Jason Allen looks at mixing and processing devices, from rack-mounted black boxes to mixing desks with all the trimmings.

When you’ve added up all of the microphone and line inputs your installation requires, plus allowed a channel or four for expansion, then calculated all of the separate mixes that need to go out to loudspeakers, foldback, relays and recording devices, you’ll have a good idea of what kind of architecture your processor or mixer will need.

The catch-all term ‘architecture’ covers the capabilities of a mixing or processing device, from the amount of physical inputs it can connect to, the amount it can process, how many mixes it can create from them and of what type, and how many physical outputs they can be sent to.

If that looks like a lot of variables, it’s because it is, and subsequently, there are a vast range of products on the market built to deal with almost every variation of input and output structure. There are four main product groups in this category, all with their own strengths and weaknesses. These are:
•    Fixed architecture, rack-mounted mixers and processors, designed for mostly automatic operation.
•    Open architecture, rack-mounted mixers and processors, designed for mostly automatic operation.
•    Fixed architecture analogue mixing desks, designed for manual operation.
•    Fixed architecture digital mixing desks, designed for a combination of manual and automatic operation.

Dancing about Architecture
The terms ‘Open’ and ‘Fixed’ when used in reference to a device’s architecture define whether the unit is built with a ‘Fixed’ signal path or an ‘Open’, customisable one. The classic example of a ‘Fixed’ architecture is an off-the-shelf, simple analogue mixing desk. Signal path goes from a mic preamp, to a channel EQ, is available for some pre-fader auxiliary sends, then to a fader, then on to group or main mixing busses. That’s always its path, and it can’t be changed. The same is true in digital mixing desks, with a few exceptions to the rule for digital plug-ins.

‘Open’ architecture exists purely in the digital realm, and almost exclusively in rack-mounted programmable DSP units with few, if any, hardware controls as standard. These devices will come with built-in or modular I/O. All processing has to be programmed from a software interface. The order and type of processes applied to incoming signals are then completely up to the system designer. Open architecture DSP units are perfect for spaces such as ballrooms and convention centres, where the nature of the room itself can change – a ballroom being divided into four separate meeting spaces, for example.

Used in combination with external touch screen or other wall-mounted hardware controls, Open architecture DSP units are extremely powerful tools. But they usually require manufacturer or distributor training in order to use effectively. If you are looking to expand your installation abilities, most of the major DSP manufacturers and their distributors such as BiAmp, Symetrix, BSS, ClearOne, and Peavey run regular or custom training sessions for new users. 

Pre and Post
All input sources need to be processed and then summed to a bus for output. A bus is any combination of input sources mixed together and sent to an output, but this can happen at various points in the signal path. On a fixed architecture device such as an analogue mixer, a pre-fade bus such as an auxillary bus is available after the inputs go through the EQ, but before they get to the volume fader. This creates a bus where the channel’s volume is independent of where the fader level is set. Pre-fade busses are essential for setting up foldback mixes to wedges or in-ear-monitors, as the level that the performer wants in their foldback will always be different to what the mix requires to the audience.

Post-fade busses are any busses that come after the fader level in a traditional analogue mixing desk structure. In addition to the traditional main outputs of the mix groups, stereo and mono busses, post-fade busses are also used as send to any effects units. Matrix busses are also post fade, as they’re busses that you can only send mixes of other busses to!

While they’re not common in analogue mixing desks, matrix busses are found in most digital mixing desks and rack-mounted processors, both fixed and open architecture. Their main purpose is to send different levels of group busses to different parts of a distributed PA system. A typical case is a theatre with a left-centre-right configuration PA, front fill and delay speakers. Mixing a musical on this system would see you send radio microphones mainly to the front fill, centre cluster and delays, a band or orchestra to the L/R and delays, and sound effects or other material everywhere. The easiest way to achieve balance in this style of mixing is to send those sources to groups first, then mix them from the groups to matrix busses.

In the Box
So after you’ve established how many independent sources and how many independent mixes of them you need, how do you decide which of the four main categories of mixer or processor are best for your installation? It all comes done to who is using the system, how and what for. Let’s start with fixed architecture rack mounted processors and the applications they’re usually found in.

Fixed architecture rack-mounted processors usually have a small-ish number of inputs and around the same number of outputs. Eight inputs and eight outputs is a good median number, but you find lots of variations; six in to four out, 12 in to eight out, and so on. The advantage of these units is that they are cheap and simple. Most feature relatively straightforward volume pots on the front panel, which can be easily labelled and operated by anyone.

