When it comes to audio, “just a phase” isn’t trivial. Misaligned signals can cause anything from odd stereo imaging to silence. AudioTools Server uses automated phase analysis to catch these issues early, ensuring clean, accurate mixes before they go to air.

This phase will correct itself eventually. Your audio, however, will need some help from you.

More Than Just A Phase

“It’s just a phase” is something that we like to tell ourselves when someone or something starts behaving differently. Your toddler suddenly decides they no longer want to eat anything that is green - “Oh, it’s just a phase”. Your teenage daughter dyes her hair pink - “It’s just a phase she’s going through”. The phrase suggests that whatever the thing is, it’s nothing to worry about, and it will soon pass. While this may be the case with your little one’s eating habits or your teenager’s choice of hair color, when it comes to audio, “phase” means something different, and “bad phase” can have serious implications for the quality of your audio content.

Why phase matters

As the saying goes, timing is everything. Especially for correct audio phasing.

When it comes to audio, the word “phase” (or “phase offset”) is generally used to describe the relationship between two or more waveforms. In the case of a periodic waveform, like a sine wave, audio that is “in phase” will have the peaks and troughs of each signal perfectly aligned. The opposite of this would be where the peaks of one waveform correspond to the troughs in the other. This is referred to as “180 degrees out of phase”. 

The relative phase of the audio plays an important role in how it will be perceived by the listener. Let us consider a stereo (two-channel) recording of a section of speech. If the two channels are, as expected, perfectly aligned, the audio will sound like a single coherent source. If the two channels are then offset from each other, instead of sounding like a single source, the audio will start to exhibit an unnatural chorusing (or doubling) effect, and the image will become more diffuse (wide). The issue can become even more pronounced if this stereo audio is reproduced over a single speaker. In that scenario, the two audio channels will be summed together, with any offsets between the waveforms resulting in audible phase cancellation errors. This is most noticeable if the two inputs are 180 degrees out of phase, as the two input signals will cancel each other out, resulting in silence! 

Automated Detection

Phase errors can be introduced into the audio in a number of different ways. In the old analog days it could be due to misaligned tape heads, or wiring errors. These days it is more likely going to be due to delays having been introduced between channels or as a result of misconfigured processing. An example of the latter would be where the channels were processed individually by an audio plug-in rather than as a single group (where the relative phase of the input signal would be preserved).

So how can you stop phase errors ruining your audio? Given that you are currently reading this blog, it may come as no surprise that AudioTools Server has the solution you need. The AudioTools Quality Analysis module available with the software includes a Phase Analysis option. The Phase Analysis process can be used in a couple of different ways. The first is to produce a report indicating any regions where the phase between the two input channels falls below a user-defined threshold. This can be useful for highlighting potentially problematic sections of audio. The other option is to use the module to produce an overall phase score, which is useful for identifying more inherent phase issues.

What sets AudioTools Server apart from other QC tools is that the results of an analysis can then be used to drive actions as part of a wider workflow. When it comes to phase detection, a practical example of this would be in the detection of mono or stereo audio.

Mono versus Stereo

Determining whether the audio is mono or stereo should be a simple case of counting the number of channels, shouldn’t it? 1 channel = Mono, 2 channels = Stereo, right? Well, actually, it’s not quite as simple, as mono audio can sometimes slip through, masquerading as stereo. This is generally referred to as “dual mono”, where the mono signal is duplicated across both channels of a stereo signal. While this may seem innocuous enough, the presence of dual mono can have a big impact when it comes to processes like stereo-to-5.1 upmixing.

Dual-mono audio inputs make for a very poor upmix.

When upmixing to 5.1 (or higher), the relative phase of the audio signal determines whether that part of the audio will be assigned to the center channel, or whether it will be assigned to the left/right or surround channels. If the input signal is dual mono, then little or no signal will be assigned to the non-center channels, resulting in a 5.1 mix where all of the audio is coming out of the center channel only. As you can imagine, this doesn’t make for a particularly compelling mix.

This is where the Phase Analysis function comes in. In addition to flagging potential phase errors, Phase Analysis can also be used to identify how alike two audio channels are. If a 100% match is detected, it would indicate that the audio is likely dual-mono. By feeding the results of the analysis into a Logic Step, we can then configure a workflow to take appropriate actions. In the case of detecting mono audio on channels destined for a stereo to 5.1 upmix, this could take the form of rejecting the incoming file entirely, or using a “mono-to-stereo” upmix process to inject some width into the audio mix before creating the 5.1 output. This ensures a more even spread of audio in the final mix.

So the next time that somebody tells you it’s “just a phase”, remember that when it comes to audio, phase is something that needs to be taken seriously!

To learn more about Phase Analysis and the other Quality Control tools available with AudioTools Server, please contact your AudioTools Server representative. We’d love to hear from you!