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

Ross Johnson Ross.Johnson at homemail.com.au
Wed May 19 19:49:22 PDT 2010

Thomas Vecchione wrote:
> 2010/5/19 Jörn Nettingsmeier <nettings at stackingdwarves.net 
> <mailto:nettings at stackingdwarves.net>>
>     On 05/19/2010 01:21 PM, Thomas Vecchione wrote:
>     > Or you inverse the phase of your second example;)
>     >
>     > While you example is not necessarily bad, it is limited.  The
>     basic concept
>     > of editing at a zero point is not bad at all and comes in
>     particular from
>     > working with ProTools and other tools for me where there is (Or
>     was last I
>     > worked with it) a very audible difference(click) between editing
>     at a zero
>     > crossing and not doing it.
>     i don't doubt that. my points are:
>     a) there is nothing magical about zero crossings.
>     b) you might as well edit at any <arbitrary value> crossing. what
>     helps
>     preventing clicks here is the fact that both sides of the splice
>     are at
>     roughly the same value. what helps even more is for both splice
>     sides to
>     cross <arbitrary value> in the same direction (unlike in my example,
>     which was intentionally nasty in that regard).
> True, but cutting at the zero crossing is infinitely more reliable 
> than trying to find the -.000234 crossing.  That is why people use 
> zero for a reference as it typically has a nice reference line in many 
> DAWs that require this for this exact reason.  That line COULD be at 
> -.000234 if they wanted, but then it becomes harder to visualize a DC 
> offset as well.
You also don't always know what you will eventually splice to, so from a 
clip or sample library point of view having a known reference is easiest.

There is also the theory/fact that, because almost any splice, at zero 
or any other matched level, will introduce some discontinuity (i.e. some 
harmonic artifacts), then it is better that it occur at the zero level, 
where the level of those artifacts will also be zero or at least minimal.

The problem is that the true local DC zero at the cut will seldom be the 
same as that after the splice, as Jörn's worst-case graphic illustration 
showed. The level of any harmonic artifacts that emerge will be equal to 
that local DC shift. So, ideally IMO, the method used to splice would 
choose the best local DC zero-crossing points to cut each piece such 
that the resulting local DC shift that results from the splice is as 
close to zero as possible. In Jörn's example that would simply be to 
choose the next (180 degree later) zero crossing in the trailing piece.

Inverting the phase is not a good thing to do as an alternative for this 
purpose, except in some limited situations. You would, for example, want 
to maintain the phase when splicing drum clips, and most percussive sounds.


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