More on Pre/Post ringing

There is an excellent discussion thread on digital sampling at DIYAudio. In one of the posts, the following is explained:

"Pre/post ringing : most obvious in the classic step response of a steep, linear phase (usually FIR) filter. ... Let me just say now, for the record, that the pre/post ringing we often encounter in digital audio is nothing more than an artifact of the Gibbs phenomenon which simply states that abrupt transitions or discontinuities in one domain, cause ringing in the other.

Band-limited systems unfortunately fit in this category and there is no better example of a SHARPLY band-limited system than digital audio. Don't like the ringing? Sure we can eliminate it but the price you pay will be aliasing, pure & simple, no way around it. And that's a BAD tradeoff."

What is described above is the fact that with a linear phase filter, reducing the ringing will require the use of slow roll off filters allowing higher frequency images to reflect back into the audio band (this is called Aliasing).

More modern DACs such as the one we used in this project are addressing this problem with so called "minimum phase filters" and to completely eliminate any aliasing, the concept of "Apodizing filters" are used together with minimum phase filters.

We previously explained the digital filters of the WM8741 here.