Denafrips DACs might not actually be NOS?

[GoldenOne]

So I've got an Ares 2 in for review from a friend at the moment, and whilst testing it I've noticed that the NOS mode is...well...not actually NOS
(All images are taken from 768khz recordings of balanced output of Ares 2 in NOS mode)

Impulse response looks like this:

 


The rise/fall rate changes depending on sample rate, indicating it is indeed DSP/OS and not a slew rate limitation

44.1khz:

 

192khz:


And lastly, putting a 1khz sine @ 44.1khz sample rate yields a result that doesn't exactly look like NOS to me:



If anyone else here has a denafrips DAC and an ADC that can do 192khz or higher, could you check if yours is the same? Lmk if you need test files

[Ben-M]

This should be super interesting 😃

[etane]

Very interesting.  I did own the Musician Pegasus for close to a month before returning it.  To my ears, the NOS mode didn't sound very NOS. 

 

BTW, enjoyed your MQA video.

[The Computer Audiophile]

I'll ask Vinshine Audio.

[numlog]

try recording a 20khz sine

[The Computer Audiophile]

Perhaps @Miska has measured one of these?

[skipspence]

Coincidentally I just thought the same🤔 on maybe calling @Miska

[The Computer Audiophile]

Hi Guys, I heard back from Alvin, who spoke with Denafrips. He says, according to Denafrips, the NOS is indeed non-oversampling. He couldn't provide more information because Denafrips doesn't want to reveal anything proprietary with respect to how its DACs operate. 

 

I'm not sure where this leaves us, but it's all I have right now. 

[GoldenOne]

1 minute ago, The Computer Audiophile said:

Hi Guys, I heard back from Alvin, who spoke with Denafrips. He says, according to Denafrips, the NOS is indeed non-oversampling. He couldn't provide more information because Denafrips doesn't want to reveal anything proprietary with respect to how its DACs operate. 

 

I'm not sure where this leaves us, but it's all I have right now. 

I guess maybe they could call it 'non oversampling' on a technicality in this situation given as the FPGA is just making things move in a linear fashion to the next sample.
Technically not adding extra PCM samples perhaps but still functionally oversampling/interpolating.

We know it's definitely not a slew rate or analog low-pass filter limit because the square wave transfer speed changes depending on sample rate.


The linear interpolation behaviour can be seen quite clearly on a 15khz sine:




For those who are unfamiliar this image shows what is happening in the denafrips impulse response vs what would happen in true NOS/Zero-Order-Hold behaviour (blue line):
 

- Sample 1 is received by the DAC, nothing happens and it holds at 0 until the next sample.
- Sample 2 is received by the DAC, it immediately moves up to the value of sample 2 and holds until the next sample
- Sample 3 is received by the DAC, it immediately moves down to the value of sample 3
- Sample 4 is received, same value as sample 3 so voltage stays where it is.
 

BUT, that isn't happening on the denafrips.
Instead, immediately after sample 1 arrives, it begins moving up toward sample 2. The only way this could happen is if additional samples had been interpolated in-between samples 1 and 2, ie: it was oversampling.
It is using a filter that introduces no ringing, but it is nonetheless still oversampling.


In reality a NOS impulse will look something more like this because most R2R dacs will have some form of analog reconstruction:

[The Computer Audiophile]

3 minutes ago, GoldenOne said:

I guess maybe they could call it 'non oversampling' on a technicality in this situation given as the FPGA is just making things move in a linear fashion to the next sample.
Technically not adding extra PCM samples perhaps but still functionally oversampling/interpolating.

We know it's definitely not a slew rate or analog low-pass filter limit because the square wave transfer speed changes depending on sample rate.


The linear interpolation behaviour can be seen quite clearly on a 15khz sine:




For those who are unfamiliar this image shows what is happening in the denafrips impulse response vs what would happen in true NOS/Zero-Order-Hold behaviour (blue line):
 

- Sample 1 is received by the DAC, nothing happens and it holds at 0 until the next sample.
- Sample 2 is received by the DAC, it immediately moves up to the value of sample 2 and holds until the next sample
- Sample 3 is received by the DAC, it immediately moves down to the value of sample 3
- Sample 4 is received, same value as sample 3 so voltage stays where it is.
 

BUT, that isn't happening on the denafrips.
Instead, immediately after sample 1 arrives, it begins moving up toward sample 2. The only way this could happen is if additional samples had been interpolated in-between samples 1 and 2, ie: it was oversampling.
It is using a filter that introduces no ringing, but it is nonetheless still oversampling.


In reality a NOS impulse will look something more like this because most R2R dacs will have some form of analog reconstruction:

 

This is interesting. 

 

1. I wish we had more information from Denafrips. 

2. I wonder what the pros and cons are of doing it the way Denafrips appears to be doing it.

3. If it's not a traditional NOS design, I wonder why and if this is better or worse than a traditional NOS design. 

 

All interesting stuff!

 

[GoldenOne]

16 minutes ago, The Computer Audiophile said:

 

This is interesting. 

