Phasing

[jndietz]

4 questions I have been meaning to ask for a long time:

1:Is there a point (when using 2 different mics) when the sound is phase-free?
(except from the point where you would put the mics next to eachother)
When I listen and I move the mic : the further I move, the lesser Higher frequency phase I have and the more lower frequency phase I have.
When i'm listening I start to walk back and back and back until I'm 5 meters from the (in this case) Guitar cab.
This can't be right can it?

2: Are there any negative aspects to moving the .wav file in time so the waves match?

3: How about I have 2 out of phase mics a SM57 up close and a Studio Projects C1 up far.

The SM57 is missing some natural high's 7k-13k area and I'd like the C1 to capture those.
So basicly what I do is I low-pass filter the SM57 @ 7k and I high-pass filter the C1 @ 7k. So the C1 alone is just some ''sissling'' but adds a more natural sound to the guitar.

Now my question: If the mics were out of phase, would they still be out of phase if I used this system

and question 4:
How about putting overheads in phase with the snare by moving them in time ?


I know these are lots of : Try it and listen to it, but i'd like to know the theoretical difficultys/problems/impossibilitys

1) The only point where mics will not phase is when they are right next to each other. Otherwise, when one microphone is further back than the other, it WILL phase, whether it be five inches or five feet.

2) The only negative aspects that I can think of when moving the actual waveform is that you could accidentally get it out of time REALLY BAD. That isn't even a big issue, though, as you can always play around with it until its back in time. So, I suppose that moving the waveform could take more time, if anything.

3) Try it and find out!!!

4) Try it!!!!!!!! TRY TRY TRY!

 

[bennychico11]

1:Is there a point (when using 2 different mics) when the sound is phase-free?
(except from the point where you would put the mics next to eachother)
When I listen and I move the mic : the further I move, the lesser Higher frequency phase I have and the more lower frequency phase I have.
When i'm listening I start to walk back and back and back until I'm 5 meters from the (in this case) Guitar cab.
This can't be right can it?

Yes and no.
Again, phase is not a bad thing...it's a natural occurence in our world. However, when working with two mic if you move the microphone far enough away, it has no longer become a problem phase wise, we now call it an echo. It's moved so far back that we don't have a problem with comb filtering, the sound just starts to sound late to our ears. The human ear notices delays as small as 3ms, maybe even less.

2: Are there any negative aspects to moving the .wav file in time so the waves match?

none that I personally have found. I do a lot of post production work where there is a shotgun or camera mic and a body mic on the person talking. It's usually impossible for the audio guy on location to check for phasing problems. So when I get it I just look for a common transient and move it back. I still get the presence of the body mic and have the ambience of the far mic.

3: How about I have 2 out of phase mics a SM57 up close and a Studio Projects C1 up far.

I can't say, try it and find out. Phase is frequency specific...so it will all depend on which frequencies are where when the microphones are picking them up.

How about putting overheads in phase with the snare by moving them in time ?

Again, try and see. It's something you should watch though....whenever you multiple mic something. Generally I find if you're only using two mics, it's okay to move the track physically. But if you're doing something like drums and moving the overhead tracks AND the room mic tracks and the HH track so that everything lines up with the snare...are the tom tracks now off? You're trying to just get as close as you can without screwing the sound up even further.

as far as the discussion with cpl_crud....it's getting too out of my league to talk, so I'll back down for now.
Be sure to tune in next week kids and we'll talk about nuclear physics!

 

[Farview]

I have found that moving the overheads in time with the snare will cost you some 'depth' in the drum mix. I know people who do it...it's just a choice, no right or wrong.

If you listen to the drums on almost and album Steve Albini produced, you will hear phase 'problems' all over the place. It doesn't seem to hurt sales as long as the vibe is right.

Just remember, phase isn't a problem unless it gets in the way of getting the sound you want. It can also be a tool for getting the sound you want, it just all depends.

 

[zeolivierm]

Impressive thread...

After reading the first part (the following was....harder...) I have this idea:

In my home studio, I want to record my acoustic guitar.

I place MIC1 in front of my guitar, and MIC 2 placed to record my computer which makes some noise.

MIC1 will record mainly guitar, and some of my computer noise

MIC2 will record mainly my computer, if I have a nice unidirectional setting ?

Playing with phases, (setting opposite phase) could I "erase" the computer noise ?
I hope my english makes it clear enough for you to understand my question...

I'll try this weekend...

Cheers,
Olivier

 

[Nick The Man]

lol thats interesting .... i guess it would work but you might loose some acoustic too im guessing

 

[bennychico11]

Playing with phases, (setting opposite phase) could I "erase" the computer noise ?
I hope my english makes it clear enough for you to understand my question...

I'll try this weekend...

