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Phần 3: Cân bằng trong âm thanh.
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.
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.
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.
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.
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.
The rule of thumb for audio systems is: Connect all shields, ground everything, and balance wherever possible.