Today's post is a bit of a
continuation from last time's look at the different types of upsampling anti-imaging playback filters using my
Raspberry Pi 3 "Touch",
piCorePlayer and
SoX. As you can see from that discussion, across the audiophile equipment spectrum, manufacturers utilize all kinds of digital filter settings in their gear. Each company ends up choosing compromises between how much frequency roll-off, how much imaging, how much temporal/phasic anomaly each would accept. And of course no matter what a company chooses, there are ways of advertising the decision as "good"; whether it be on the basis of frequency spectral accuracy, temporal accuracy, or just claims from pure subjectivity - "
it just sounds better"!
The end goal of audiophilia is a bit like the modern interpretation of
Goldilocks (and the Three Bears)... We're all trying to figure out for ourselves what is "just right" as we wade through the commercial and mainstream audiophile literature, unofficial blogs and forums, mix-and-match speakers with amplifiers, try out different accessories perhaps, and the like. So too it seems with digital filters and all the variants attached to the DACs we buy.
Remember that the only reason we're even talking about this is because of that 44.1kHz (and to a lesser degree 48kHz) samplerate such that the Nyquist frequency is at 22.05kHz; relatively close to the usual 20kHz upper limit of hearing acuity that the younger ones among us
might be able to perceive. This is literally the only reason for all the hand wringing and millions of spilt keystrokes over the years around filtering by audiophiles (the few who still obsess over this...)
These days, we essentially have 2 major options for filter "types" among the DACs out there...
Linear phase (the default for most mainstream DACs, Chord) or
Minimum phase (
Apple,
MQA,
Pono) - pick your "poison" :-). Of course within each phasic variety we have different levels of steepness and allowance for ultrasonic imaging. We intuitively know that due to the biological phenomenon of auditory masking, maximum phase (where the group delay is pushed forward so "pre-ringing" is accentuated) is not desirable. But is there another choice?
Yes, there is of course... We can try to figure out a "just right" state with
intermediate phase settings. Accepting that
maybe there's some value to ensuring that pre-ringing isn't an issue even with some of the worst audio recordings out there, while maintaining awesome frequency and temporal accuracy - let me show you my choice for the filter that I listen to daily with the Pi 3 streamer...