A Rethink of THD/THD+N (or why both tell us almost nothing)

Discussion in 'Audio Science' started by ChaChaRealSmooth, Jul 6, 2020.

  1. Armaegis

    Armaegis Friend

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    New dumb idea: generate tone(s) based on a piece of gear's distortion profile. Use that for evaluation and purchasing decisions rather than listening to the equipment itself.
     
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  2. atomicbob

    atomicbob dScope Yoda

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    You do not need expensive equipment to test a new weighting algorithm. It can be done with the graphs containing FFT presented in the measurements posted, a scale or calipers, a pencil, piece of paper and a spreadsheet, calculator or even a slide rule.

    Example:
    1. print out distortion graph for amplifier of interest (FFT does not have any weighting applied)
    2. determine dB below 0 for each harmonic desired
    3. remember to convert dB back to linear before performing weighting calculations
    4. apply new weighting algorithm and calculate new metric
    5. convert back into dB or % as desired
    6. Read impressions on amplifier of interest. Best information is usually in the first page or two of impressions post.
    7. Correlate new metric to impressions
    8. Repeat for all amplifiers of interest.

    This only requires time and effort on your part, if you are interested.

    example graph for harmonics 2 through 6 (green circles on right channel)
    20201012 Bachelor GE5654W A04 THD+N  THD  nth-HD FFT 300R - 4+HD+N with 60Hz 0dBu annotated.png


    Now if you were intending that I do the following:
    1. spend 20 to 40 hours coding in a new algorithm into the analyzer, including testing new algorithm measurement accuracy
    2. remeasure a number of amplifiers, requiring 2 to 4 hours for each amplifier for a single measurement (remember amps should be thermal equilibrated before measurement)
    3. gather impressions information
    4. spend time making correlations
    5. post results for instant consumption

    Well, that would be annoying. I've already put in significant time on the 4+HD+N, which I believe is a useful metric, one that may be frequency swept for creating amplifier profile graphs with this metric. I am already on record as abhorring simplification of complex behavior to simple figure of merit numbers. There is another site on the internet that believes in that bullshit idea.
     
  3. mkozlows

    mkozlows Friend

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    Crutchfield does that for speakers.
     
  4. Ox Cart

    Ox Cart Facebook Friend

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    I find crap factor compelling, but in this specific example even the worse amp's crap factor is already -103dB. Is that really going to be noticeable on real world listening?
     
  5. atomicbob

    atomicbob dScope Yoda

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    Even the crap factor is still an over simplification. Look at the blue lines in the relevant post and note the greater downward slope of the preferred amp over the slight downward slope of the perceived lesser amp. Also remember to include the low NFB vs high NFB design criteria for the two amps. It is easy to achieve low distortion with high NFB. Much harder to achieve with low to no NFB.
     
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  6. Ox Cart

    Ox Cart Facebook Friend

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    Yep, the distortion profile is different. I'm just trying to figure out the hypothesis here.

    I can buy the idea that some (maybe many) prefer amplifiers that manifest audible distortion following a decending amplitude profile. I'll also buy the idea that audible crap factor is a demerit. What I can't get my head around is that the distortion profile of an amp with overall inaudible distortion should matter. Is there something I'm missing?
     
  7. atomicbob

    atomicbob dScope Yoda

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    The basilar membrane can operate over a 120 dB dynamic range. It also perceives sound differently inside a critical bandwidth than outside. For a give effective rectangular bandwidth (ERB) the operating dynamic range is about 30 dB. But different ERB may be operating at very different nominal levels. It is possible for the basilar membrane to detect 80 ~ 100 dB lower in one ERB far from another ERB. We can thank Bell Labs for a lot of work in discovering these principles. One of my mentors worked there for 26 yrs.
     
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  8. Cspirou

    Cspirou They call me Sparky

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    Last edited: Jun 28, 2021
  9. Marvey

    Marvey Super Friend

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    It shouldn't.

    SINAD, THD+N, or even THD itself with the N were never meant as sound quality or transparency evaluators. They are really just numbers based from a simple steady state sine wave measurement, intended to assess one single aspect of the technical performance of an amp. In the late 1960s and early 1970s, a good number to shoot for was 0.01% THD for 1kHz into an expected load.

    Most of the great audio designers are dead or retired. I've had the pleasure of meeting many of the big names (and lesser known ones behind the scenes) on the account of where I used to live in CA (the San Fernando area used to be the Audio Mecca). I can tell you they just don't look at 1kHz THD, but also many other factors. There is a reason why my new Triple-Sine THD measurements are done with a bandwidth of 90kHz, even though we can't hear that high up.

    There is a lot of secret knowledge with respect to measurements on how something may sound which I am rediscovering today. I am beginning to sense that distortion harmonic distribution matters more than ultimate THD, especially when THD is good enough past a certain threshold. It's weird, but what I suspect is what are seeing are indicators of transient behavior which cannot be measured well (that is processed mathematically with error so we can visualize them properly - at least not without quantum computers).
     
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  10. gepardcv

    gepardcv Almost "Made"

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  11. yotacowboy

    yotacowboy McRibs Kind of Guy

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    https://www.superbestaudiofriends.o...on-pass-diy-h2-injector-for-loaner-tour.8660/
     
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  12. lemon07r

    lemon07r Rando

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    I really want to know for myself as well, I really cant stand them. I liked my starfields more than my dusks cause of the weird BA sound it had.
     
  13. uncola

    uncola Friend

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    "subjectivist objectivists" I feel like is basically accepting the value of measurements but also accepting fringe audio science hypotheses but only where actual research by smart experienced people is done.. and ignoring snake oil stuff that only exists to increase profit margins based on zero testing or research.

    pedja seems like he fits in with that
    https://www.monoandstereo.com/2008/04/interview-with-pedja-rogic-of-audial.html
     
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  14. Elmer Danilovich

    Elmer Danilovich MShenay trying to pretend to be an old man

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    I have to agree, that was an outstanding read and hope we will see even more manufacturers whose line of thought are similar to his
     

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