There appears to be argument amongst studio engineers as to the 'best' display mode. One channel (left or right) is fed to one input, and the other to the second input. Not very many mixer manufacturers have even bothered making phase meters, and broadcast studios (FM radio, TV, etc.) most commonly use a 'vector-scope' - essentially a two channel oscilloscope connected in X-Y mode to display a lissajous pattern. If a channel is inverted or a stereo microphone pair has one mic out-of-phase, this will show up with the pointer in the red area. Because the audio content of the two channels can be very different, there will inevitably be times when the correlation of the two channels is less than 'perfect', and this is shown on the meter. Ideally, a stereo signal will remain in the green section most of the time - this indicates that the left and right channels are basically in phase. Just so you have some idea of what these meters might look like, see the photo below. It uses a now obsolete CMOS Schmitt trigger IC (MC14583) and appears to be wired in such a way as to be as confusing as possible. One circuit is from SSL (Solid State Logic) and dates from around 1984 or thereabouts. There are many, many questions posed on forum sites, and one schematic pops up a few times. The figure below presents an illustration of the concept.This is one of those projects that came about from a reader's question, and it piqued my curiosity to the extent that I had to see what was available (virtually nothing for DIY) and how much interest exists for something like this. Therefore, in such cases, the segments of time where the phase difference is constant are referred to as phase coherent segments. Situations may exist where the relative phase between the two waves may constantly vary with time. The figure below shows the scenario where the relative phases are not constant, or phase incoherent with respect to each other. In such scenarios, the relative phase or phase difference is not constant and hence the incident and disturbed waves are no longer coherent in phase – or become phase incoherent. In practical situations, waves (e.g., electromagnetic waves, acoustic waves) may face disturbances due to the surrounding environments (e.g., reflection, refraction, scattering, diffraction etc.,) and may lead to a change in phase. Phase coherent waves are particularly useful in producing stable interference patterns. Thus, it can be concluded that there exists a constant phase difference and hence a perfect phase coherence between the two waves. However, the phase of orange wave relative to the black wave (or vice-versa) does not change as a function of time and they produce crests and troughs at the same time. In the above figure, two sinusoidal waves of the same frequency, wavelength, and amplitude are generated and time-shifted with respect to each other. This means that the two waves are perfectly coherent in phase with changes in time. For instance, when two sinusoidal signals or sine waves are resonating at the same frequency and are time-shifted relative to each other, their relative phase does not change with respect to time. Phase Coherence is a phenomenon where a constant phase difference exists between any two signals or waves of the same frequency.
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