Cheers,
Olivier

lol, ironically I posted something awhile back that was similar to this....take a look at it:
https://homerecording.com/bbs/showthread.php?p=1917328

basically...no, you'll never get rid of the room/computer noise. If this was possible people would have been doing it for years. With two microphones pointed in different direction capturing different wave forms with different frequencies in them and at different amplitudes...it's impossible. Room noise is so random and your room is not going to be perfect.
Give it a shot, but I think you'll find that the noise will just be amplified when you sum it to the guitar mic.

 

[bennychico11]

okay, here's some samples...both audio and visual.

First, here in order from top to bottom are a Top Snare mic, Bottom snare, Left Overhead and Right Overhead tracks:


This recording has a couple problems with it with regards to phasing.
The top two tracks are recording the same instrument and in the first few cycles of the wave form we can see they are out of phase. When the top mic's wave is going upward, the bottom mic's is going down. This will produce comb filtering which you can listen to below. This is the reason why when tracking you should reverse the polarity of the bottom snare mic.
We can also see that because of the distance from snare to overhead mic there is a delay in our wave form. Measuring this we find it's about 2ms. Not terrible, but we can easily fix this. And here is what it looks like corrected:



graphically we can see that the bottom snare has been inverted so that it somewhat resembles the top snare. This helps with our comb filtering. You can see there are still some peaks and valleys in our bottom snare that don't correlate with the top snare. There is no way to ever 100% correct this and in fact this is what helps give the bottom part of the snare it's sound...so don't worry about it. When looking for phase problems graphically, find a common transient in the two wave forms and correct using that as a reference.
You can also see I moved the overheads physically BACK in time to correct the delay. If you DO do this on something like overheads, double check with EVERY track so that you don't make the problem worse somewhere else. Like Farview said, this can sometimes cause a loss of depth.

(side note*) This isn't something you have to do everytime and of course you should mix with your ears and not with your eyes. But sometimes it's beneficial to use both when troubleshooting and when learning how something like phase works/sounds.


Now, the audible difference.
Here is just the snare track BEFORE we corrected the phase problem:

and after:

Listen to the differences. In the first one the ringing/comb filtering should stand out like a sore thumb while the second one you actually hear more of what the snare is supposed to sound like. Like I mentioned above, there is still some ringing in there that may not be to your liking, but EQ can help with this.
This is just a simple example but I hope it has helped
-B


--EDIT--
You may also notice that after we reversed the polarity of the snare track, the bleed from the kick drum track now is out of phase with the other mic. This 'may' be undersireable, and if so it's even more reason to use a gate or a Strip Silence type of feature on the snare

 

[Nick The Man]

hey thanks ALOT man thats awesome

yeah that helps a ton


cool cool

 

[Nick The Man]

i dont have a phase reverse button anywhere.......

 

[Farview]

i dont have a phase reverse button anywhere.......

If you are using software, it might be an offline process called 'invert'. Cubase and Nuendo have phase reverse buttons on the channel strips.

 

[Nick The Man]

If you are using software, it might be an offline process called 'invert'. Cubase and Nuendo have phase reverse buttons on the channel strips.

im using sony vegas .. which isnt really specified for audio recording . but ill check

 

[Nick The Man]

heyyyyy it does have "invert phase"


cool thanks!

 

[boingoman]

Just remember, phase isn't a problem unless it gets in the way of getting the sound you want. It can also be a tool for getting the sound you want, it just all depends.

That doesn't get said enough.

 

[cpl_crud]

Put the guitar and hte computer as far apart as possible.

Make sure that Mic 1 and Mic 2 are the same distance from their respective sources. Try to use a cardioid on the computer mic, as it will recect the most direct guitar noise.

Flip the phase on mic 2, and lower it's volume to about the level of the copmuter noise you hear in mic 1.

However, the main freuency that you are going to eliminate is the harmonics of teh distance between the two mics, not the computer nose in general. You're probably better off going with a noise gate or a noise reduction system, like Dolby (Which, from memory uses phase in a complex algorithym to remove some noise)

 

[Farview]

Dolby is an encode/decode thing. It doesn't get rid of noise that you are recording, it gets rid of the noise of the recording medium. (i.e. tape hiss)

 

[boingoman]

Dolby is an encode/decode thing. It doesn't get rid of noise that you are recording, it gets rid of the noise of the recording medium. (i.e. tape hiss)

Yeah, basically the HF of what you record gets boosted a lot by the encoder, making it much louder than the hiss. The decoder reverses the treble boost, restoring the original frequency response, but with the hiss now at a much lower level.

 

[killthepixelnow]

Great threat, I've learned a lot of phase invertion... or it was polarity invertion? *_*
PD: I get the idea, just kidding

 

[bennychico11]

Great threat, I've learned a lot of phase invertion... or it was polarity invertion? *_*
PD: I get the idea, just kidding

read this post if you want to dive into it further:
https://homerecording.com/bbs/showthread.php?t=210825