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Thread: "What The Specs Don’t Tell You… And Why"

  1. #51
    Member Sponsor [Technical Expert] DonH50's Avatar
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    Quote Originally Posted by jkeny View Post
    The other thing that Mallinson mentions in his video is something he calls "nonlinear excess phase" of sigma delta modulators & that this is a variable delay related to signal & "every sigma delta modulator will eventually oscillate at some high signal level". "Unconditional stability or modulators that don't oscillate" he also states is audible because these various characteristics can be simulated in FPGA turned on/off

    I don't believe he mentions any measurement but I'm sure this oscillation should be easy to measure? I wonder if anyone has an example of such measurements?

    The other thing of great interest is just how far down these audible effects are - the noise floor modulation @ -100dB
    I tend to shy away from questions of audibility. Elsewhere an assertion was made that artifacts -140 dB down audibility corrupt the sound heard; I cannot say my ears are anywhere near that good (nor is most test equipment).

    Stability of delta-sigma loops is pretty complicated. I have run into various issues designing them but do not claim to be an expert, especially for audio converters that tend to be much more complex than the RF types I have usually designed (or helped design). Two popular introductory references are by John Candy & Gabor Temes (a collection of tutorials and IEEE papers) and the more recent book by Steven Norsworthy et. al. (see below). They are both pretty old now and there are a ton of other books, natch.

    The oscillation will show up in spectral analysis but there are several types of oscillatory mechanisms. Some are signal-dependent, some relate to the DC level, and some are related to basic loop stability. So, you have to know what you are looking for, and they are not always immediately obvious.

    Most delta-sigma loops are digital or sampled-analog (I have also designed continuous-time (CT) modulators and they add more worms to the can). Sampling is a non-linear phenomenon and so the phase shift is nonlinear and related to the architecture (including modulator/demodulator loop design) and sampling rate. Since a delta-sigma modulator is a feedback system they can get very complicated in design and analysis. Multiple bits, multiple loops, cascading loops, etc. Stability can be a nightmare, and simulations to ensure unconditionally stability of a nonlinear sampled system are not fun. And likely to miss that one magic combination that the real world finds immediately after power-up of the fabricated device...

    That said, a lot of those issues have been addressed over the years, and I would not have expected them to be significant concerns, at least not stability, these days. But then I have expected a lot of things that haven't really worked out for me...

    HTH - Don

    Oversampling Delta-Sigma Data Converters: Theory, Design, and Simulation
    James C. Candy (Editor), Gabor C. Temes (Editor)
    Publisher: Wiley-IEEE Press; 1 edition (September 2, 1991)
    Language: English
    ISBN-10: 0879422858
    ISBN-13: 978-0879422851

    Delta-Sigma Data Converters: Theory, Design, and Simulation
    Steven R. Norsworthy (Editor), Richard Schreier (Editor), Gabor C. Temes (Editor)
    Publisher: Wiley-IEEE Press; 1 edition (October 28, 1996)
    Language: English
    ISBN-10: 0780310454
    ISBN-13: 978-0780310452
    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
    Don's Technical Articles on WBF

  2. #52
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    Quote Originally Posted by DonH50 View Post
    I tend to shy away from questions of audibility. Elsewhere an assertion was made that artifacts -140 dB down audibility corrupt the sound heard; I cannot say my ears are anywhere near that good (nor is most test equipment).
    Well the audibility question is the one that most interests me - measurements which hardly relate to audibility are of little interest. However, I'm also interested in whether the noise floor fluctuations @ -100dB is an indicator of an issue somewhere else which is audible or whether it itself is the causal audibility factor. I believe an issue in the field of psychoacoustics which is possibly relevant to the noise floor modulation relates to how various types of modulation affects our perception. There are 3 basic affects termed Comodulation Masking Release (CMR), Modulation Detection Interference (MDI) & Comodulation Difference Detection (CDD). The effect of CMR is that the threshold audibility of a signal is lowered when another signal at a remote frequency is coherently modulating. In other words we can hear a signal buried in noise better if there is another signal modulating at the same phase but spectrally remote from the signal frequency. MDI is where a spectrally remote non-coherent modulating tone (or noise) can detrimentally affect the perception of modulation of a probe tone. CDD is somewhere between these two
    Natural sounds often exhibit correlated amplitude modulations at different frequency regions, so-called comodulation. Therefore, the ear might be especially adapted to these kinds of sounds.

    You'll find an interesting demonstration of CMR here

    Stability of delta-sigma loops is pretty complicated. I have run into various issues designing them but do not claim to be an expert, especially for audio converters that tend to be much more complex than the RF types I have usually designed (or helped design). Two popular introductory references are by John Candy & Gabor Temes (a collection of tutorials and IEEE papers) and the more recent book by Steven Norsworthy et. al. (see below). They are both pretty old now and there are a ton of other books, natch.

    The oscillation will show up in spectral analysis but there are several types of oscillatory mechanisms. Some are signal-dependent, some relate to the DC level, and some are related to basic loop stability. So, you have to know what you are looking for, and they are not always immediately obvious.

    Most delta-sigma loops are digital or sampled-analog (I have also designed continuous-time (CT) modulators and they add more worms to the can). Sampling is a non-linear phenomenon and so the phase shift is nonlinear and related to the architecture (including modulator/demodulator loop design) and sampling rate. Since a delta-sigma modulator is a feedback system they can get very complicated in design and analysis. Multiple bits, multiple loops, cascading loops, etc. Stability can be a nightmare, and simulations to ensure unconditionally stability of a nonlinear sampled system are not fun. And likely to miss that one magic combination that the real world finds immediately after power-up of the fabricated device...

