Adventures In Audio
Finally, Pro Tools gets new pan laws!

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of Adventures In Audio.

Thursday January 6, 2011

How do you tell the difference between an engineer and a producer? Simple - get them to talk about pan laws and see who wants to talk all day, and who gives up after two minutes.

Pan laws are in the detail of sound engineering, but they are important and anyone involved in recording would benefit from a basic understanding.

Once upon a time there was mono. Then someone invented stereo.

In mono, every instrument goes to the same channel. In the early days of stereo you could switch instruments left, right or center (center is the same as sending both to left and right at equal levels.)

And then some bright spark invented the panoramic potentiometer, or panpot as we usually call it.

With this, you could direct an instrument to any point in the stereo sound field.

But there was a problem, and still is...

Suppose you sweep an instrument all the way from hard left to hard right. Assuming you keep the fader constant, will its level change?

Well that depends on the pan law.

Let's call the level of the signal S; the level of the left output channel L; and the level of the right output channel R.

So if you pan all the way left, L = S and R = 0.

If you pan all the way right, L = 0 and R = S

What if you pan center? Then what should happen is L = S/2 and R = S/2

In terms of decibels this means that when the pan control is pointing to the center, the level in each channel is minus 6 dB compared to the original signal. Adding two identical signals together always results in an increase of 6 dB, which brings us back to where we were in level.

But there is a difference according to whether you are creating work that will end up in mono, which in broadcasting still is important, or work that will end up in stereo.

In fact, the -6 dB center position works properly only for mono. You can pan a signal anywhere you like (assuming the in-between settings of the panpot are in proportion) and it will always be the same level in the mono mix.

But these days we nearly always think of stereo as our end point. Surely therefore a signal should appear to be the same level in the stereo image wherever you pan it.

Well if you had an anechoic listening room and totally identical loudspeakers speakers, the same would apply.

But most listening environments, at least on loudspeakers, have a certain amount of reverberation. When this is taken into account it is better that the drop at the center position of the pan control should be only 3 dB (according to early research by Disney, or 4.5 dB according to later research by the BBC). This is because differences in the reverberation decorrelate signals from the left and right speakers that would otherwise be identical.

If you want to look this up further, look at how correlated and uncorrelated signals add together. It's fun ;-)

And since many people listen on headphones, where the left and right signals add actually in the brain, whether anyone has considered the response of the human nervous system with respect to pan law, I don't know.

Anyway, back to Pro Tools.

In the olden days of Pro Tools, i.e. Version 8 and before, the pan law was always -2.5 dB in the center. But now you can select it to be -3, -4.5 or -6 dB.

Clearly this will give tweak heads plenty to play around with, and of course mixes made on older systems will now sound subtly different.

It all adds pleasure to the recording process!

Image: CC BY 3.0

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