Lobby > Amp and DAC Measurements
FFT Process Gain in Spectrum (Distortion and FR) Measurements
jkeny:
--- Quote from: Solderdude on September 03, 2015, 02:42:20 PM ---Hi John,
Welcome to the forum...
It is customary in this forum to introduce yourself here:
http://www.changstar.com/index.php/topic,1784.0.html
Most will know you though... I reckon... as some use your products.
The noise floor in Merv's measurements is determined by the used soundcard and usage of a 1:10 probe.
You are correct though that the real noise floor is well above the 'seen' noise floor in the plots.
All the THD spikes from a complete audio signal spans along the whole frequency range lifting the actual noise floor to levels as determined by the highest harmonics (and jitter) and depending on the 'density' of the spikes the 'energy content' of those spikes will add like random noise adds.
In other words the actual noise floor could actually be raised about 20dB above that of the highest THD spike as found when -10dBFS to -20dBFS.
Still, I believe (based on observations of noise floors of actual recordings) that the noise floor in recordings usually still is well above the noise added by a decent DAC.
Based on atomic bobs measurements I estimate the real life noisefloor of the Yggy to be between -100dB and -110dB, still well below any recording I know.
--- End quote ---
A second reply to your post, Ian - do you think these areas you mention would be of interest in a new topic discussing measurements & the various aspects to them?
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.
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. As you say "All the THD spikes from a complete audio signal spans along the whole frequency range lifting the actual noise floor to levels as determined by the highest harmonics (and jitter) and depending on the 'density' of the spikes the 'energy content' of those spikes will add like random noise adds."
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!
Solderdude:
--- Quote from: jkeny on September 03, 2015, 03:56:24 PM ---do you think these areas you mention would be of interest in a new topic discussing measurements & the various aspects to them?
--- End quote ---
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.
--- Quote from: jkeny on September 03, 2015, 03:56:24 PM ---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.
--- End quote ---
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
--- Quote from: jkeny on September 03, 2015, 03:56:24 PM ---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!
--- End quote ---
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 ?
jkeny:
--- Quote from: Solderdude on September 03, 2015, 05:49:21 PM ---...
Perhaps a mod can move this part to a more suitable thread ?
--- End quote ---
Yes, Frans, that would be good & sorry for mistakenly calling you Ian :)
ultrabike:
Done. Carry on :)
--- Quote from: jkeny on September 03, 2015, 11:50:38 AM ---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
--- End quote ---
Thanks for bringing this up. That is a very good point, and I might have overlooked that in my own interpretation of the results.
jkeny:
Thanks for splitting this off into it's own thread.
I just discovered Solderdude's thread "Musings on Future Measurements" & will have a read of it so I don't duplicate anything already covered
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