The input and output levels of audio equipment can be divided into two categories:
If a signal voltage is small it may be of similar level to the level of
noise in the circuit, (hum, hiss, clicks etc). Mic level signals
are amplified to line level before being processed or routed onwards This improves the ratio of wanted signal to unwanted noise . The
signal to noise ratio would be even better if processing was carried
out at a large voltage but the equipment would be inconveniently large
and expensive. The compromise solution is to transfer the signal around
an audio system at a level of 0 dB, (approximately 1 volt).
This is known as line level, (or zero level).
Microphone inputs
There are usually separate microphone and line-level input sockets for
each channel of the mixer. Mics generate small voltages, (from 0.1mV
to about 100mV depending on the intensity of the sound and the type of
mic). Since the signal voltage is low a good quality connector, (eg. XLR)
is essential for connecting a mic to a mixer input. Jack plugs and sockets
are adequate for line level signals.
Some capacitor mics have high enough output levels to overload a mixer's
channel amplifier if connected to the mic input. Such mics should be
connected to the mixer's line input.
Microphone impedances
There are two main impedance categories:
Mics used for professional sound operations nowadays have a nominal 200 ohm impedance. The majority of these pro. mics will operate successfully into a preamplifier with a higher impedance than the mic. A 200 ohm mic will, for example, work with a pre-amp with an input impedance of 600-1800 ohms.
The rule to remember here is:
"Low into High"
These are mainly cheap domestic-quality mics. Some are moving coil mics
of 50 kilohms impedance; some are crystal mics of 2 Megohms (2 million
ohms) or thereabouts.
They should only be connected to amplifiers with
"HIGH" impedance input sockets.
Overload margin
All amplifiers have a maximum permissible voltage swing at the output.
If this maximum voltage swing is exceeded the peaks of the signal will
be cut off or "clipped".
To avoid clipping it is necessary to operate with the average signal well below the maximum permissible level. The peaks of the signal will not then be clipped. The range of levels (the "dynamic range") between the normal operating level and the level where clipping occurs is called the overload margin or, ("headroom").
When a channel is contributing a signal to the mix, its
fader should be kept near a nominal setting (usually 10dB
from maximum). The group fader, (if any) and the master fader
should be similarly set. The channel gain, ("trim") potentiometer, ("pot") should be adjusted to give a peak reading for the loudest sound.
If the signal is peaking too high on the meter, the channel gain pot should be adjusted - not the fader. The channel fader is reserved for altering the amount of signal each channel contributes to the mix.
Phantom power
A capacitor microphone requires a d.c. voltage to polarise its diaphragm
and to power its internal "head" amplifier. The usual voltage is 48
volts.
Modern mixing consoles contain d.c. power supply units that can supply any number of capacitor mics through a system known as phantom powering. The phantom supply is sent along the mic cable to the capacitor mic. A common arrangement is for the positive terminal of the d.c. supply to be fed through two identical resistors to the two signal wires of a balanced line. The negative terminal of the d.c. supply is connected to the mic cable's braided or lapped screen.
Since both wires are at the same potential (both at +48v) non-capacitor mics may also be safely connected to a phantom powered socket, provided the mic and cable are balanced.
Low pass filters
Filters that reject high frequencies are less common in recording studios. They may be used to reduce high frequency overspill eg. to remove the sound of cymbals picked up by a mic. used to cover a bass instrument. In broadcast studios low-pass filters are used to remove hiss from programmes transmitted over lines.
Equalisers
An equaliser is a variable filter used to modify the frequency response of an audio system.
Equalisers provide both boost or cut to low frequencies, high frequencies and midrange frequencies.
Low frequency equalisation
When low frequencies are boosted, the response must be returned to level before 50 Hz otherwise any mains hum, rumbles etc. will be emphasised. The bass boost is therefore usually in the form of a broad resonant curve that lifts low frequencies eg.(200 - 300 Hz) before returning to a level response by 50 Hz. This arrangement would, for example, allow a male voice or low-pitched instrument (such as a cello) to be emphasised without increasing low frequency noise.
Mid frequency equalisation
The mid-frequency range of 1 to 6 kHz is boosted to provide "presence" ie. to make this part of the signal stand out in the balance. The effect is similar to moving closer to the microphone but without the hazards of close-mic. working.
Mid frequency cut, (rarely known as "absence") has the opposite effect; it reduces parts of the signal that are too prominent.