 

1. I wish we had more information from Denafrips. 

2. I wonder what the pros and cons are of doing it the way Denafrips appears to be doing it.

3. If it's not a traditional NOS design, I wonder why and if this is better or worse than a traditional NOS design. 

 

All interesting stuff!

 

1) Yep definitely. I understand they won't want to disclose exactly how their DACs are working. Given they are indeed exceeding the accuracy of their resistors it shows that certainly some kind of compensation is both at play and working and they wouldn't want to spill their secret sauce, but it would be good to get this cleared up given as it's one of the selling points of the product

2) I guess the pro would be that it might allow for more flexible correction techniques or subsample correction with the FPGA where it might not be possible with NOS?
The con being that because it is mostly randomly (but of course in a signal dependent manner given as it depends where the samples are as to where the interconnecting interpolated line moves to/from) creating additional tones lasting 1 sample duration, this COULD maybe have an effect on noise floor with musical content?
With true NOS the transition is always just going to be vertical/near-vertical and so will remain constant. But then on the denafrips interpolation it would be random according to the signal being played.
Measurements seem fine, but given as FFT relies on content being present for long durations (hence why you can use it to look below the noise floor), the effects of what denafrips is doing here might not actually be particularly visible on an FFT.

To be honest I think the best person to ask might be @Miska. What in your opinion would be the major drawbacks of using fully linear interpolation like what seems to be happening here?
And are there any particular benefits?

[The Computer Audiophile]

2 minutes ago, GoldenOne said:

1) Yep definitely. I understand they won't want to disclose exactly how their DACs are working. Given they are indeed exceeding the accuracy of their resistors it shows that certainly some kind of compensation is both at play and working and they wouldn't want to spill their secret sauce, but it would be good to get this cleared up given as it's one of the selling points of the product

2) I guess the pro would be that it might allow for more flexible correction techniques or subsample correction with the FPGA where it might not be possible with NOS?
The con being that because it is mostly randomly (but of course in a signal dependent manner given as it depends where the samples are as to where the interconnecting interpolated line moves to/from) creating additional tones lasting 1 sample duration, this COULD maybe have an effect on noise floor with musical content?
With true NOS the transition is always just going to be vertical/near-vertical and so will remain constant. But then on the denafrips interpolation it would be random according to the signal being played.
Measurements seem fine, but given as FFT relies on content being present for long durations (hence why you can use it to look below the noise floor), the effects of what denafrips is doing here might not actually be particularly visible on an FFT.

To be honest I think the best person to ask might be @Miska. What in your opinion would be the major drawbacks of using fully linear interpolation like what seems to be happening here?
And are there any particular benefits?

 

Agree, hopefully Jussi has time to add an opinion here. 

 

By the way, I like your approach to investigating this one. Curiosity is always best when we don't have all the information, rather than finger pointing and assuming something sinister. Bravo. 

[GoldenOne]

41 minutes ago, The Computer Audiophile said:

 

Agree, hopefully Jussi has time to add an opinion here. 

 

By the way, I like your approach to investigating this one. Curiosity is always best when we don't have all the information, rather than finger pointing and assuming something sinister. Bravo. 

Absolutely. 
I just do this cause I'm curious and find it interesting and am not at all pointing fingers. Just wanting to know more :)

To be clear it DOES look like what they're saying on the tin isn't strictly true, at least not in the traditional sense. And I think that that might not be a good thing.
But it's certainly not as clear cut as in other situations where it's just a binary "is it lossless" question.

There could be more at play here and honestly I know enough to know when I don't know enough. This is one of those times and I'll leave it to people with better understanding than myself to suggest the answers. I just wanted to start the conversation. It's an interesting middleground area between software/digital stuff in terms of oversampling/DSP, but also hardware in terms of controlling and compensating a resistor ladder.

Miska is most likely the best person to ask about the OS/Software side of things. But I don't know if we'll ever know exactly what's going on here in terms of the hardware side for sure.
I just found it interesting and thought others might too.

Also, seems like in the @John_Atkinsonstereophile review they presented some interesting behaviour when he tried to measure linearity. It was oddly deterministic. So that could perhaps further suggest something going on in terms of DSP.

[John_Atkinson]

2 hours ago, GoldenOne said:

Also, seems like in the @John_Atkinsonstereophile review they presented some interesting behaviour when he tried to measure linearity. It was oddly deterministic. So that could perhaps further suggest something going on in terms of DSP.

You can find the complete set of measurements of the Denafrips processors, in both OS and NOS operational modes, at

https://www.stereophile.com/content/gramophone-dreams-40-denafrips-terminator-ares-ii-measurements

 

Regarding the peculiar behavior I found when testing linearity error, a reader commented that what I found was consistent with the processor inverting the LSB of the input word. A coding error? Or a byproduct of something else that is happening?

