Rigor in tracking low level measurements involving PCs

BE718

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No they are sequential measurements second to second. We could get rid of them and that is what we do but I like to analyze them to find smaller effects. And they are only there from device under test.

So the measurement interval simply dependant on the FFT resolution?

Well the FFT will effectively "average" that random noise so you are going to struggle to see anything useful I suspect.

So that leaves looking at the time waveform. Again, just looking at the TW directly you are probably going to struggle to see anything useful, however a technique we use with vibration analysis is auto correlation. It looks for repeating patterns in the waveform and picks them out. It's very effective at finding stuff buried in the waveform.

I will post some links and more information about it.
 

BE718

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Yes, Frank, although FFTs are a very valuable & sensitive tool, they have their limitations & recognising these limitations is worthwhile when using any tool to avoid falling into the same old traps, as you say.
A nice analogy for FFTs is a long exposure photograph - the elements that remain fixed during the exposure are sharply defined, the moving elements in the frame are blurred & indistinct. If it's the moving elements that we are interested in then long exposure photographs are not the best method to analyse or even sense these aspects exist.

Noise floor modulation due to the effect of a musical signal should be quite detectable. What I'm thinking is to band limit a musical signal and look at the frequencies around it.
 

jkeny

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Noise floor modulation due to the effect of a musical signal should be quite detectable. What I'm thinking is to band limit a musical signal and look at the frequencies around it.

When you say band limit a musical signal, what do you mean, exactly - a pass band of a couple of frequencies or what?
What surrounding frequencies do you mean & how would you expect them to be affected & displayed on FFTs?
 

jkeny

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Just thought it might be useful to read what Rob Watts of Chord has to say about noise floor modulation:
Good question. Noise floor modulation is very difficult to measure, as ADC's (even the very best) have very significant noise floor modulation. Firstly, what is noise floor modulation? When a sine wave signal is used in a DAC, you get different types of distortion - harmonic distortion (distortion of integer multiples from the sine wave fundamental) enharmonic distortion (distortion products that are non integer) and changes to the noise level. So for example you may have a DAC that produces noise at -120dB with -60dB sine wave (traditional dynamic range test) but the noise with a 0dB sine wave maybe -115dB - thus the noise has increased by reproducing a higher level sine wave - in this case the noise floor (seen by doing an FFT measurement) would increase by 5dB.

Now noise floor modulation is highly audible - it interferes with the brain's processing of data from the ear - and immeasurably small levels of noise floor modulation is audible. I know this as I have listened to noise floor modulation at around -200dB - these numbers are derived from simulation - and heard the effect when the noise floor modulation mechanism was switched on and off.

The problem with noise floor modulation is measuring it, as ADC performance is far worse than Hugo's, and certainly worse than Dave's. With my old Audio Precision (AP) I used to use a fixed frequency passive notch filter to remove the fundamental, then fed the residual into the AP. From this one could determine noise floor modulation, but the AP was not good at resolving small noise floors. But around October 2014 AP launched the APX555, and this had a clever system to enable more accurate measurements of distortion and noise floor. What this instrument does is the employ two ADC's per channel, and an automatic notch filter, so one ADC uses notched out fundamental, and another ADC for the fundamental. The instrument then stitches the two plots together in the digital domain.

It also had a very high purity analogue oscillator - the system has residual THD at 2.5v of -150dB. Since I need a high purity analogue source to test the pro ADC project, and since Dave at that time exceeded the old AP measurement capacity, once AP launched the APX555 I went out and purchased one.
 

BE718

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Just thought it might be useful to read what Rob Watts of Chord has to say about noise floor modulation:

Seems Im thinking along the same lines.

However I would like to know the details behind his assertion that noise floor modulation is highly audible. The details of what he has done to test this assertion and external references that support it.

On the face of it his claim of hearing something at -200 dB is total b******s! Not possible to hear or measure from my understanding of the world. However they may be something that I dont understand or know - but it certainly needs explanation. I simply dont understand it. This is below the noise floor of any DAC or ADC and thermal noise. The quote is either out of context or he lives in a cryogenic chamber and has supermans hearing..
 
