Media College (Chương 1 : Âm Thanh) (3,4) Print
Written by tuyenphuc   
Saturday, 13 March 2010 08:47

Phần 3: Cân bằng trong âm thanh.

Balanced Audio

This tutorial explains how balanced audio systems work. It is suitable for people who have a basic understanding of audio cables and connectors, as well as simple wave interactions (such as how waves from different sources interfere with each other). If you don't understand these things, take our introduction to audio tutorial first.

What is Balanced Audio?

Balanced audio is a method of minimizing unwanted noise from interference in audio cables. The idea is that any interference picked up in a balanced cable is eliminated at the point where the cable plugs into a sound mixer or other equipment.

Balanced audio works on the principle that two identical signals which are inverted 180° out of phase will cancel each other out. The cables used in such systems are designed to carry two versions of the signal and manipulate the relative phases of these signals to eliminate noise.

This will make more sense when we look at how balanced cables work, but first we need to take a step backwards and look at unbalanced audio cables.

Unbalanced Audio Cables

Traditional unbalanced cables use two lines to transmit the audio signal - a hot line which carries the signal and an earth line. This is all that is required to transmit audio and is common in short cables (where noise is less of a problem) and less professional applications.

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Note: Internal componentry (in sound mixers etc) is also unbalanced.

Unbalanced Audio Connectors

Unbalanced audio cables are commonly associated with the 1/4" phono jack connector and the RCA connector. However any single-pin connector used for audio is unbalanced. 3-pin XLRs can also be used for unbalanced cables. For more information about these connectors, including how to wire them, see Audio Connections.


1/4" phono


RCA

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Balanced Audio Cables

Balanced audio cables use an extra line, and consist of a hot line (positive), cold line (negative) and earth. The audio signal is transmitted on both the hot and cold lines, but the voltage in the cold line is inverted so it is negative when the hot signal is positive. These two signals are often referred to as being 180 degrees out of phase with each other. This terminology can be confusing — it does not mean one signal is delayed until it is out of phase, it means one signal is effectively flipped upside down.

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When the cable is plugged into an input (on a mixer or other equipment) the hot and cold signals are combined. Normally you would expect these two signals to cancel each other out, but at the input stage they are put "back in phase" (i.e. the inversion is reversed) before being merged together, so they actually combine to form a stronger signal.

Removing Noise

Along the length of the cable, noise can be introduced from external sources such as power cables, RF interference, etc. This noise will be identical on both hot and cold lines. This is known as a common mode signal - a signal which appears equally on both conductors of a two wire line.

So the hot and cold lines carry two signals: A desirable audio signal which has an opposite voltage on each line, and unwanted noise which is the same on both lines.

This is where the trick of balanced audio kicks in. At the input stage when the inverted audio signal is re-inverted to make both desirable audio signals the same, the unwanted noise is inverted (i.e. put out of phase). Viola - all the unwanted noise is cancelled out, leaving only the combined original signal.

Combining Balanced Cables

The standard connector for balanced audio is the 3-pin XLR. For details on wiring various configurations and connectors see Audio Connections.

Unfortunately there is no official standard for wiring balanced audio cables, but the most common configuration is:

Pin 1: Shield (Ground)
Pin 2: Hot
Pin 3: Cold

Mixing Wiring Configurations

Using cables or equipment with different wiring configurations in the same system is a recipe for trouble. You may well find that audio signals start canceling each other out and leave you with nothing.

Many sound mixers have a "phase invert" switch on each channel. This swaps the phasing of the hot and cold pins to solve the mismatch problem.

Obviously the best plan is to keep your wiring consistent. Use the configuration above and you shouldn't experience too many problems.


Last Word

The rule of thumb for audio systems is: Connect all shields, ground everything, and balance wherever possible.



Phần 4: Sound Mixer.

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Sound Mixers: Overview

A sound mixer is a device which takes two or more audio signals, mixes them together and provides one or more output signals. The diagram on the right shows a simple mixer with six inputs and two outputs.

As well as combining signals, mixers allow you to adjust levels, enhance sound with equalization and effects, create monitor feeds, record various mixes, etc.

Mixers come in a wide variety of sizes and designs, from small portable units to massive studio consoles. The term mixer can refer to any type of sound mixer; the terms sound desk and sound console refer to mixers which sit on a desk surface as in a studio setting.

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Sound mixers can look very intimidating to the newbie because they have so many buttons and other controls. However, once you understand how they work you realize that many of these controls are duplicated and it's not as difficult as it first seems.

