Where there is an AUX return facility each AUX return input has a return level control. Switches enable the return signal to be routed to any of the Groups. Equalisation is usually provided and the return signal can also be panned.
A solo facility may also be provided for the AUX output and the AUX input. These solo buttons are used when adjusting the level, quality and position of the treated signal, (within the mix).
The AUX output, ("AUX SEND") solo button can be used to hear the "dry" sound. The AUX input, ("AUX RETURN") solo button can be used to hear the treated sound.
Echo Send and Echo Return
In small, multi-purpose, mixing desks the AUX outputs serve to provide foldback, echo and other auxiliary effects. In larger or specialised mixing consoles separate outputs may be provided for echo and foldback.
If there is an ECHO SEND facility, the echo send signal is derived from the input channels or group. These signals are combined on a bus and passed by way of an overall level control to the ECHO OUT. The sending circuit may have simple equalisation plus an insert point, (eg. for connecting a compressor).
The ECHO RETURN facility is just an input channel with a line level input, simple equalisation, a fader and switches to route the echo return signal to an appropriate group.
Echo return may also be routed to foldback. Artistes find it easier to perform while listening to a treated sound rather than a dry one.
Monitor echo
Some mixing consoles have the facility to add echo to the monitor feed. (This is quite separate from any echo applied to the main mix.) Signals from selected channels are fed via echo send pots to the MONITOR ECHO SEND. The signal from this output drives the echo unit and the treated signal is returned to the mixing desk through the MONITOR ECHO RETURN jack. The return channel usually has pots for adjusting the proportion of treated signal fed to the monitor or foldback systems.
A little echo on the monitor speakers or headphones gives a more natural sound, making it easier for sound engineers and producers to assess balances early in the mixing process. Noises such as clicks and pops also become more noticeable with added echo.
With this system, the sound heard on the monitors may not be the same as that being recorded or transmitted. Frequent use should therefore be made of the individual solo buttons to assess the signals being recorded.
Foldback
Foldback is the system whereby artistes and technicians in a studio receive, (by loudspeaker or headphones) a feed of signals selected at the mixing console.
In a recording studio, the sound folded back includes a feed from the artistes' microphones, enabling them to monitor their performances.
In a broadcast studio it is sometimes desirable that a performer hears other contributors but does not hear himself or herself. The version of the programme sound that omits one source, (eg. the presenter's own voice) is known as clean feed.
Talkback
A talkback circuit enables a sound engineer or producer, (in the control room) to talk to artistes and technicians, (in the studio).
When all studio mics are faded out, the talkback (TB) may be relayed to the studio through a loudspeaker. However, when a studio mic is faded up the feed of TB to the studio loudspeaker must be cut. This is to prevent the TB being picked up by the 'live' microphone and recorded or transmitted. Another possibility, if loudspeaker talkback is not cut when a studio mic is live, is howlround.
The studio loudspeakers are muted by relays activated by micro switches on the backstops of the faders of the mixing console. When mics are 'live' studio personnel must wear headphones, ("cans") to hear talkback from the control room.
It may be possible to route talkback to the output of the mixer. This enables spoken identification to be recorded on tape. In radio and T.V. studios it allows one studio to communicate with another through the audio output line. Of course, this is only possible in rehearsal conditions; under transmission conditions a relay prevents the accidental transmission of talkback.
Panning
Panning is the deliberate shifting of a sound image between the positions occupied by loudspeakers connected for stereo or quadraphonic reproduction.
The pan pot is designed so that the sum of the intensities from the two loudspeakers remains constant as the signal is panned from one loudspeaker to the other. With the control central,
the output level of the left hand channel needs to be -3dB with respect to its output when panned fully left. Similarly with the right channel.
Besides panning those signals routed to the main outputs, pan pots
may be included in other signal paths. If an input channel signal can be routed to a stereo monitor output, then there will be a monitor pan control to vary the position of signals routed to that output.
Routing input channels to groups
The main function of a mixing desk is to combine signals from a number of sources.
Mixing is made easier if a number of input channels can be routed to a mixing bus. The mixing bus is made the input to a group channel. A number of input channels are thus 'funnelled' into one group channel. All the individual mics covering a drum kit, for example, could be controlled with one group fader.
Routing switches may be..
On 4 and 8 group desks, group assignment switches usually take the form of push-buttons. These sometimes have caps that are illuminated when depressed. With this arrangement an input signal can be routed to any or all groups simultaneously.
The only limitation is that a great deal of space is required if the desk has 16 or more groups.
One solution to the problem of panel space is to use a pin matrix situated at one end of the mixing desk. Two signal wires arrive at the matrix from each input channel and one bus bar returns the signal to each group mixing amplifier. Pins are inserted into the matrix to make the desired connections. The system is low-cost and reliable. A drawback is that there is no visual indication of the selected routing at the input channel.