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fas42

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On the face of it his claim of hearing something at -200 dB is total b******s! Not possible to hear or measure from my understanding of the world. However they may be something that I dont understand or know - but it certainly needs explanation. I simply dont understand it. This is below the noise floor of any DAC or ADC and probably thermal noise. The quote is either out of context or he lives in a cryogenic chamber.
My guess is that what he was hearing was when "the noise floor modulation mechanism was switched on and off. " IOW, not the -200dB stuff, but the impact of the "modulation mechanism" on the test setup - he was hearing the "motor", that theoretically should not have got in the way ...
 

BE718

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My guess is that what he was hearing was when "the noise floor modulation mechanism was switched on and off. " IOW, not the -200dB stuff, but the impact of the "modulation mechanism" on the test setup - he was hearing the "motor", that theoretically should not have got in the way ...

Yes, I understand this from the quote. Perhaps I should have said hear the effect of something at -200dB. How did he generate a noise floor modulation in real equipment at -200dB? How did he know it was actually at this level? What about the masking of signals that are up to 200 dB higher? Sorry but its non-sensical at the moment. It would need an awful lot of explanation. At the moment its akin to homoeopathy.
 
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jkeny

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Yes, I understand this from the quote. Perhaps I should have said hear the effect of something at -200dB. How did he generate a noise floor modulation in real equipment at -200dB? How did he know it was actually at this level? What about the masking of signals that are up to 200 dB higher? Sorry but its non-sensical at the moment. It would need an awful lot of explanation. At the moment its akin to homoeopathy.
I believe the answers to your questions are:
- he is stating that noise shapers are on of the biggest offenders in causing noise modulation & these figures of -200dB & later -350dB are the calculated noise floor of his 17th order noise shape- it's all about low level signal linearity & the apparent sensitivity of auditory perception to low level signal linearity

From here
Depth perception is indeed mostly down to the reverberant signal to direct sound signal ratio. But another important cue is due to HF absorption of the air, so brighter sounding sounds are perceived as closer. Quite how the brain determines that the frequency response of signal is degraded I don't know; perhaps the brain subtracts the frequency response of reverberant to direct to get a another depth cure, or perhaps its simply bright sounds is inferred as being closer.

For sure, if the reverberant signal is attenuated or distorted with respect to the larger direct signal then depth perception suffers. Indeed, in all of my career of listening and then explaining the results of listening tests, small signal non-linearity is the only factor that I have correlated with depth perception. With noise shapers you get distortion of very small signals, in that signals below the noise shaper noise floor are lost and signals that approach the noise shaper noise floor have reduced amplitude. This is why fundamental linearity measurements are important, as they show the amplitude of small signals changing with level. Using FFT's with the result calibrated at -60.000 dB, then re run the test at -120 dB you check that the level is actually -120.000 dB (with results adjusted for noise). Using my Pulse Array DAC's you get pretty much perfect results (no consistent error). Conventional delta sigma have measurable errors, R2R DAC's have even more, as its impossible to match resistors accurately enough.

In the bad old days, when my designs had significant fundamental linearity errors, improving this problem did lead to better depth perception. Moreover, when I used to design cables in the 80's, one could always hear a correlation between conductor purity, metal surface oxides and depth perception. In this case, the oxides in metal to metal interfaces (crystal to crystal inside the wire and contact to contact) create small signal non linearities, with resistance being a tiny bit higher for small signals than larger signals. But although it was easy to hear depth being degraded, you could never measure this distortion.

Before the Dave project, my target for noise shapers was better than 200 dB distortion and noise performance. that's around 1000 times more capable than high end noise shapers. But with Dave I had ten times more gate capacity than ever before; also whilst listening to different noise shapers when developing Hugo I could hear large changes in depth perception. This indicated that my assumption that 200 dB was good enough was not correct.