Applications

Some of the most common uses for sound mixers include:

  • Music studios and live performances: Combining different instruments into a stereo master mix and additional monitoring mixes.
  • Television studios: Combining sound from microphones, tape machines and other sources.
  • Field shoots: Combining multiple microphones into 2 or 4 channels for easier recording.

Channels

Mixers are frequently described by the number of channels they have. For example, a "12-channel mixer" has 12 input channels, i.e. you can plug in 12 separate input sources. You might also see a specification such as "24x4x2" which means 24 input channels, 4 subgroup channels and two output channels.

More channels means more flexibility, so more channels is generally better. See mixer channels for more information.

Advanced Mixing

The diagram below shows how a mixer can provide additional outputs for monitoring, recording, etc. Even this is just scratching the surface of what advanced mixers are capable of.

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Sound Mixer: Channels

Each input source comes into the mixer through a channel. The more channels a mixer has, the more sources it can accept. The following examples show some common ways to describe a mixer's compliment of channels:

12-channel

12 input channels.

16x2

16 input channels, 2 output channels.

24x4x2

24 input channels, 4 subgroup channels and two output channels.

Input Channels

On most sound desks, input channels take up most of the space. All those rows of knobs are channels. Exactly what controls each channel has depends on the mixer but most mixers share common features. The list below details the controls available on a typical mixer channel.

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Input Gain / Attenuation: The level of the signal as it enters the channel. In most cases this will be a pot (potentiometer) knob which adjusts the level. The idea is to adjust the levels of all input sources (which will be different depending on the type of source) to an ideal level for the mixer. There may also be a switch or pad which will increase or decrease the level by a set amount (e.g. mic/line switch).

Phantom Power: Turns phantom power on or off for the channel.

Equalization: Most mixers have at least two EQ controls (high and low frequencies). Good mixers have more advanced controls, in particular, parametric equalization.

Auxiliary Channels: Sometimes called aux channels for short, auxiliary channels are a way to send a "copy" of the channel signal somewhere else. There are many reasons to do this, most commonly to provide separate monitor feeds or to add effects (reverb etc).

Pan & Assignment: Each channel can be panned left or right on the master mix. Advanced mixers also allow the channel to be "assigned" in various ways, e.g. sent directly to the main mix or sent only to a particular subgroup.

Solo / Mute / PFL: These switches control how the channel is monitored. They do not affect the actual output of the channel.

Channel On / Off: Turns the entire channel on or off.

Slider: The level of the channel signal as it leaves the channel and heads to the next stage (subgroup or master mix).



Subgroup Channels

Larger sound desks usually have a set of subgroups, which provide a way to sub-mix groups of channels before they are sent to the main output mix. For example, you might have 10 input channels for the drum mics which are assigned to 2 subgroup channels, which in turn are assigned to the master mix. This way you only need to adjust the two subgroup sliders to adjust the level of the entire drum kit.

Sound Mixers: Channel Inputs

The first point of each channel's pathway is the input socket, where the sound source plugs into the mixer. It is important to note what type of input sockets are available — the most common types are XLR, 6.5mm Jack and RCA. Input sockets are usually located either on the rear panel of the mixer or on the top above each channel.

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There are no hard-and-fast rules about what type of equipment uses each type of connector, but here are some general guidelines:

XLR

Microphones and some audio devices. Usually balanced audio, but XLR can also accommodate unbalanced signals.

6.5mm Jack

Musical instruments such as electric guitars, as well as various audio devices. Mono jacks are unbalanced, stereo jacks can be either unbalanced stereo or balanced mono.

RCA

Musical devices such as disc players, effects units, etc.

Input Levels

The level of an audio signal refers to the voltage level of the signal. Signals can be divided into three categories: Mic-level (low), line-level (a bit higher) and loudspeaker-level (very high). Microphones produce a mic-level signal, whereas most audio devices such as disc players produce a line-level signal. Loudspeaker-level signals are produced by amplifiers and are only appropriate for plugging into a speaker — never plug a loudspeaker-level signal into anything else.

Sound mixers must be able to accommodate both mic-level and line-level signals. In some cases there are two separate inputs for each channel and you select the appropriate one. It is also common to include some sort of switch to select between inputs and/or signal levels.

Input Sockets and Controls

The example on the right shows the input connections on a typical mixer. This mixer has two input sockets — an XLR for mic-level inputs and a 6.5mm jack for line-level inputs. It also has a pad button which reduces the input level (gain) by 20dB. This is useful when you have a line-level source that you want to plug into the mic input.

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Some mixers also offer RCA inputs or digital audio inputs for each channel. Some mixers provide different sockets for different channels, for example, XLR for the first 6 channels and RCA for the remainder.

