Hey, I'm wondering which real time spectrum analyzer might work best with Reason?

IMHO Ableton Live

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You can use the Reason Vocoder as a spectrum analyzer. Just split your output signal with a spider and route 1 signal into the vocoder mod input and 1 to the hardware interface. Advantage is, that you don't need any further software. No rewire and so on.

Oh yeah... I remember seeing something to do with that on Youtube... if you searched you could probably find it.

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tried it but I find that the vocoder max's out, especially in the bottom end, so you can't get a really clear picture.

Did you raise the number of frequency bands?
And you can adjust it by changing the attack and release.

Freq bands yes. Attack/release no, i will see if it helps.

I wired it up between the mastering combi and the main output, and then tested it by doing a freq sweep with the EQ in the mastering combi. It was quite responsive in the low freq but no so in the highs.