Mark Foley
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Oct 16, 2007, 9:36 AM
Post #2 of 3
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I'm assuming you are referring to the Canon MA 300 adapter....
The MA 300 has two lines to transmit the audio signal to the camera - a hot line which carries the signal and an earth line. Since noise can be easily injested, only very short cable runs should be used.
Unbalance and Balanced terminology can be a bit confusing...but here goes....Balanced audio is preferred in long cable runs because they 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
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.
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Mark
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