 

John Atkinson

Technical Editor, Stereophile

 

[Hensema]

That explains a lot. NOS doesn’t sound as I would expect. Therefore I always make use of HQPlayer oversampling for the Denafrips Pontus. Hopefully this thread shakes things up a bit for Denafrips. Maybe a DSP board with real NOS in the future? 
 

Linear interpolation is oversampling, but not an oversampling filter. So, the button should not be called OS/NOS. It’s just wrong in case of linear Interpolation.

[idiot_savant]

Linear Interpolation gives a squared sinc frequency response - if you look at John Atkinson's measurements, you can see it gives you -1.5dB @ 10kHz, and -6dB (ish) @ 20.0k. NOS gives you a sinc frequency response

Doing the sums:

PI/4 ( so 11.025k @ 44.1k sample rate ) SIN(PI/4) / (PI/4) gives us -0.91dB droop  for a NOS

PI/2 ( so 22.05k @ 44.1k sample rate ) SIN(PI/2) / (PI/2) gives us -3.9dB droop  for a NOS

 

PI/4 ( so 11.025k @ 44.1k sample rate ) (SIN(PI/4) / (PI/4))^2 gives us -1.82dB droop  for a linear interpolate

PI/2 ( so 22.05k @ 44.1k sample rate ) (SIN(PI/2) / (PI/2))^2 gives us -7.84dB droop  for a for a linear interpolate

 

So from everything we've seen here, it's a linear interpolate.

 

Your friendly neighbourhood idiot

[GoldenOne]

2 hours ago, idiot_savant said:

Linear Interpolation gives a squared sinc frequency response - if you look at John Atkinson's measurements, you can see it gives you -1.5dB @ 10kHz, and -6dB (ish) @ 20.0k. NOS gives you a sinc frequency response

Doing the sums:

PI/4 ( so 11.025k @ 44.1k sample rate ) SIN(PI/4) / (PI/4) gives us -0.91dB droop  for a NOS

PI/2 ( so 22.05k @ 44.1k sample rate ) SIN(PI/2) / (PI/2) gives us -3.9dB droop  for a NOS

 

PI/4 ( so 11.025k @ 44.1k sample rate ) (SIN(PI/4) / (PI/4))^2 gives us -1.82dB droop  for a linear interpolate

PI/2 ( so 22.05k @ 44.1k sample rate ) (SIN(PI/2) / (PI/2))^2 gives us -7.84dB droop  for a for a linear interpolate

 

So from everything we've seen here, it's a linear interpolate.

 

Your friendly neighbourhood idiot

Thanks for this.

Just checked and yep the ares 2 rolls off quite a bit more than the may

 

[idiot_savant]

DId someone say the R2R was compensated digitally somehow? If so, chances are that correction stuff works at a fixed rate, so they have to generate the data for this rate somehow, so there must be some processing going on.

 

Oh, and I'm sorry, my terminology was wrong - When I said NOS, I really meant NOS sample-hold.

 

Anyhow, linear interpolates are usually poor choices for this sort of thing - very droopy in band, yet not very good stop band either...

 

[semente]

On 5/14/2021 at 2:45 PM, idiot_savant said:

Your friendly neighbourhood idiot

Haven’t been around that frequently, didn’t realise that you were back.

Good news.

[idiot_savant]

Thanks for that, I’ve been away, but I have a weakness for graphs…

 

which brings me into the linearity measurement, which has puzzled me - the only thing I can think of is it’s doing some kind of gain ranging, which is misbehaving here? It certainly seems to be responding to the amplitude of the incoming signal. 
 

needless to say, gain ranging is great for test equipment, not so good for music…

 

your friendly neighbourhood idiot

[idiot_savant]

I’m a bit surprised no more discussion that denafrips are possibly cheating at measurements in addition to clearly doing DSP they claim not to?

 

your friendly neighbourhood idiot

[idiot_savant]

 

 

 

I don’t see how this is off topic, providing requested analysis to graphs in the thread?

 

your friendly neighbourhood idiot

[davide256]

53 minutes ago, idiot_savant said:

 

 

 

I don’t see how this is off topic, providing requested analysis to graphs in the thread?

 

your friendly neighbourhood idiot

Seems to me that would be a discussion between tester (Stereophile) and Denafrips. Playing whack-a-mole in copy cat forum discussions isn't

scalable for any manufacturers resources. Hopefully the points raised here also make it to the Stereophile article comments... where they are also likely

to raise questions with other like reviewers considering future Denafrips reviews

[semente]

24 minutes ago, davide256 said:

Seems to me that would be a discussion between tester (Stereophile) and Denafrips.

 

I thought that the tester was the OP @GoldenOne. From my understanding JA only chimed in later, although he was also a tester.

[davide256]

5 minutes ago, semente said:

 

I thought that the tester was the OP @GoldenOne. From my understanding JA only chimed in later, although he was also a tester.

The OP is a Youtube audio reviewer with 12K subscribers... they have a venue thats more suited to attract a manufacturers attention. There is also a review of the Pontus on this site... comments there are more likely to get a response as prospective purchasers will be affected. A random forum discussion... not  likely to get a response