Hi,
This does make a fair bit of sense.
Someone had their 3 shredded wheat.
https://www.youtube.com/watch?v=wWLBuAW1lEE
Short Answer
At AMR/iFi we always seek to minimise the impact of digital filters on the music, finding the results to our ears, more "analogue" and more "realistic".
Which is why we have the Bit-Perfect and Minimum Phase filter options for PCM and DSD respectively; which are mainly reduced in filter length and complexity.
This is subjective and is what we like, that is all.
Much Longer Answer
Digital Filters - Tapping and Mixing Away
Digital filters have multiple delay taps and mix the signal from the multiple taps to create a filter function.
A representation of how this works is here:
http://www.sakurasystems.com/graphics/fig5.gif
A representation of the acoustic effect of using such a filter is here:
http://www.sakurasystems.com/graphics/fig6.gif
These graphics are taken from Ryohei Kusunoki's article "Non-oversampling - Digital filter-less DAC Concept" (the original source and standard for Non-OverSampling that started the whole ball rolling for AMR, Audio Note, Zanden et al.)
It is worth reading and studying for avid students of NOS :
http://www.sakurasystems.com/articles/Kusunoki.html
From Zero to Infinite Order Filters - balancing transient and frequency response
There is invariably a tradeoff between filter complexity and filter action. This trade-off affects the transient/impulse response and the frequency response.
The least complex digital filter is a zero order digital filter, also known as "no filter" or Non-Oversampling.
At AMR/iFi we call this type filter "Bit-Perfect" because it is the only digital filter that is bit-perfect (it does what it says on the tin). It has a perfect impulse response but equally it has absolutely no filter action.
The most complex digital filter is an infinite order digital filter. Such a thing does not exist, but it sure is interesting as a concept. Such a filter would have a perfectly rectangular filter function (meaning immediatly above the cut-off frequency attenuation is infinite) and it would "ring" forever (literally, for an infinite amount of time) and the chain of speakers shown above would be infinitely deep.
In the real world, digital filters generally tend to be neither very low order (though much of the legendary digital gear of yesteryear used very simple custom digital filters with very low filter complexity (Wadia Digimaster, Pioneer Legato Link, Luxman Fluency, Ayre MP etc.).
One of the infamous Pioneer "Battleships" with Legatolink technology
We remember this technology fondly as it was very smooth sounding indeed. Sonically, it was not too far off the infamous Phillips/Marantz "Battleships" with the TDA1541A.
And it had this:
Note, just because a a digital filter has low filter complexity does not mean implementing it has low circuit complexity - often such filters require extra DSP or FPGA cores. Put it this way, the Pioneer Legatolink was reserved for their top-tier models.
The low complexity digital filters are rarely well documented, but both Luxman Fluency and Pioneer Legatolink only have three taps. The minimum phase filter in the iDSD has more taps than that, but still very low filter complexity, as said, Bit-Perfect does not filter at all in the digital domain.
A common digital filter, the SM5842, cascades:
- a 169 tap filter with
- a second 29 Tap filter and
- a third 17 Tap filter.
This makes for very high filter complexity and an equally complex impulse response. This filter is similar to the standard filter on the iDSD.
There are designs that like to create filters with even more taps than that, tens of thousands of taps have been implemented and purported as much better than the shorter and less complex filters.
DSD incidentally in its purest form has no digital filters and if using the iDSD micro, it has only an 8 Tap analogue filter, which at 2.8MHz makes for an excellent impulse response. This excellent impulse response however is "purchased" at the "expense" of lowered resolution and more ultrasonic noise compared to standard PCM. Still we find it likely that the impulse response improvement makes up a large part of the facination of DSD, especially at higher rates where resolution and ultrasonic noise are much improved.
In the words of Linkin Park, "In the End" (read: it boils down to what your ears like)
To conclude:
1) DSD has much better impulse response than (CD) PCM (though at a price) and we feel a large part of the attraction of the sonic qualities of DSD is this improved Impulse response.
2) Complex filters have a poorer impulse response but better filtering/frequency response. This may or may not be preferred. The nano and micro and Pro iDSDs are at the opposite end of the design sprectrum with minimal filtering or even no filters.
At AMR/iFi the route of less complex and less manipulative filters agrees most with audible sense of how music is real and we will continue to work on systems that incorporate them, rather than going after the most complex digital filter in the world. But this is just one company giving you just their take, other opinions obviously exist.
It is in the end up to the listener to decide what they prefer and to make their purchasing choice according to their own taste.
Just like specialist breweries and branded beer, both exist and both are eminently drinkable.
Please drink responsibly and in moderation
!
