. Furthermore, these spare monitor channels may be used to handle extra effects returns if you run out of conventional effects return inputs. It's worth noting at this point that an effects return is just another kind of input channel, and you could equally well use it to feed synths into your mix. The names are largely for convenience.
Subgrouping
Now that you've got used to the idea of the monitor channels changing roles when you come to mix, it's time to look at the group routing system, which also has another part to play. During recording, the groups are used to route signals to tape, but when you mix, they can be routed back into the stereo mix. Why would you want to do this? Imagine you have a backing vocals recorded over four or five tracks of your multitrack tape. To change the overall level of the backing vocals, you have to change the level by moving several faders at once, which is both cumbersome and inaccurate. A more effective approach is to create a subgroup of the backing vocals, by routing those channels not to the L,R stereo mix but, instead, to a pair of groups. In this way, the whole stereo backing vocal mix can be controlled by just two group faders. Some consoles have the groups permanently routed to the stereo mix, while others provide 'Groups to Stereo' buttons for each group fader, and these generally route all odd‑numbered group faders to the left and all even‑numbered ones to the right. A better system, which is usually missing from budget consoles for cost reasons, is to provide group Pan controls. If you have group Pan controls, you can create mono subgroups and still pan them anywhere in the stereo mix. If you don't have group Pan controls, you always have to use up two group faders for every subgrouping operation, apart from those where the end result will be panned either hard left or hard right.
In a typical mix, you might create subgroups from elements like drums, backing vocals and keyboards, which reduces the number of faders which need to be moved during the mix. Note that any effects that are to be added to these subgroups using the Aux sends should be returned to the same subgroup, otherwise the effect level won't change when the group fader is moved. Figure 4 (on previous page) shows the signal flow at mixdown. In this example, the monitor channels are being used as extra line inputs and the signal flow shows how subgroups actually work.
Summary
Your first encounter with a multitrack mixer may be daunting, but you can take comfort from the fact that one channel is exactly the same as the next, and once you've got your head around the basic routing system, you're almost home and dry. Take the time to study the block diagram of your particular mixer — there's always one in the back of the manual — and try to get a feel for what can be routed where. Most mixers work in the same way, but there are small differences between models that it pays to be aware of.
Finally, when choosing a mixer as part of your system, don't look at your system as it is now, but as it might be in a year or two's time. If you don't, you're bound to run out of channels as your MIDI system expands or as you add more tape tracks. If you have a very large MIDI setup, consider whether buying a separate rack submixer would make more sense than buying one big multitrack console, and ask whether expander modules are available for the mixers on your short list. If you've moved into digital multitrack you have the option of using multiple machines to provide more tracks, but will your choice of mixer allow for this? You may even be considering mix automation, in which case, is there a system that can be added to your mixer at a later date? You can never predict every future requirement, and even if you could, you probably couldn't afford to buy a mixer that would cover every eventuality, but a little forethought will help you choose something that you aren't likely to have to sell at a loss in six months' time when you've outgrown it. Next month, I'll be looking at how the mixer and multitrack are connected.
Key Terms
MUTE: the vast majority of serious studio mixers have Mute buttons on their input channels. These turn off both the channel signal and any post‑fade Aux (effects send) sends. Pre‑fade (foldback) sends are not normally affected.
PFL AND SOLO: PFL is short for Pre Fade Listen, a system that allows any selected channel or Aux send/return to be heard in isolation over the studio monitors. Because PFL is pre‑fade (monitored prior to the channel fader), the level is independent of the channel fader position. When a channel's PFL button is pressed, all the other 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. PFL is generally used in this way to set up the individual channel input gain trims. Solo also isolates the channel in the monitor mix, but this time the signal is post fader, which means that what you hear is the actual level of the signal in the mix. Most solo systems also retain the Pan position of the signal being checked, which is why the term 'Solo In Place', or SIP, is also commonly used. On a studio console, the main stereo output feeding the master stereo recorder, is not interrupted when PFL, Solo or SIP are used.
AUX SENDS: mixers invariably incorporate both pre‑fade and post‑fade Aux sends. Aux sends provide a means to set up an independent mix of the channel signals, either for feeding effects or for providing a foldback mix. Pre‑fade sends aren't affected by changes in the channel fader position, which makes them ideal for setting up foldback mixes. Post‑fade sends are derived after the channel fader, so if the channel fader is adjusted, the Aux send level changes accordingly. This is exactly what is needed when adding effects such as echo or reverb, because we normally want the relative levels of the dry signal and the effect to remain constant.
INSERT POINT: an Insert point is simply a point at which the signal flow may be interrupted, allowing an external signal processor (a compressor or gate, for example) to be connected. Most consoles use stereo jacks as insert points which means that a Y lead (stereo jack at one end, two mono jacks at the other) is needed to connect the external device. Alternatively, the insert points may be wired to a normalised patchbay. Insert points are usually provided in the Input channels, the Groups, and at the main L/R stereo outputs.
AUX RETURNS: An Aux return is 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. Though these are included for use with effects, they can be used to add any line‑level signal (such as a tape machine, CD player or MIDI instrument) to the mix.
Performers' Cue Mix
So, what exactly does the performer hear when overdubbing? In a small project studio, where only one or two people are recording at a time, it's often sufficient to use the control room mix (usually based on the monitor mix during recording) to feed the performers' headphones. At its simplest, this may mean plugging a pair of cans into the console's phone output, but a separate headphone amp with multiple outputs is a better choice.
For larger sessions, where different members of the band want to hear different mixes, you have to use the monitor section pre‑fade (foldback) Aux sends, and on a console that has four pre‑fade sends, you can set up four different mono foldback mixes. It may also be useful to use some of the Input channel pre‑fade sends as part of the foldback mix.
Of course you'll need a headphone amplification system that can handle four different mixes to do this, but in practice, the smaller studio can usually get away with just two different mixes — one 'rhythm heavy' and the other 'vocals heavy'. Professional consoles may have stereo pre‑fade sends, but most budget consoles don't, and it's unlikely that a budget in‑line console will give you four different pre‑fade monitor Aux sends, for that matter! While it's easiest to use the monitor channel pre‑fade sends to set up a monitor mix, some engineers also use the Aux sends on the main input channels, which allows more control when adding the sounds of the individual performers into the headphone mix. It's also customary to add reverb to the singer's headphone mix just to help encourage a good performance. This need not be recorded to tape.
Most low‑cost commercial headphone amps allow for two different input mixes to be handled, and each phone output can be be set to 'listen' to either of the two available mixes. There'll be more about foldback monitoring and headphone amp systems in future issues of SOS.
Real World Mixing
Though both in‑line and split consoles have separate Monitor channels to provide an off‑tape control room mix while recording, it seems that a great many people don't use them for that purpose at all. Instead they operate their mixers in so‑called 'all‑input' mode.
To illustrate by example, let's assume you have an 8‑track tape machine. Providing you have enough Input channels, you can leave the outputs of your tape machine permanently connected to the first eight channels of the mixer, routing these directly to the stereo L/R mix, and use the remaining Input channels to handle the signals being recorded. This does away with the need to switch the off‑tape signals between the Monitor and Input channels, and also means that you can build up your mix, complete with added effects, as you record. In effect, your control room mix also becomes your final mix.
The now‑redundant monitor channels may be used as inputs for MIDI instruments being sequenced in sync with the multitrack recorder or as effects returns. The only limitation of working this way, other than having to ensure you have a mixer with enough input channels, is that the monitor channels can't be routed via the groups to create subgroups — they always feed directly into the stereo L/R mix. Figure 5 illustrates this way of working.