Digital Room Correction and Bass Artefacts

[cliveb]

Having set up DRC using a RPi with HiFiBerry DIGI2 Pro + DSP card, I have an issue.

When setting it up (having used REW to calculate the required filters), I was of course using source material with significant bass content to audition the results, and pleased with the way it tamed the bass humps in the room.

But once I started listening in earnest, noticed some curious low frequency artefacts during quiet moments, a kind of grumbling that varies in amplitude. The closest analogy I can give is that it sounds a bit like turntable rumble. Thinking I might be imagining it, I tried playing something while toggling the filters on & off, and the artefacts are definitely only present when the filters are active. It's quite subtle and only at very low frequencies. I can't hear it on (admittedly inexpensive) headphones, only on my active Meridian speakers.

The filters are fairly extreme, with some significant gain settings and high Q:

Filter  1: ON  PK       Fc    54.3 Hz  Gain -19.2 dB  Q 2.831
Filter  2: ON  PK       Fc    71.1 Hz  Gain   9.0 dB  Q 3.982
Filter  3: ON  PK       Fc    75.2 Hz  Gain  -5.4 dB  Q 7.677
Filter  4: ON  PK       Fc    89.3 Hz  Gain  -5.5 dB  Q 11.271
Filter  5: ON  PK       Fc     148 Hz  Gain -13.3 dB  Q 13.429
Filter  6: ON  PK       Fc     202 Hz  Gain  -6.2 dB  Q 5.000
Filter  7: ON  PK       Fc     225 Hz  Gain   6.3 dB  Q 5.000

My question for the experts here is whether these kind of grumbling low frequency artefacts are a known issue with DRC.
Perhaps they are to be expected with such extreme filtering?
Maybe the code that HiFiBerry uses to implement the filters isn't as good as it ought to be?

I'm in a bit of a quandary: with the filters turned on, the bass response is nice and smooth, but now I've heard those artefacts, they are impossible to ignore. With the filters turned off, the bass has some pretty obvious humps that I would like tamed.

[foxesden]

Hi,

Most of your filters are attenuating the bass but filter 2

Filter 2: ON PK Fc 71.1 Hz Gain 9.0 dB Q 3.982

is adding a lot of gain in between filter 1 and 3 which are attenuating bass.

Filter 1: ON PK Fc 54.3 Hz Gain -19.2 dB Q 2.831
Filter 2: ON PK Fc 71.1 Hz Gain 9.0 dB Q 3.982
Filter 3: ON PK Fc 75.2 Hz Gain -5.4 dB Q 7.677

So is the intention to boost this frequency. If it is then it will only target very specific frequency and possibly give a weird effect. I would suggest lowering this value and see if it gets rid of the artefacts.

[cliveb]

Hi,

Most of your filters are attenuating the bass but filter 2

Filter 2: ON PK Fc 71.1 Hz Gain 9.0 dB Q 3.982

is adding a lot of gain in between filter 1 and 3 which are attenuating bass.

Filter 1: ON PK Fc 54.3 Hz Gain -19.2 dB Q 2.831
Filter 2: ON PK Fc 71.1 Hz Gain 9.0 dB Q 3.982
Filter 3: ON PK Fc 75.2 Hz Gain -5.4 dB Q 7.677

So is the intention to boost this frequency. If it is then it will only target very specific frequency and possibly give a weird effect. I would suggest lowering this value and see if it gets rid of the artefacts.

All these filter settings were what REW calculated based on the room measurement, so that's how I set them up.
But I agree that they are a bit weird. 9dB of gain just 4Hz away from 5.4dB of cut does seem pretty strange.

I have tried removing filter 2 and slightly reducing the amount of attenuation on the neighbouring filters - we'll see how that goes.

I'm beginning to wonder if I'm in danger of getting trapped in a cycle of fiddling around trying to find the best compromise. Having to keep swapping between HiFiBerryOS to set up the filters and piCorePlayer to properly audition the results is a pain in the rear. Just listening to music would be a better use of my time.

At the moment, the low level bass grumbling caused by the DSP room correction is more annoying than the lumpy response of the room. Wouldn't surprise me if I end up removing the DSP card completely and just accepting the bass nodes in the room!

[BoxOfCables]

I'm beginning to wonder if I'm in danger of getting trapped in a cycle of fiddling around trying to find the best compromise. Having to keep swapping between HiFiBerryOS to set up the filters and piCorePlayer to properly audition the results is a pain in the rear. Just listening to music would be a better use of my time.

Like you I started out with a Hifiberry DSP using REW, I didn't get any bass issues like you have, but the board and the dsptoolkit were very unreliable. I did spend more time fixing issues than listening to music. The board recently died, and I've discovered that it's pretty simple to implement the filters through SoX which can be done with a file upload through piCorePlayer web interface. Even a lowly pi3B can cope with running the filters without issue.

