Topic: need help synching AUD/SBD sources...Sample inside....

[Todd R]

Drift ALWAYS occurs (though it may be minimal in rare cases and not need correction) when you use two different digital recorders with different internal clocks that are not synced together. 

In general, I agree with the overall advice, and there is a lot of good info in this post.  But drift isn't always from two different clocks.  Drift also can occur due to the difference between the speed of electrons (constant practically speaking) and the speed of sound, which varies.

The speed of sound is dependent on temperature and humidity, so the inherent time lag between sources (in milliseconds) could be different early on in a set in a cold, uncrowded club compared to later in the set with a croweded, hot, sweaty, humid club -- leading to drift later in the set even if the clocking is the same and the the two sources were synced at the outset of the recording.  Generally, this probably isn't worth accounting for since it would have too great of an effect, but the OCD might want to re-sync periodically throughout the show or time-stretch (or better yet figure out exactly where the club is heating up and accelerating the speed of sound and time stretch only during that period).

Overall, differences in clock timing is probably the biggest effect leading to drift (at least for solid state recorders -- I think slight variances in DAT tape speed was probably a much bigger issue back when), but changes in the speed of sound can also lead to drift.

[su6oxone]

Overall, differences in clock timing is probably the biggest effect leading to drift (at least for solid state recorders -- I think slight variances in DAT tape speed was probably a much bigger issue back when), but changes in the speed of sound can also lead to drift.

Maybe that explains why I had an impossible time with one show, trying to stretch two sources to perfectly match.  I spent some two hours trying to get the drum stick clicks to perfectly match but it was impossible and I gave up eventually and ended up just deleting them (the drum stick clicks) and it sounds fine.

[stevetoney]

He must not have been running two bitbuckets on the output, or perhaps one source was from the digital outs and one from analog.

I swear he said he ran 1 channel as sbd and 1 as audience through a single UA-5.

Oops, I missed that.  Sorry, was thinking there were two sets of sources.

[junkyardt]

im using Audacity. I stretched the sources out.I think theyre fine. Just need a second opinion. its one of those things, where I have listened to it so many times, that i cant anymore. the wave forms look fine, but like I said, just need a second opinion...

Oh I see, yeah I've been in that situation a lot before.  The best way to be sure, in a really OCD way, is to check periodically throughout a show for single snare drum beats or better yet, the drummer 'timing intros' where he hits the sticks together (click click click click) at the start of a song.  Those will almost always be off if it's not timed properly and are great to use because they tend to be isolated sounds before the song starts.  If you check those in the beginning, middle, and end, and they're all perfectly single clicks, then you are good to go.  Timing with other sounds works too but is less precise, in my experience.

another problem i have run into which makes it even trickier is when the SBD itself has some natural echo on it. then even a perfect matrix will naturally have some amount of echo to it of course, but it gets really difficult to tell how much echo constitues the 'right' amount from the natural echo only with the sources being perfectly in sync, and how much constitutes too much echo with some extra entering in from a slightly off sync job.

[easy jim]

Drift ALWAYS occurs (though it may be minimal in rare cases and not need correction) when you use two different digital recorders with different internal clocks that are not synced together. 

But drift isn't always from two different clocks.  Drift also can occur due to the difference between the speed of electrons (constant practically speaking) and the speed of sound, which varies.

The speed of sound is dependent on temperature and humidity, so the inherent time lag between sources (in milliseconds) could be different early on in a set in a cold, uncrowded club compared to later in the set with a croweded, hot, sweaty, humid club -- leading to drift later in the set even if the clocking is the same and the the two sources were synced at the outset of the recording.  Generally, this probably isn't worth accounting for since it would have too great of an effect, but the OCD might want to re-sync periodically throughout the show or time-stretch (or better yet figure out exactly where the club is heating up and accelerating the speed of sound and time stretch only during that period).

You're right that changing environmental conditions will affect how sound travels in space, and thus the AUD/mic feed. 

In my experience, however, the amount of that change has always been negligible and I have never felt a need to correct for it - particularly when the SBD and AUD tracks I recorded were clock-synced when captured.  So, for practical purposes, I believe it is a reasonable operating principle that drift between two non-synced digitally recorded sources will be constant over time, due to the different internal clocks, and that environmental factors are negligible. 

One example where there was significant environmental change, but no noticeable drift from those environmental factors, was a PBS set I recorded at Joshua Tree Music Festival last May.  The set began a half hour before sunset and ended in the dark.  The air temp. was probably ~ 100 deg. and there were not too many people at the beginning of the set.  By the end of the set, air temp. had probably dropped at least 25 deg. to the mid 70s.  A lot more people also filled in the concert bowl area as the sun dropped.  Humidity was pretty constant - bone dry.  I shifted and aligned the SBD tracks to the AUD tracks, but found no noticeable drift over the course of the recording. 

http://www.archive.org/details/pbs2009-05-16.matrix.flac16

I could see where you might get something more noticeable in conditions like the summer in the SF bay area, when warm and drier daytime conditions change to more humid and colder temperatures as the fog rolls in with the sunset.  But, I think you'd need to have something like significant change in environmental factors coupled with an Allman Bros.-like 3+ hour long set before drift from any environmental factors becomes noticeable.

[Todd R]

Yep, totally agreed that temp/speed of sound related drift is pretty negligible and can be ignored.  I can't imagine even with extreme changes it would affect things by more than 5-6ms -- just enough to drive the OCD a bit crazy, but no practical/discernible difference.

Just getting it out there that there are other sources of drift.  The matrixes I did back when I ran DAT were far more difficult, even compared to doing matrixes of non-clock synced sources from solid state recorders.  Maybe the clocks were just that much more variable on older DAT decks compared to the newer decks, but it always seemed to me that it must be that variable tape speed was a big issue for drift.  Who knows?  Maybe someone out there with a V3 with a WC out and a Mytek with a WC in, and two old DAT decks can test out the theory for me.