At this stage I should indicate that there are actually two entirely different distortion mechanisms that are important to noise shaper designs (there are other things but this is the most important). These are:

1. Noise floor modulation. This is where the noise floor modulates with signal level - so large signals give greater noise than small signals. The brain is remarkably sensitive to this problem, and can easily hear unmeasurably small levels; it perceives it as smoothness or warmth to the sound when the problem gets smaller. With gross noise floor modulation you hear it as grain in the treble. Now all DAC's (except mine) have very large and measurable noise floor modulation, which explains why they sound harder (they normally add some 2nd harmonic to fatten up the sound to hid the innate lack of warmth). Additionally, reducing noise floor modulation improves instrument separation and focus, and you reduce the loudest instrument taking your attention problem.

2. Fundamental linearity - how accurately small signals are reproduced (small signal linearity). Now this only affects depth perception, it does not do anything else. And I have not come up with any change in depth that wasn't explained by small signal linearity.

Now a noise shaper has both problems - making a better noise shaper with say 220 dB performance than 200 dB will have lower noise floor modulation and better small signal linearity.

So when I designed a 220 dB noise shaper and listened to it, I could easily hear better depth - and maybe a bit smoother. So better than 200 dB is a good rule of thumb for noise floor modulation, but certainly not for depth. Indeed, over a 90 day period I constantly improved the noise shapers and came to the absolutely remarkable and frankly amazing conclusion that there was no limit to how good the noise shaper needs to be in order to resolve depth perception. I ended up with an very complex 17th order noise shaper that had 350 dB performance.

With 90 days of work and getting an improvement every day in depth, I could hear now a cavernous depth perception. This is very exciting, as its long been a major problem of mine that one can hear depth so accurately in real life, but high end audio sounds flat as a pancake. Listen to a real organ and choir in a cathedral - if you are 100 feet away and shut your eyes, it sounds uncannily 100 feet away. I am now starting to get something like that depth perception with loudspeakers - but I guess the weak link now is the ADC, which is my next design challenge.

Now when I first got to 350 dB performance (that's the best I can do with available FPGA and flip flop speed) I was simply amazed that the brain was so sensitive. How can it detect such small errors? Perhaps its down to something else, and the noise shaper number is merely a proxy for something else going on in the analogue domain. But recently (a month ago) I upgraded internal digital (no analogue consequences at all) noise shapers within Dave from 220 dB to 350 dB - and lo and behold depth did get much better. So that suggests that the brain really is sensitive to absolutely no error for small signals. But it still perplexes me that minute effects can be so easy to hear.

But this brings me full circle. I have always been puzzled that cable small signal non linearity was never measurable and if its true that -350 dB performance is necessary then this proves why we can't measure cable effects - they are simply too small, and the brain is just too sensitive.

Funny that work in noise shapers can solve a puzzle with metal purity with wires...

And from here he seems to state that we are far more sensitive to low level linearity than is commonly accepted
The perception of depth is a weird phenomena and something we take for granted. I am on holiday in Catalan, Spain and yesterday visited the monastery at Montserrat. We went for a walk, and was about a mile out viewing the monastery; the bells peeled out. The perception of depth was stunning, shut ones eyes and you can hear the bells a mile away with amazing accuracy.

Then we were lucky enough to hear the choir in the basilica. I was 150 feet away, and again, shut ones eyes and the sound was 150 feet away. It was glorious.

But the amazing thing is how the brain manages to compute depth from very tiny cues and get it to such accuracy and we take it entirely for granted.

So far early versions of Dave has been shown at a few shows and listeners have reported back about how unusual the depth perception is with Dave. Now this is due to the DAC resolving accuracy of very small signals - for some reason any small non-linearity of small signals upsets the brains ability to determine depth. What is curious is that there seems to be no limit to how accurate the linearity needs to be; Dave's noise shapers are accurate to -350 dB and this was the performance required by depth perception. Indeed, the brain may be sensitive to even smaller levels, but 350 dB is the best I can do with current FPGA's. But if you had said 2 years ago that one would need 350 dB performance from a noise shaper to get proper depth perception I would have said you were completely mad, as this is ridiculously small levels. But I have done thousands of depth listening tests, and always came to the same conclusion - very very small errors are significant. No doubt the sound science brigade will be on my back about this; but sound science is about observation not pet theories; and the observations are saying that something very weird is going on about depth perception (something which our understanding of how the brain achieves this level of accuracy is very limited).