Setup and mixing is now commonly via software, but it’s usually really simple. These units are great for connecting a couple of static mics, a radio mic or two and some playback, putting them in a useable state and leaving as-is. The customer has just enough control to change a couple of things, and there’s just enough processing to handle a simple job. They’re perfect for when all operation is going to be by inexperienced users, but on a small enough scale so that each user knows how they can manually adjust a couple of levels if need be.

Out of the Box
Open architecture DSP units are perfect when the system is going to be used by a lot of people with little to no knowledge, and/or for a lot of different things. Going back to our hotel ballroom example, a system in that environment needs to be operated by catering staff, venue management and occasionally by in-house or hired-in AV techs. The PA could be used for conferences, multiple meetings, musical performance or simple BGM. As a basis, an open architecture system in ballroom/conference environment will be controlled from one or more touch screens and other push-button hardware mounted throughout, set-up to load different DSP states on activation.

A system like that needs a lot of programming and tuning to function automatically in each state. Open architecture DSP units usually have no hardware control whatsoever on the front panel, and all system programming and design is done off-site on software, which is then used to load the file into the unit and tune and test while on-line.

Open architecture DSP offers the advantage of complete flexibility and often input and output modularity. Most major DSP units have a card-based input/output system in which the central unit is fitted with whatever combination of I/O is need – 24 in, 12 out analogue, plus digital, for example. These units can also access Ethernet transmission protocols such as Dante and AVB, as well as older systems like CobraNet. This means that I/O can be in many locations, all linked via Cat5 over a network. Systems like this offer serious savings compared to the expense and complexity of trying to achieve the same with analogue cabling. 

On the Desk
If your venue is going to be used by customers with any kind of familiarity with mixing desks, a simple analogue desk could be the best option. They’re cheap, easily available and well understood by most musicians or AV staff. Even unfamiliar users can be trained if the desk and requirements are small enough. A small mixer with up to 16 input channels, three or four auxillary busses, up to four groups and a stereo bus shouldn’t pose a challenge in most school, church and community settings. There’s usually a line output built in to go to a recorder to boot.

If set-up correctly, most users will only ever have to move some labelled faders up and down to get what they want. This is more likely to work if there’s a digital speaker processor in-line after the desk to handle the broad system tuning and have a limiter across the speakers to prevent damage. If the PA doesn’t consist of much more than a left/right system, most amplification is voice, and it’s usually used to do exactly the same thing, this is the way to go.

Digital vs Analogue
Digital mixers are now available in almost every size and shape. Once only in large formats for major gig production, all major brands now have affordable, small channel and bus count units available for every conceivable job. Considering the extra processing, space and cost saving, it’s worth seriously considering a digital solution for any install. A digital mixer offers the best of both worlds between manual operation and automatic recall of set states.

Some customers may baulk at the perceived ‘complexity’ of a digital mixer, but most of the new, small format digital mixers from manufacturers such as Soundcraft, Yamaha, Allen & Heath and Behringer are a lot like analogue desks in their layout and operation. If someone has a conceptual or technical problem in operating these newer models of digital desk, they’re likely to have the same problem with an analogue unit.

An optimal use-case for digital mixers is if the unit is going to be used by a handful of trained staff for a variety of tasks, with the occasional automated use of simple tasks by non-trained staff. This means, for example, the trained staff can use all of the processing, effects and advanced mix capabilities to mix a musical, and then the system can be simply reset from either the unit or an integrated controller mounted into a wall that puts the system back to ‘lecture’ mode. Going digital also gives you access to a lot of useful add-ons, such as simple digital recording, Ethernet-based signal transmission to and from stage and control of the whole system via WiFi. There’s also the security functions – users can be locked out of specific functions according to their name and password set-up, much like a computer.

Summing it Up
The best choice for the heart of your system will be determined by a mixture of what your customer needs, what they’re comfortable with and what level of control they want. Broadly speaking, the more people who are going to operate a system, the less control should be available to them. On a sliding scale, at one end we have a performing arts venue with small team of trained staff who are always present when the system is in use, and at the other we have a multifunction centre used by wildly different people every day where there is rarely, if ever, a skilled technician available to supervise.

There’s a solution for both ends of the market and everything in between. If you’re unsure of the best way to go, talk to your suppliers, as they have the experience and lots of previous jobs they can direct you to for inspiration.