Zoom H6 line out phase inversion problem

[Paul R Johnson]

I’m really struggling to get this time alignment accuracy quest. Audio is so rarely time aligned to anything near the accuracy needed to cause an accidental polarity reversal to wreck things unless you are trying to null out audio, and with multiple mics this is usually impossible anyway. Time alignment is wrecked totally at that precision when we are looking at say a band on stage. Anybody who attends real concerts hears delays as part of the ‘sound ‘. I suspect the reason drums evolved into centre stage is to keep the snare drum in time for the majority. A few of my videos have a close camera on the drummer, and sync wise the distance to the furthest camera can be quite delayed, so syncing to a snare down beat that can be seen, is easy. I find that in premiere, the close cam must be spot on, but if that makes that point impossible at frame rate locked audio sliding for the distant cams, I just have those a tad late, never early and it works. I could easily pop into audition and tweak the timing but it’s just not important. When we work with more than 8m distance, as in the concert halls and churches I work in often, I’m not sure trying to correct it works. Organs are the biggest culprit. Not all of a rank is in the same place. Often over the years they’ve been extended and between a B and the next C the actual pipes could be a long way and even facing different directions, as in along the church or across. This is the real work and I’m sure this all helps the stereo field to sound realistic.

If you are shooting a string quartet you have one angle where the violin should arrive first as it is closest. On the other side the cello should arrive first and the wide shot on centre should have them the same. If you were there. In the video your audio could be A/B, a bit rare nowadays, but more likely a variant of X/Y. Maybe the best solution is for sound to be delayed a tad compared to picture to create depth?

I’ve never seen a commercial orchestral concert with big delays to simulate a person at the back, but equally on the huge close miked orchestras we see now, the close mics all get processed to add realism, usually reverb and small delays.

The question is the old one. For the viewer, when is NOW?

[Geoffrey Cox]

Originally Posted by Don Palomaki

No. I'm saying audio recordings are also colored. And "almost" can cover a lot of territory

And no recording, audio or video, is an exact representation of the live event. as experienced by the viewer/listener in his/her seat.

This all brings to mind the ads for AR speakers (Acoustics Research) from the 1960s. Behind the stage curtain AR3a vs a live performer. Point source vs point source in a large venue and the best of gear. But a far cry from a pair of speakers vs a 90 piece orchestra.

And a lot of current music is highly engineered - about the only purely acoustic instruments are the drum set and voice and even they are processed, reverbed, and compressed to satisfy the artistic intents of whose making the final calls on the released work

Is it bad? (a mater of taste), is it wrong? (its art so wrong is a matter of opinion)

Except for few wackos and those in chains an unhappy audience is unlikely come back for more.

My point about the last bit is not to dissatisfy the audience but to delight them by doing something they don't expect. I think all really great art does this in one form or another.

[Geoffrey Cox]

Originally Posted by Paul R Johnson

I’m really struggling to get this time alignment accuracy quest. Audio is so rarely time aligned to anything near the accuracy needed to cause an accidental polarity reversal to wreck things unless you are trying to null out audio, and with multiple mics this is usually impossible anyway. Time alignment is wrecked totally at that precision when we are looking at say a band on stage. Anybody who attends real concerts hears delays as part of the ‘sound ‘. I suspect the reason drums evolved into centre stage is to keep the snare drum in time for the majority. A few of my videos have a close camera on the drummer, and sync wise the distance to the furthest camera can be quite delayed, so syncing to a snare down beat that can be seen, is easy. I find that in premiere, the close cam must be spot on, but if that makes that point impossible at frame rate locked audio sliding for the distant cams, I just have those a tad late, never early and it works. I could easily pop into audition and tweak the timing but it’s just not important. When we work with more than 8m distance, as in the concert halls and churches I work in often, I’m not sure trying to correct it works. Organs are the biggest culprit. Not all of a rank is in the same place. Often over the years they’ve been extended and between a B and the next C the actual pipes could be a long way and even facing different directions, as in along the church or across. This is the real work and I’m sure this all helps the stereo field to sound realistic.

If you are shooting a string quartet you have one angle where the violin should arrive first as it is closest. On the other side the cello should arrive first and the wide shot on centre should have them the same. If you were there. In the video your audio could be A/B, a bit rare nowadays, but more likely a variant of X/Y. Maybe the best solution is for sound to be delayed a tad compared to picture to create depth?

I’ve never seen a commercial orchestral concert with big delays to simulate a person at the back, but equally on the huge close miked orchestras we see now, the close mics all get processed to add realism, usually reverb and small delays.

