Are Reconstruction Filters Audible? (Part 3)

I previously talked about filters and the new headphone rig I put together after taking a long break from audio as a hobby. Now I'm going to talk about the bizarre series of events that made me a believer in the negative effects of linear-phase filters.

Like a lot of people these days, my main listening rig is at my desk and my computer doubles as a transport. Last time I was listening to music daily, foobar2000 was state of the art and I was using Windows.

Now I'm a Mac user, but iTunes doesn't do it for me. After looking around a bit I found some software that was interesting feature-wise but absolutely horrible UI- / usability-wise. Then I found Roon. Roon is sold as SaaS on a subscription basis and has clients / remotes for nearly every platform.

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Roon is really good. It's cross-platform and most likely runs on OpenGL. Their UI is custom, as far as I can tell, but it's well done. Whoever is behind it understands that people who invest in audio gear tend to be OCD.

For example, if you open the play queue and scroll it to look at past or future tracks, a button pops up that, when clicked, returns the play queue to exactly where you found it. Functionally that button is completely unnecessary. But man is it satisfying to be able to tidy up like that.

But what's really interesting about Roon is 1) it has extensive DSP capabilities and also integrates with Tidal. Tidal is great because they stream CD-quality audio (FLAC).

The DSP capabilities include EQ (which I use), headphone cross-feed (which I can't stand) and up-sampling capabilities.

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The CMA400i DAC/AMP which I use (and love) does not, unfortunately, expose its AKM4490 filter settings. They really should, and many other products in that price range (and cheaper) do.

But it does have a "pure" DSD mode that bypasses the chip's internal filtering completely.

DSD vs PCM is another really interesting conversation, but I won't go into that now. Having tried all format and up-sampling options in the first couple months of using it, I noticed that I tended to gravitate towards DSD256 with a minimum phase filter.

As discussed previously, I had reason to believe that a minimum phase filter would sound better, but it was hard to say whether or not the preference was just "in my head".

So, without questioning it too much, I listened this way for months. Long enough, in my opinion, for my ears to adapt to it.

The Penny Drops

One morning I sat down at my desk to get some work done and fired up my rig. Almost immediately I thought something was wrong with the sound. The front panel lights on the CMA400i show what format is being played and to my surprise it still indicated DSD256.

So I went through the list of things that might be wrong. Was the volume too high? On a system with very low distortion, things don't usually sound "loud" even when they are. Nope, the volume was just where I left it.

Were my headphones on the fritz? Something wrong with the amp? I couldn't believe it, given the build quality of both and the fact that they're both new.

So I started to dig into Roon's DSP settings. And there it was -- for reasons I can't explain, the reconstruction filter had be switched to linear phase.

Switched it back to minimum phase and bam! Back to audio nirvana.

That really blew my mind. A lot of people argue that the only valid tests of audio quality are experiments done double blind. But these same tests have convinced the world that MP3 sounds just as good as FLAC.

Eliminating bias is certainly important however, especially when trying to understand subtle differences.

In this case, my expectation was that I was listening to my rig exactly as I had left it, including with a minimum phase filter.

But I was able to instantly tell the difference.

Linear Phase Sound?

So what does a linear phase filter sound like, especially given the fact that, given our current understand of the ear-brain mechanism, a difference should not be audible?

I describe it as a general feeling of unease -- and no, that's not a description of sound. I've heard other people describe it a high-frequency harshness. I don't hear that. Nor do I hear differences in imaging (using headphones that's not really that relevant).

To be even more vague, I feel like the music is arguing with me. It's trying to convince me of something. It continuously draws attention to itself.

Having said that, the absence of that problem is very audible. With a proper filter, I don't hear anything except music. I forget that I'm listening to music, but in a good way.

It's a kind of clarity and ease that has shown me, for the first time, just how good digital audio can be.

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