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Is this argument against DI + Mic valid?

P

PhilaCPJ

May 24, 2005
27
1
49
Metro Philadelphia
#1
Supposedly if you mic a bass cab and simultaneously track a DI, you'll get something called "cumulative spectral subwave delay", where the transient points of the waves may line up, but issues with phase and velocity still exist. The DI box gets a full waveform, while the mic capsule acts as a filter (depending on its resonant frequency) and produces a partial waveform. Somehow when the two waveforms are finally added, the bass becomes allegedly spotty and thin.

I don't know enough about physics as it relates to audio to truly understand this. Any engineers or audio geeks capable of explaining it better?
 
#2
Each will give a very complex representation of the waveform. Anytime you use multiple devices on a source, there will be phase differences, be it 2 mics or a mic + DI. If the result is a thin sound than it's likely due to phase cancellation. However, blending the two signals doesn't necessarily result in a thin sound and actually the opposite is often true, ie, the small differences give a fatter sound. Phase cancellation occurs when the signals are ~180 degrees off.

BTW, a DI almost always uses a transformer, which has it's own resonate frequency and doesn't present a pure picture of the bass either.
 
#4
Denyle Guitars said:
Each will give a very complex representation of the waveform. Anytime you use multiple devices on a source, there will be phase differences, be it 2 mics or a mic + DI. If the result is a thin sound than it's likely due to phase cancellation. However, blending the two signals doesn't necessarily result in a thin sound and actually the opposite is often true, ie, the small differences give a fatter sound. Phase cancellation occurs when the signals are ~180 degrees off.
Click to expand...

Hmmm....interesting. I'd always thought that was true: a DI + Mic + squashing the hell out of the signal gives you a huge bass tone that sits way up front. Haven't actually tried it yet, although supposedly Geddy Lee does something like that. I think he said he runs a clean signal and a dirty signal from a SABDDI into a compressor and mixes them to get the desired result.

What are you looking to get out of the DI vs. the mic? DI for the bottom and the mic for the mids and highs that cut through the mix?
 
#5
I'm no expert, but I would have thought that if the transients are aligned that means the signals are in phase - plain and simple.

Velocity? WHAT? I don't get that bit at all. Can anyone shed some light on that for me?

The DI box gets a full waveform, while the mic capsule acts as a filter (depending on its resonant frequency) and produces a partial waveform.
Click to expand...

Is the theory that this "cumulative spectral subwave delay" only occurs when you use BOTH a DI and a mic? And that when doing so the microphone somehow alters it behaviour? That would require that the microphone is "aware" that a DI box is tapping the same signal - which is of course ludicrous.

Most of us know that aligning the microphone for correct phase is important if you plan to combine the signals. We would also be aware that the mic signal is going to be very different simply because of the fact that it's a transducer.
 
#6
Anytime you mix two signals from two different devices you will get some phase cancellation. This is due to the distance from the source to the devices being different, therefore misaligning the waveforms. If you have the time to adjust your sources as to find a pleasing mix of the two then phase cancellation be damned - what sounds good sounds good. If you don't have time to carefully position your mic (with a DI) or multiple mics then you may possibly wind up with a thin sound due to the phases being off, thereby canceling frequencies. It is possible to fix this in a DAW using a phase correction plug-in (not a phaser) to move the phase of one of the signals until you get a more pleasing tone.

I run a small studio so I am reasonably knowledgeable, but I am certainly not an expert. Generally speaking most bands I record really don't have the budget for me to spend time moving a cab mic around or screwing with a phase plug trying to get a pleasing mix of cab and DI so I generally steer bands away from it. But in the rare occurence that they have the money, and therefore the time, I am happy to do it. It can sound wonderful, but a good DI generally sounds fine in nearly all mixes so it all depends on how much time / money you want to spend and what kind of results you want to get!
 
#7
If you want the mix of cab and Di you can always track both and find a transient and line it up in your DAW for both tracks. The phase won't be perfect (eqs and other devices can change the phase of specific frequencys as opposed to the entire signal) but it will be close enough not to matter to most. Also it is much faster than trying to move the mic around if you're in a time crunch.

Oh yeah, most of the phase change from a microphone is caused by its distance from the speaker not the electronics.
 
#9
i think most of the important things to put into practise have been said already, so just remember in theory that, although physics MAKES SENSE, when it actually comes to putting it into practise everything is a different ballgame.

If it sounds good, keep it! The inherent flaws in playing, and equipment (seen scientifically) are what gives music its character.
 
#10
it's worth remembering that phase issues aren't some black magic that's created by electronic recording processes. they exist in the world, in any acoustic space. room ambience and room reflections create phase issues all the time. whenever you find a room (or part of a room) boomy or thin sounding, you're experiencing phase issues--the direct sound and various reflections either reinforcing each other or subtracting from each other.

ime, the sound from these multiple signals with different timings is more likely to be fatter than thinner. Sound in a live room tends to be richer than sound in the middle of a field, or an anechoic chamber--places that eliminate phase effects. i'm no recording expert, but the few times i've recorded or played live with both a mic and and d.i., there were no weird sounding issues, and no need to fuss around much with mic placement. it just sounded good. in general, we've liked the sound best when mixed pretty heavily toward the d.i., and with just enough speaker/room sound to fatten and liven things up.
 
#11
Just a query is this targeted at both live and recording situations? Because if its live your sound check should be sufficient time to pick up ANY problems (especially as they shouldn't be likely to be there as a mic and a D.I will be gettin significantly differently signals). Remember a mic is being placed after a processed sound which would then be different meaning it would interact with the other DI'd waveform as if its a totally different signal and phase problems USUALLY occur when the 2 signals are almost perfectly the same. In a studio situation i would say no problem whatsoever as you can easily fiddle about with ANY apparent phase problems after recording.
 

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