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Post Index » STAGE TECHNIC » Pro Sound


Media College (Chương 1 : Âm Thanh) (3,4) - Page 4 PDF Print E-mail
Written by tuyenphuc   
Saturday, 13 March 2010 08:47
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Media College (Chương 1 : Âm Thanh) (3,4)
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How is Phantom Power Generated?

Phantom power can be generated from sound equipment such as mixing consoles and preamplifiers. Special phantom power supplies are also available.

Does Phantom Power Affect the Audio?

No, it does not affect the quality of audio at all and is quite safe to use. However it is recommended that you do not supply phantom power to microphones which do not require it, especially ribbon microphones.

Sound Mixers: Channel Equalization

Most mixers have some of sort equalization controls for each channel. Channel equalizers use knobs (rather than sliders), and can be anything from simple tone controls to multiple parametric controls.

The first example on the right is a simple 2-way equalizer, sometimes referred to as bass/treble or low/high. The upper knob adjusts high frequencies (treble) and the lower knob adjusts low frequencies (bass). This is a fairly coarse type of equalization, suitable for making rough adjustments to the overall tone but is not much use for fine control.

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This next example is a 4-way equalizer. The top and bottom knobs are simple high and low frequency adjustments (HF and LF).

The middle controls consist of two pairs of knobs. These pairs are parametric equalizers — each pair works together to adjust a frequency range chosen by the operator. The brown knob selects the frequency range to adjust and the green knob makes the adjustment.

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The top pair works in the high-mid frequency range (0.6KHz to 10KHz), the lower pair works in the low-mid range (0.15 to 2.4KHz).

The "EQ" button below the controls turns the equalization on and off for this channel. This lets you easily compare the treated and untreated sound.

It is common for mixers with parametric equalizers to combine each pair of knobs into a single 2-stage knob with one on top of the other. This saves space which is always a bonus for mixing consoles.

Notes about Channel Equalization

If the mixer provides good parametric equalization you will usually find that these controls   are more than adequate for equalizing individual sources. If the mixer is limited to very simple equalization, you may want to use external equalizers. For example, you could add a graphic equalizer to a channel using the insert feature.

In many situations you will use additional equalization outside the mixer. In live sound situations, for example, you will probably have at least one stereo graphic equalizer on the master output.

Sound Mixers: Auxiliary Channels

Most sound desks include one or more auxiliary channels (often referred to as aux channelsinput channel's audio signal to another destination, independent of the channel's main output. for short). This feature allows you to send a secondary feed of an

The example below shows a four-channel mixer, with the main signal paths shown in green. Each input channel includes an auxiliary channel control knob — this adjusts the level of the signal sent to the auxiliary output (shown in blue). The auxiliary output is the sum of the signals sent from each channel. If a particular channel's auxiliary knob is turned right down, that channel is not contributing to the auxiliary channel.

 

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In the example above, the auxiliary output is sent to a monitoring system. This enables a monitor feed which is different to the main output, which can be very useful. There are many other applications for auxiliary channels, including:

  • Multiple separate monitor feeds.
  • Private communication, e.g. between the sound desk and the stage.
  • Incorporating effects.
  • Recording different mixes.

Mixers are not limited to a single auxiliary channel, in fact it is common to have up to four or more. The following example has two auxiliary channels — "Aux 1" is used for a monitor and "Aux 2" is used for an effects unit.

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Note that the monitor channel (Aux 1) is "one way", i.e. the channel is sent away from the mixer and doesn't come back. However the Aux 2 channel leaves the mixer via the aux sendaux return input. It is then mixed into the master stereo bus. output, goes through the effects unit, then comes back into the mixer via the

Pre / Post Fader

The auxiliary output from each channel can be either pre-fader or post-fader.

A pre-fader output is independent of the channel fader, i.e. the auxiliary output stays the same level whatever the fader is set to.

A post-fader output is dependent on the fader level. If you turn the fader down the auxiliary output goes down as well.

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Many mixers allow you to choose which method to use with a selector button. The example pictured right shows a mixer channel with four auxiliary channels and two pre/post selectors. Each selector applies to the two channels above it, so for example, the button in the middle makes both Aux 1 and Aux 2 either pre-fader or post-fader.



 

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