Linearity Talk

Discussion in 'Tales from the Bully Pulpit' started by winders, Jun 9, 2018.

  1. atomicbob

    atomicbob dScope Yoda

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    What happened that everyone is so hung up on ENOB? I don't see any usefulness in this particular measure unless it is extremely low, which will be seen in other attributes measured.
     
  2. Elnrik

    Elnrik Super Friendly

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    It's another end-all, be-all Amir measurement.
     
  3. Vtory

    Vtory Illogical Spock

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    I am feeling that ENOB is quite misleading for average consumers.

    1. ENOB is not different from SINAD (linear transformation), which is (provisionally) equal to THD+N (note: SINAD is different from SNR as already discussed).

    upload_2018-6-10_20-50-12.png

    2. But psychologically, 15 bits of ENOB and 0.002% of THD+N sound very different. The latter looks like "overkill" while the former reads "not enough".

    Of course as many already indicated, Amir-bits are nowhere close to ENOB. Also I like atomicbob's great statement (see the below)
     
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  4. atomicbob

    atomicbob dScope Yoda

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    Also remember that THD+N is an oversimplification in quantifying a complex spectrum into a single number.

    *edit* (I'm repeating myself. Typing as the message above was posted.)

    Please refer to this for examples.
     
  5. ultrabike

    ultrabike Measurbator - Admin

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    Oh!!! Now I see what you are saying (BTW, I just came back from my son's bday party so I was a bit distracted).

    Yes, that analog link there can be a bit misleading.

    Check this publication also by Analog Devices:

    http://www.analog.com/media/en/training-seminars/tutorials/MT-003.pdf

    You can see that SNR and SINAD are not the same thing, and they are not related by that full scale and whatever scaling factor. SNR is S/N. SINAD is S/(N+D) where N+D = N+THD = THD+N.

    The equations are actually approximations though (they actually depend on input signal in part because distortion does), and like @atomicbob said, hard to really say exactly how things are going to sound just by looking at the numbers. But if the basic numbers are really fouled up, don't expect things to sound great either.

    EDIT: Regarding the article, I think I've used that 500 MSPS 12-bit DAC (or similar) in the past. ENOB is important for development (multibit DAC/ADC or not). And 12-bit with 10.5-bit ENOB is somewhat to be expected. If they said 12-bit with 3-bit ENOB I would be concerned because that is pretty shitty. All of the sudden, your system that was supposed to have a range of such an such meters, cannot get you more than a few centimeters (500 MSPS is not exactly audio range). There are no lies. The customer in that AD article was clueless and the dude gave a polite customer-is-always-right marketing like response. On a more serious interchange, that kind of customer remark would be a cause of concern. If it really happened it was definitively a face-palm moment.
     
    Last edited: Jun 10, 2018
  6. ultrabike

    ultrabike Measurbator - Admin

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    The article is saying that ENOB = (SNR - 1.76) / 6.02 applies to an ideal ADC or DAC, which would have no distortion because it's ideal. We are not measuring ideal DACs, so we use SINAD, not SNR.

    The correction factor in the SINAD equation is for measurements done at levels bellow full scale. You do have enough information to use the second formula because the THD+N @ Schiit's site is at full scale. So you do not need to apply the correction factor because it cancels out: 20*log10(Full scale/ Full scale) = 0.

    I'm a firm believer that measurements mean quite a bit. But they will not tell you everything there is to know, nor do they replace subjective evaluation of performance.

    As far as Amir, he is aimless. The fact that he is using linearity and a 0.1dB threshold to arrive at bits is enough for most to understand he is clueless.
     
    Last edited: Jun 10, 2018
  7. Vtory

    Vtory Illogical Spock

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    Btw, solely based on loop-back results, isn't it too rushing to the conclusion to blame ap555's incorrectness? Bad loop-back results can be derived by either internal structures or real fuckery (previously faced a similar issue with one dac-adc interface). To perfectly rule out the former, Amir needed TWO ap555's imo, or some tricks (e.g. using recorded signals/responses). But he didn't say anything. So as usual, my bet is that he intentionally or unintentionally ignored the real story.

    Surely it's blamable if we consider AP's fucking price. Haha.

    PS. Also just wondering. Did you test loopback for Avermetrics AverLAB, @purr1n ?
     
    Last edited: Jun 10, 2018
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  8. ultrabike

    ultrabike Measurbator - Admin

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    I'm sure Amir would get far better Amir-bitness with 12 daisy-chained ap555s in parallel with 13 dScopes and AC coupled with 7 ap2722's.
     
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  9. johnjen

    johnjen Doesn’t want to be here but keeps posting anyways

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    MOAR is ALWAYS better!
    It's the Uhmerikan way!

    JJ
     
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  10. purr1n

    purr1n Finding his inner redneck

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    Geek mode on. @ultrabike / @atomicbob: I was thinking a bit more about this and I do wonder if the method we've been using to derive the THD portion of THD+N (or SINAD) might be incorrect. We've been using THD at 0dbFS. This assumption would work if we assume that THD near the noise floor would be the same as at full output. Should we not take THD at the lowest level where either the signal or distortion components get buried? I was sort of thinking how those camera measurement sites measure dynamic range.

    I mean, take at look at @atomicbob's Yggdrasil A2 1kHz sine measurements at -100db. They are pretty insane. And if we take a look at the FFT, there's barely a blip - distoriton components of the 1kHz are well under -150dbFS. Now we could say that AC mains components would lower the number of bits; howeve, the 60Hz and its harmonic components are not going to mask a 1kHz signal! This just points out how complex things really!

