Phasing theory RE DI'd AND mic'd bass guitar

[bobbydj]

So I'm starting to think I need to record bass slightly differently. The preferred method of most TapeOppers is a mix of DI and mic, right? Fair enough. I've certainly tracked like this in pro-studios whilst in bands - the DI being used for the low end, whilst the cab kicks out the clank and thrutch and fizz etc etc. I can see the science of this.

However, I've seen posts that talk of phase occurrences between DI and mic signals. Up to now I've only really understood phase as an effect produced by multi-micing a source, where 2 or more mics are at different distances from the source. Sound waves hitting capsules at different times, thus different parts of the wave starting to cause cancellation resulting in a thinning of the overall sound. Crude as that description is, I feel as though I'm up to speed on phasing for present purposes.

What I don't really get is how a direct signal and a mic'd signal can phase. Is the thinking that the DI is a very immediate wave with little time lag between signal and fold-back, while the mic is very, very slightly later? Or what? That's the only way I can think of how phasing may result. Admittedly, maybe the mic's signal is ahead of the DI - perhaps because of the DI passing through a box before hitting the board. But then the mic's signal also has a similar path, it would seem.

Anyway, I want to try mixing DI and mic signals to enhance sounds (whether guitar or bass or whatever) so I'm trying to anticipate phasing issues.

Can anyone throw a bit of crude Ladybird Book science at me please? Be gentle. The grey matter isn't what it used to be, and what it used to be isn't all that either. Cheers. Oh - also I'm working with a M*ck** 16*4 VLZ Pr* so don't have any fandango phase cancellation switch or whatever they are.

[swingdoc]

I'm gonna talk now about something of which I know nothing of...so....
I would add the following....
Phase cancellation probably couldnt be adequately explained in this setting as a "here's why it happens" theory..
Probably related to the fact that the bass fundamentals are so low in frequency with larger waves, they have more time and "opportunity" to cancel given any strategy for the mixing of signals.
Probably have to use ears here more than most scenarios, since there are no rules of thumb. Try flipping phase, with / without...which ever sounds better, is most likely the correct way for that signal chain.
If you dont have phase reversal switches, I would worry too much about it anyway..
You could probably move the signals in and out of phase, by how far you place the mic away for the bass cabinet.
The signal probably goes in and out of phase based on which stinking note is being played...a low open E vs a high C for example..
Since the bass phase bounces in and out randomly anyways (with two varying siganl sources), compression and leveling the signal has become the common place /standard mantra for the bass.
You're likely to get more consistant results manipulating these parameters (compression), than trying to capture bass phase perfection..

Damn... a caffiene buzz (red bull) at 12:30 am....

[Shawn Simmons]

Phase is a relationship of time, i.e the same signal arriving at your ears at different times. A DI signal is usually "earlier" in time than a signal from a mic on a speaker. For bass guitar, I minimize this time issue by putting the DI and mic signal as close "in time" as I can. I mean that I put the DI very close to the input of the amp and use a very short cable between them. That way, the DI signal is maybe, and I mean maybe, a millisecond earlier in time than the mic signal. I also jam the mic in the speaker as close as I can.

If this doesn't make any sense, it's cause I've been drinking. *hic*

Shawn

[AGCurry]

Phase IS a function of time. It is also a function of frequency, in that if I delay a pure sine wave of, say, 60 cycles per second, by 1/60th of a second, the two signals will be in phase.
It will take a certain, measurable amount of time for the act of touching a string to be converted and processed and to arrive at a microphone which is in front of a speaker. By far the greatest part of that time consists of the mechanical delay in the speaker, and the time it takes for the sound waves to travel from the speaker to the microphone at about 700 miles per hour. So, yes, phase problems are possible. A fast speaker and a close microphone will minimize the problem.
Keeping the cable length short? Well, let's see... electricity travels at the speed of light - 186000 miles per second... not much effect there.

[grrrayson]

I think your responses so far summed up the substance of what you were looking for, but I do have a few small reiterations and amendments...

1. as electricity does travel at the speed of light, it will go from the source through a 30 ft. cable through the wall jack through the pre through the console through an elco to a tape machine and all the way back through that through an amp and into the phones and be in perfectly in time for all we can ever tell. an electrical audio signal can always be counted on as travelling instantanouesly--just be wary of the relatively few pieces of gear that create latency. so in your case it's quite natural that the miked signal will be the late one. you were very correct.