    That said, a lot of those issues have been addressed over the years, and I would not have expected them to be significant concerns, at least not stability, these days. But then I have expected a lot of things that haven't really worked out for me...

    HTH - Don

    Oversampling Delta-Sigma Data Converters: Theory, Design, and Simulation
    James C. Candy (Editor), Gabor C. Temes (Editor)
    Publisher: Wiley-IEEE Press; 1 edition (September 2, 1991)
    Language: English
    ISBN-10: 0879422858
    ISBN-13: 978-0879422851

    Delta-Sigma Data Converters: Theory, Design, and Simulation
    Steven R. Norsworthy (Editor), Richard Schreier (Editor), Gabor C. Temes (Editor)
    Publisher: Wiley-IEEE Press; 1 edition (October 28, 1996)
    Language: English
    ISBN-10: 0780310454
    ISBN-13: 978-0780310452
    Thanks, Don, much appreciate your input & experience in delta sigma design issues.
    Manufacturer digital products www.Ciunas.biz
    "The greatest obstacle to discovery is not ignorance – it is the illusion of knowledge." - Daniel J. Boorstin.
    "Your bias negates your pretensions to scientific credibility"

  3. #53
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    I came across similar multitone measurements being used by this guy

    Name:  sample-spectrum.jpg
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    Quoted text from the webpage:
    "As you might have noticed, the output of the device has extra tones that look like grass in between the original tones. They are the result of distortions in the signal. Now we measure the energy of these extra tones + noise. This give us one number that is the total distortion + noise, or TD+N. Comparing two DACs this way will tell you which one sounds clearer. It’s simple.
    Higher is bad. Lower is good. It’s like golf!
    This measurement doesn't tell you everything there is to know about a DAC. But, it measures what I consider to be the most important factor in determining sound quality for a DAC, its clarity. So I call it the Sound Clarity Score.
    It also can be used to measure more than just DACs. So, we are going to measure the Clarity Score for everything that we can think of, and publish the results on this blog. Hopefully, we will elevate our knowledge of computer audio, bust some myths, make some people angry, and in the process bring some clarity to the world of computer audio .

    He also credits the origin of the concept:
    "I also should mention I didn’t come up with this measurement in vacuum. This test was inspired by the work of Deane Jensen (who was trying to achieve the same goal for vacuum tube amplifiers) and others. You can find Deane's paper here.
    Manufacturer digital products www.Ciunas.biz
    "The greatest obstacle to discovery is not ignorance – it is the illusion of knowledge." - Daniel J. Boorstin.
    "Your bias negates your pretensions to scientific credibility"

  4. #54
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    IN following up Jensen's paper "Spectral Contamination Measurement" I came across Jon Risch's pdf where he uses a different set of multitones chosen so that the IMD products don't overlap "What I wanted was a spacing ratio that would make as many of the products spaced away from one another and from the original tones as much as possible. Eventually, I found that the ratio known as Phi, or the Golden Ratio, did the trick."

    The PDF also includes his AES paper "A NEW CLASS OF IN-BAND MULTITONE TEST SIGNALS" explains the technique further

    The full paper is here
    Last edited by jkeny; 04-19-2016 at 06:06 PM.
    Manufacturer digital products www.Ciunas.biz
    "The greatest obstacle to discovery is not ignorance – it is the illusion of knowledge." - Daniel J. Boorstin.
    "Your bias negates your pretensions to scientific credibility"

  5. #55
    Member Sponsor [Technical Expert] DonH50's Avatar
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    TD+N is a new one on me -- I think that is SINAD (signal to noise and distortion) to most of us. I have seen THD+N but that is not the same as SINAD (or TD+N if you prefer). Most audio analyzers use THD+N and that generally provides a rising curve at low power as the noise rises relative to the signal so is somewhat misleading IMO.

    I skimmed the paper and it looks interesting. The IEEE ADC Standard (1241, IIRC -- my boss was on the committee and I helped review it) discusses the methodology for generating test tones that bin perfectly and are non-overlapping. It relies on relatively prime numbers. That also obviates FFT windowing, making testing much simpler and more exact. I use that method in all my various Mathcad and Matlab programs used to generate the various plots in my little articles here as well as for real work (single and multitone).
    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
    Don's Technical Articles on WBF

  6. #56
    Quote Originally Posted by DonH50 View Post
    TD+N is a new one on me -- I think that is SINAD (signal to noise and distortion) to most of us.
    SINAD is strictly the inverse of THD+N; just swap the numerator and denominator. SINAD is measured with a single tone just as with THD+N. TD+N is based on multitones and includes the harmonics of all the fundamentals and the IM products of each tone interacting with the others. If you limit the number of tones, it is not too difficult to keep the IM products away from harmonics. This can let you look at specific bins of the FFT and calculate THD+N and twin tone IMD products.

  7. #57
    Member Sponsor [Technical Expert] DonH50's Avatar
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    Not in my world, but my world is not usually audio measurements. SINAD includes harmonic and non-harmonic spurs; THD+N includes total harmonic distortion plus noise, no other correlated spurs. It is true a lot of test equipment treats SINAD and THD+N the same, which was sort of my earlier point. And we do multi-frequency SINAD, and sometimes the signal is pretty complex when it is serial data or a communications or telemetry signal.

    But, now I see the distinction between TD+N and THD+N, thank you! Obvious in hindsight... To me, SINAD and TD+N would be the same, but we're down to semantics, I think. SINAD leads to the ENOB (effective number of bits) for a data converter (ADC or DAC); THD+N does not (again, using the definitions I have used for ages and have always had to report).

    The IEEE Standard is what I use to bin signals in the FFTs without overlap, or sometimes the old relatively prime method (which works out to pretty much the same thing).

    No worries - Don
    Don Herman
    "After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
    Don's Technical Articles on WBF

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