The central frequency, about which mid-frequency equalisation operates, may be alterable.
If it can be altered, the central frequency may be
Equaliser overload
An equaliser is placed between a channel's microphone amplifier and the channel fader. If the equaliser's controls are set flat its output cannot be overloaded unless its input is overloaded
(ie. unless the mic amplifier output is overloaded).
If, however, there is a full level of signal coming out of the mic.amplifier then applying equalisation boost may result in the equaliser's output being overloaded. The channel fader comes after the equaliser. Bringing the channel fader down will not , therefore, prevent the equaliser output being overloaded. It will prevent the signal reading too high on the Peak Programme Meter (PPM). Meter readings are therefore not infallible indications that all is well with signal levels.
Some mixers have Light Emitting Diodes (LEDs) at the output of the equalizers to warn the operator that either the mic. amplifier or the equaliser is overloading.
Equaliser overload is avoidable if the channel fader is kept up near its optimum working point and the mic. input is kept down.
Channel faders
Channel faders in professional sound mixers are of the straight-line slide type. This is the only type of fader you are likely to meet with except in miniature mixers which may have gain 'pots'.
The straight-line slide type faders have continuous resistive tracks of carbon compounds or resistive plastic. Slide faders have the operational advantage that a number of faders can be adjusted simultaneously.
A well-made fader has:
Another switch may be fitted and connected to a lamp that illuminates the fader when it is faded up.
A further switch may be fitted to enable a tape replay machine, cartridge player or CD player to be started remotely when its channel fader is opened. This facilitates one-person operation.
Yet another fader-mounted micro-switch causes loudspeakers in the studio* to be cut when a microphone channel is opened, thus preventing feedback.
Another facility is "overpress." When a fader is closed, further finger pressure on the fader knob, towards the operator and against a backspring, operates a switch that activates the pre-fade listen.
* A studio is the room where microphones are set and
artistes perform.
The separate room where the mixing
console and recording equipment are located
is better described as the control room.
Pre-fade listen (PFL)
Pre-fade listen allows the operator to check, with the fader closed,
that:
The mixed PFL signal is typically routed to either:
The take off points for PFL are before the channel faders and pan pots.
Therefore a PFL signal
Auxiliary outputs
Input-channel signals may be tapped off before or after the channel faders and routed to outputs separate from
the main mixer output. These alternative outputs are called auxiliary outputs.
An auxiliary output can have a number of
uses, for example:
Foldback feeds need to be independent of channel fader settings so, if an auxiliary output is used for this purpose, the signal is derived pre-fader. On the other hand, signals fed to effects units need to follow the fader levels. Therefore if an auxiliary output is used to send a feed to an effects unit, the signal is derived post-fader.
A large mixing console usually has a separate output dedicated to supplying foldback to the studio, in which case the auxiliary output(s) may be permanently post-fader. Small mixers, that do not have separate outputs for foldback, are often fitted with switches that allow the signals fed to one auxiliary output to be derived either pre-fader or post-fader.
Example:
The Mackie MS1202-VLZ has two auxiliary outputs labelled AUX1
and AUX2 .
AUX2 is always post-fader.
AUX1 may be selected to send
signals derived pre-fader or post-fader by pressing the
AUX1 SELECT button (below the phones jack). When this button is up all the signals routed to
AUX1 OUT jack (socket) from input channels will be derived pre-fader. Alternatively, if
the button is down, all the
signals routed to AUX1 OUT will be derived after the channel faders.
Actually this little mixer has "Channel Gain" pots instead of straight line faders, but the principle is the same. It has two stereo auxiliary returns.
[The Mackie MS1201-VLZ mixer.]
Example:
The Mackie MS1402-VLZ also has two auxiliary outputs and has two stereo auxiliary returns.
AUX2 send is always post fader.
AUX1 send can be derived post or pre-fader.
This mixer has 60mm slide faders.
[The Mackie MS1402-VLZ .]
Auxiliary returns
With most mixers, the signal fed to an effects unit, (e.g. a reverb unit) by way of an auxiliary
output is returned to the mixer by way of an AUX RETURN facility.
Small mixing desks may not have inputs dedicated to auxiliary applications, in which case the treated signal is returned to the mixer by way of a mono or stereo input channel.
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Voltage (V)
The difference in potential between two points in an electrical circuit.
Expressed in volts.