Input Gain

When a signal enters the mixer, one of the first controls is the input gain. This is a knob which adjusts the signal level before it continues to the main parts of the channel. The input gain is usually set once when the source is plugged in and left at the same level — any volume adjustments are made by the channel fader rather than the gain control.

Set the gain control so that when the fader is at 0dB the signal is peaking around 0dB on the VU meters.

Other Controls and Considerations

Phasing: Some equipment and cables are wired with different phasing, that is, the wires in the cable which carry the signal are arranged differently. This will kill any sound from that source. To fix this problem, some mixers have a phase selector which will change the phasing at the input stage.

Phantom Power: Some mixers have the option to provide a small voltage back up the input cable to power a microphone or other device. See Phantom Power for more information.

Phantom Power

Phantom power is a means of distributing a DC current through audio cables to provide power for microphones and other equipment.

The supplied voltage is usually between 12 and 48 Volts, with 48V being the most common. Individual microphones draw as much current from this voltage as they need.

A balanced audio signal connected to a 3 pin XLR has the audio signal traveling on the two wires – usually connected to pin 2 (+ve) and pin 3 (-ve). Pin 1 is connected to the shield, which is earthed. The audio signal is an AC (alternating current), whereas phantom power is DC (direct current).

The DC phantom power is transmitted simultaneously on both pin 2 and 3, with the shield (pin 1) being the return path. Since the DC voltage on the ‘hot’ and ‘cold’ pins (2 & 3) is identical, it is seen by equipment as “common mode” noise and rejected, or ignored, by the equipment.

If you put a volt meter on pins 1 & 2, or pins 1 & 3, you will see the 48v DC phantom power, but if you meter pins 2 & 3 (the audio carrying wires) you will see no voltage.

The DC voltage can be harnessed however, and used to power mics, mic-line amps, or indeed a video camera (in this case the DC voltage would travel up the video cable – and would need special equipment to filter this voltage).

Phantom powering is defined in DIN standard 45 596 or IEC standard 268-15A

Note: Audio signals transmit as AC current, whereas powered equipment requires DC current to operate. Phantom power is a clever way of using one cable to transmit both currents.



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.



Sound Mixers: Channel Assigning & Panning

One of the last sets of controls on each channel, usually just before the fader, is the channel assign and pan.

Pan

Almost all stereo mixers allow you to assign the amount of panning. This is a knob which goes from full left to full right. This is where the channel signal appears on the master mix (or across two subgroups if this is how the channel is assigned). If the knob is turned fully left, the channel audio will only come through the left speaker in the final mix. Turn the knob right to place the channel on the right side of the mix.

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Assign

This option may be absent on smaller mixers but is quite important on large consoles. The assign buttons determine where the channel signal is sent.

In many situations the signal is simply sent to the main master output. In small mixers with no assign controls this happens automatically.

However you may not want a channel to be fed directly into the main mix. The most common alternative is to send the channel to a subgroup first. For example, you could send all the drum microphones to their own dedicated subgroup which is then sent to the main mix. This way, you can adjust the overall level of all the drums by adjusting the subgroup level.

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In the example pictured right, the options are:

  • Mix: The channel goes straight to the main stereo mix
  • 1-2: The channel goes to subgroup 1 and/or 2. If the pan control is set fully left the channel goes only to subgroup 1, if the pan is set fully right the channel goes only to subgroup 2. If the pan is centered the channel goes to subgroups 1 and 2 equally.
  • 3-4: The channel goes to subgroups 3 and/or 4, with the same conditions as above.

For stereo applications it is common to use subgroups in pairs to maintain stereo separation. For example, it is preferable to use two subgroups for the drums so you can pan the toms and cymbals from left to right.

You can assign the channel to any combination of the available options.

In some cases you may not want the channel to go to the main mix at all. For example, you may have a channel set up for communicating with the stage via an aux channel. In this case you don't assign the channel anywhere.

Sound Mixers: PFL

PFL means Pre-Fade Listen. It's function is to do exactly that — listen to the channel's audio at a point before the fader takes effect. The PFL button is usually located just above the channel fader. In the example on the right, it's the red button (the red LED lights when PFL is engaged).

Note: PFL is often pronounced "piffel".

When you press the PFL button, the main monitor output will stop monitoring anything else and the only audio will be the selected PFL channel(s). This does not affect the main output mix — just the sound you hear on the monitor bus. Note that all selected PFL channels will be monitored, so you can press as many PFL buttons as you like.

PFL also takes over the mixer's VU meters.

PFL is useful when setting the initial input gain of a channel, as it reflects the pre-fade level.

