This is just the sort of thing I need to try and understand this stuff.
I can understand that we need to avoid aliases during the ADC process, because they not only pollute the audible range, but will also cause IMD. When it comes to potential images created during the DAC process, if they only occur *above* Nyquist, does that matter? They will waste a bit of power further down the chain, but will be inaudible anyway. I thought that a proper brick wall filter was essential in the DAC, but am I wrong to think that?
Images can induce IMD as well. Like this:
I agree, resampling from 44.1kHz to 96kHz with a proper re-sampler and resampling filters is the way to get better results and usually better sound as well ;). The challenge here is that there is no off-the-shelf ASRC with a stopband that begins at Fs/2. Every single chip I looked at...
www.audiosciencereview.com
The first screenshot is the Realtek's original filter. The second and third screenshots are the filters from Windows resampler. You can see there are two signals, a tone and a white noise and the tone created IMD at 2kHz in the first screenshot. But then no reasonable music files contain high amplitude single tone at 21 to 23kHz and the IMD product at 2kHz is 100dB below the original tone, so kind of meaningless. You can also see that the tone seems to have higher amplitude than the white noise but you can look at the waveform view in Audition, the white noise is not 40dB quieter than the tone. Because white noise contains all frequencies evenly spreading throughout the whole spectrum, a tone on the other hand has all energy on a single frequency, and many people don't know how to read these things because for example, the white noise plots by Amir are often aligned to 0dB, it is just a graphical setting and has nothing to do with the actual noise amplitude.
Now read another post:
Is 44.1kHz fine as a delivery format and we don't need anything more? Just about the only objection from reasonable people I see is that it's hard to filter content in a DAC with that sample rate... Now, I only have a very superficial understanding of how a (modern) DAC works. From what I've...
www.audiosciencereview.com
You can see that the steepest filters (apodizing and brickwall) have the worst passband ripple (frequency response below 20kHz). ESS's linear phase fast roll off, brickwall and apodizing filters are basically the same: They are all linear phase and have the same group delay (latency). The differences are passband ripple, attenuation depth and steepness. If you have to make one of them better, then it means the other two parameters will become worse. ESS linear phase fast roll off filter for example has the flattest passband ripple below 20kHz and deepest attenuation (120dB), the trade off is it can only have full attenuation beyond 24kHz. You may think that all filters only have 110dB attenuation by looking at the noise spectrum, it is not. See the Realtek examples from the previous example I posted. The 120dB attenuation can be identified by using a singe tone instead of noise, and I showed both. Stereophile also shows similar measurements as well, the tone is 19.1kHz though, a lower frequency, so less likely to induce strong imaging.
Sidebar 3: Measurements
www.stereophile.com
Here is an experiment you can try, I mean really download the files and use the software mentioned and try it out, don't just read, otherwise it is not possible to understand how these trade offs are made:
Some of you probably have seen a lot of filter plots like these on DAC datasheets: How about making these things without using specialized software like Matlab, Octave and such? They are not particularly easy to use. So many people just "see" them rather than try to figure out the meaning...
www.audiosciencereview.com
It is also important and beneficial too see measurement data from other sources as well, don't just rely on one or two websites. Here are some examples of third party measurements:
I used the word "tricks", but you should find the word "marketing" as well.
www.audiosciencereview.com
