Fixing Timing Issues With Simultaneous DI and Mic Recordings

Being in the business of pro audio, we here at PAL believe that supplying our customers with the best possible value and equipment is only half the goal. The other half is making sure that they are given the best possible resources to get the most out of their gear by providing a number of guides, reviews and tips. Last week, we took an in depth look at the studio microphone along with several key facets as they relate to recording such as the most common variations, their frequency charts, inherent problems and their solutions along with plenty of more integral info. To check out part one of our Microphones 101 series, hit the link right here. Now yesterday, we brought our readers the first half of our bass recording guide. We discussed the basic principles of recording a low end frequency instrument along with the two different methods of recording – through DI (direct input or direct injection) or with a traditional mic and amp setup. We touched on the different pros and cons of each, the variations of the DI box, selecting a perfect mic to record an electric bass along with a few tips on mic placement and their effects.

Yesterday, we left off on the two different methods of bass recording and each of their pros and cons in order to help out our readers decide what would work best for them. Recording through DI – that is, directly into the mixer or computer – will give you a clean, consistent and unadulterated bass signal that is easy to work with in DAW programs and can be compressed and equalized much clearer as well. The problem with a DI is that it might sound too clean for some. Also, unless your bass pickups are of the active variety, you will need the help of a DI box in order to get things properly going. The other method is the much more traditional mic and amp setup. This method is a bit harder to work out as it requires a loud enough bass amp, a suitable low-end retaining microphone, proper mic placement and a better ear for using compression and EQ, although the natural colorations and tone added by the amplifier (which is usually the main reason people take this route) is pretty much impossible to attain through purely DI. Yes, you can use amp models to give that DI signal some natural amp coloration but it will still sound a slightly off from the real deal at best or completely unnatural and way too digital at worst. So which one should you choose? Well, if you’re feeling up for something slightly more challenging that will give you a truly amazing track (when done right), how about going with both? In fact, this is the route taken by many of today’s top producers for bass recording. By using these two recording approaches simultaneously you can truly get the best of both worlds – but it’s not without its difficulties. The main problem is phase matching; pretty much making sure both tracks are in sync with each other. Essentially, there are two ways you can go about fixing this problem; fixing it beforehand so that you can record live with both signals already in phase or waiting until the tracks have already been recorded to fix it. Let’s take a look at the tougher one first.


Live Phase Matching

The main problem with doing simultaneous DI and mic recording is that the miked signal is always delayed a bit after the DI’ed signal. The reason for this is simply physics; it takes approximately one millisecond per foot traveled for sound to go from the speakers to the microphone, causing the DI’ed signal and the miked signal to be slightly out of phase with each other. When two more or less identical tracks are mixed out of phase it results in comb filtering – a pattern of peaks and dips in the frequency response which almost always results in a weaker overall sound. Instead of giving you a solid, sharp track, you get something that ends up being – for lack of a better word – fuzzy. Well, all of this info will do you no good unless we know how to remedy this problem. The good news is that one of the main methods of fixing phasing issues for simultaneous recording is actually pretty easy – delay! By using an outboard delay line (such as a pedal or preamp) you can artificially delay the DI signal until it matches the miked one. Easy, right? Well, the problem with this method is that it probably won’t work for you depending on several reasons. First thing is you will have to have a unit capable of very high resolution delay times which already rules out analog delays since they usually can’t give you anything below a millisecond or so. To make things worse, digital delays typically need at least half a millisecond of processing time, even if the delay is set to its very minimum. This means that in order for this method to work, the mic can’t be placed anywhere closer than six inches away from the speakers – a real problem considering sometimes a mic will have to be placed as close as an inch from the speaker. But don’t toss that outboard delay line off to the side just yet as they can make other phasing processes much easier, (which we will touch on later).

A good number of digital mixers already include a short high resolution delay line on every channel specifically made for dealing with these very slight time-alignment problems. Some mixers will also allow you to set individual channel delays with single sample accuracy while other mixers include built-in insert effects processors which can give you a high enough timing resolution. If your mixer does happen to have access to an inboard delay line with enough resolution, the first thing you will need to do is to connect both the mic and DI lines into separate channels on the mixer. Once you have each line on individual channels, you will need to record a consistent bass track using both channels simultaneously in order to help you find the appropriate delay time needed. Try to make the recording long, clear and consistent without too many shifts as this recording will simply be used as a guide to figure out the appropriate delay needed. Don’t make it harder on yourself by doing anything other than playing one consistent and clear note for about two minutes or so. Now that you have your test tracks recorded, make sure you pan both channels to the center in order to make it easier to detect certain comb filtering problems.  Also, set them both at the same approximate audible level (which may be different than the actual volume setting of each). The next thing you need to do is reverse the polarity of the DI track by hitting that channel’s phase invert switch. Now you can begin experimenting with the DI channel’s delay time setting.

Start off by selecting the minimum amount of delay possible. Keep on upping the delay with the smallest increments possible until you suddenly hear a drop in volume. Move the delay up and down around this point until you find the delay setting that causes the greatest drop in volume. If you don’t happen to hear any sort of drop in volume, you probably don’t have a high enough resolution delay, which means you will have to skip down to the post recording method described below in order to fix phasing. Alright, once you have found this delay setting, try to find an even lower point in volume by moving the in very small increments either closer or away from the speakers. Moving about a quarter inch or so at a time should be small enough to detect this lowest volume point. Alright, now that you found this minimum volume point, simply hit the phase invert switch once again to undo it, and you should have both tracks perfectly in phase, ready to simultaneously live record and DI’ed and miked bass line! Just don’t forget to balance your channels appropriately before you record.


Post Recording Phase Matching

Certain DAW software such as Logic Pro can make phase matching a much simpler task











But what if you don’t have a mixer capable of creating the appropriate amount of delay resolution? You still have options and this one is actually a bit easier than the one mentioned above but you won’t have the benefit of being able to fix the problem before recording.  All you have to do is set it up so that you can record the DI and mic signals onto individual tracks so that you can align them later on while mixing. Sure, it will take an extra step during the mixing phase as opposed to the delay method mentioned above (which takes care of track phasing before recording), but it will still sounds much better than simply doing nothing. If you’re using an analog or digital tape multi-track recorder, you can fix the phasing issue by using a delay line in a way similar to tracking (read: manually). Some digital multi-track tape recorders also provide the option for delaying individual tracks which can make fixing phasing issues that much easier since it won’t require the use of any outside delay processor.

For those of you equipped with digital recorders that use a hard disk, memory cards or other digital storage media, you can either start by displaying the waveform (if your recorder has this option) or send both tracks to your DAW software. If you don’t happen to own any DAW software (but somehow own a DI box, a recorder, amps and mics), you can use certain free audio editing software such as Audacity (which, unlike DAW, only feature simple editing tools such as cut, paste, etc). Anyways, regardless of the method, once you have both waveforms in front of you try zooming in until you can clearly see the initial timing of the attack for each track. You should notice that the mic track starts slightly after the DI. Now, all you have to do is use your particular program or device’s editing features in order to line up the mic track with the DI track. And just in case you’re wondering why not just simply use a delay plug-in to fix the problem, plug-ins introduce latency which will pretty much only add to the problem instead of fixing it.

While this may seem like a huge investment in time, it’s actually a lot easier once you get the hang of it and completely worth it when you start noticing how much stronger a properly phased track will sound.



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