The question is the old one. For the viewer, when is NOW?

In my case the specific project is simply a person talking to camera (reciting poetry to be precise) or at least having the camera pointing directly at them so sensitivity to synch is high. But my main reason for wanting to go finer than frame rate is the point I made about timbre upthread - this is definitely affected by alignment finer than frame rate.

To be clear, the phase inversion happening with the Zoom is more anecdotal than a real problem as such.

[Don Palomaki]

Mixing multiple mics of the same material in the same audio track can result in comb effect cancellations and they would be sensitive to time alignment. Room acoustics can add to this. Proper mic placement can reduce this issue.

[Greg Miller]

Originally Posted by Paul R Johnson

Audio is so rarely time aligned to anything near the accuracy needed to cause an accidental polarity reversal

Time (mis)alignment can cause a polarity reversal only with a symmetrical waveform of fixed amplitude, and only at some particular frequency when t = n*([period of f)]/2), where n = an odd integer. (At other frequencies it will create a comb filter.)

Time misalignment will never cause a polarity reversal of something asymmetrical, such as a rim shot, clap of a slate, etc. (In the case of transients like those, misalignment will just cause a short same-polarity echo.)

[Patrick Tracy]

Originally Posted by Paul R Johnson

I’m really struggling to get this time alignment accuracy quest. Audio is so rarely time aligned to anything near the accuracy needed to cause an accidental polarity reversal to wreck things unless you are trying to null out audio, and with multiple mics this is usually impossible anyway. Time alignment is wrecked totally at that precision when we are looking at say a band on stage. Anybody who attends real concerts hears delays as part of the ‘sound ‘. I suspect the reason drums evolved into centre stage is to keep the snare drum in time for the majority. A few of my videos have a close camera on the drummer, and sync wise the distance to the furthest camera can be quite delayed, so syncing to a snare down beat that can be seen, is easy. I find that in premiere, the close cam must be spot on, but if that makes that point impossible at frame rate locked audio sliding for the distant cams, I just have those a tad late, never early and it works. I could easily pop into audition and tweak the timing but it’s just not important. When we work with more than 8m distance, as in the concert halls and churches I work in often, I’m not sure trying to correct it works. Organs are the biggest culprit. Not all of a rank is in the same place. Often over the years they’ve been extended and between a B and the next C the actual pipes could be a long way and even facing different directions, as in along the church or across. This is the real work and I’m sure this all helps the stereo field to sound realistic.

If you are shooting a string quartet you have one angle where the violin should arrive first as it is closest. On the other side the cello should arrive first and the wide shot on centre should have them the same. If you were there. In the video your audio could be A/B, a bit rare nowadays, but more likely a variant of X/Y. Maybe the best solution is for sound to be delayed a tad compared to picture to create depth?

I’ve never seen a commercial orchestral concert with big delays to simulate a person at the back, but equally on the huge close miked orchestras we see now, the close mics all get processed to add realism, usually reverb and small delays.

The question is the old one. For the viewer, when is NOW?

I'm not quite sure what you're referring to. Do you think it's a goal to change the acoustic perspective with the camera angle? I would never do that, I would pick one acoustic perspective that represents an idealized audience experience.

In the case of my Zoom H5, I'll slide the onboard mic tracks to account for the fact that it takes a bit more time for sound to arrive from the backline compared to the direct board feed. Sometimes that really matters when there's some common signal between them, in which case polarity and time have to be considered.

In the case of a drum kit, aligning, or not, the close mics to the overheads will affect the tone. There's no one right way to do it, so I go based on how it sounds. Different mics at different distances and angles will never be in phase across the audio spectrum, so I put them where they sound right to me. It just works out that I tend to prefer it when the arrival times are at least approximately compensated for.

[Geoffrey Cox]

Originally Posted by Don Palomaki

Mixing multiple mics of the same material in the same audio track can result in comb effect cancellations and they would be sensitive to time alignment. Room acoustics can add to this. Proper mic placement can reduce this issue.

This is why am using the Zoom, so the tracks are separate and can be tweaked to get rid of any comb effect. I started off using a small mixer and going straight into the camera but came up against the comb effect. And because it is outdoors and it occurs when I go hand held and get close in (i.e. inside the 3x distance between mic and source) it is very hard to detect on the headphones till you get back in the studio when it is too late.

[Paul R Johnson]

I'm sorry guys - you've headed away from common sense practical issues into science fantasy.

