from phasing to phase alignment

In the recent article about audio signal coloration I’ve already talked about the importance of the signals phase response in respect to the perceived tonal spectrum and today I’m going straight ahead towards phase alignment and how a signal delay relates to the phase response. But first let’s have a look at some nice youtube stuff showing Jonathan Little on demonstrating his Little Labs IBP phase alignment tool.

Having a closer look to what the IBP actually does, it turns out that it basically offers some different methods and configurations on how to “align” already shifted signals back into phase. The main method combines two 1st order all-pass filters in a serial fashion altogether with some additional twist. The basic phase shifting shows up as in the following diagram (90/180 phase adjustment engaged):

phase shift

example phase shift in the IBP

There are some further alignment options such as the typical polarity switch (also referred to as phase inversion) or a sligthly more sophisticated phase frequency center switch. The interesting thing about the IBP is that even if some rather constant phase shifting approaches (which are possible in the digital domain) might appear to be more reasonable or accurate in theory the IBP approach still maintains a more interesting or  musical compensation at the end (as mentioned by some users and judged by their hearing).

Such alignments can be used for rather “artistic” mixing treatments and audio signal coloring as well. As a simple example, adjusting it properly to the upper frequency range one can easily achieve some serious amounts of transient smearing or, applied to the lower frequency part, the bass range can be decoupled to some extend (and so some bass enhancers are taking advantage of such effects as well).

The digital implementation of the IBP offers yet another method for phase alignment which is just a plain digital delay w/o any further bells and whistles. The ultra short delays (basically below around 4ms where they are indistinguishable from the source by human hearing) are adding a more drastic and different phasing effect. Technically speaking, one could easily create  an allpass filter by utilizing a simple delay line too and so both effects appears to be just like two sides of a coin.

In this way all the well-known time shifting based effects (such as chorus and phaser just two name the two) are implying heavy phase distortion but I’m not going to follow that specific lane any further here. Instead, in an upcoming article I’ll gonna show you that there is just a little gap (circuit wise) on how to extend the already discussed concepts into a quiet simple but already musical sounding delay unit.

Comments

  1. IBP/IBP Jr hardware: $545/$365
    IBP software: $99 plus UAD card
    Voxengo PHA-979: $80

    …PhaseBug: FREE – http://www.betabugsaudio.com/plugs.php

  2. Good video, thank you

  3. susiwong says:

    Something like the IBP might look a bit unglamorous at first, but it grows on you the more you use it.
    Possible applications go far beyond the obvious dual mic technique, getting rid of environmental noise by recording a dummy track and mixing it in behind the real recording with altered phase, getting the most out of double- or triple-tracked guitars and so on.
    Highly recommended !
    Ymmv,
    susiwong

  4. Great post Herbert! I definitely look forward to playing with your forthcoming plugs on the subject. As far as the pure delay approach is concerned, I guess the way you split the audio band will also have a dramatic impact on the results as well. Linear-phase or minimal phase band-split? How steep should the filter be at the crossover frequencies (6dB/oct, … 24dB/oct)? Quite an exciting subject I must say.

    Best,
    Thomas

Trackbacks

  1. […] 23, 2010 In the recent article from phasing to phase alignment I had a closer look to what the Little Labs IBP actually does and hinted that in principle it could […]

  2. […] from phasing to phase alignment […]

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