Audio PHASE monitoring

At Murraypro, we believe PHASE monitoring to be so important

that we will not design, or supply, an audio metering Unit without it.

A brief Monograph on the importance of Audio Phase, by Tony Drummond-Murray of Murraypro Electronics.

What do we mean by "PHASE"?

In a correctly connected system, (both) the channels of a stereo system will be wired so that a positive excursion on the generator of LEFT and RIGHT channels results in a simultaneous positive excursion at the two inputs of any subsequent downstream device. If the connections to pins 2 & 3 of an XLR3, for example, are reversed on one channel, then the positive excursion will be converted to an apparent negative excursion downstream of the reversal, on that channel alone.

Why is it so important?

Consider the case of a simple "crossed microphone" stereo setup where the directional information, differentiating between left and right channels, is derived by the apparent phase (or timing delay) between the two transducers. At medium and high frequencies there will be an appreciable difference between the output of the left and right transducers when the source is displaced to one side of the setup; largely because the actual wavelength of the audio source material is an appreciable fraction of the distance between the two transducers (ignoring factors such as microphone directivity). Lower frequencies are (largely) carried on both channels because there is very little difference between the perceived phase of low frequency sources, due to their much longer wavelength.

AUDIO DISASTER strikes when a phase reversal on one channel (alone) occurs because any signal, which was common to left and right, now effectively cancels this common component on the two channels. Since this common information invariably comprises largely low, and very low, frequencies, the cancellation of this element results in severe bass frequency loss. The stereo image is severely upset too, and becomes most unnatural.

During a busy Editing Session, perhaps leading to an immovable TX deadline, the actual quality of the audio (rather than just the presence thereof) may not always be fully appreciated whilst the subtleties of editing are foremost in the mind. Too often, the embarrasing phase problem only becomes apparent whilst reviewing the finished session, or even worse, whilst on Transmission!

Murraypro's PHASE Detection

With a conventional stereo signal, processing circuits can be devised that produce the "algebraic sum" and "algebraic difference" from the left and right channels. This is easily achieved mixing L + R [=Sum] and L-R [=Difference]. The internal "-R" signal is easily produced by electronically inverting the "R" channel information.

With conventional stereo signals, the Sum is always greater than the Difference.

An accidental inversion on one channel changes all this! Consider an accidental inversion on the right channel, which converts R to -R. Applying the previous summation of the two components in the same sum and difference mixing matrix gives:-

L+ (-R) [=Sum] and L-(-R) [=Difference]

Effectively, the Sum and Difference have swapped over!

(The equasion is equally true should the "L" signal invert)

Traffic Light logic

Using circuitry to detect this INCREASE in Difference channel amplitude, with respect to that of the Sum amplitude, gives an extremely reliable indication of the presence of a stereo phase problem. Murraypro use this change to drive a dual colour LED, whereby the normally green illumination snaps to red immediately an analomoly is detected. Additional processing is applied to prevent nuisance triggering on noise or very low amplitude signals.

Murraypro does not believe that attempting to measure the instantaneous phase angle of one channel, with respect to the other, is worthwhile. The highly complex nature of real audio signals precludes this as a meaningful process, except on steady state signals such as 1KHz tone, which has rather limited entertainment value to the listener! (.....and anyway, just how do you measure the phase of a bowed string, or human voice?). The Sum and Difference stereo phase concept is mathematically elegant and essentially independent of the instanteous phase angles of the complex input signals.

The picture shows that an inversion on one audio channel will effect no change in measured level on either channel, and only the "Traffic Light" logic LED status will give an indication of a problem.

Simple and elegant!

© Murraypro August 2007