2. for the record, no console anywhere has a button that can alter the phase of a signal. rather it alters the polarity. it doesn't delay the signal in time (unless someone built a console with built-in time based processing on every channel that I am unaware of). phase (which occurs with the actual sound wave in the air) translates into polarity (which occurs with electrical signals) after it leaves the microphone transducer. so what that button does is make the voltage positive wherer it was negative, and thus make the electrical signal look like it what it would were it arriving at the mic 180 degrees out of phase, or half way through the cycle of that frequency in relation to an identical sound source (leaving a vacuum in the air that pulls on the mic diaphragm instead pushing on it equals, for example, a negative voltage where there would have been a positive one coming out of ther mic). phase cancellation is actually easy as pie to explain with a piece of paper and a pencil.

3. perhaps you don't have any POLARITY (yes, I have no problem conjecturing that almost every manufacturer is incorrect in calling them "phase switches") reversal switches on you M*****, but if you get really hardcore into this phase stuff you can acheive the same polarity reversal by rewiring cables. if you are at all handy with a soldering iron, it's not too tough.

so one method that I've seen guys use when they can't get things perfectly IN phase is that they try to get it perfectly OUT of phase, sounding as thin and tinny as possible, and then they reverse the polarity--voila! perceived perfect phase alignment is acheived electrically. if you aren't too happy with your luck getting things in phase you might want to try this. build a cable with the polarity flipped. get your sounds with a normal cable, and if it doesn't work getting it in phase, get it as much out of phase as you can, and then swap the normal cable with the polarity reversed one. even if it doesn't work, it would probably be very educational. I have actually seen it done to good effect.

4. this a little far out for me to get into, but I don't think phase is so much a relationship in time as it is an effect of relationships in time.

5. you would probably be extremely interested to check out the little labs website--they make devices to specifically deal with this problem. I can't afford this stuff, but even reading about it helps clear things up.

sorry if I'm too wordy, but anyone accusing me of being an audio nerd on a site like this can f*** off.

hope this helps!

love

grayson elliott

[corinpills]

being a bass player, I certainly can't give you the mathe on this phenomenon ( we just count to four and start over, you know), but I can assure you that you're onto something here. This does happen and it takes keen ears to pinpoint why the bottom is getting weak at certain points ina song.

On our most recent recordings, we used the combo of DI and mic'd amp that you describe. I go for a very McCartney- esque tone with flatwounds ona hollowbody bass through a tube amp mic'd at a distance of about 6 feet (per teh prescription of Geoff Emerick). The tone is killer, but we noticed taht it would thin out at certian points in songs- and it seemed to center around the low F on my bass. Analyzing the sound waves, there was a discenable time lag between the mic'd signal and the DI and it was exacerbated on some notes.

And as I told Roger the engineer; this is a really good opportunity for someone to design a "mondo- hyper phase alligner" pedal and sell it for big bucks to bass players.

[soundguy]

someone already designed it. Its called the Little labs IBP. If youve never used one, you'll never understand how you lived without one after you get it.

The DI signal and mic signal are arriving at your machine at different times. The latency is what is causing your phase relationships to get screwy. Flipping the phase aint gonna do much of anything to solve the problem unless you have aligned the two signals so they are exactly 180 degrees out of phase. The IBP will allow you to correct for a little bit out of phase, which in reality is what you are dealing with %90 of the time you have a phase problem in the first place. And while the polarity switch is just that, the IBP is actually a filter which attacks the issue of phase sounding like junk. I used to work for a mixer that used to EQ on the fly for phase and without a doubt, it was the most amazing shit I have ever seen in my life, and I dont imagine there would be a way for another recording engineer to impress me the way bill daly has. The guy is one of the best mixers to ever walk on planet earth. I digress. EQ can be used to solve phase problems. And before you contest that, Ive seen it, Ive heard it, I STILL cant believe it. But that is sorta what is going on with the IBP, I think its an all pass filter if I remember correctly. I think the new ones also have a reamp circuit, so while it may cost a few bucks, it really is an exceptional value. careful if you get one though, you'll want two more.

dave

[cgarges]

I read a comment by George Massenburg that said "if your console doesn't have a polarity switch, you're not using a real console." I found that amusing, but for those of you who don't, here's a trick that can keep you from plugging and unplugging standard vs. polarity-reversed mic cables all day long. Wire a couple of polarity reverse points into your patchbay. This way you can check phase relationships by repatching quickly from the control room. Simply take a short piece of wire and connect the positive of one point on the bay to the negative of another, and connect the negative of the first point to the positve of the second, as well. If you don't have extra points on your bay available, make some short polarity-reversed patch cables for your bay (if your bay is balanced) and that again will keep you from having to run back and forth between your studio and control room. And by the way, I believe that phase relationships are everything in recording.

