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Anatomy Of A Studio (Part 1)

The Mixer

A brand new series that delves into the intricacies of studio equipment. Part 1: The Mixer.

A new series that should help you come to terms with each of the devices commonly found in a studio. This month we focus on the 'nerve centre' of any studio setup — the mixing desk by outlining the function of every control to he found on a typical recording mixer.

This article is intended as an introduction to studio equipment connection and the roles played by the various pieces of equipment within a studio set-up. We shall discuss the interconnection of tape recorders, the mixer and signal processors, paying particular attention to the location of equipment within the signal path through the mixing desk and the type of cables used to wire it all together.

The block diagrams may be used as a universal guide, regardless of the number of 'tracks' of your own recording setup, be they 2, 4, 8 or 16 tracks.

To illustrate the various simple connections of different pieces of equipment to the mixer would be doing things the easy way round. In order to fully understand the reasons for linking equipment to specific points on a mixer, it will be necessary to explain in detail each of the main sections which make up a typical mixer.

The mixing desk plays a very important role within the studio and is comparable to the human nervous system, in that it deals with a host of devices around and connected to it. As the focal point of the studio, its job is to mix and route signals from the mic and line inputs to all the various tape recorders, signal processors, amplifiers etc. The following text deals with just one channel and one group of a mixer.

Input Functions

1 Input select, Pad switch and Gain control are all part of the mixer input stage. There are normally two inputs on each channel, Microphone and Line, the difference between these being the gain required to bring the signal up to the mixers own internal operating level, microphones would normally require more gain than line level signals. Besides the signal level, input impedance also differs, low for mics (typically a few hundred ohms), high for line levels (typically a few hundred Kilohms). The type of connector used would probably be jack or XLR for the mic input phono or jack (occasionally XLR) for the line input.

2 Equalisation, (tone controls). On basic mixers you may only find treble and bass controls which provide cut and lift of the input signal at frequencies predetermined by the manufacturer. EQ can become quite extensive on more costly mixers and would probably include middle frequency controls, with the more advanced mixers having separate sweepable frequency controls, sometimes referred to as 'parametric' equalisation.

This type of EQ control allows more careful adjustment of the sound, typically up to 15 dB of cut or boost at a wide range of frequencies, which would be sweepable from around the lower mid, say 400 Hz to the upper mid around 2kHz.

EQ Out: a simple switch which allows the incoming signal to bypass the equalisation section. This facility can be used when you need to compare the original sound, with the equalised sound.

Figure 1.

3 Insert point, normally found immediately after the EQ section, is usually accessed via a stereo jack socket or two phonos on the rear panel of the mixer. When a stereo jack is used the 'send' (output) is usually wired to the ring of the jack and the 'return' (input) to the tip (Figure 1). However, having said this, certain mixers may be wired the other way around. An insert point enables you to connect an effect or signal processor (noise gate or compressor etc) to one particular channel only. This serves two purposes: one, it allows you to process only the sound within that particular channel, and two, it saves the main auxiliary/echo sound bus for use with an overall effect such as echo or reverb. These being effects which are more likely to be required on several channels simultaneously.

Bus (Busses) This word often crops up in mixer manufacturer's handbooks. It is a term used to describe an electrical path that several signals may take, and should be imagined as a major road with many small streets running onto it. The channels being the streets and the Auxiliaries and Group outputs being the major roads.

Auxiliaries, are possibly one of the most confusing areas to be found on all mixers, often due to manufacturers' use of different names for this particular section. Any number of auxiliaries may be found on a mixer, even the most basic usually have at least two. There are two types of auxiliaries, Foldback (pre-fade) and Echo Send (post-fade). Electrically, the main difference is that foldback appears before the fader, see diagram, therefore being audible whether the channel fader is up or down, whilst echo send appears after the channel fader, therefore it's only audible when the fader is up.

Block diagram of a typical mixer Input Channel.

4 Foldback is normally used to send a separate mix of the sound to a set of headphones. This enables, for example, a vocalist to hear the backing music whilst overdubbing their own voice part. It can also be used as an echo send. Try plugging a pair of headphones into the foldback output socket, turn the main monitoring down and adjust the fold-back level controls on each channel to produce a satisfactory mix.

