[Ardour-Users] click free editing and zero crossings?

Thomas Vecchione seablaede at gmail.com
Thu May 20 09:20:37 PDT 2010

On Thu, May 20, 2010 at 11:44 AM, John Emmas <johne53 at tiscali.co.uk> wrote:

> Consider the case of two identical waveforms (i.e. theoretically capable of
> being perfectly edited).  Start off with the simplest case (a continuous,
> uniform frequency such as a sine wave).  Detecting two corresponding zero
> crossings and producing a flawless edit should be easy enough.  But uniform
> frequencies don't make for interesting waveforms.  Most waveforms will have
> a fundamental frequency, modulated by harmonics.  In a small window either
> side of the preferred zero crossing (the one corresponding to the
> fundamental frequency) there'll often be further zero crossings caused by
> the harmonics.  Detecting two that match mightn't be so difficult if the
> waveforms are (literally) identical.  But if they're (say) two different
> takes of the same bit of music, there's a good chance that the harmonics
> will have slightly different phase and timing relationships in each case.
>  This makes it no more reliable to edit at a zero crossing point than to
> edit at any other arbitrary value.  Of course, if you can see the waveforms
> visually, that's a great help - but seeing them would be a great help
> anyway, no matter where you wanted to place your edit.
You have stated above the limitations of edits without fades, not why
zero-crossing edits are any harder than edits at non-zero crossings, which
was my question about your assertion.

> The only thing that's intrinsically helpful about zero crossings is that
> crossing point is guaranteed to exist in all alternating waveforms.
Not really.  What is intiriniscally helpful about zero crossings is the UI
design in programs where these types of edits are felt to be needed help you
identify exactly where the zero point is, whereas any other point there
isn't a fun line that goes through it and identifies where -.00245 is.  As
such you can easily find the 'zero crossing' and edit at that point.

As Paul correctly points out the concept of a 'zero crossing' edit is a bit
of a minomer because you will rarely get sample values equal to 0.  All that
it is important for though is really the visual point of reference when
making the edit to be assured that your two points are going to be close in
value, taking out one possible cause for a drastic change that causes a
click like sound when reconstructed to a waveform from the samples.

It is merely one step in many that helps assure a clean edit, and I think
that is what people are missing here.  If you DON'T do this step, and DON'T
utilize fades from zero at the start and ends of the regions(Which as I
stated before are the default in Ardour), then you are that much more likely
to get a 'click', in fact it is pretty well a guaranteed unless you happen
to be lucky.  It is a very common occurance in a few basic protools classes
I know of at the uni near me and one of the first things taught to their
students as a result, but as I said it is only the first step in several if
fades are not utilized.

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