In Part I we covered how the signal gets into a mixing board and is manipulated by the input channel controls. Part II will cover signal path basics and a little mixing theory. If you look at the following figure, your first response is probably to run away screaming. But to an audio engineer, this is simply a road map of how the signal flows in the board. Taken one path at a time, it’s not really that complicated. In the same way that the top of a big mixing console may be intimidating because of its hundreds of knobs and switches, much of a signal path is duplication of the same simple functions.
The left half of the diagram was covered in Part I and included the pre-amp, equalization, and auxiliary sends. If a mixing board has 24 inputs, then there will be 24 of the input paths. 48 inputs will mean 48 paths, and so on. Now we’ll trace the signal on it’s way to the loudspeaker. If you follow the red traces from the left to the right, you’ll find the most direct path for the sound on it’s way to the speakers or recording console. The blue traces show alternate paths that allow you to do external effects. For instance, if you want to hook up a digital reverb, you would hook up the output signal from the auxiliary 1 send to the input of the reverb, process the signal inside the reverb, and then return it via the auxiliary return path.
Notice that if you send it to an outside device, it must be returned back to the signal path or the processing won’t be heard. This is called parallel processing since there’s both an original signal path, and a parallel processed path that follows along side it. By changing the level of the auxiliary returns, you can select how loud the overall effect will be, while changing the auxiliary send on each input channel strip will affect how loud the processing is on a particular instrument. Reverbs and delay devices fall into this category. This allows you to add just a little reverb to a voice on one channel, while adding more reverb on a guitar channel, while using a common reverb device.
The other processing category is serial devices. This includes compressors, limiters, and equalizers, as well as enhancers and distortion devices. In order for them to function properly, you must break the signal path and insert the processor in series with it (the black signal path on the diagram).This is usually done with an “insert” cable, basically a “Y” cable with a stereo plug on the common end, and two mono phone plugs. The tip of the stereo plug is wired to the top of one of the mono phone plugs, while the ring of the stereo plug is wired to the tip of the other mono plug. The stereo plug is inserted into the “insert” jack on the mixing board, with one phone plug sent to the input of an effect device, and the other phone plug returning the signal from it. Thus, 100% of the signal is passed through the effect loop. If you shut off the effect device, you can loose your signal entirely, so most compressors and equalizers have a “bypass” switch that passes the signal through unaffected, rather than really shutting it off.
You can patch various effects into different parts of the signal paths, depending on what you want to happen. For instance, equalizers are usually patched into the output bus inserts, so that overall room acoustics can be dealt with. Compressors can be used on individual input channels, allowing you to compress just the bass guitar, or a single vocal. Signal enhancers such as the BBE can be used in either place, in the output section to process the whole mix, or maybe just a vocal or two to make it stand out in the mix.
Subgroup buses allow you to group signals in a logical manner. To send a signal through a subgroup, the L-R bus is un-selected, and one or more subgroups are selected on the input channel. For instance, all backup vocals could be routed to bus 1 (see the green path) and then back to the output faders. Then you could patch in a compressor that will only affect backup vocals. If you want to bring up all the harmonies in the mix, you only have to bring up one subgroup slider instead of 4 or 5 different input channels. All keyboard can be sent to a common subgroup, and the bass guitar and lead guitar can each get their own subgroup bus. This is especially handy if a guitarist has several instruments that may change during a set. You now can hook up each guitar to a separate input channel for individual equalization, and then to a common sub-group bus for overall level control.
The left and right outputs are the final level control for the signal before it goes to the speakers or recorder. This is the master volume control that allows you to change the level of everything at once. Small mixing consoles usually place these controls on the right side of the board, but once you get above 24 channels, the output section is generally placed near the center of the board to allow the engineer to sit near the center of the speakers, and to get to more controls without moving his or her chair.
All mixing consoles have these basic functions, and many can be even more complicated. Up to this point we’ve assumed that our console will be used for live performance. Live mixing is its own challenge because of the immediacy of the situation. What if you could practice mixing the same song over and over until it was perfect? That’s essentially what happens in a taped mix-down situation. But what really gets hectic is a complicated mix where you want the horns to fade up on the intro, and then stop right before the bass guitar. You’ve got to mute the background vocals right before the sneeze, and then get it back before the doo-whops.
Many mixes like this require 2 or more people, using both hands and sometimes a foot or two. And if someone makes a mistake or forgets a cue while mixing down to stereo, its rewind time. Then there’s the fun of finishing the mix of a lifetime, and getting a call from the producer the following week asking if you can recut it and bring down the guitars “just a wee little bit”? Getting all those faders, pots, and switches back in the same starting position is tricky enough, let alone trying to remember if you mute the guitar during the first or second bridge.
There must be some way to make this easier using computer based technology… And there is. Join me for Part III, when we’ll explore the fun of mixing on an digital console with moving faders and snapshot memory, the Midas M32R.
Copyright Mike Sokol 2016
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