5 PFL is the abbreviation for 'pre-fade listen'. Its function is to do exactly what it says, namely, to allow you the privilege of hearing the sound before actually moving the channel fader up. This is a very useful facility which enables you to locate sounds and check on their 'existence' before bringing them into the mix proper. The operation of a PFL button normally mutes all other sounds and routes the selected channel to the monitor output; it does not affect the main recording mix. As you can see from the diagram, the PFL function comes before the channel fader, and it is for this reason that you are able to hear it, regardless of the actual channel fader position.

6 Channel fader. Normally a linear slide control rather than a rotary control, as there are several advantages in using slider control. It provides an easily understood graphic display of the relative channel levels, is ergonomically simple to relate to, the word 'fader' sounds and feels right for the action and its resulting effect. Perhaps most important is the ability to fade a pair of channels smoothly, which is an absolute necessity when working in stereo.

The use of a channel fader can be compared with that of a doorway, in that it allows you to see more or less of the sound in that particular channel. Electrically, it's simply an attenuator; when the fader is fully up at the top of its track, there is no attenuation, but as you pull it down, the attenuation increases until the sound is heard no more. This is why faders are often calibrated with minus figures marked down the side and increasing in value as they near the bottom of the fader's travel. The larger the number, the greater the attenuation, so, the quieter the sound.

Adjustment of the channel faders varies the balance of the sounds in the mix, and for most people this is the point where the creation of the final mix happens.

7 Echo send, as the name suggests, sends the sound on that particular channel out on the echo send bus to an echo unit. It should be remembered, however, that any effect or signal processor may be connected to this type of auxiliary, eg. Reverbs, Flangers, Digital Delays, Compressors etc.

Many mixers have two or three of each type of the auxiliaries described, with some having a switch which allows each auxiliary to operate in either the pre or post-fade mode.

8 Channel output, is simply an output of the sound in that channel taken from a point after the fader to an output socket which is usually located on the rear panel.

The purpose of a channel output is to provide a separate 'direct' output before being mixed with other sounds onto a main group output. This is very useful when group outputs are in short supply and when the sound in the channel is a 'one off', ie. it's not destined to be part of a drum kit sound - a bass guitar would be a good example. A drum kit would probably have several microphones around it and therefore use several channels on the mixer, which would require mixing together onto a 'group', whereupon they would then be routed to the desired track on a multitrack tape recorder.

9 Pan: short for 'panorama', A rotary control that allows you to position the sound in that channel anywhere within a stereo mix, from extreme left through to extreme right and all positions in-between. Often used creatively in a mix for moving sounds around in stereo between the speakers. A simple illustration of movement in the horizontal plane. It must be remembered that each channel of a mixer is mono and it is not until the signal reaches the pan control that stereo positioning of the sound can take place.

10 Group select switch. This type of switch is normally only found on mixers of four or more outputs. Its purpose is to enable you to select which of the various group outputs you want to route the signal to. Once the sound passes the pan control it may be routed to any pair of output groups by means of these switches.

In the initial recording stages of a multitrack recording, these switches allow you to group together certain sounds, eg. all the mics around a drum kit or mics used on a vocal harmony backing.

In a final mixdown situation all the channels being mixed together would be switched to the same group output to produce the final stereo mix, say groups 1 and 2. If you require a cassette mix at the same time as you are mastering onto your two-track machine, you should also switch all the channels to groups 3 and 4. This will result in two identical mixes, one for the mastering machine and one for the cassette machine. It is a useful way of saving time and retaining quality when a cassette copy is required.

Block diagram of a typical mixer Output Section.

Group Output

11 Input, the signal input to the group comes from the main group bus. By the use of the pan and group select switches, signals are routed onto the main group bus and out to the tape recorder and monitor speakers.

12 Auxiliary return (Echo return). These are inputs used for returning a sound from an echo unit or other effects device. The echo return is a simple level control that adds a variable amount of the processed signal with that of the main group signal. Sophisticated echo returns have EQ and pan controls which enable more creative possibilities to be realised. Mixers which lack echo returns are not necessarily poor mixers, if enough spare channels are available then these can be utilised as echo returns and do in fact provide even greater facilities, eg. EQ, foldback and echo send yet again, on the echo return itself. Further information on this type of routing can be obtained from the article on 'Tape Echo Techniques' in the February issue of HSR.

