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Author Topic: Mic / SB Matrix Time/Phase Shift  (Read 1080 times)

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Offline d5

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Mic / SB Matrix Time/Phase Shift
« on: March 26, 2017, 10:57:00 AM »
Apologies if this has been covered before, but I did a search and while it's certainly mentioned in passing, I didn't see a thread directly addressing this.

I've recently started running 4 tracks; stereo audience microphone and stereo SB and I've spent some time calculating what I predicted the time shift would be and then actually making adjustments and it comes fairly close.

On one particular recording, the mic's were set about 2 meters from the stage and approximately 4 meters from the mains. On paper this seems to be about a 11ms shift (11ms = 12/343) to earlier for the aud/mic's. When I make the adjustment by visually comparing the tracks, I come up with about 9ms.

I suppose there are all sorts of phase problems with doing a matrix and this only compensates certain aspects, but was hoping to get other opinions on approaches. Also, i'm using Audacity to make the adjustment. Also, visually comparing tracks is not exactly a science, wondering what others do here.

Thanks,
John
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Re: Mic / SB Matrix Time/Phase Shift
« Reply #1 on: March 26, 2017, 06:35:19 PM »
Time = Distance/Speed of sound (1,126 ft/s)


sample:

4 ch matrix mix: stereo pair is located 30 ft. f/ the PA and SBD mix.

30 (distance)/1126 (speed of sound) = 26 ms (time)

add 26 ms sample delay to the sbd mix.

Offline ScoobieKW

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Re: Mic / SB Matrix Time/Phase Shift
« Reply #2 on: March 27, 2017, 10:11:10 AM »
Time = Distance/Speed of sound (1,126 ft/s)

At sea level, with a certain barometric pressure. Due to these variables, a millisecond per foot works to get you close, then lining up by eye is helpful.

if your mics are onstage or close, often a phase flip of the board channels will be enough, take a listen and judge for yourself.
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Offline Gutbucket

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Re: Mic / SB Matrix Time/Phase Shift
« Reply #3 on: March 27, 2017, 12:37:20 PM »
In addition to air-travel time/distance, digital processing and recording will cause some small but significant delay of signal.  That can potentially affect things on both sides of the equation  - on your side for the recorders, which is probably only significant if you are using different recorders for AUD and SBD, and on the SBD side due to a digital board or FOH processing which is probably more common.  It won't be much but it can be several milliseconds.

Calcs help get close, visual alignment helps get close.. yet the best way I know is to get close via those methods then make a final alignment decision by ear while nudging things back and forth a bit listening for what works best.  For me stage banter is the most optimal material to use for detecting alignment issues by ear, we are super attuned to hearing detail and echo on human voice so as to be able to extract speech content, and there generally is no other conflicting content while the band members are talking to the audience so any misalignment becomes easier to hear.
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Offline goodcooker

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Re: Mic / SB Matrix Time/Phase Shift
« Reply #4 on: March 27, 2017, 12:48:55 PM »

I follow the "roughly 1 ms per foot or less" rule then line up by ear. You can hear comb filtering in the bass frequencies when you are off. When the snare  drum sounds tight and the kick drum and bass guitar have good presence with no smear I call it good. I also utilize the wave scope in Wavelab. There may be a plug in for Audacity.

If you are using a coincident stereo mic and the SBD is run in mono you can line them up just by looking at the scope which will show a straight up and down line indicating mono compatibility.
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Re: Mic / SB Matrix Time/Phase Shift
« Reply #5 on: March 28, 2017, 02:48:08 AM »
Food for thought: In what plane is the source you're aligning? The distance from the upstage kick drum to the downstage lead vocal mic could be several meters.

I often go by drum stick clicks on a quiet song count-in, as the drums are often mic'd with more channels than all the vocal and instrument mics combined.

I also feel that within 10 ms, delay won't be noticed, and probably not within 20 ms, realistically.

