dj! (112dB) wrote:I don't think you can properly check the frequency response of a M/S processor using stereo white noise.
I'd personally propose (at least for this particular case) to define the combined amplitude response (a function of input pan) as the L2 norm of the amplitude responses of the output channels. The phases won't interact, and amplitude summing works the same as what one would get with normal circular panning law, and frequency dependent stereo width control doesn't affect the results (assuming it preserves the combined level in the L2 sense).
With such a definition it's meaningful then to measure frequency response with pan-potted mono white noise (with pan position determining which direction we want to measure... obviously one can sweep over the stereo field and graph frequency response as a function of direction or whatever).
As for stereo white noise directly, especially in the case of headphone listening where phase-interactions don't exist? At least I can't come up with a definition of stereo frequency response that would make sense for such measurements.