Getting back to Dave and classical music. Sure classical music is not close miked, and so perceiving depth would be beneficial to that genre. But depth is often added in recordings by adding reverb. Also, its about small signal linearity which is useful for detail resolution as well as depth perception. But because Dave's depth perception is so much deeper than other DAC's its easy to latch onto that aspect of performance - its about four times deeper with recordings that have good depth than Hugo for example. But there is a lot more to Dave than just depth.

BTW, you heard the Dave DAC in that audition of Regen Vs Intona - did you (or others) notice a superior sound stage depth?
 
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BE718

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I'm afraid that doesn't answer anything. There are a lot of statements and claims there, but I don't see a lot to back it up. Soundstage is very dependant on the room acoustics and speaker dispersion which will always be far more a gross effect than the linearity of the DAC.

Ok, I may be missing the point here, gladly admit that, but I'm still struggling with the significance of a - 350dB noise floor modulation in a dac with a noise floor that is much higher, with an amp with a noise floor that is much higher, with a room with a noise floor that is much higher again.

4x better depth than other dacs........mmmmmmmmm.........really.............

There was no direct comparison to other dacs, and although the system sounds good there was nothing overtly special about the sound stage.

Frankly the more I'm reading the more incredulous I'm becoming. This guy reckons he can hear and quantify the purity of metals ....ok.....
 
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jkeny

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OK, I'm not going to get into trying to "prove" what he hears - it seems to me that is a perennial argument on this forum.
And I'm not going to get into the Room Vs source argument & which has more effect on sound stage but I would suggest that if a source is changed & the room isn't & a difference in sound stage is heard then it is logical to conclude that the source is the determining factor

I think this fixation on the headline of -350dB is mistaken - it's not a primary isse - I look on it like this - something else is happening that is affecting our audibility, it's just that noise shaping down to -350dB is needed to stop this "something else" - we don't yet know what this "something else" is & this is just the first indicator of it. A bit like RF noise & it's audible effects - we certainly don't hear RF noise but it does have many & varied consequences in the electronics & it's secondary effects are therefore audible.

"4x better depth than other dacs........mmmmmmmmm.........really............." yea, that's why I asked you about the group audition using the DAVE DAC - to check this statement. I haven't heard a DAVE DAC myself so just wondered.

However, I have heard the effects of lower, more stable noise floor (through the use of battery PS) giving rise to better solidity of the sound stage - mainly as a result of lower level signal audibility i.e. the sonic tails are better defined. This is a perceptual thing, which is what Watts is saying, I believe - in other words it goes hand in hand with perceiving the sound as more realistic.

Edit: Just regards the statement "the significance of a - 350dB noise floor modulation in a dac with a noise floor that is much higher, with an amp with a noise floor that is much higher, with a room with a noise floor that is much higher again." If this were adhered to wouldn't it make a source with 2-3% THD a perfectly acceptable performance because many speakers introduce this level of THD?
 
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BE718

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OK, I'm not going to get into trying to "prove" what he hears - it seems to me that is a perennial argument on this forum.
And I'm not going to get into the Room Vs source argument & which has more effect on sound stage but I would suggest that if a source is changed & the room isn't & a difference in sound stage is heard then it is logical to conclude that the source is the determining factor

I think this fixation on the headline of -350dB is mistaken - it's not a primary isse - I look on it like this - something else is happening that is affecting our audibility, it's just that noise shaping down to -350dB is needed to stop this "something else" - we don't yet know what this "something else" is & this is just the first indicator of it. A bit like RF noise & it's audible effects - we certainly don't hear RF noise but it does have many & varied consequences in the electronics & it's secondary effects are therefore audible.

"4x better depth than other dacs........mmmmmmmmm.........really............." yea, that's why I asked you about the group audition using the DAVE DAC - to check this statement. I haven't heard a DAVE DAC myself so just wondered.

However, I have heard the effects of lower, more stable noise floor (through the use of battery PS) giving rise to better solidity of the sound stage - mainly as a result of lower level signal audibility i.e. the sonic tails are better defined. This is a perceptual thing, which is what Watts is saying, I believe - in other words it goes hand in hand with perceiving the sound as more realistic

So we are none the wiser. What is the "something else" and if we don't know what it is how can you state it is the noise floor modulation that is the cause? How have you correlated it?

