[sdiy] THAT2180A

[Richie Burnett]

Sin & Cos curves over the first quadrant give panning with constant power summation. This is apparent from the trig identity: sin^2 + cos^2 = 1

(Whether or not constant power summation is the correct panning law for a particular listening environment is a subject of debate and personal taste. Things like mono compatibility can influence the decision.)

As for an analogue circuit to generate the panning law, that is what the first part of that schematic does. The switchable resistors allow the attenuation at the "panned center" position to be adjusted to achieve various different panning laws. I wouldn't consider it a hack though. It's an engineered circuit that performs the required function cheaply with acceptable accuracy.

FWIW, I would generate the curves digitally too. Either using a look up table on a PIC or polynomial approximation on a DSP.  It really doesn't need to be that accurate as panning is quite a subjective quantity like volume. If someone pans something three-quarters to the left, and it doesn't sound far enough over they'll turn the pot a bit further round. And if it now sounds a bit too quiet they'll nudge up the volume a bit too until it sounds how they think it should.

-Richie,

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