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Matching The Phase Of Mic & DI

In a recent tech tip we discussed multi-miking techniques and how signals arriving from different mics at different times (due to placement) could cause phase cancellation (Getting the best recorded sound from acoustic guitar 2/16/05). We also mentioned that this is a more common problem when combining miked signals with direct signals, and that there are techniques for aligning them to avoid cancellation. If you recall, we stated that some acoustic guitars have an onboard transducer with a direct out that can be mixed with the miked sound, and that in some cases can produce a desirable outcome. However, the combination of miked and direct signals is more of an issue with bass than it is with acoustic guitar. Since the techniques are the same, we’ll discuss them in terms of bass, since that is the more prevalent usage, and also, because we don’t want bass players to feel left out.

There are two main methods for recording bass guitar: miking an amp, which picks up the amp/speaker combination’s character, and recording direct, which avoids the amp by feeding the instrument directly into your mixer or multitrack recorder, via a Direct Injection (DI) box or other interface, for a cleaner sound.

To get the best of both worlds, some engineers use both of these approaches simultaneously. The problem is that the miked signal always ends up being delayed slightly in relation to the DI signal, because of the time sound takes to travel through the air from the speaker cab to the microphone — approximately one millisecond per foot traveled. This delay causes the two similar signals to be out of phase with each other, so that mixing them together results in comb filtering, which will weaken the combined sound.

However, you can compensate for this delay in order to retain a strong sound – and there are ways to do this both before and after the recording has taken place.

During recording, the simplest solution to match the phase of the mic and DI signals is to use an outboard delay line to delay the DI signal in order to match its phase with that of the already-delayed mic signal. For this to work, you’ll need to have a fine delay-time resolution available to you, which rules out most analog delays as these can’t usually resolve delays of less than a millisecond or so. Furthermore, digital delays will typically need at least half a millisecond of processing time, even when set to their minimum delay time, so they’ll work only if the mic is placed six inches or more away from the speaker — sometimes, the amp’s mic sits only about an inch away from the speaker cone.

However, a number of digital mixers include a short high-resolution delay line on every channel, which is ideal for time-alignment purposes. For example the Yamaha O-series mixers allow you to set individual channel delays with single-sample accuracy. Other digital mixers include built-in insert effects processors which offer a high enough timing resolution.

If you do have access to a delay line that can handle it, then the first step is to patch both mic and DI line into separate mixer channels. Pan the channels to center, as this makes it easier to detect comb-filtering problems. If you’re using an outboard delay line, patch it into the direct path’s insert point or between the DI box and the mixer. Alternatively a similar routing could be implemented within a digital mixer using built-in effects processing, though it would be worth placing delay processors into both channels (with the mic channel’s delay time set to zero) in order to remove the processing delay of the insert from the equation.

Once you’ve done this, get the bassist to play continuously on one string, at a consistent level. Compensating for the processing delay of users of physical modeling processors, such as the Bass Pod from Line 6 have an additional factor to take into account. Both the digital conversion and the physical modeling process take finite amounts of time — a few milliseconds, in fact — which means that the signal emerging from the outputs could be even later, compared to the DI signal, than the signal from a miked cabinet would have been. Line 6 has dealt with this issue on the Bass Pod by incorporating a dedicated DI output on the unit itself. This simply offers a clean throughput of the input signal, which is delayed to time-align it with the physically modeled signal. The difference between using this output and a traditional DI box split is dramatic, as the Pod’s 9mS or so latency places it firmly in comb-filtering territory. Using the time-aligned DI, you can freely balance the clean and amp-simulated signals against one another without producing any notching or tonal coloration — the two signals simply add.

Once you’ve got audio coming into the two channels, solo each in turn and set them for the same approximate level. Then temporarily reverse the polarity of the DI audio out using that mixer channel’s phase invert switch. Now it’s time to experiment with the DI channel’s delay-time setting. Starting from the minimum delay available, slowly increase the delay using the smallest increments possible. At some point, the overall level of the combined mic and DI signal will start to drop, and you should try to find the delay setting that causes the greatest drop.

If you get no volume drop, then either the delay line can’t achieve a short enough delay, or the resolution isn’t sufficiently fine. In which case you’d be best advised to record the mic and DI signals onto separate tracks and then to attempt to fix any comb-filtering problems later (see below).

Having found the minimum volume point, try for an even lower volume by moving the mic in very small increments closer to or further from the speaker cone — working in quarters of an inch is a fairly safe bet. If you have no one to watch the meters for you while you move the mic, then it can be handy to write down meter levels between moves to keep a track of things.

If you’ve followed these instructions, you ought now to have almost completely compensated for the delay in the miked signal, so it’ll be time to undo the phase change on the DI channel. The two signals should now be well in phase, and ought to be stronger together than they were before. All that remains is to balance the channels appropriately.

If you don’t have a suitable delay to hand while tracking, then you should record the mic and DI signals to separate tracks so that you have the option of aligning them later while mixing. If you’re recording to analog tape, or to a digital multitrack tape machine, then such fixing might have to be done with a delay line in a similar way as when tracking. Some digital tape machines, notably the popular Alesis ADAT HD24, provide facilities for delaying their individual tracks, which can simplify the correction process considerably as no external delay processor is required.

With hard disk recorders and software based DAWs, their waveform displays and non-destructive editing functions can be used instead of delay processing in order to line up the audio. All you have to do is zoom in the waveform display view far enough so that you can clearly see the initial attack of both tracks — you’ll see that the audio recorded from the mic starts slightly later than that recorded from the DI. Simply use your particular program’s editing facilities to line up the mic track with the DI track. (For software based recording, inserting a delay plug-in into the DI track will not work since plug-ins introduce latency, the very thing we wish to avoid.)

Eliminating comb filtering might take a little time to do satisfactorily, but it can greatly improve your bass guitar sounds. So set aside a few minutes to try out some of the techniques described above — you might well be surprised at how much better your recordings sound in the end.

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