Thanks
@amirm for this very nice introduction article, I'm sure this will help a lot as an anchor for many questions that may arise from your measurement reviews.
Can you help me to visualize and understand the reconstruction filters?
To test for the filter response, we feed the DAC random white noise, which naturally has infinite bandwidth. The response of the low pass filter becomes obvious once we capture the output of the DAC and convert it to the frequency domain using FFT. Figure 16 shows an example of this as I change the filter settings in the DAC.
Figure 16: example of different DAC output filters.
The audibility impact of such filters is likely very low to non-existent so I don’t put a lot of value on this test.
In the presented graph we can see frequency response with white noise signals (*) but how does this equate to
proper input signals? [EDIT: Can we assume the white-noise is a fully legal signal with only <FS/2 components?]
What are the (aliasing) effects, if any, due to the different reconstruction filters on
legal signals?
For instance, I would expect that a properly formed 44100Hz
input would have
zero frequency components above 22050Hz when properly reconstructed, is that correct?
The question is then: which of the filters presented constitute a
proper reconstruction? Intuitively I would say the blue one (steep decline towards FS/2), while e.g. the green one seems... wrong?
Thanks, and keep the articles and reviews coming!
Regards, Erik
[Edited, thanks Ray!]
(*) White noise can be an
illegal signal when the levels are set to high (causing inter-sample overs)
As example of such an illegal signal, I used Audacity to create 44100Hz white noise (0dB) and up-sampled it to 384kHz and indeed you can see FFT frequencies extend all the way up to 192kHz: