Double Filter - Roon and DAC (PS audio perfectwave II)

Audio Gear Talk PS Audio
1

I’m a little torn about using upsampling with Roon. If I upsample, Roon applies a conversion filter - be it precise linear phase, precise minimum phase, smooth linear phase etc.

But my DAC also applies one of 5 filters - described in the manual as follows:

  • Filter 1 MP APOD. Minimum phase Apodising filter.
    Low pre-ringing, minimal group delay, minimized post ringing, good phase vs. frequency, sharper filter cutoff.

  • Filter 2 MP SOFT. Minimum phase soft knee filter.
    Low pre-ringing, minimal group delay, minimized post ringing, good phase vs. frequency, soft cutoff.

  • Filter 3 LP APOD. Linear phase Apodising filter.
    No group delay, perfect phase vs. frequency, minimal post ringing, some pre-ringing, sharper filter cutoff.

  • Filter 4 LP SOFT. Linear phase soft knee filter.
    No group delay, perfect phase vs. frequency, minimal post ringing, some pre-ringing, softer filter cutoff.

  • Filter 5 MP 1/2B. Minimum phase recursive Half Band symmetrical filter.
    Minimized pre and post ringing, good group delay, good phase vs. frequency response, sharp cutoff.

It’s an old PS audio perfectwave II incidentally.

So… I’m not sure if the filters are the same. And should I be trying to choose the same filter? If I mix filters, will that worsen the sound? Or does it not matter?

Any thoughts would be much appreciated.

Cheers
Gareth

2

Can you hear any difference when you try the different settings?

3

Hi Gareth,

I own a Perfect Wave Dac MK II myself and asked myself a similar question, i.e. which filter number corresponds to which filter type and which filter is used by the “auto” mode for different sample rates.
I posted my results over at the PS Audio forum some time ago, so the following text might look familiar.

First off, the user manual is plain wrong about the filter order.

The following table is from the Wolfson WM8741/8742 DAC Chip data sheet.

Depending on sample rate, the chip offers slightly different sets of filters and even (for some strange reason) in different order.

  • “Low” refers to SR 44.1 and 48 kHz
  • “medium” refers to SR 88.2 and 96 kHz
  • “high” refers to SR 176.4 and 192 kHz

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b02638573b4588ac936a1eaa620172f79eb6648f694×431 26.2 KB

By measuring frequency and impulse response at different sample rates and for all filters including the auto setting I can make the following clarifications:

  • Filters 1-5 as selected on the PWD display correspond to the order in the table from the data sheet shown above for the respective sample rates.
  • For sample rates 44.1 kHz and 48 kHz low mode filter 3 (minimum phase half-band filter) is activated using the “auto” setting
  • For sample rates 88.2 and above, medium/high mode filter 4 (minimum phase apodizing filter) is activated using the “auto” setting

Testing has been done with latest/final firmware version 3.0.4, but I guess, the same is true for earlier firmware versions.

I use filter 4 (linear phase anodizing) for low mode (44.1 and 48 kHz) as it is the technically most correct one. Filters 1-3 are soft roll of filters and let unwanted ultrasonic aliasing noise pass through. Filter 5 introduces phase shift in high frequencies. The sonical differences are minimal though. A question of taste.

If you upsample in Roon, one would initally think, the filter used by the PWD becomes less important, as cut off frequencies typically are half the sample rate (fs/2) which is far beyond the hearing range for 88.1 kHz sample rates and above.

Digging deeper into the DAC Chip data sheet, that turns out not to be the case. Only Filter 3 (linear phase brickwall) in medium and high mode actually cuts off at fs/2 with a steep filter. All other filters are designed to cut off around 20 kHz with a more or less soft roll off, regardless of sample rate.

So if you’re playing proper high res material, usage of filter 3 is mandatory, otherwise you’re actually filtering out the native ultrasonic part of the music. Again, the perceived effect is minimal in my experience.

