- May 26, 2017
Same signal, only change was changing the input padding by 10db. DR shouldn't be changing here, should it?
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 offUnweighted THD+N has dropped and likely by a little more weighted, so the DR figure will increase.
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.
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.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).
So if it's only using the value of the fundamental compared to the noise floor, then the DR figure seems wrong.No, the only use made of the fundamental is subtracting it to find the N+D figure in dBFS.
Understood, thought that's what you said earlier, but the last message didn't make sense.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.
No DUT I've ever measured except a specifically calibrated interface has ever had the max output line up with -0.1dbfs, so the number displayed is virtually always wrong.I think it's useful, or I wouldn't have bothered to implement it in the first place. It provides at least half the answer, since it gives the bottom end of the span required for an A weighted dynamic range figure. If the top end, i.e. the maximum level within the distortion threshold, is at -0.1 dBFS it already does the whole job. Likely to be the case when measuring an ADC, for example. If not, a figure for the maximum level needs to be obtained by the user, there is no way for REW to know or guess what that might be.
Cool, was paining me seeing irrelevant figures most of the time on generated measurementsI have replaced the RTA "DR" figure by A-weighted noise plus distortion in the current Y axis units.