    [​IMG]
     
    Last edited: Jun 11, 2018
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  11. ultrabike

    ultrabike Measurbator - Admin

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    One could do THD at a few levels other than 0 dBFS. Likely we would have to look at distortion relative to the 1 kHz signal and not full scale.

    The number of FFT bins may have to increase as well because some non-negligible distortion terms relative to the input signal my be somewhat buried in the noise. I know non-negligible and buried in the noise don't seem to go together, but again, I mean non-negligible relative to the level of the input signal.

    Also, 0 dBFS is not the same across DAC products. Some may actually map 0 dBFS too low (way lower than clipping) or too high (well into clipping).
     
    Last edited: Jun 11, 2018
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  12. Darren G

    Darren G Friend

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    ^^^ this

    In real music listening it's rare I am listening to a full 16 bits of dynamic range, and if I am, the lower levels are probably drowned out (i.e., at any volume levels I could hear this range, my ability to hear the gnats fart would be comatose), so don't care much about these 17+ bits of measurements or theoretical limits, but yea, I suspect Schiit was just referencing the hardware potential.

    p.s., 10db is 10x the power, the assault of pressure on the ears. It adds up fast when you put into that perspective.
     
  13. winders

    winders Know-it-all boomer, prob racist

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    If that is what they are doing, it's kind of like Ford saying the GT-350R has a top speed of 250 MPH. With that being only in a vacuum but not mentioning that to anyone.

    That would be false advertising and they would get in legal trouble over it when owners discovered their cars could not go 250 MPH.

    Schiit specifically says:

    "Yggdrasil is the world’s only closed-form multibit DAC, delivering 21 bits of resolution with no guessing anywhere in the digital or analog path."

    Which, along with Amir's numbers, contributed to me asking the questions that I have. It seems that there is no way Yggdrasil is delivering 21 bits of resolution. At least not based on what I have been reading here.
     
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  14. ultrabike

    ultrabike Measurbator - Admin

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    That is not false advertising. 21-bit is the number of quantization bits the DAC has. It's like TI's 24-bit DACs. They are not 24-bit ENOB.

    That AD paper you linked where the AD representative was told by a customer that the number of bits was a lie had more to do with the poor understanding of how DACs and ADCs work by the customer, than with poor or misleading specs.

    In layman's terms, that customer was an idiot (given it was his job to understand these things).
     
    Last edited: Jun 11, 2018
  15. ultrabike

    ultrabike Measurbator - Admin

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    And regarding your 250 MPH analogy, do you expect the GT-350R to do 250 MPH on all types of terrain, slopes and while turning? Do you think tires mater? How about doing 250 MPH with strong winds against you? Really man, get that there are non-idealities that will affect performance.

    As an engineer I can tell you I fully expect ENOB to not match quantization bits of an ADC or DAC.
     
  16. winders

    winders Know-it-all boomer, prob racist

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    Now who is confusing ENOB with resolution?

    Schiit is not saying the DAC chips offer up to 21 bits of resolution. They say the DAC delivers 21 bits of resolution. By what measure is that even close to being true in real world situations? Because I don’t see that based on the linearity screenshots posted by anyone.

    I am not trying to be a dick here. I am trying to understand what is reality.
     
  17. ultrabike

    ultrabike Measurbator - Admin

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  18. nachocheese70

    nachocheese70 Facebook Friend

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    There’s always danger in trying to compare between different disciplines. In mechanical terms, maximum speed is a final measurement (along with 0-60, 1/4 mile time, ...). Based on all the discussions above, these measures would be closer to audio measurements such as ENOB or THD.

    I would say Schiit 21 bit is a physical property, and the closest Ford GT350R analogy would be it is a 5.2L Ti-VCT NA V8 powered car.
     
  19. ultrabike

    ultrabike Measurbator - Admin

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    @winders, resolution can have a lot of meanings. But since you are using that stupid AD link let's get it clear for you.

    In that paper they define N in the equation SNR = 6.02 × N + 1.76 dB as resolution. That SNR equation completely neglects thermal noise and non-linear distortion. It only includes quantization noise. On a 21-bit DAC, with no thermal noise or distortion taken into consideration, the SNR is 128 dB according to that equation (which is extremely ideal to the point that it can only yield a rough estimate, and in fact in some case a pretty useless one).

    In other words, for them "resolution" is the number of quantization bits. Which in Schiit's case is indeed 21.

    Schiit says their SNR is > 119 dB probably because they are including thermal noise. They may not be including non-linear distortion which would make that number SINAD.

    Again, my understanding is that "resolution" according to that link for the Yggdrasil is 21-bits, which the Yggdrasil does. Like that link said, the DAC or ADC "resolution" (defined by the quantization levels of the device) will not match actual ENOB performance. Because in the real world there exists such things as thermal noise and non-linear distortion that further degrade "resolution".
     
    Last edited: Jun 11, 2018
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  20. ultrabike

    ultrabike Measurbator - Admin

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    Furthermore, stop going on the defensive with "now who is confusing ENOB with resolution?".

    I'm not posting here to show you how awesome sauce my understanding of the subject is. I'm trying to help you understand what it is going on to the best of my abilities and given time constraints. I'm only human and my patience with those sorts of responses is limited.
     
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