And if you err, err on the side of SBD source happening first. It's weird otherwise.
« Last Edit: March 28, 2017, 02:49:47 AM by morst »
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Offline d5

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Re: Mic / SB Matrix Time/Phase Shift
« Reply #6 on: March 28, 2017, 02:00:03 PM »
Food for thought: In what plane is the source you're aligning? The distance from the upstage kick drum to the downstage lead vocal mic could be several meters.

The stage is wide, but not deep; there isn't really room for a performer to be directly in front of the drums.

My mic's are close to center and usually about 5' to 7' back from middle, upfront stage vocal mic depending where i decide to clamp. I have flexibility here, but found this range sounds pretty good. I the 3rd picture, this would be approximately above the von Trapp beer coster  :cheers:, but about 2' to the right. My mic's are about 12' to 14' from the main speakers. The decks are a pair of 702's linked, so they start on a common sample and share the word clock from the master deck. I thought I was hearing some smearing and "think" that the time shift helped to clean this up, but I didn't do a good A/B comparison. The time shift i settled on was 5 ms as this lined up visually for vocals.

Note: The pic's are from 2 different shows; first is from Hot Creekin' Acoustics on Feb 11; they have percussions, no formal drum kit, but pic shows the right side of the stage. The other two pic's are from Marks Brothers on Mar 18.

Thanks,
John
« Last Edit: March 28, 2017, 05:01:13 PM by d5 »
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Offline Life In Rewind

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Re: Mic / SB Matrix Time/Phase Shift
« Reply #7 on: March 29, 2017, 08:23:46 AM »
Besides the waveform visual - take note of your levels when making the sync - you'll start to have levels increasing as you get closer to "in sync" - watch for peaks that you didn't have before.
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Re: Mic / SB Matrix Time/Phase Shift
« Reply #8 on: March 29, 2017, 09:42:52 AM »
Calcs help get close, visual alignment helps get close.. yet the best way I know is to get close via those methods then make a final alignment decision by ear while nudging things back and forth a bit listening for what works best.  For me stage banter is the most optimal material to use for detecting alignment issues by ear, we are super attuned to hearing detail and echo on human voice so as to be able to extract speech content, and there generally is no other conflicting content while the band members are talking to the audience so any misalignment becomes easier to hear.

This is a great recommendation!  :cheers:

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Re: Mic / SB Matrix Time/Phase Shift
« Reply #9 on: March 29, 2017, 09:58:08 AM »
Calcs help get close, visual alignment helps get close.. yet the best way I know is to get close via those methods then make a final alignment decision by ear while nudging things back and forth a bit listening for what works best.  For me stage banter is the most optimal material to use for detecting alignment issues by ear, we are super attuned to hearing detail and echo on human voice so as to be able to extract speech content, and there generally is no other conflicting content while the band members are talking to the audience so any misalignment becomes easier to hear.

This is a great recommendation!  :cheers:

I agree - I find, visually - once you're zoomed in, it gets harder to decide which little jaggy "peak-lets" to use when picking a sync point - since the peak looks less extreme at high zoom levels - its not as obvious.
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Offline Gutbucket

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Re: Mic / SB Matrix Time/Phase Shift
« Reply #10 on: March 29, 2017, 10:52:49 AM »
And if recording using near-spaced or wide-spaced mic setups, the peaks will quite often not visually align perfectly when zoomed way in due to the slight time of arrival difference from the spacing between the mics.  The waveforms are visually confusing way down at that level.
volition > vibrations > voltages > numeric values | numeric values > voltages > vibrations > virtual teleportation time-machine experience

"Narrow or widely spaced microphone configurations are preferred. It is well-known experience that pure coincidence microphone concepts are not able to produce a satisfying natural spatial impression, due to the lack of adequate interchannel temporal relations (time-of-arrival, phase, correlation)" -Günther Theile
"The mix of the Double M/S signals with a large A/B configuration of omnis results in the spacious sound that is often desired. This option also provides decorrelated low-frequency signals." -Helmut Wittek

Offline d5

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Re: Mic / SB Matrix Time/Phase Shift
« Reply #11 on: March 29, 2017, 07:23:16 PM »
Thanks for all the great suggestions, I plan to work a bit more with this one particular recording. I came up with an 11 ms shift using calculations, but visually 5 ms seems about right. I think the next step will be to take some samples and try a range of shifts, something like 3, 5, 8, 13, 21 so I can do a true compare.

