Measurements Are Hard

Discussion in 'Measurement Systems and Standards' started by Marvey, Sep 29, 2017.

  1. SineDave

    SineDave Friend

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    Amir was one of my partner's boss' actually - from what I've heard he's a good guy, but his first foray into forums (whatsbestforum) is equally full of nuttiness. The story goes that when he retired, he had enough cash to burn so he started a dealer\integrator business called Madrona Digital and co-founded WBF. At some point, WBF and he parted ways, and the new audiosciencereview forum was born. Strange stuff. When you retire wealthy, methinks you need something to stay sane.
     
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  2. Marvey

    Marvey Loves sex and records

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    You mean how @Auditor would travel to our dojo, proclaim his cultural superiority and superior Kung Fu, and then get beaten to a pulp by the groundskeeper?
     
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  3. Ringingears

    Ringingears Honorary BFF

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    I was a VP* for four years. I assure you I was a badass. I know hard to believe. But we had "Kung Fu". Our measurements of knife lengths were accurate to two decimal places. :cool:

    *High School Vice Principal
     
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  4. Cspirou

    Cspirou Friend

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    Found another post where he takes measurements of the Modi 2 with power regeneration.

    https://audiosciencereview.com/foru...eview-and-measurements.1829/page-2#post-46217

    Measurements are far better and it’s obvious that bad USB power is the culprit. The next post he goes on to say the Modi 2 is a poor design. Here’s the thing though, the iFi iDAC2 that he regards so highly is $350. A Modi 2 + Wyrd stack is $200 and fixes most problems he’s complaining about. A Modi 2U with separate linear power is $150 and fixes a lot of those issues as well. At the $99 price point of the Modi 2 I don’t know if you can solve the issues of USB power without increasing the price significantly. I mean the USB gen 5 upgrade is $150 alone, which is literally impossible on the Modi 2 since the gen 5 is not USB powered.

    Tried to find Modi 2/2U measurements on SBAF and didn’t see them. Were they on changstar?
     
  5. Marvey

    Marvey Loves sex and records

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    I never posted Modi 2 measurements (Modi 2 is actually my instrumentation DAC for headphone measurements).
     
  6. Thad E Ginathom

    Thad E Ginathom Friend

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    Oh god... Amir on Hydrogenaudio. Don't even think about it: it was horrible.

    I have learned a lot from actual scientists/engineers versus digital-mythologists on the internet, and one of the last things that I learned was not to read those pantomime Oh Yes It Is Oh No It Isn't threads now.
     
  7. Marvey

    Marvey Loves sex and records

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    The iFi DAC 2 has an internal battery which gives it advantages and disadvantages. The disadvantage is that the battery can run out of juice and the iFi will be relegated to using the power provided by the USB rails. The advantage is that the battery allows the iFi to not be dependent on USB power. This is a great idea, but it does add cost. As you said, the iFi DAC2 is $350. The results of the Modi 2 would likely carry over to other USB powered DACs, O2, SDAC, etc.

    The problem is with USB.

    An USB hub (we are talking inside a computer), that numerous USB will ports attached to, has a only a certain amount of bandwidth and power that must be shared among all of the devices plugged into the USB ports. Sometimes a computer, like a laptop might have one hub with two or three USB ports. A gaming PC might have four hubs with eight or more USB ports available. So to begin with, it's hard of hard for us how to tell WTF which device is plugged into which USB port, and if the juice (or bandwidth) provided to each device is sufficient. For example, I know that plugging in the Focusrite 2i2 with an XMOS USB based DAC doesn't work, from at least one of my computers. Another issue is how USB works. USB doesn't use DMA (direct memory access that bypasses the CPU). USB data transfers still require the CPU to do some work. Finally, we might have have a good idea what the OS might be doing with the power management of USB. I have a SD card reader that seems to turn off and on on its own on one of my computers. A way around this for Windows is to turn off PC power management for each hub.

    Finally, let me illustrate an example. This was the initial measurement I took of the SDAC. I never posted it. You want to know why? Because it looked WRONG. This is where experience comes into play. Maybe for a discrete R2R DAC, it might be believeable. But this was very odd for an AKM chip based DAC. And knowing the guys at Grace, I knew they were not incompetent - that it would be me that was doing it wrong (or something was odd with the computer or Windows).

    bad SDAC.png

    This is exactly why measurements are hard, and why I would hestitate to come up with strong conclusions with such fine measurements, not at least without more corroborating data. One thing I like here is that @Hands, @atomicbob, and I have been doing measurements on the same devices that we can cross compare. Such a system (peer review) should be in place for measurements if we are truly interested in audio science.
     
    Last edited: Sep 30, 2017
  8. atomicbob

    atomicbob Friend

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    If we wanted to pursue this a bit further, I propose that a single device, either an amp or DAC be measured by all the participants for cross-calibration. Maybe a magni3 or SDAC. Ship the very same unit around to all participants for measurements. Have at least one common measurement to all, such as THD+N, THD with a spectrum shown. IMD would also be nice.

    Wishing out loud, I wish Quantasylum, HpW and ARTA would consider adding automated THD and THD+N sweeps over frequency measurements with graphs.
     
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  9. atomicbob

    atomicbob Friend

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    Here is this morning's ambient SPL shown with Noise Criteria lines. Measured using an ACO Pacific 7046 mic, True Systems mic preamp and
    ADI-2 Pro ADC into Smaart 7. Calibrated with a B&K 4231 mic calibrator.
    201701001-1040 SSL residual noise 16 dBA 7046-28439.png
     
    Last edited: Oct 4, 2017
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  10. gixxerwimp

    gixxerwimp Friend

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    Just a clarification, the micro iDAC2 is purely USB powered, and it's the micro iDSD(BL) that has an onboard battery.

    I run the iDAC2 at work with a 1st gen iPurifier (came free as a bundle, don't think it does SFA) out of a PC and the iDSD at home on battery only out of an android phone. I can't really hear much difference between them.
     
  11. Marvey

    Marvey Loves sex and records

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    I thought it did because of its size!
     

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