[cliveb]

... I've discovered that it's pretty simple to implement the filters through SoX which can be done with a file upload through piCorePlayer web interface. Even a lowly pi3B can cope with running the filters without issue.

Do you have any links to guidance for doing this?
I'd always assumed that my Pi3 LMS server wouldn't have the necessary grunt to run DRC filters, but now you've reassured me I'd like to give it a try.

[BoxOfCables]

Do you have any links to guidance for doing this?
I'd always assumed that my Pi3 LMS server wouldn't have the necessary grunt to run DRC filters, but now you've reassured me I'd like to give it a try.

You need to create a text file (you can use Notepad etc.) containing the something like the following (this will become "custom-convert.conf" on the server):

flc flc * *
	[sox] -q --multi-threaded --no-clobber -t flac $FILE$ -t flac - equalizer 99.6 2.70q +3.6 equalizer 283 1.00q -2.9 equalizer 2293 1.00q +1.4

Each entry for "equaliser" can hold one filter from REW. The first value is the Hz, the second number is the quantisation (Q) and the third is the gain (positive or negative). This one has three filters, it applies only to flac files. You would need additional blocks for different formats.

In REW, using a generic equaliser will give you a filters file like below (this is not the one to match the convert above, don't get confused ). You can see it has the Hz, gain and Q. So all you need to do is pick those out and write them into the "custom-convert.conf" format.

Filter Settings file

Room EQ V5.19
Dated: 21-Dec-2018 15:23:09

Notes:

Equaliser: Generic
Dec 21 15:06:19
Filter  1: ON  PK       Fc    29.9 Hz  Gain  -2.8 dB  Q 6.589
Filter  2: ON  PK       Fc    38.7 Hz  Gain  -2.3 dB  Q 3.502
Filter  3: ON  PK       Fc    63.3 Hz  Gain  -2.7 dB  Q 2.000
Filter  4: ON  PK       Fc    80.4 Hz  Gain  -2.4 dB  Q 36.106
Filter  5: ON  PK       Fc     120 Hz  Gain  -7.5 dB  Q 3.864
Filter  6: ON  PK       Fc     122 Hz  Gain   9.0 dB  Q 12.341
Filter  7: ON  PK       Fc     142 Hz  Gain  -8.6 dB  Q 33.829
Filter  8: ON  PK       Fc     145 Hz  Gain -11.2 dB  Q 25.092
Filter  9: ON  PK       Fc     147 Hz  Gain   9.0 dB  Q 5.743
Filter 10: ON  PK       Fc     172 Hz  Gain -10.2 dB  Q 10.848
Filter 11: ON  PK       Fc     183 Hz  Gain   7.0 dB  Q 28.041
Filter 12: ON  PK       Fc     221 Hz  Gain  -5.1 dB  Q 4.909
Filter 13: ON  PK       Fc     280 Hz  Gain   9.0 dB  Q 5.000
Filter 14: ON  PK       Fc     297 Hz  Gain -17.8 dB  Q 5.000
Filter 15: ON  PK       Fc     315 Hz  Gain   5.6 dB  Q 5.000
Filter 16: ON  PK       Fc     405 Hz  Gain  -3.3 dB  Q 5.000
Filter 17: ON  PK       Fc     415 Hz  Gain   9.0 dB  Q 3.357
Filter 18: ON  PK       Fc     481 Hz  Gain  -3.1 dB  Q 1.000
Filter 19: ON  PK       Fc     950 Hz  Gain  -2.2 dB  Q 5.000
Filter 20: ON  PK       Fc    1192 Hz  Gain  -5.8 dB  Q 5.000

Once you have your text file ready with the filter values, head to the piCorePlayer web interface, click the "LMS" tab and upload your file using this input:



Select the file, then hit upload and it will whirr away for a bit and restart LMS, and you'll have your filters applied. If you want to undo them, simply upload a blank document.

[JanVana]

But once I started listening in earnest, noticed some curious low frequency artefacts during quiet moments, a kind of grumbling that varies in amplitude. The closest analogy I can give is that it sounds a bit like turntable rumble.

Hello Clive,

I have a Raspberry 4, Amp100 and a DSP module running. I have the same issue with low frequency rumble on some Hifiberry DSP Profiles. I asked hifiberry for support, but unfortunately they can't help me. Have you found a solution.

Thanks for your help. Regards.
Jan

[cliveb]

Hello Clive,

I have a Raspberry 4, Amp100 and a DSP module running. I have the same issue with low frequency rumble on some Hifiberry DSP Profiles. I asked hifiberry for support, but unfortunately they can't help me. Have you found a solution.

Thanks for your help. Regards.
Jan

Jan, I saw your post on the HiFiBerry forum and responded there.
But to summarise: ditch the DSP card and just use SOX to apply the filters you need.

I strongly suspect that the algorithms used in the DSP card are flawed.