# Review and Measurements of Audio-gd NFB28.28 DAC and Headphone Amp

#### Bluespower

##### Active Member
My DACs are so stressed I give them one day of a week holiday. They are asking for more but that is all I can do.....
Have nice holidays.

#### Krunok

##### Major Contributor
Well one step at a frequency of 2.8 mhz that means one step takes 1/2,8mhz s to come (t=1/f).
So one step take 0,35us to come.
So the 140 steps wilo take 50us to grow up'the level to the max. Because dsd is one bit after the other to grow the level of the signal.
50us means a frequency of 20khz. F=1/t
If you make more steps the time to reach the 141th step wil be more than 50us so it s impossible.
If you make a low level it Ill be rounded to one of the 140 steps.
For exemple for a low level the 50th step will take 17us to'come that means it s less than 50us so it s ok for 20khz.

In dsd for 20khz you have only 140 steps.
In pcm 16 bit you have 65536 steps.

If just calculate 2,8Mhz/20000khz it gives you directly the number 140.
Is it logic?

It seems very strange i agree but that is'how'i'understand dsd. One bit after the other every 0,35 us.
Well, you can't have both, can you? At 44.1kHz sample rate you don't catch more than 2-3 sample points of 20kHz wave either. What exactly is your point here?

#### Bluespower

##### Active Member
Well, you can't have both, can you? At 44.1kHz sample rate you don't catch more than 2-3 sample points of 20kHz wave either. What exactly is your point here?
You'loose precision on the amplitude of the sine. The amplitude will be rounded with less precision with dsd but the shape will be more accurate. For example a -43db sine you'would be rounded to -40 db if -40 is in the 140 steps. In pcm you ve got much more steps.

#### andreasmaaan

##### Major Contributor
You'loose precision on the amplitude of the sine.
The amplitude errors appear as noise - not as inaccuracies in the signal. This noise is shaped, giving greater than 20 bits of resolution in the audio band.

#### Krunok

##### Major Contributor
The amplitude errors appear as noise - not as inaccuracies in the signal. This noise is shaped, giving greater than 20 bits of resolution in the audio band.
I don't think he gets that PCM and DSD formats cannot really be directly compared.

#### Krunok

##### Major Contributor
.... what about my holiday? .... ... ..
Why would you need holiday? You're not stressed. You actually enjoy putting people to misery around here..

#### elira

##### Member
How hard is to make a test where you play noise and then check how accurate the output is? That way we would have a bunch of different frequencies at the same time. My guess is that such test is not trivial to do.

#### andreasmaaan

##### Major Contributor
How hard is to make a test where you play noise and then check how accurate the output is? That way we would have a bunch of different frequencies at the same time. My guess is that such test is not trivial to do.
A multitone test is much easier and I don’t see what noise would show up that 32 (or more) equal-amplitude tones wouldn’t?

But if you were going to try with noise, my first thought is that you’d need to do a null test with it.

#### elira

##### Member
A multitone test is much easier and I don’t see what noise would show up that 32 (or more) equal-amplitude tones wouldn’t?

But if you were going to try with noise, my first thought is that you’d need to do a null test with it.
Well, if a DAC can accurately reproduce the noise then it's obvious it can reproduce any signal. It is not obvious at least to me how being able to reproduce 32 sine waves translates to accurately reproduce any signal.

#### andreasmaaan

##### Major Contributor
Well, if a DAC can accurately reproduce the noise then it's obvious it can reproduce any signal. It is not obvious at least to me how being able to reproduce 32 sine waves translates to accurately reproduce any signal.
What kind of noise?

#### dc655321

##### Addicted to Fun and Learning
Well, if a DAC can accurately reproduce the noise then it's obvious it can reproduce any signal.
It would be anything but obvious - how would you distinguish the input signal from any DAC-generated artifacts, non-linearities, etc?

#### elira

##### Member
What kind of noise?
I'm not sure what kind of noise would provide the most information, but if such test is possible then changing the noise would become trivial. It should work with any audio file.

