Polarity
Overview
The term "Polarity" as applied to audio is used to describe the relationship of the electrical audio waveform to the original acoustic waveform.
- The audio waveform is considered to have "correct polarity" if the positive voltage section of the waveform corresponds to the positive air pressure section of the acoustic waveform. This is also referred to as "non-inverted" polarity.
- It is possible for the polarity of the waveform to be "inverted" by audio circuitry or connections. In the case of balanced connections, this is intentional and transparent because the inverted signal of the balanced output is re-inverted at the balanced input. In other cases; it may not be intentional and may be caused by an inverting amplifier or wiring issues. The audibility of polarity inversion can vary greatly with program material and the listening environment.
Related Subjects
History
In the early days of audio, it was widely held that signal polarity was "not important" because inverting the polarity had no obvious audible effect. As audio technology progressed, advances such as the elimination of transformers and the development of "DC" audio circuitry made the effects of incorrect signal polarity more apparent. As a result; correct signal polarity is more likely to be maintained throughout the recording process.
Basics
The effect of incorrect signal polarity is that the speaker cone will move away from the listener when the original acoustic wave had a positive pressure and towards the listener when the acoustic pressure was negative.
Signal polarity is sometimes referred to as "absolute phase;" but this can be confusing as the term phase is used to describe a relative time relationship or delay between two signals. If a symmetric periodic waveform such as a sine wave is shifted 180 degrees in phase, it will appear to have the opposite polarity. But it actually has the same polarity it had prior to the phase-shift; it is only because of the similarity of the positive and negative half-cycles that it appears similar to the same waveform with inverted polarity.
Complex waveforms typical of music audio are quite different in this respect, with very different positive and negative half-cycles, and in many cases the portion of the waveform that is positive or negative may be more or less than one-half of the cycle as a result.
For more related information; see waveform.
Practical Applications
It is not possible to change signal polarity with connections between unbalanced inputs and outputs; but it is possible with connections between balanced inputs and outputs, or unbalanced outputs and balanced inputs. This is due to the presence of a “non-inverted” and “inverted” signal in balanced systems.
To avoid polarity inversion in balanced analog interconnects; care must be taken to connect the non-inverting output to the non-inverting input, and the inverting output to the inverting input. In most cases; this does not require any special attention if the output and input use the same terminals for non-inverted and inverted signals. A problem is more likely to arise when “mixing” XLR and ¼” connections; or using adapter cables to connect balanced and unbalanced equipment.
To avoid polarity inversion in balanced to unbalanced or unbalanced to balanced adapters or adapter cables; care must be taken to use ‘’only’’ the non-inverting output to connect to the signal conductor of the unbalanced connection. How the contacts that carry the inverted signal are connected (or in some cases; not connected) varies with factors such as whether the balanced equipment is sending or receiving the unbalanced signals and the design of the output circuitry.
For more information see adapter.
The balanced version of the AES digital audio uses XLR connectors. Although it is possible wiring issues could cause the signal polarity to be electrically inverted, the format of the signal is polarity independent. As a result, there is no effect on either the polarity of the encoded audio or the functionality of the signal transmission.