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[Bug 20327] Continuous splice flag

From: <bugzilla@jessica.w3.org>
Date: Tue, 11 Dec 2012 05:09:52 +0000
To: public-html-bugzilla@w3.org
Message-ID: <bug-20327-2486-AfZynXJP1u@http.www.w3.org/Bugs/Public/>
https://www.w3.org/Bugs/Public/show_bug.cgi?id=20327

--- Comment #6 from Pierre Lemieux <pal@sandflow.com> ---
> I feel like we should wait until there is more implementation
> experience before we add something like this.

Ok with me.

> it isn't clear to me how perceptible the difference would be since the content is supposed to be identical.

I have uploaded at the link below a WAV file containing a single 440 Hz sine
wave with a single 5 ms equal-power crossfade generated using Pro Tools --
simulating a splice with identical content on each side.

https://docs.google.com/open?id=0Bz7s0dhnv-7HZHhSdXg1dWxLOE0

An equal-gain crossfade would be inaudible as the original signal would be
recovered exactly.

> It seems odd that crossfading algorithms would introduce
> significant artifacts when the two inputs are identical. 

The optimal cross-fade algorithm depends on the nature of the content on each
side of the splice.

s_splice = a_left s_left + a_right s_right

For identical content, one should use equal-gain crossfade (a_right = a_left -
1), as you noted earlier, so that the exact signal is recovered.

s_splice = a_left s_left + (1 - a_left) s_left = s_left

For uncorrelated content (<s_left s_right> = 0), equal-power crossfade
(a_left^2 + a_right^2 = 1) can be used so that, assuming that the power of
s_left and s_right are equal, power remains constant across the splice.

<s_splice^2> = a_left^2 <s_left^2> + a_left a_right <s_left s_right> +
a_right^2 <s_right^2> = (a_left^2 + a_right^2) <s_left^2> = <s_left^2>

> I feel like only an extremely tiny fraction of users
> would actually even use this

Well, every DAW out there typically offers both equal-power and equal-gain
crossfades.

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