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<blockquote data-quote="Arthur Skudra" data-source="post: 56107" data-attributes="member: 131">Re: Dual Channel FFTDual channel FFT&#039;s make some assumptions regarding the data being fed into it:&nbsp; 1) Both the measurement and reference inputs of whatever interface is being used must treat the individual inputs the same in terms of frequency/phase response, 2) The timing between the two channels must be stable, or synchronous.&nbsp; Get those two assumptions right to begin with, and you&#039;re starting down the road towards making valid measurements with a dual channel FFT.Smaart 7.3 kinda lets you do what I think you&#039;re describing with their internal &quot;loopback&quot; feature, however the clocks *must* be synchronous output vs. input, which limits which audio interfaces will work with this feature, and also lengthens the propagation time giving you approximately 40-50 ms additional delay time for the signal to be eventually routed back as &quot;reference&quot; into a measurement engine.&nbsp; This also limits you to using ONE interface to handle ALL inputs and outputs (for the sake of clock synchronization).&nbsp; Nevertheless, Langston raises very valid points about differences in the signal between what the dual channel FFT thinks in &quot;real&quot; and what is &quot;reality.&quot;&nbsp; If you want to do the internal loopback feature in Smaart 7.3, then use a very high quality audio interface, and not the cheap built-in sound card on your computer, in addition to clock synchronization, what &quot;goes out&quot; must match what &quot;comes in&quot; in order to make the measurement valid.</blockquote>

[QUOTE="Arthur Skudra, post: 56107, member: 131"] Re: Dual Channel FFT Dual channel FFT's make some assumptions regarding the data being fed into it: 1) Both the measurement and reference inputs of whatever interface is being used must treat the individual inputs the same in terms of frequency/phase response, 2) The timing between the two channels must be stable, or synchronous. Get those two assumptions right to begin with, and you're starting down the road towards making valid measurements with a dual channel FFT. Smaart 7.3 kinda lets you do what I think you're describing with their internal "loopback" feature, however the clocks *must* be synchronous output vs. input, which limits which audio interfaces will work with this feature, and also lengthens the propagation time giving you approximately 40-50 ms additional delay time for the signal to be eventually routed back as "reference" into a measurement engine. This also limits you to using ONE interface to handle ALL inputs and outputs (for the sake of clock synchronization). Nevertheless, Langston raises very valid points about differences in the signal between what the dual channel FFT thinks in "real" and what is "reality." If you want to do the internal loopback feature in Smaart 7.3, then use a very high quality audio interface, and not the cheap built-in sound card on your computer, in addition to clock synchronization, what "goes out" must match what "comes in" in order to make the measurement valid. [/QUOTE]

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