Rudy Trubitt wrote:
>I've done a few things with two recorders and assembled them into a
>single mix in editing. As Dan says, there is drift. I had about a 45
>minute concert which drifted by an un-usuable amount from beginning to
>end. Here's how I fixed it:
>
>I began by measuring the difference in time from a sharp transient near
>the start of the recording to another transient another near the end. I
>then used this ratio and calculated a small sample rate conversion,
>followed by a header change. (for instance, convert one file from 44.1k
>to 44.087 kHz, then do a header change (not a sample rate conversion)
>back to 44.1 (this makes it play back at an ever-so-slightly higher
>pitch that before). At that point, the two 45 minute stereo files were
>really, really close in length, almost no drift. I tried time aligning
>the front and rear, but ended up liking the sound of the rear channel
>about 15 milliseconds behind the front, so as Dan points out, a bit of
>drift when you've got a big offset is not going to break anything.
I did a similar thing with 4-channel experiments that only had a
15-foot front-back distance, so the drift was much more significant
to the sound effect. I used the Pitch 'n Time plug-in (essential for
my dance work) to adjust the rear channels to the same tempo as the
fronts, then offset the tracks 15 ms.
With the wide "Rich Peet" style rear spaced omnis, the drift of the
rear recorder is much less significant. I just recorded an orchestra
and I didn't use any stretching, just a re-synch after about 20
minutes.
-Dan Dugan
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