When upsampling in roon there is no native ultrasonic content in the resulting signal so the choice of filter in the PWD is less important. I would recommend filter 3 as well to avoid double filtering at 20 Khz.

In my experience upsampled content sounds more open and spacious, but less direct and dynamic. So depending on content and mood, I toggle it on/off.

Hope that helps und enjoy the music!

Regards, Roland

3 Likes
4

Thanks, Roland - that is an incredibly impressive response!

On another topic, I’ve just discovered the sound is appreciably better setting the PerfectWave DAC volume to 100, then using my amplifier volume control (McIntosh 6700).

I do wonder if the old PerfectWave volume controls are not as good as the newer DACs. This was hinted at in a video of Paul McGowan where he referred to the superior volume controls of the new DACs (which seemed to mean post 2014 or so).

It may be that bits are being stripped in the conversion process at lower volumes. Again, this is hinted at in an old review of the PerfectWave by Darko.

Did you have any thoughts on this?

Regards
Gareth

5

Sorry, one other quick question. With the Roon upsampling, do you use precise, linear phase?

To my uneducated eye, this seems to correspond most closely to Phase 3 on the PerfectWave.

6

Hi Gareth,

to answer the second question first, yes i use the precise , linear phase setting for upsampling in roon as well and upsample to 176.4 KHz for 44.1 kHz source material and to 192 kHz for 48 KHz content. On the PWD i use filter 3 and i am quite happy with the results soundwise.

The digital volume control on the PWD loses resolution at low volumes, just as any other digitally implemented volume control as well. Depending on the resolution of the source material and the internal resolution available for digital volume control of the dac, there is a certain level of attenuation possible without losing information, below that you’re actually stripping off bits. With the PWD this window is quite narrow, if i interpret the data sheet correctly, as the digital volume control operates only with 24 bits of resolution (Roon does it with 64 bits).
Two more things to consider with digital volume control are dither being added to mask systematic noise resulting from digital attenuation with random noise and DAC chip linearity at low levels, which also distorts the digitally attenuated signal during the conversion to analog.

Newer DAC chips most likely have improved alot in this regards, so digital volume control is less an issue with those.

So if you’re using an amp with volume control, you’re doing the right thing to keep the volume fixed at 100 on the pwd an set the volume in the analog domain.

Regards,

Roland

7

Interesting - thanks, Roland. So you use a custom upsampling with 44.1 (redbook) - going to 176.4 rather than 192 because it’s an even multiple. Understood.

Thanks and regards
Gareth

8

you’re welcome! And i’m happy to know there are still fellow users of the PWD MkII around!

There is a setting to automatically upsample to the highest even multiple, so no need to configure this individually for every base sample rate.

One more thing, in case you’re connecting to the PWD via USB:
The DAC reports to accept 32 bit resolution, so Roon converts from 64 bit to 32 bit after upsampling and outputs this to the DAC.
However - again according to the data sheet of the WM8741 chip - the incoming 32 bits are immediately truncated to 24 bits for internal processing of the chip. It’s a 24 bit DAC after all.
So i recommend modifying the advanced audio device settings in Roon to limit the bit depth to 24 bit to avoid the above mentioned truncation in the DAC. Roon does an audibly better job in reducing the bit depth from 64 bit to 24 bit then the DAC chip in truncating 32 bit to 24 bit.

9

Interesting - no I abandoned USB a while ago. I used to connect my ROCK directly to my Bluenode via USB. Way too noisy.

Using the Bridge 2 in the PerfectWave DAC, connected via the ethernet to my ROCK. Using the balanced, XLR outputs to the amp.

Cheers
Gareth

10

Hi Gareth,

the bridge II is a nice solution. I have a bridge I, but ultimately removed it from the PWD since it never really worked satisfactory - and didn’t get any Roon ready update.

It might be worth taking a look at the signal path anyways. If it shows 32 bits as output bit depth to the bridge II, reducing this to 24 bits in audio device setup might be worth a try.

Regards, Roland

11

Yes, I upgraded mine - the artwork still doesn’t work (!)