Two mics on a snare and the polarity button does obvious stuff, but in a stereo recording of a quartet, a miswired cable does not shout loudly about polarity errors and often you just don't even notice until you see the stereo meter display being unusual. There really is no point to this when every person with an HDMI connected display will be watching with the picture slightly misplaced from the audio, and as we've already said, sound behind is quite normal, but picture behind horrible to watch. My point is simple. We are all used to latency and have our own minimums before we start to get cranky, but below a certain point, while we could adjust, there simply is no point and in real life. at 50fps what are we talking about? A subject to camera distance of about 20ft? We all cope with that sort of latency in real life every day. Are we not generating a problem to fix that doesn't need fixing?

I'm just not that interested in a 'problem' that really isn't one.

[Geoffrey Cox]

I think the times when subframe adjustment really does matter sonically have been clearly articulated. They may not apply to what you do Paul but it is often useful to understand why things matter to what others do. If such fine adjustments of audio were meaningless, why do audio sequencers allow much finer adjustments than even 100 frames per second? And a video NLE is also an audio sequencer.

[Paul R Johnson]

Because when I am mixing a multi mic orchestra project with stereo clusters and spot mics it is very important. In video this kind of sonic cohesion technique is simply not where you should be even attempting it. We are talking about aligning audio and video in a multi cam shoot. The very nature of the source material usually dictates any audio from camera positions is for for effect, as in audience reaction or atmos. Distant capture rarely sound right spacially and always lacks clarity, so your premixed replacement audio is the primary source. Micro timing of a compromised recording is somewhat pointless.

The balance and blend of the audio tracks is paramount. Syncing these with the picture edit makes this audio track the key track, and the picture just needs to match it and frame rate is sufficient. It annoys me when there are video screens in the edit that are delayed by just a few frames, or often now half a dozen!

[Patrick Tracy]

I never got the impression that it was about actually using camera audio in the finished video. I just use it as a reference for getting different angles roughly aligned before fine tuning the alignment to some visual event, then muting the camera audio. Well, I'll also use it to rough in the finished audio, but I fine tune that by comparing it to the video. Either way, the camera audio gets muted.

[Geoffrey Cox]

As is often the case the topic has changed from my original one which was not about using camera audio except in an emergency situation. It was about a probable fault with the Zoom recorder. I posted it in the dedicated audio thread as I thought it would not interest folk elsewhere. A unit that inverts the phase of an output is bad as it will cause problems for anyone who works closely and especially creatively with audio at some point.

[Greg Miller]

And we still don't know whether the Zoom inverts the polarity of the electrical output, or whether it inverts the polarity of the recorded file. You could ascertain that as follows.

1.) Get a single mic that is known to work with the Zoom and a few other recorders, without needing any adapters that might possibly flip polarity. (An omni would be my first choice, because that would eliminate any phase shifting design to obtain some directional polar pattern.)

2.) Record some asymmetrical sound (perhaps a stick on a slate), preferably in open air so there will be no confusing reflections. Check the waveform and confirm that the asymmetry is clearly visible.

3.) Repeat this test on each of the recorders (always using the same mic in the same acoustic environment).

4.) Now compare all the waveforms. Presumably all of them (ignoring the Zoom) will be the same, so that can be called the "reference" polarity.

If the Zoom waveform is the same polarity as the other recorders then the polarity of the Zoom file is correct. In that case its electrical output must be inverted.

OTOH if the Zoom waveform is flipped from the other recorders then the polarity of the Zoom file is flipped, and the Zoom's electrical output is in phase.

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At any rate, this situation might be annoying, and certainly is "wrong" from a theoretical standpoint. But it's trivial to invert the waveform in your editing software, right? If so, then it's not really causing you any problem in this scenario.

[Patrick Tracy]

Or use an XLR splitter known to be wired correctly and record the exact same sound to the Zoom and another device.

[Paul R Johnson]

I agree with Geoffrey that any recorder that a recorder that doesn’t follow convention is bad, but the ‘problem’ is something far easier to fix than in the past, because the fix is a click and spotting it needs to be done can often be confirmed visually. If you take your microphone collection, I’d guess quite a few have reversed polarity. Hopefully each model is consistent, but it seems rare now to see the statement in the spec saying a positive pressure on the diaphragm produces a positive voltage on pin 2 (or 3). Shure had this reversed on their products for a long time, then moved to the same system as others. We don’t get any problems from using old mics do we? Nowadays we hear a problem and easily fix it. Just because we can shift waveforms tiny increments in time, or invert polarity my feeling is we’re curing things that usually are not even a problem.

I’m amazed the inverted polarity was even noticed.