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<blockquote data-quote="Langston Holland" data-source="post: 56101" data-attributes="member: 171">Re: Dual Channel FFTHi Mark:You obviously have a solid grasp on what&#039;s going on and your logic is spot-on, but you&#039;re missing something simple (one of my specialties).Here&#039;s the issue - the pink noise stimulus in your computer is slightly different than the pink noise stimulus that you&#039;d measure even with a hardwire loopback of the measurement channel. There is at least one reason for this and possibly two:1. The pink noise returned to the measurement channel went through D/A and A/D conversion that the internal pink noise did not.2. Depending on the OS and sound card, every time you start your measurement software the roundtrip latency may change slightly.Forcing your reference channel pink noise to go through the same D/A, A/D and roundtrip latency will force the measurement to yield the truth, the whole truth and nothing but the truth about the behavior of the DUT. Nevertheless, the difference between an internal reference stimulus and the one that went through the same converters and latency as the measurement channel are typically insignificant for acoustic work.A final note is that some measurement programs offer companion sound devices that do not bother to route the stimulus through the D/A, A/D stages and yet are able to get at the whole truth. This is because the hardware&#039;s behavior is known to the mfg and they account for the changes that occur internally.</blockquote>

[QUOTE="Langston Holland, post: 56101, member: 171"] Re: Dual Channel FFT Hi Mark: You obviously have a solid grasp on what's going on and your logic is spot-on, but you're missing something simple (one of my specialties). Here's the issue - the pink noise stimulus in your computer is slightly different than the pink noise stimulus that you'd measure even with a hardwire loopback of the measurement channel. There is at least one reason for this and possibly two: 1. The pink noise returned to the measurement channel went through D/A and A/D conversion that the internal pink noise did not. 2. Depending on the OS and sound card, every time you start your measurement software the roundtrip latency may change slightly. Forcing your reference channel pink noise to go through the same D/A, A/D and roundtrip latency will force the measurement to yield the truth, the whole truth and nothing but the truth about the behavior of the DUT. Nevertheless, the difference between an internal reference stimulus and the one that went through the same converters and latency as the measurement channel are typically insignificant for acoustic work. A final note is that some measurement programs offer companion sound devices that do not bother to route the stimulus through the D/A, A/D stages and yet are able to get at the whole truth. This is because the hardware's behavior is known to the mfg and they account for the changes that occur internally. [/QUOTE]

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