It would be anything but obvious - how would you distinguish the input signal from any DAC-generated artifacts, non-linearities, etc?
Well, that's the kind of things that should be absent in the output. If the test quantifies the difference between the output signal and the original input (in this case noise) then we would know how accurate is the reproduction. I'm not saying the test is going to be easy to do, it may not even be possible.

#### andreasmaaan

##### Major Contributor
I'm not sure what kind of noise would provide the most information, but if such test is possible then changing the noise would become trivial.
I’m not convinced. If you could tell us which type of noise and why this would be most revealing, that would at least give us something to discuss (forget practical considerations about how to do the test, I’m just interested in the theory)

#### elira

##### Member
I’m not convinced. If you could tell us which type of noise and why this would be most revealing, that would at least give us something to discuss (forget practical considerations about how to do the test, I’m just interested in the theory)
Well, it's kind of brute force, you pick a bunch of random values it should get you many of the possible transitions a DAC would need to generate between two samples.

#### Bluespower

##### Active Member
Well, it's kind of brute force, you pick a bunch of random values it should get you many of the possible transitions a DAC would need to generate between two samples.
If'there is less noise on the'output than in the input'it will be a problem. Does some noise disappear with the'filter'or the'dithering? Or with feedback on the opamps?

#### elira

##### Member
If'there is less noise on the'output than in the input'it will be a problem. Does some noise disappear with the'filter'or the'dithering? Or with feedback on the opamps?
An ideal DAC should have no problem generating a random wave. With current DAC implementations the results of the test may be useless, probably a bad idea considering that DACs modify the input with filters and stuff.

#### Killingbeans

##### Addicted to Fun and Learning
Why then do R2R DAC manufacturers or proponents not provide their own alternative tests that show up the alleged disadvantages of DS DACs?

Surely at least one R2R manufacturer or enthusiast would by now have done the tests that show this? The reason they haven’t is that they can’t
I've been asking myself those same questions. Soekris, for instance, claims the same advantages on his/their webpage, but has no references to measurements or scientific papers: Here

Go to a recording studio with a highly praised R2R DAC and a good measuring DS DAC. Record some complex music with the mixer set for maximum transparency and dynamic range. Play back the captured file via the DACs and record the output with the same mixer settings. Do a digital nul-test.

#### andreasmaaan

##### Major Contributor
I've been asking myself those same questions. Soekris, for instance, claims the same advantages on his/their webpage, but has no references to measurements or scientific papers: Here

Go to a recording studio with a highly praised R2R DAC and a good measuring DS DAC. Record some complex music with the mixer set for maximum transparency and dynamic range. Play back the captured file via the DACs and record the output with the same mixer settings. Do a digital nul-test.
I have a huge amount of respect for Soekris as a designer but when I asked them which measurement they could propose that would highlight any performance advantage of an R2R DAC, there was no reply (I can’t be certain they saw the message but they did reply to other questions further down the same thread).

Re your suggestion, why do we need a recording studio here? Why not just take an excellent recording of any piece of music, record the outputs from two DACs (one R2R and one DS) and then compare them by nulling them?

And even then, what would the result tell us about which were superior? All we would see would be the difference, but there would be too many variables to know what caused it, or indeed even which DAC had reproduced the input more accurately.

Moreover, which ADC would we use? The best ones and those almost ubiquitously used in top recording studios are (to my limited knowledge) DS ADCs.

#### Bluespower

##### Active Member
I have a huge amount of respect for Soekris as a designer but when I asked them which measurement they could propose that would highlight any performance advantage of an R2R DAC, there was no reply (I can’t be certain they saw the message but they did reply to other questions further down the same thread).

Re your suggestion, why do we need a recording studio here? Why not just take an excellent recording of any piece of music, record the outputs from two DACs (one R2R and one DS) and then compare them by nulling them?

And even then, what would the result tell us about which were superior? All we would see would be the difference, but there would be too many variables to know what caused it, or indeed even which DAC had reproduced the input more accurately.

Moreover, which ADC would we use? The best ones and those almost ubiquitously used in top recording studios are (to my limited knowledge) DS ADCs.
Well we found that imd test with a lower level second tone can be done as easily as normal imd test. So why not making it? Ok you say there is no sense of'making it but it would validate your theory or not and then there won't be debate anymore.