From: Mike Giacomelli <giac2000_at_hotmail.com>
Date: Fri, 13 Feb 2015 15:32:32 -0500

Sorry for the delay, I was traveling and did not have access to a PC
to type a real message on.

>You expressed your doubts that this has a discernable effect on audio.

Does it? According to the manual it looks as though it should not, as
the two filters have cutoffs above 20 kHz. Even without knowing the
precise phase response, it seems very unlikely that it will be
meaningfully different in the audio band. The setting appeared to me
to be dictated by marketing rather than any actual effect. This is
not the case with all DACs, I have certainly seen some which give you
the (completely useless) option to hard code a high shelf EQ band into
the reconstruction filter, but it doesn't look like this is the case
here.

>It my be, that this has no discernable effect on audio, but I for once
want to least have the option to decided this for myself.

>http://gerrit.rockbox.org/r/#/c/467/

That is a fair point. I almost rejected that commit as well, but I
didn't fully understand what it did so I deferred to the device
maintainer. Looking at it now, that option is also somewhat
pointless, since the slow rolloff option essentially duplicates what
our EQ can do while the fast roll off looks uniformly better.

>In the same line as 44.1/48kHz playback, software balance,
parameteric eq, surround filters and Doom.

Its not like those things. The mixer sampling rate, software balance,
and EQ have no correct setting. They depend on the hardware that the
device is connected to and the input files that user intends to play.
Therefore we cannot anticipate what to use, but a user can, and so
they should reasonably be configurable. In contrast, reconstruction
filter coefficients have an optimal value that minimizes distortion
and frequency error. This value will be correct for all users, all
headphones, and all source files as it does not depend on any of those
things. My recommendation was that you determine the correct value,
and use it, because you, the device maintainer, most completely
understand the hardware. Most likely the datasheet hints at what it
should be, probably by defaulting to the setting the designers
intended you to use.

>When it comes to audio we generally try to expose all hardware
capabilities, if only to make audiophiles happy, because that's our
"core business".

I'm all for exposing capabilities (although I also dislike how many
settings we have), but not for settings that are actually useless, and
also extremely confusing/misleading. I have a doctorate in a related
field and it took me several hours of googling through AES conference
proceedings to fully grasp what the underlying concept of this setting
was. This is not like setting crossfeed or tone controls or whatever,
its more analogous to exposing DMA settings or PLL dividers to a user:
 extremely technical and not a great idea to screw around with
(although in this instance Wolfson seems to have realized that and
completely neutered the setting).

Mike
Received on 2015-02-13