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HDV:What You NEED to Know
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It's often asked what the difference
between balanced and unbalanced audio is as relates to video camera recording.
It's very simple. Just think of mirrors. A balanced audio system uses a means of 'mirroring' an audio signal, complete with the second signal being the exact opposite of the 'actual' audio signal. Known as being "180 degrees out of phase," the audio phase is inverted in the line, and then later coupled in the input device. The two signals of opposite polarity cancel noise. Cables don't balance or unbalance the line, but rather carry a balanced or unbalanced signal. Connecting an unbalanced cable to a balanced output gives you an unbalanced line. Connecting a three conductor cable to an unbalanced source such as a keyboard or line out from a bass guitar rig does not balance the line. This short excerpt assumes your output source is balanced, and your input source is as well. Most mics are balanced output, and most mixers have balancing input. If it's not, you'll find some alternatives for assuring balance throughout the system. Whether for video recording or not, the cable solution for any kind of recording is identical. The image below demonstrates the cable makeup of both balanced and unbalanced cables.
Unbalanced cables do not
offer the 'mirror' or "Common Mode Rejection" that a balanced cable
offers. This means you can't run the cable for long distances, as
interference can/will overtake the audio signal, as the signal degrades
over distance. A balanced line uses the
'mirrored' audio that is 180 degrees out of phase, to filter/reject
external interference. Another way to articulate this is to say that the
hot/cold conductors are of opposite polarity.
This is why it's critical that for cable runs requiring audio integrity, the cables should always be balanced. This means balanced at the source (microphone/other output device) and balanced on the input (sound card, camera input, mixer). Usually, although not
always, cables/devices are wired as: Sometimes pins 2 and 3 are reversed. This is why you see Phase Invert on most hardware devices and some software applications, it allows the pins to be inverted in case of a cable mismatch. Bear in mind that just because a cable has 3 pins or TRS connectors on it, doesn't mean it's a balanced line cable. If you don't know the source, or if you aren't sure...unscrew the cable and be sure that the connections are wired to separate posts on the connector. If you have a device that only accepts unbalanced inputs, consider using a balancing transformer at the end of the cable run. This keeps the signal balanced all the way down the cable, and converts it to unbalanced at the last possible moment, providing signal integrity.
Always ground
everything, always monitor/check audio with headphones prior to
recording. Whenever possible, use balanced devices down balanced cables and balance
all cable
runs. Use converters at the source for unbalanced output devices such as
guitars and keyboards, and keep the signal balanced to the mixer or
recording device input. This will substantially lower the noise floor in
any recording system/setup whether in a studio, wedding chapel, or
sporting event.
This page is an edited excerpt
from "Instant Digital
Audio" by Douglas Spotted Eagle |
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