Topic: Mic / SB Matrix Time/Phase Shift

[nulldogmas]

I did mean shifting one source, not one channel.

In this particular case there shouldn't be any drift because both sources were recorded at the same time on the same multi channel recorder, so the recordings are exactly the same length and only issue to deal with is the very slight time delay of the microphone source. I visually found it to be about 20ms, small enough that I can't hear any echo effect, but it was recommended to me that I try to account for that.

After this I do plan to tackle a matrix of two sources recorded on different devices though. It still seems odd to me that there would be a drift in digital recordings, but I will try to check for it.

If both your sources went into the same recorder there won't be any drift.

Yup. And if one entire source is 20ms behind, then trimming 20ms from the beginning of it should make the two align perfectly all the way through, no worries about them being "different lengths."

[Gutbucket]

I visually found it to be about 20ms, small enough that I can't hear any echo effect, but it was recommended to me that I try to account for that.

It still seems odd to me that there would be a drift in digital recordings, but I will try to check for it.

20ms should be clearly audible, especially over headphones.  Maybe not so much as a distinct echo, more of a smearing/flammy/flangy/chorus type effect.  But because that sound that can occur naturally in a reverberant recording from reflections, it will be mostly be apparent via comparison with the same in full alignment (or with either source on its own).  In other words, you may need to play with it while listening to identify it clearly as misalignment.

Different digital clocks run at slightly different rates, but are generally quite consistently so over time.  So once two different sources that were recorded using different clock are initially time-aligned.. and stretched to correct for the rate difference between the two clocks, they generally will remain in sync before and after the actual sync points used to align them.

In this situation both sources were recorded using the same clock, so only initial time-alignment was required for them to remain aligned at all other points.

[morst]

If both your sources went into the same recorder there won't be any drift.

Just to be fully nit-picky; If the speed of sound changes substantially over the course of the recording, be it from temperature or humidity or air pressure changes, there could potentially be measurable drift, though a much lower order of magnitude.
But I don't ever bother compensating for that... I chalk it up with clock jitter as something too small for me to hear in a rock and roll context.

[Gutbucket]

Never really considered that before, but yes that is technically true! ..when the sound pickup locations are spaced apart, because the change in the speed of sound effects the travel between them. 

+T for the super nano nit!

[EmRR]

Sometimes 'aligned' invariably means SOMETHING will display phasing artifacts, and you're picking the least evil version.  This discussion should involve the notion of pre-delay, the timing between a dry sound (source/board) and the reverb return (ambient mics, or in reverb, the sound returning after it's bounced and returned).  A lack of pre-delay implies the room has no size, no matter the amount of reverb decay.   As someone who does this for a living in recording and broadcast, I've never picked the version that's perfectly aligned but then, my mix sources are more and varied and the room versus direct sound is not balanced more toward the room as it is when you and I make an audience/board recording such as is being discussed here.  Even then, any reduction I do in the timing difference is only enough to clean up any sense of echo and tighten it up, but never all the way to matched, usually out in the 15-20mS range, past flanging into echo.  I just make the room 'smaller' as needed to get clarity, and a good sense of space.   There's an additional next-level argument to be made for measuring tempo per song, and adjusting the timing so the delay is tempo related, creating an on-beat short slapback to accentuate the sense of space, with it being tucked in musically, rather than competing.  Overkill, but effective.  It's what you do with reverb pre-delay in a mix of dry instruments and vocals when mixing a record.  Higher the tempo, shorter the timing difference.

Related tangent.  Occasionally I get a mono board tape with no ambience, and often can find a short decay room ambience that gives a little sense of space without obviously being reverb. Usually sounds more like EQ than ambience. Experimenting with pre-delay is critical to the result.  The level of it is often surprisingly low to be both effective and non-obvious, frequently -20 to -36 relative, and you can still clearly hear it come and go with a mute toggle.

[Gutbucket]

+T  Good insights. Another reminder that in audio ears beat eyes.

[hoserama]

Reminds me of an Adele show I did in 2016. They had a main stage and b-stage, and each had it's own PA system. Each on opposite ends of the arena floor. So when she would walk between the two stages, the sound guy would switch PAs. Which changed the timing on the audience recording relative to the IEM recording(s), which stayed constant. It wasn't hard to figure out the two alignment settings when the PA switches were between songs, but there was 1-2 switches while music played that were a lot of fun to try and figure out.