I think if I can get one where I feel is "best" sounding, it'll make future work easier; I do tend record at this club quite a bit.

Thanks again!
John
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Re: Mic / SB Matrix Time/Phase Shift
« Reply #12 on: April 02, 2017, 11:36:25 PM »
I also feel that within 10 ms, delay won't be noticed, and probably not within 20 ms, realistically.

In addition to goodcooker's statement about the "roughly 1 ms per foot" general rule of thumb and then using your ears, I agree with what morst posted.  Many years ago when I was in college, I recall an acoustics professor of mine telling us that the human ear and brain won't notice a delay if it's less than 20 ms.  As long as you have the SBD and mics lined up relatively close, it should sound just fine. 

Offline Gutbucket

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Re: Mic / SB Matrix Time/Phase Shift
« Reply #13 on: April 03, 2017, 09:32:45 AM »
Slight misalignments of under 20ms are generally too short to be perceived as a delay or echo but may be perceived in other ways - image shifts, image blurring, image width, tonal changes, depth changes, apparent local performance space size, stuff like that.  One reason a unaccompanied talking and drumstick clicks are good signals for detecting slight misalignments is that those kinds of spatial effects are more easily identified as such using those simple "well known" clean sounds heard in relative islolation, whereas for musical signals those attributes might well be part of the original sound of the instrument. 

Unconscience attention is drawn to the earlier of the arrivals, so if a SBD slightly proceeds an AUD the image may sound slightly closer and more present, whereas if the AUD precedes it may sound a bit wider and less center prominent. 

Also, because short delays are not perceived as being a delay, they can be useful for creating stereo interest or for differentiating channels from each other.  One simple but effective pseudo-stereo technique is the application of a short delay in one channel verses the other.  That can be useful when mixing in a single monophonic channel of room ambience or reverb, or making less apparent a short repair consisting of one channel copied to the other to cover an intermittent flaw or dropout.  For my surround playback stuff I sometimes introduce a slight delay into the rear facing mic channel(s) if there isn't enough front/back separation and the recording needs some help keeping the stuff in front from leaking into the surround channels too much without otherwise having to lower the level in those channels.  Likewise, I might introduce an ever so slightly different delay to each surround channel so that a single monophonic channel of room and audience ambience conveys something of a similar openness and richness to using multiple recorded ambient channels.

Hope all this doesn't come across as pedantic, I'm sure may of you guys know this stuff well.
volition > vibrations > voltages > numeric values | numeric values > voltages > vibrations > virtual teleportation time-machine experience

"Narrow or widely spaced microphone configurations are preferred. It is well-known experience that pure coincidence microphone concepts are not able to produce a satisfying natural spatial impression, due to the lack of adequate interchannel temporal relations (time-of-arrival, phase, correlation)" -Günther Theile
"The mix of the Double M/S signals with a large A/B configuration of omnis results in the spacious sound that is often desired. This option also provides decorrelated low-frequency signals." -Helmut Wittek

Offline danlynch

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Re: Mic / SB Matrix Time/Phase Shift
« Reply #14 on: April 03, 2017, 02:05:15 PM »

Yeah, be careful with that "slight delay between channels" stuff.  I pulled a board feed at a local venue (later mixed with mics) and the traveling FOH was a little too heavy in that channel-delay trick.  In post, when I laid out the tracks in Soundforge, I didn't initially notice the delay and after mixing with the mics and using the 1 foot per ms formula, the result was echo-garbage.  After spending a little time trying to figure out WTF, I noticed that the SBD tracks weren't aligned and fixed it (eventually).  It also woke me up to the idea that I was doing this stuff by rote / formula / habit and maybe should be less robotic next time.

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