The issue here is the credibility of the statements, and the information that backs up the claims and statements. It's not a fixation. I would love to see the guy blind test cable metal purity based on sound stage depth.

Yes and I told you that I didn't notice, and I didn't hear anyone else remark on how extraordinary the soundstage was.

Not wishing to get into an argument, but how did you measure the noise floor of your DAC. Can you post the measurements of with without battery? Can you post the measurements of how the modulation changed? Afaik you don't possess the kit to measure it. So have you attributed what you claim to hear to something you haven't quantified?
 
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jkeny

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So we are none the wiser. What is the "something else" and if we don't know what it is how can you state it is the noise floor modulation that is the cause? How have you correlated it?

The issue here is the credibility of the statements, and the information that backs up the claims and statements. It's not a fixation. I would love to see the guy blind test cable purity based on sound stage depth.
What I'm saying is that Watts & others have focussed on noise floor modulation as a possible explanation for some issues of audibility that remain elusive to measure. Now, this is speculative & all would state that - it's based on observation & theory rather than direct measurement. So take what I & others are saying as a premise not a claim

Yes and I told you that I didn't notice, and I didn't hear anyone else remark on how extraordinary the soundstage was.
I know, I wasn't challenging you. Not having heard a DAVE DAC myself, I wondered about his statement. As I said, it concurs with my observations about low level signal quality & realism of sound & soundstage

Not wishing to get into an argument, but how did you measure the noise floor of your DAC. Can you post the measurements of with without battery? Can you post the measurements of how the modulation changed? Afaik you don't possess the kit to measure it. So have you attributed what you claim to hear to something you haven't quantified?
If all I did was change to LifePO4 battery from PS, & I had roughly measured the PS noise Vs Battery noise which showed the battery far quieter, why would I not assume that powering a DAC from this battery would be noisier? Do I have measurements of noise modulation? No, I don't. Do I hear a lower noise floor? Yes, I do, Do I now hear sonic tails that were masked before? Yes, I do. Is the sound more realistic with a more solid soundstage - yes!

My conjecture from the above is that a lower noise PS is the main cause of the changes in audibility. I further conjecture that using a battery (which has a capability of 120Amps instant current & 60Amps continuous current @3.3V) with an internal impedance of 8mOhm through short stout wiring is most likely giving a very stable power supply into such a low current device. This together with other experiments which I have done have led me to strongly consider lower noise modulation as a factor for better sound
 
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dallasjustice

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It's easy to test out different types of dither. I've found there's no audible difference between shaped dither and TPDF. However, others claim they can hear TPDF versus other types. I can't say what they can hear but hearing small differences in noise floor seems pretty fantasistical to me.
 

fas42

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It's easy to test out different types of dither. I've found there's no audible difference between shaped dither and TPDF. However, others claim they can hear TPDF versus other types. I can't say what they can hear but hearing small differences in noise floor seems pretty fantasistical to me.
It's all about how one tests for the effect. Get some very, very low material, apply the different dithers, and then boost the output level to an unrealistically high acoustic volume. I've done this, and the differences are dramatic, the "right" dither can completely transform how it sounds, in a positive sense.

As a counter to that, some time ago I listened to an ambitious system with a CD player which could apply switchable dithers. Careful listening made it clear that the actual mechanism in the player applying the dither was doing more damage than anything else - only with it completely switched off did the sound quality lift to a good standard.
 

BE718

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What I'm saying is that Watts & others have focussed on noise floor modulation as a possible explanation for some issues of audibility that remain elusive to measure. Now, this is speculative & all would state that - it's based on observation & theory rather than direct measurement. So take what I & others are saying as a premise not a claim

I know, I wasn't challenging you. Not having heard a DAVE DAC myself, I wondered about his statement. As I said, it concurs with my observations about low level signal quality & realism of sound & soundstage

If all I did was change to LifePO4 battery from PS, & I had roughly measured the PS noise Vs Battery noise which showed the battery far quieter, why would I not assume that powering a DAC from this battery would be noisier? Do I have measurements of noise modulation? No, I don't. Do I hear a lower noise floor? Yes, I do, Do I now hear sonic tails that were masked before? Yes, I do. Is the sound more realistic with a more solid soundstage - yes!