13 Group insert; similar to the insert point on the channel. When in a final mixdown situation the group inserts on the pair of group outputs would usually be linked to a signal processor device to apply an overall effect such as compression, reverb or a stereo graphic equaliser, to the entire sound.

14 Group fader: the final level control in the signal path before it leaves the mixer. When recording it would be used as an overall level, controlling a group of sounds ie. all the mics around a drum kit. You can then move the level of the whole drum kit up and down but still retain individual control on each drum mic, by using their corresponding channel fader.

In a final mixdown situation all the channels would be routed to groups 1 and 2 and would be used as a stereo pair. Here the group faders function as the overall stereo level controls feeding the group output sockets and main monitor mix.

15 Remix select, is a switch used to route the signal coming from the tape recorder into the monitor mix section. The switch would usually connect the main group output signal to the monitoring, thus enabling you to read group signal levels on the meters and also create a rough 'working mix' of the music over the monitor speakers. When the remix position is selected, signals from the tape recorder are routed into the same system.

16 Monitor level. Each 'group' on the mixer will usually have a monitor level, which allows you to set the listening levels of each group when you are creating a monitor mix. With an eight track system you would perhaps have eight groups each with a monitor level, similarly you would have four level controls on a four track system. You should think of the monitor levels as another separate mixer built within the main one.

17 Monitor pan: operating in the same way as on the channels, this control allows the sounds to be panned around in the monitoring only and does not affect the main recording signal.

18 Stereo tape select. This switch allows you to listen over the monitors to either the sound at the main group output, ie. that which you have created on the monitor mix, or to the sound routed back from a stereo tape recorder, and would be used to check the sound quality of the stereo 'master recording' against that of the main group output. It is sometimes referred to as the 'B return'; 'A' being the sound sent to the tape recorder and 'B' that which has been recorded and then sent back.

19 Master monitor level, controls the overall listening level of the monitored sound. After setting up the monitoring mix on the various group monitor levels, the master level can be used to adjust the overall sound to your own personal listening level. This control normally affects both the headphone level and the main monitor output signal.

20 Headphones and Monitor output, both derive their signal from the master monitor level control. Manufacturers of some mixers link the operation of the headphones socket to that of the main monitor output, the result being that when you plug a pair of headphones into the headphone socket it cuts off the main sound on the monitor speakers. This is often a very useful feature which saves you having to turn the main monitor speaker volume down when you want to quickly check sounds over headphones.

Foldback master is used to vary the overall level of the foldback mix. The output from the foldback socket is normally fed to an amplifier which would then drive a number of headphones or speakers, so that the musicians themselves can hear a 'mix' of what they are playing.

Echo send master is basically the same as the foldback master except that it would usually be linked to an echo unit or other effect device. Remember that the channel and group faders have an effect on the echo send levels.

This concludes our trip through the mixer. It's impossible to deal with any specific make of mixer, but we do hope that this article will help you to understand more fully the signal paths and routing possibilities of your own unit. There are dozens of different mixers on the market, some with very comprehensive facilities others with very basic, all will contain many of the individual sections discussed and some will include facilities not mentioned at all in this article.

If you are not in the slight bit technically-minded, don't be put off by the diagrams. By reading the text and following the signal directions you should be able to come to terms with the diagrams. And if your own mixer doesn't include some of the sections then just imagine a line between one section and the next, bypassing the 'bit' your mixer lacks. The mixer is the nerve centre of any studio and once understood, every other piece of equipment seems to fall into place soon after.


Read the next part in this series:
Anatomy Of A Studio (Part 2)

Previous Article in this issue

RAM Micro RM16 Mixer

Next article in this issue

Rogers LS7 & Studio 1 Speakers

Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.


Home & Studio Recording - Jun 1984

Donated & scanned by: Mike Gorman


Sound Fundamentals


Anatomy Of A Studio

Part 1 (Viewing) | Part 2 | Part 3

Feature by Paul Gilby

Previous article in this issue:

> RAM Micro RM16 Mixer

Next article in this issue:

> Rogers LS7 & Studio 1 Speake...

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