DR calculation in RTA seems wrong

Discussion in 'Official REW (Room EQ Wizard) Support Forum' started by nyt, May 13, 2019 at 9:41 PM.

  1. nyt

    nyt Member
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    Same signal, only change was changing the input padding by 10db. DR shouldn't be changing here, should it?

    [​IMG]

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  2. John Mulcahy

    John Mulcahy REW Author

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    Unweighted THD+N has dropped and likely by a little more weighted, so the DR figure will increase. The test signal levels are too high for a DR measurement, DR should be measured with a -60 dB signal for a result per AES17 - distortion shouldn't be a large contributor, but applying a test signal ensures there is no muting causing invalid results in the absence of a signal. From the help:

    An A-weighted dynamic range figure is also shown alongside the THD data. The figure assumes that the system being measured is capable of reproducing a sine wave at -0.1 dBFS (where 0 dBFS is a full scale sine) at distortion of better than -40 dB. If the actual maximum output is known the DR value used should be amended accordingly. For a meaningful result the system should be driven with a 997 Hz sine wave at -60 dBFS, per AES17-2015.
     
  3. nyt

    nyt Member
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    That's what I would expect, however THD+N and N are actually increased in the measurement listing the higher DR figure, so something seemed off

    0.0013% THD+N and 118.8 dBa DR

    vs

    0.00083% THD+N and 111.9 dBa DR

    However, I see the DR is calculated based off of what you say instead of what's actually being measured. This seems kind of strange since it's unlikely the max output of the DUT and the max input of the measurement interface will be exactly the same levels...
     
  4. John Mulcahy

    John Mulcahy REW Author

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    The percentage figure is higher but the test signal, which it is a percentage of, has dropped by more so the absolute level of THD+N has dropped. Bear in mind also the DR is an A weighted figure.
     
  5. nyt

    nyt Member
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    I edited my earlier post.

    The only change was on the input side of things. I understand it's not going to be completely precise by 10db, but it's the same signal, just one measurement is padded 10db more than the other. The noise floor has possibly come up by less than 10db in the second measurement, hence the noise floor difference.

    I see the issue with DR is that it's calculated while expecting the signal to be able to reach -0.1dbfs instead of reporting on the actual DR of the measurement.

    This seems kind of strange since it's unlikely the max output of the DUT and the max input of the measurement interface will be exactly the same levels...
     
  6. John Mulcahy

    John Mulcahy REW Author

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    DR is from THD+N, at the lower signal level the harmonics have dropped by slightly over 10 dB. The idea of using - 60 dBFS as the test signal is that harmonics would usually be in the noise floor, then the noise figure is the determining factor.

    A DR figure per AES17 requires a separate measurement of the maximum level (highest while achieving THD+N better than -40 dB).
     
  7. nyt

    nyt Member
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    So REW is performing the DR calculation by assuming the max level is -0.1dbfs? This seems like it will rarely ever be accurate. Wouldn't it make more sense to display SNR there? It seems without the ability to enter the max level for the DR calculation that it's mostly useless.

    Also, the harmonics level difference is almost exactly 10db, and is visible in the THD calculation, 0.00059 vs 0.00060 percent. The different pads will not be exact, but it's very very close as you can see by the overlay. The noise however is slightly below 10db difference.

    [​IMG]
     
  8. nyt

    nyt Member
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    Maybe still misunderstanding, does REW add 60db to the fundamental signal to calculate dr and just caps it at 0dbfs? Assuming the user has found the max level, then reduced output by 60db? Or does it use the peak sample value?
     
  9. John Mulcahy

    John Mulcahy REW Author

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    No, the only use made of the fundamental is subtracting it to find the N+D figure in dBFS.
     
  10. nyt

    nyt Member
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    So if it's only using the value of the fundamental compared to the noise floor, then the DR figure seems wrong.

    The second image has an increased dynamic range, yet it is reported lower.

    The spectra from the second image converted to red and overlaid on the first clearly shows the increased DR, yet DR was listed at about ~112db as opposed to the ~119db in the first image.

    [​IMG]
     
  11. John Mulcahy

    John Mulcahy REW Author

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    We seem to be talking at cross purposes. The DR figure is not stated relative to the fundamental and "the value of the fundamental compared to the noise floor" has no bearing and no use is made of it. The fundamental is subtracted from the signal, what is left is noise and distortion. It doesn't matter what level that fundamental was (beyond its effect on distortion), it is removed. The remainder, noise and distortion, is weighted and gives an absolute dBFS figure that is expressed relative to the maximum dBFS level, or in REW's case relative to -0.1 dBFS.
     
  12. nyt

    nyt Member
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    Understood, thought that's what you said earlier, but the last message didn't make sense.

    Generating a DR figure in this way is only meaningful if the max output of the DUT lines up with -0.1dbfs on REW's input. Chances of this happening are pretty slim. It would likely be better to calculate the SNR of the RTA measurement, or calculate the DR using a historical peak value as the max output instead of -0.1dbfs, or both :)

    Just my $0.02, but the current DR calc does not seem very practical. It's pretty much just a nonsensical measurement based on the noise floor at this point.
     

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