A development of the pin matrix system uses solid state switching. One button represents a channel and another represents a group. When buttons are selected the routing is carried out by solid state switching and there is usually a visual indication given at the channel.
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Routing group outputs
The outputs of the group channels can be routed to track outputs for multitrack recording and the main outputs, (L and R) for stereo recording.
Routing group outputs to a multitrack recorder
To avoid extensive re-plugging during multitrack recording, the ability
to route any group to any track output is essential.
The group fader is followed by a buffer amplifier which feeds the signal to a track select switch. The track select switch directs the signal from the group to the to the chosen track output. The track output is patched to an input of a multitrack recorder.
Routing groups to main or "master" output
While multitrack recording is progressing, the sound engineer and
producer may want to hear an approximation of what the final mixdown will sound like. One arrangement is to route the group outputs through a groups to master switch.
The groups to master switch enables the outputs of any selected groups to be routed to the main outputs, (e.g. odd numbered groups to the main output LEFT; even numbered groups to the main output RIGHT).
Group insert points
After the group mixing amplifier there is usually an insert point, (which may be used to insert a compressor in a group output, for example). The group fader comes after this insert jack. A buffer amplifier then feeds the signal through the track selection switch to the track output.
Main Outputs
The output amplifiers of the mixing desk are capable of delivering a high level into a load, which is often 600 ohms but may be 75 ohms. An output level of about 21 dBu is typical.
When using a mixer for recording, it is important to match the mixer's output level with the input level of the tape recorder. Professional tape recorders have input gain controls. These can be adjusted so that the tape is fully recorded with an input level, to the machine, of zero dBs.
Visual monitoring
The level of audio signals fed from the output of a mixer must be maintained within certain limits. If the signal is too high, distortion will occur; if the signal is too low, the signal-to-noise ratio will be poor. Visual monitoring enables the operator to keep output levels within these limits. Visual monitoring also allows channel gains to be balanced. It can also provide a quick check for certain faults.
V.U. meters
V. U. (Volume unit) meters may be used to provide visual monitoring of signal levels. The V.U. meter is an a.c. voltmeter with a specified needle rise-time and overshoot. Its performance standard was set by the American Standards Association. Not all meters with V.U. scales conform to the A.S.A. specifications so do not have too much faith in levels indicated by V.U. meters, especially in semi-professional equipment. Normally the reading of 0 V.U. for steady tone, (e.g. 1kHz sinewave) is the only V.U. indication worth relying on.
The meter scale is calibrated in decibels. It usually extends below -20dB and up to +3dB. The scale above 0 V.U. is coloured red. You should understand that 0 V.U. does not stand for 0dB. The standard level chosen for feeding signals to analogue recording machines is +4 dBu.
0 V.U. therefore usually corresponds to +4 dBu.
Peak Programme Meter (P.P.M.)
The PPM is an instrument designed to indicate the peak levels of audio signals. It is a voltmeter driven by an amplifier. It detects the peak values of signals and holds each value long enough for the meter needle to rise to the correct reading on a scale. The needle then falls back slowly enough for the eye to register what the peak was. The rise and fall times depend on the charging of a capacitor which then discharges through a resistor.
In monitoring audio signals, two levels are important..
The advantage of a steady tone for test purposes is that the average level is always a fixed proportion of the peak level. In programme material this is far from the case. A percussive sound, for example, being transitory, has a high peak value but a low average value. If the peak value alone was considered in adjusting the recording level, average-level signals would be under-recorded. Fortunately, distortion lasting only a few milliseconds is not objectionable. The charging time of the capacitor (around 2.5 mS) is selected so that the needle of the PPM does not respond to individual transient peaks. The "peak" indicated by a PPM is not an instantaneous peak value but an average value measured over a brief period.
The scale of a PPM is approximately linear and this, together with the less erratic needle movement, makes the PPM easier to read than a VU meter.
With the B.B.C. type PPM, (below) 4 corresponds to 0 dBu.
Barographs
Barograph, (bargraph) displays have become almost universal for mixers but mechanical meters are still found on tape recorders and some old mixers.
Cheaper barographs consist of rows of Light Emitting Diodes (LEDs). The resolution of the meter depends on the number of LEDs.
Liquid crystal displays can give almost continuous gradations.
Barograph displays often cover a greater range than mechanical meters. They might read as low as -50 dBs and can thus indicate very low level noise.
The amplifiers associated with barograph displays give the meters a PPM or VU type response. Some amplifiers can be switched between the two modes.
Barograph displays frequently incorporate a peak hold facility. Peak hold indicates an excessive signal level that might otherwise have gone unnoticed.
Vertical barographs take up less horizontal space than mechanical meters, so it is possible to put one meter in line with each channel.
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