Can you help me understand this phase issue?

[audioluche]

Hi,

I have a phase issue that i dont understand when i record some bass.
I recorded an album with a band a couple of monts ago and for the bass, i used a JDI going to a Drawmer 1960 and the amp was miked with a royer r121 going thru a api 3124.
The amp is a Hiwatt.

Both tracks were sounding great and complemented each other really nice. The phase was sample locked and the tracks lined up in pro tools perfect (well i did not looked at this at that time because everything sounded fine but it looks like this when i checked today.)

They decided to add another song to the album this week and when i recorded the bass with the same setup, this time the amp was delayed by approx. 2 to 3 ms.
The sound was bad, phasey when you listen to the 2 tracks toghether.
I tried to lined them up in PT but as close as i can get, i still hear a little comb filter effect in the mid range.

What i found to be curious is that the other time the 2 bass tracks looked almost as if it was the same track (waveform was verry close) but this time, the waveform is verry different beetwen the mic and the di.

One thing that come to me as a source of disparity is that i run a speaker cable (in the walls) between the head in the control room to the box in the studio. I am now in a new studio but i was doing the same thing in the other one.
So i did a test and recorded the bass with the head in the control room and with the head in the studio.

There was a little difference in the timing between both tracks but still, i could not get it to sound as "phase locked" as the first time. And the waveform still looked very different.

I also had the same issue in my new studio with another bass player, i could not get the di to lock with the bass amp.
Of course i use the same converter (192 i/o).


So my question is: Do you have a clue on what could cause this (the first time the bass was really sample locked)?
And do you know if i can get my two tracks more accurate with a plugin like the one Eventide does for playing with the phase.

If you have any opinion on this case, fell free to share, i'm still scratching my head!!!!

[Shawn Simpson]

What you're describing in the later attempts sounds more normal to me for using the bass amp and di tracks together. There will be a slight timing shift between the two signals because of the time of arrival at the mic and there will be some phase shift from the additional electronics in the path. That being said, it's not always unpleasant to have that little bit of phase shift as it will sometimes help the bass fill out. In the case of your first attempt, where you said the waveforms looked almost the same, are you certain you weren't just feeding the DI to two different paths and recording it? The reason I ask is because those waveforms will have a pretty marked difference if one is from a mic recording the amp and the other is directly off the pickup.

Don't take that as a slight against your engineering skills either...we've all made mistakes like that at some point. Before we start addressing a potential hardware issue, you should be sure it wasn't an error between the chair and the keyboard.

[Naagzh]

In the case of your first attempt, where you said the waveforms looked almost the same, are you certain you weren't just feeding the DI to two different paths and recording it?

Totally sounds like it to me. We've all done stuff like this!

You said the waveforms were "very close". How close? Solo and play both tracks. Flip the phase on one track, and play again. If you get no sound, then you'll know that the two tracks are cancelling out each other perfectly, because they are the same signal, recorded twice.

But hey, at least you didn't hear any phase issues between the two tracks originally! Your ears are pretty good!

[audioluche]

Hi Shawn,

Thanks for your input. Yeah, i know, it sound weird but it's really the case! One track is the DI and the other one is the amp, they really sound different, trust me!
I agree with you that there should always be a little time shift between the two but i dont understand that (this time) it is so obvious and sound that bad, while it was sounding really good the other time and it was almost perfect phase wise. And i dont remember having that (that much!) phasey sound in the past so i just question myself about this...

This is why i was wondering about having the amp in the control room and the box in the studio. 30 feet of wire dont seems much to me but maybe i'm wrong?

Thanks!

[Shawn Simpson]

I'm sure there's somebody on here who could do the math to figure it out, but I'm almost certain that a 30' cable isn't going to induce that much delay in the signal. It might pickup some noise and/or lose some signal integriby if the cable sucks, but I just can't imagine there being a significant delay added.

[Andre Knecht]

Shawn is absolutely correct — electrical signals travel at about two-thirds of the speed of light, in wires.

It’s not the cable(s).

IHTH.

[Vaporhead]

Are you sure that the mic is the same distance from the speaker as it was the last time you recorded the bass? Mic placement can have drastic effects on phase relationships.

[audioluche]

Yes, the mic was about 2or 3 inches from the speaker, both times.
Thats why i dont understand, same setup, totally different phase relationship. What can cause this?
I dont know....

Thanks!

[c-tone]

A difference of even 1 cm can drastically change the phase relationship between a mic and another source. I find it nearly impossible to recreate a mic setup on amps after moving them, even after marking the front of the grill and or taking pictures, etc.

This may not account for the entire problem you had, but it is certainly a huge factor.

[c-tone]

Also, what version of Pro Tools were you using? There was an issue in an earlier version where tracks could get shifted in time.

What doesn't make sense is how your first tracks were exactly lined up. It seems like there should always be a bit of delay from the mic signal vs. DI

The amp was fed directly from the DI's 1/4" output right? No re-amping or anything like that, correct?