Sure, I understand the use of longer FFT runs (more points) for raising the spurie from above the FFT "grass" but it's worth mentioning about the noise floor as it seems to be a common mistake made with FFTs - misinterpreting the "grass" as the device's noisefloor.

Thanks for that, Ian - yes, didn't want to make too big a deal about the noise floor thing, just point it out.
Considering that the current model of hearing has the concept of filter banks (frequency bins) at it's core, it may well be that the pscychoacoustics of hearing actually are somewhat hinted at by the bins of an FFT than a standard noise floor measurement. Of course it's too simplistic an analogy but it does hint at some of the sort of dynamic variabilities that might be at play in the perception of hearing?

I will proceed to intro section & write something short

do you think these areas you mention would be of interest in a new topic discussing measurements & the various aspects to them?

There already are a couple of threads discussing the merits and validity/relevance of measurements.
From what I understand is that the measuring crowd here is currently trying to get all their noses and measurement techniques in the same direction.
More uniform as it were.

For instance you mention that the actual noise floor could be 20dB above the highest THD spike found when -10dB to -20dB signal is used.

Those aren't exact numbers, but more ballpark figures in the range of what I think it might be.
The difficulty lies in providing proof as we need other methods of testing.
The surrent set of measurement methods is not able to show these effects using real music.
Have some ideas but these require lots of math (not my strong point) and very powerfull and expensive ADC's

I'm interested in the IMD products that we see in plots of two tone signals & their implications for real world behaviour of a DAC. For instance a two tone plot shows IMD products as a certain level but multitone plots always show higher spuria, if the IMD products now overlay one another.

I would be interested in following up on these thoughts & learning something about this in another thread if you or others found it worthwhile to do so!

IM products are very dissonant and aren't masked as easily as harmonics.
The raised noise floor consists of all noise products.
IM products, however, can also exist below fundamentals so are interesting.

I believe Marv/Purrin/Merv already experimented with multiple tones, 2 tones of various amplitudes and frequencies and also looked into this by measuring this in headphones.


This doesn't really belong in the Yggdrasil thread though.
Perhaps a mod can move this part to a more suitable thread ?

Done. Carry on

When looking at SNR from FFTs shouldn't the FFT gain be taken into account? In other words the noise floor seen on the FFT is the noise floor of the FFT bins NOT the noise floor of the device & the FFT process gain needs to be taken into account to calculate the noise floor of the device & thereby it's correct SNR.

Have a look at http://www.designnews.com/author.asp?doc_id=236273&dfpPParams=ht_13,industry_aero,industry_gov,industry_machinery,aid_236273&dfpLayout=blog where FFT gain is calculated as FFT gain = 10*log(M/2) where M equals the number of points processed in the FFT plot.

The 6384 pts used in these plots gives a FFT gain of about 39dB - thus a 120dB "noise floor" seen on an FFT plot is actually 81dB real noise floor

Thanks for bringing this up. That is a very good point, and I might have overlooked that in my own interpretation of the results.