Millivolt (mV)
One thousandth of a volt (0.001V). 
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Decibel (dB)
A unit used to compare two powers, (or two voltages).
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Socket
Communications engineers call a signal-carrying socket a jack. The plug which fits into the jack is called a jack plug. Jacks can be arranged in horizontal rows to form a jackfield or patchbay.
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Impedance (Z)
The total opposition of an electric circuit to an alternating current.
Expressed in ohms,( R or Omega ).
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Moving coil microphone (dynamic mic.)
A mic in which air pressure variations are converted to an electrical signal by causing a tiny coil to move in a magnetic field.
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Crystal microphone (piezoelectric mic.)
Certain crystals produce an electric potential across two faces when distorted.
This piezoelectric effect is sometimes used in mics where sound quality is not important, (e.g. in communications).
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Mix
The signal at the main output of a mixing console formed by combining
numerous input signals.
A stereo mix is usually routed to a recorder.
Often a separate mix is fed to the monitor speakers.
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Channel gain control ("input gain" or "trim")
A pot, (variable resistor) for adjusting the gain of a preamplifier.
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Gain
The ratio of the power out of an amplifier to the power in.
Expressed as a ratio or in dBs.
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Capacitor microphone (condenser mic.)
A mic with two electrically charged plates separated by a narrow gap.
Variations in air pressure cause the size of the gap to vary. This changes the charge-carrying capacity of the plates and thus generates a small voltage.
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d.c. (direct current)
An electrical current in which electrons flow in one direction only.
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Polarise
To set up an electrical charge, (e.g. by maintaining a potential difference
[voltage] between two plates separated by a nonconductive material).
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Amplifier
Apparatus for increasing the intensity of an electrical signal.
Amplifiers, ("amps") that increase voltage, are called voltage amplifiers.
Mic amps are voltage amplifiers. Amps that produce a considerable increase in current, (e.g. speaker amps) are called power amps.
A voltage amp should be connected to a high impedance "load".
A power amp should be connected to a low impedance "load".
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Screen
The conductive shield surrounding the signal carrying wires of a cable.
In mic cables it reduces the intensity of interference which can induce noise in the signal wires.
A cable screen may be braided copper, lapped aluminium or conductive plastic.
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Balanced
A cable system with two signal wires surrounded by a screen
that acts as a common return path.
An unbalanced system has only one, (coaxial) signal wire.
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Overspill
A sound intended for only one mic is sometimes picked up by adjacent mics.
This can occur when a number of musical instruments are playing together.
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50 Hz
This is the frequency of the UK's a.c. mains supply.
a.c. mains can induce an audible hum in sound equipment. A filter set to boost the response at
50 Hz would clearly make matters worse.
The frequency of the a.c. power supply in the USA is 60Hz.
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Close-mic working
Placing a mic close to an instrument or vocalist.
Close miking with certain types of mic leads to bass frequencies being
emphasised.
This is called, "the proximity effect" or, "bass tip-up".
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Sibilant
Sibilance is the emphasis of 's' and 'ch' consonants in speech and vocal music.
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Bandwidth
That band of frequencies over which the power of a signal falls by half,
(-3dB).
Expressed in hertz (Hz) or octaves.
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Patched
Connected with patch cords.
Patch cords are flexible screened cables terminated by jack plugs.
They are used to interconnect equipment temporarily.
In broadcasting they are often called "double enders".
Outboard equipment connected to a mixer by way of patch cords is said to be "patched in".
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Parameter
A characteristic that can be varied.
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Equaliser (EQ)
A variable filter used to modify the frequency response of an audio system.
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Amplitude
The peak value of a wave in either the positive or negative direction.
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Q
A measure of the sharpness of a peak or notch in a filter's response curve.
High Q gives a sharp peak or notch.
Low Q gives a broad peak or notch.
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Octave
A frequency ratio of 2:1
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Slide fader (straight line fader)
A variable resistor in which the conductive wiper (attached to the fader knob) moves along a straight resistive track.
A "long throw fader" has a long resistive track.
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Bus bar (bus)
A conductor used to connect similar points in a system.
Example: an earth bus.
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Foldback
A system for feeding programme sound from the mixing console to the performers. Performers listen on headphones or carefully positioned loudspeakers.
Foldback is derived pre-fader so that artistes do not hear levels changing as the engineer adjusts fader settings.
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More about mixers
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