This seems rather intriguing, but I'll have to read up on what it actually is
This seems rather intriguing, but I'll have to read up on what it actually is
Is it essentially taking two time-domain wave forms, scaling/aligning them and then computing the differential between the two? Then performing further analysis on the resultant differential waveform?
i found this in the RME user manual. Unfortunately Audacity does not go below 0.1 Hz for the simple high pass filtering. Maybe some other software could do it. Or maybe Mani would dare to take a soldering iron to his RME?
@pkane feature request
The combination of relatively high input impedance (47 kOhm) with 47 μF coupling capacitors
results in an extremely low cutoff frequency of (calculated) 0.07 Hz.
The ADI-2/4 Pro SE can actually use this low cutoff frequency effectively - many other converters
could not, because the analog input section has a low DC offset, which is detected by the ADC,
and shows up in the recorded audio signal as a fixed DC component. If this offset is too high, click
noises occur at the beginning, at the end, during editing etc.. The ADI-2/4 Pro SE is factory ad-
justed to minimum DC offset, typically -90 dBFS - problem solved.
The DC offset is usually eliminated in the digital domain via a high-pass with 1 Hz cut-off frequency
within the ADC. However, the ADC of the ADI-2/4 Pro SE does not provide such a filter. Therefore
a filter developed by RME is used, in the FPGA, which has a cut-off frequency below 0.5 Hz and
very low phase deviation. This filter is active by default. The DC component is thus reduced to
below -130 dBFS.
The feature request was for a very low frequency high pass filter to allow correction for DC blocking filters like the RME ADC uses.
Are there files that demonstrate the need for such a filter? I’m still not sure why the DC removal setting that’s already there wouldn’t do exactly what you are asking.
See the result I got comparing your RME loopback recording to a 0.7hz high passed Original2.wav (post#159). It improves the results quite a bit. Mani wants to evaluate only the DAC perfomance, so this helps get at that, although of course it won't correct for all of the error that the ADC contributes.
I’m still confused. Did you see my comparison using original2 and my ADI-2/4 loopback recording? I don’t see any obvious difference in DC that would require additional filtering. What am I missing?
it isn't DC. It is that the RME ADC has a 0.07hz high pass filter. So filtering the reference file before comparing gives a better match. So this is an alternative to shorting the RME capacitors like @KSTR has done. It isn't a must have feature for DW since you only have to filter the reference once and can do this with sox. Just would be a little more convenient.
I am away from that computer. You see the spectrum of the delta curve in post #159 where everything is below -100dB. I think peaks were like -80dB or -85dB without correcting the reference. By the way I can't say enough time how great your software is. The people on this site are unbelievable.
The file is here:
drive.google.com
Well, so where are there any two files, original and DAC/ADC recorded, that demonstrate these "wildly different results"? The one link to the file you posted doesn't give me a way to analyze it, since DeltaWave requires two files so the delta can be computed. This one appears to be the reference file. I also need the loop-back recording(s), aka, comparison file(s).
I see what you are saying but if you want to isolate DAC effects and remove ADC effects (mainly the low frequency effects from the 0.7Hz RC filter), how do you do that if you don't model the ADC HP filter and correct for it? Or else short the ADC caps. In my opinion, for purposes of what Mani is trying to do, it is perfectly valid to filter the reference with a filter that is based on a known circuit that is described in the RME user manual. This different from the challenge on the gearspace thread where people were attempting to compare overall loopback performance.
