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The musicians are all in tune, but are your microphones in time?

If you use more than one microphone, it's likely that their timing is out. Does this matter? What can you do about it?

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Imagine this... you are recording an orchestra. You use the conventional setup of a basic pair of mics to cover the whole orchestra - let's say a coincident crossed pair - supplemented by spot microphones on individual groups of instruments. A pair of ambience mics at the rear of the auditorium completes the arrangement.

So the music starts. Stravinsky's Rite of Spring, which opens with a solo bassoon.

The first note strains out of the instrument in its topmost register. After three or four milliseconds it arrives at the spot mic on the bassoon section. The engineer can see the meter on that channel twitching.

The sound travels further and after another twenty or so milliseconds it arrives at the stereo pair. It spreads out into the auditorium and after a further forty milliseconds it hits the ambience mics. It reverberates around the auditorium and continues to reflect back into the ambience mics over a period of around a couple of seconds.

What we can see here is that the sound is being stretched out in time, due to the distance between the mics. Anyone sitting in the auditorium hears the sound once only, plus the reverberation. But on the recording, you hear it three times.

Clearly there is something not entirely right about this. But the solution is simple...

On a hard disk recording system it is possible to advance or delay tracks in time. So it is a simple matter to delay the spot mic on the bassoons so that it time aligns with the crossed pair. You could go further and align the ambience mics, but because of the general mass of reverberation it won't make any real difference.

The time delay between the spot mic and the stereo pair is, in this instance, twenty milliseconds. Mixing a signal with a delayed version of the same signal can result in phase cancellation. But the delay would have to be under 10 milliseconds to approach being perceptible.

Also, 20 milliseconds is short enough that the delay isn't perceived as an echo, but as a reinforcement of the earlier signal.

So although on the face of it it looks like the sound of a multi-miked orchestra would be a bit of a jumble, considering the number of spot mics in use and the various different time delays. But in practice the delays fall within a window where they are not too noticeable.

So many orchestral recordings have been made without compensation for delays that we are well used to the effect. But in the quest for perfection, time alignment will take you one small step further.

Where time alignment is also significant is where you use two mics on one instrument. A good example is a mic close to an electric guitar cabinet, coupled with another maybe a meter away. This is a good method of miking a cabinet, but here the distance is within the zone that leads to phase cancellations, so time alignment is well worth a try.

It's odd that time delay compensation of loudspeakers is considered of major important in live sound, but we hardly think of it at the microphone end in recording.

Never too late to start though...

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By David Mellor Thursday November 30, 2006
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