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I own an ASUS N56VZ, which comes with an included external subwoofer to be plugged through a 2.5mm jack. The subwoofer is working with my actual configuration, but it's reproducing all of the frequencies, and not just the low ones as it does on Windows. This obviously causes the audio quality to be very low.

How can I correctly set it up in order to have a low-pass filter over its channel? Thanks.

  • Are you sure this is an output jack for a "subwoofer"? Looks like a standard headphone jack to me, with a separate S/PDIF speaker out. The S/PDIF could be interpreted by a subwoofer with a TOSLINK (or similar) input I suppose. Are you saying there was a setting in Windows to bandpass the S/PDIF output? Anyway, be clear what you are connecting to and how you are connecting to it. – jdv Jan 17 at 20:28
  • what am I connecting to: a 2.5mm jack connector, as I wrote. In Windows, there is no setting related to that specific jack, the subwoofer automatically reproduces low-frequencies only, as simple as that (even after completely formatting the system, so it's not because of some pre-installed ASUS software managing that channel in the background). – NeutronsCollision Jan 17 at 21:19
  • Then the specs for this model don't mention that this is a "subwoofer" jack. It is just a headphone jack. It might use some trickery to determine that an ASUS branded device is to be fed a different input. This is far enough out of the range of normal that you should just raise a bug with Canonical. – jdv Jan 17 at 21:24
  • So you think that I cannot manually impose a low-pass filter over that single output in any way? – NeutronsCollision Jan 17 at 21:30
  • I wouldn't dream of restricting your ability to band-pass any signal! I am saying, however, that this appears to be just a simple 2.5mm headphone jack. The driver for that output can control lots of things, including volume, tone, shape, etc. If your experience is that, on windows, plugging in this subwoofer automatically results in a shaped signal specific to it, then there is something else going on that no ordinary headphone port driver does. – jdv Jan 17 at 21:37
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Upon further Googling, it turns out that this headphone port, and the hardware expected to plugged into it, is some special 4-connector system that is used by the OS to detect ASUS hardware.

Windows has either built-in or custom drivers for the port that understand this hardware and treat it special, and it figures this out because the hardware is not just a standard headphone jack. While it should work as one, there are related reports of this jack just not working with regular earphones in Ubuntu here and on the greater internet.

Canonical would have to implement this driver behaviour in the OS if they decided it was a supported configuration, perhaps via a feature request.

My initial musings cribbed from the comments continues below.


I don't think there is anything Ubuntu specific here, but we really don't know because we don't know what Windows or this subwoofer hardware is doing special (and, therefore, what Ubuntu could do different).

However, if the assertion is that we are using the ordinary 2.5mm "headphone" jack as output, then this is software controlled to some degree on all platforms. We can at least change the volume, and some drivers/interfaces will allow pretty sophisticated tone control all the way to faking spatial audio. Also, the assumption here is that we are not talking about the S/PDIF output.

AFAIK, the typical Ubuntu controls for the headphone port will offer volume and muting. There may be something that offers equalization or output presets. If someone knows more they can provide that as a better answer!

But, if the assertion is that some hardware, once plugged into this port, tells that port to change the equalization to the point of a low-pass filter, then there is something else going on. These drivers can detect when something is plugged in, but there is nothing I know of that would be a generic way of saying "I am a subwoofer" vs. "I am a pair of headphones".

Maybe this subwoofer hardware is special; perhaps it shorts one of the unused wires on the cable (assuming this jack supports more than a single pair of signal wires) to signal "special ASUS hardware plugged in" to the driver. If so, then the Ubuntu port driver stuff would have to know about this, and then would have to know how to equalize the signal in a similar manner. The obvious conclusion based on this question would then have to be that it does not.

This sounds like a losing battle, however. I'd look for (ALSA?) audio equalizers, perhaps with some presets, or equalization that can be made into a preset so you could low-pass the audio appropriately when this hardware is plugged in.

  • I already looked into equalizers and I've been able to tune pulseaudio in a way to make the whole 2.1 system sound better when the subwoofer is plugged. However, I haven't been able to set a specific curve for the subwoofer, in order to have it reproducing only the low frequencies, so at the moment it continues reproducing the entire frequencies spectrum, and since it isn't made for the purpose I still end up with a poor overall sound quality. By the way, thank you for your support. – NeutronsCollision Jan 17 at 23:11
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In the end, I've been able to manually tune the subwoofer by noticing that Pulseaudio's qpaeq, that I was already using in order to equalize the 'normal' speakers, does provide per-channel equalization.

qpaeq

I've set up a low-pass filter and now I've finally been able to reproduce only the low frequencies through the subwoofer. It's important to set the audio system as 2.1 in Pulseaudio's settings, since keeping the default stereo profile causes the 3rd channel, the one referring to the subwoofer, to be misssing in qpaeq.

  • great! I used your answer to configure my subwoofer (which nevertheless is in the fourth channel of a 4.0). – c.p. Feb 7 at 11:40

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