[bobbydj]

Brilliant, brilliant and bloody brilliant. Thanks for all those, esp. grrayson - that was a really big help. Cheers all.

[brian beattie]

dave
did bill daly correct phase by equally sliding all of the faders on a graphic on the questionable channel??
brian

[soundguy]

no, no graphic. The phase relationship between a mic an actor is wearing and the boom pole above their head is a *constantly* changing variable as they all move around, so the mixer would EQ the track to account for the change in tone when the wireless mics interact with the pole, which in bills world, is all the time. This guy is operating really on a plane all his own, there isnt ANYONE out there doing what this guy is doing. You can hear his work on Law And Order SVU, hands down, the best sounding production sound on TV, anywhere. Sadly, its a TV show... The EQ on his console is a parametric.

dave

[cgarges]

So I would imagine his (primarily frequency) settings are changing "all the time," too, right? Awesome. If he's doing audio for TV, it's a little easier for him (albiet time consuming) to get it right. Especially if he's doing audio editing with a Fairlight. There's a guy here in Charlotte, NC named Joe Kuhlmann who's really good at the same thing.

[soundguy]

the practices on SVU are a very very long and mulitfaceted conversation. Changing all the time, yes. however, there is absolutely NO time to do it in. The reason bill has the rep he has is that that show can throw the most fucked up situation that no mixer on earth could manage, and bill gets it on the first take, every time. Granted his crew is crack, but the guy is a legend. There is no fairlight editing or any such nonsense, the show is mixed in mono on a nagra just like every sync for picture job SHOULD be done. Its difficult to describe exactly how beyond the pale this man's performance is.

take for instance, an actor sitting in a chair, talking across a desk, reaches over his shoulder bending down into a drawer and then turns to the guy on his left, delivering dialogue the entire time. Its very common to have part of the scene on one pole, the half the line in the drawer on a wire, the turn on a second pole and the end on a wire on another actor. And the EQ is constantly sweeping to make up for the phase relationships between the different mics. To watch the show, you would never imagine in your wildest dreams that something that impossible is happening in real time, mixed to mono with sometimes only 1 blocking rehearsel. The show just sounds naturalish, you dont even begin to anticipate the work that goes into every setup. This is the kind of situation that %99 of the mixers out there would tell you is impossible to record in that fashion or fail miserably trying to do it. This is also the kind of situation that production has come to expect the mixer on the show to handle flawlessly. And he does, episode after episode. Legendary doesnt really even do the dude justice. Really. And Bill's histroy is the stuff of defining industry standards for film production. Ever see time code readouts on a slate? He put it there. Legendary just scratches the surface.

The entire time I worked on that show I was never- not one time- able to discern the sound of a single microphone, and I own all of the same mics that he does. Its a very humbling experience to work with someone as gifted as that. There is pretty much NO other human walking this earth who is doing what bill is doing. How many engineers can you say that about? crazy. Shame its TV, but thats life I suppose.

Luckily, in a recording studio you dont have to worry about the phase relationships between 3 or 4 constantly moving microphones. The amp doesnt move (or isnt supposed to) the mic you stick in front of it doesnt move, and whatever latency between the DI and cab mic (s) will always be a constant until you move your mic position. Dealing with a phase problem like that, well, its not really a PROBLEM, its just doing the work of a recording engineer. An IBP works suprisingly well, I never liked combining DI and cab until I got one of those, at least not with an SVT. If you cant afford an IBP, set a good sound with your DI, use the amp as fill and have an assistant move the mic in front of the cab while you listen until you find the best spot where the sound is fullest. The best spot for the mic filling the DI is likely not going to be the best spot if you just mic'd the cab alone without the DI. This kind of stuff is easy, move the mic 'till it sounds good. Recording sound for TV might be the hardest gig for a soundman, but thats a different conversation entirely.

dave