PFL vs Solo

PFL is similar to the solo button. There are two differences:

  1. PFL is pre-fader, solo is post-fader (i.e. the fader affects the solo level).
  2. PFL does not affect the master output but soloing a channel may do so (depending on the mixer).

Sound Mixers: Channel Faders

Each channel has it's own fader (slider) to adjust the volume of the channel's signal before it is sent to the next stage (subgroup or master mix).

A slider is a potentiometer, or variable resistor. This is a simple control which varies the amount of resistance and therefore the signal level. If you are able to look into the inside of your console you will see exactly how simple a fader is.

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As a rule it is desirable to run the fader around the 0dB mark for optimum sound quality, although this will obviously vary a lot.

Remember that there are two ways to adjust a channel's level: The input gain and the output fader. Make sure the input gain provides a strong signal level to the channel without clipping and leave it at that level — use the fader for ongoing adjustments.



Sound Mixers: Subgroups

Subgroups are a way to "pre-mix" a number of channels on a sound console before sending them to the master output mix. In the following diagram, channels 1 and 2 are assigned directly to the master output bus. Channels 3,4,5 and 6 are assigned to subgroup 1, which in turn is assigned to the master output.

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Subgroups have many uses and advantages, the most obvious being that you can pre-mix (sub-mix) groups of inputs.

For example, if you have six backing vocalists you can set up a good mix just for them, balancing each voice to get a nice overall effect. If you then send all six channels to one subgroup, you can adjust all backing vocals with a single subgroup slider while still maintaining the balance between the individual voices.

Note that if your mixing console's subgroups are mono, you will need to use them in pairs to maintain a stereo effect. For each pair, one subgroup is the left channel and the other is right. Each channel can be panned across the two subgroups, while the subgroups are panned completely left and right into the master output bus.

Sound Mixers: Outputs

The main output from most mixing devices is a stereo output, using two output sockets which should be fairly obvious and easy to locate. The connectors are usually 3-pin XLRs on larger consoles, but can also be 6.5mm TR (jack) sockets or RCA sockets.

The level of the output signal is monitored on the mixer's VU meters. The ideal is for the level to peak at around 0dB or just below. However you should note that the dB scale is relative and 0dB on one mixer may not be the same as 0dB on another mixer or audio device. For this reason it is important to understand how each device in the audio chain is referenced, otherwise you may find that your output signal is unexpectedly high or low when it reaches the next point in the chain.

In professional circles, the nominal level of 0dB is considered to be +4 dBu. Consumer-level equipment tends to use -10 dBV.

The best way to check the levels of different equipment is to use audio test tone. Send 0dB tone from the desk and measure it at the next point in the chain.

Many mixers include a number of additional outputs, for example:

Monitor Feed: A dedicated monitor feed which can be adjusted independently of the master output.

Headphones: The headphone output may be the same as the monitor feed, or you may be able to select separate sources to listen to.

Auxiliary Sends: The output(s) of the mixer's auxiliary channels.

Subgroup Outputs: Some consoles have the option to output each subgroup independently.

Communication Channels: Some consoles have additional output channels available for communicating with the stage, recording booths, etc.

Sound Mixers: Outputs

The main output from most mixing devices is a stereo output, using two output sockets which should be fairly obvious and easy to locate. The connectors are usually 3-pin XLRs on larger consoles, but can also be 6.5mm TR (jack) sockets or RCA sockets.

The level of the output signal is monitored on the mixer's VU meters. The ideal is for the level to peak at around 0dB or just below. However you should note that the dB scale is relative and 0dB on one mixer may not be the same as 0dB on another mixer or audio device. For this reason it is important to understand how each device in the audio chain is referenced, otherwise you may find that your output signal is unexpectedly high or low when it reaches the next point in the chain.

In professional circles, the nominal level of 0dB is considered to be +4 dBu. Consumer-level equipment tends to use -10 dBV.

The best way to check the levels of different equipment is to use audio test tone. Send 0dB tone from the desk and measure it at the next point in the chain.

Many mixers include a number of additional outputs, for example:

Monitor Feed: A dedicated monitor feed which can be adjusted independently of the master output.

Headphones: The headphone output may be the same as the monitor feed, or you may be able to select separate sources to listen to.

Auxiliary Sends: The output(s) of the mixer's auxiliary channels.

Subgroup Outputs: Some consoles have the option to output each subgroup independently.

Communication Channels: Some consoles have additional output channels available for communicating with the stage, recording booths, etc.

(Xin xem tiếp Phần 5: Chất lượng Âm thanh, Sound Quality).