My conjecture from the above is that a lower noise PS is the main cause of the changes in audibility. I further conjecture that using a battery (which has a capability of 120Amps instant current & 60Amps continuous current @3.3V) with an internal impedance of 8mOhm through short stout wiring is most likely giving a very stable power supply into such a low current device. This together with other experiments which I have done have led me to strongly consider lower noise modulation as a factor for better sound

That's fine, it's a theory, one which needs to be proven. No problem with that. I'm just pointing out how fantastical some of Robs claims are and how erroneously definitive some of your statements are.

Just because your power supply is quieter, which I agree a battery might well be, it doesn't automatically follow that the DAC output will have a Lowe noise floor. You would need to measure it to find out.

This also has no bearing on the noise floor modulation. The modulation, unless your psu was totally all over the place, is primarily caused by the audio signal..

So, you don't know what is going on in the noise floor of your DAC. You don't know what to attribute the improvements you said you heard to, and you definitely have no reason to attribute it to NFM.

Now, it is possible that NFM is an issue, but you need to frame your statements appropriately.

So far we have a DAC designer who should be credible but has made some pretty fantastical statements about his auditory capabilities and You have a gut feeling. This is the measurement forum after all.

Now I have an idea regarding a test. I am going to produce a file with some modifications and we will see what people can hear.

Anyway this has strayed a bit off topic, it should be another thread on its own.
 
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dallasjustice

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I don't doubt that others can be trained to hear differences. I know Bob Katz claims to hear the difference between different dithers. My local dealer, Scotty, could pick out different dithered digital volume controls blind 100%. I level matched them to be sure. He said he could hear spatial cues which tipped him off. That's the same thing Katz says. It seems like there should be a study on this. Maybe I'll search the AES library.

It's all about how one tests for the effect. Get some very, very low material, apply the different dithers, and then boost the output level to an unrealistically high acoustic volume. I've done this, and the differences are dramatic, the "right" dither can completely transform how it sounds, in a positive sense.

As a counter to that, some time ago I listened to an ambitious system with a CD player which could apply switchable dithers. Careful listening made it clear that the actual mechanism in the player applying the dither was doing more damage than anything else - only with it completely switched off did the sound quality lift to a good standard.
 

jkeny

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That's fine, it's a theory, one which needs to be proven. No problem with that. I'm just pointing out how fantastical some of Robs claims are and how erroneously definitive some of your statements are.
Rob anticipates this reaction. All my statements regarding this were/are brainstorming, not definitive

Just because your power supply is quieter, which I agree a battery might well be, it doesn't automatically follow that the DAC output will have a Lowe noise floor. You would need to measure it to find out.
Perhaps but again it seems a reasonable conclusion to me - that a lower noise PS will most likely result in a lower noise floor on the DAC output

This also has no bearing on the noise floor modulation. The modulation, unless your psu was totally all over the place, is primarily caused by the audio signal..

So, you don't know what is going on in the noise floor of your DAC. You don't know what to attribute the improvements you said you heard to, and you definitely have no reason to attribute it to NFM.

Now, it is possible that NFM is an issue, but you need to frame your statements appropriately.
I won't argue with you

So far we have a DAC designer who should be credible but has made some pretty fantastical statements about his auditory capabilities and You have a gut feeling. This is the measurement forum after all.

Now I have an idea regarding a test. I am going to produce a file with some modifications and we will see what people can hear.

Anyway this has strayed a bit off topic, it should be another thread on its own.
I thought this thread was about low level measurements & don't understand why you want to exclude this topic?
 

fas42

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I found this paper.

Yes, note that it's all about bit rates - the number of bits actually being exercised to encode the signal - which is exactly the same as using a very low level waveform. The remarkable thing is that one can go to quite ridiculously low bit rates, I think I tried just 2 bits at one stage - and my brain could reassemble what the music was about quite adequately, using the right dither - the power of the brain to extract information from noise, if the latter is sufficiently random.
 

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