5.1/7.1 Surround Sound: Center And Subwoofer Channel are not separated, but shared

[tordeu]

In Ubuntu 11.04 (and before in 10.10) I noticed very "weird" sound, when I played videos with surround sound.

1) When using the "Test Speakers" in the "Sound Preferences", Pulse Audio (?) does not distinguish between Center and Subwoofer. No matter which of those two speakers I want to test, this "plop" sound comes out of both speakers.
Somehow the Center and Subwoofer Channel seem to be shared.
What is interesting though, is that in "Sound Preferences" (in the "Output" tab) I can set the Subwoofer from Minimum to Maximum and that really affects only the subwoofer channel.

As I have not messed with any configuration files it seems that either there is a bug in Pulse Audio or (which I think is more likely) something is set up incorrectly in Ubuntu when it comes to surround sound.

So far, I could not find anything about this matter, which I find very weird. But the problem is that it is hard to search for, since most searched for surround sound, center and subwoofer lead to other problems. The only mentioning of the same problem I could find (or stumbled upon) is the 4th post in this thread.

2) Another problem, just as a side note in case anyone knows anything about it. How can I set a cut-off frequences and redirect all frequencies below to the subwoofer? I suspect that this is another reason why surround sound on Ubuntu sound "not quite right"

Thanks a lot in advance in case someone knows something about this or can point me in the appropriate direction.

P.S.: My soundcard is a ASUS Xonar D1 (PCI version)

[tordeu]

I tested a few things and the problem is ALSA, not PulseAudio.

I used speaker-test to test the individual channels, like this:

speaker-test -t wav -c 8 -s <channel>

As expected, most channels works. But testing channels 5 and 6 (Center and LFE) does not work as it should In both tests sound comes out of the center speaker and subwoofer.
(Besides: speaker-test output "LFE" if testing channel 6, but the voice says "rear center"; I don't know if that is part of the problem; that ALSA does treat the subwoofer as a rear center speaker. But of course even then, speaker-test should be able to separate them).

As I also have an internal HDA Intel and have repeated the test. With the internal HDA Intel the center and subwoofer channel work as expected. So the problem is probably limited to the CMI8788 chip on the Xonar D1.

[tordeu]

I now fixed this issue.

The problem is the stereo-upmixing which can be selected by running alsamixer. If you run alsamixer and select a stereo-upmixing setting which includes "Center/LFE" (that is either "Front+Surround+Center/LFE" or "Front+Surround+Center/LFE+Back"), the center and lfe channel will be permanently mixed together and the result will be output to both the center speaker and subwoofer.

You can check this by running the code mentioned above, like this:

speaker-test -t wav -c 8 -s 5
speaker-test -t wav -c 8 -s 6
(or in case you have a 5.1 setup, use "-c 6" instead of "-c 8")

In both cases sound will come out of the center speaker and subwoofer, even though this is supposed to test the channels individually.

If you have this problem, you will notice that this behavior is permanent and - although it is called "stereo upmixing" will occurr with surround sound content as well, which will lead to "weirdly bad sound" when watching movies, because the center and lfe channel of the movie will be mixed together and the result is sent to both the center speaker and the subwoofer, so that voices can be heard from the subwoofer as well and the bass/lfes will appear on the center speaker as well, which will make the output of the center speaker appear unclear because of the bass which is mixed into it.

I am not sure if this affects all sound cards or if it is a limited problem. But in case you have this problem and stumble upon this thread, please post that you are affected by this problem and which soundcard you have.

[ledgerline]

Tordeu,

Thanks a million for this post, I had been fighting a similar situation and this fixed it for me and wanted to reply with my specific problem in effort to help others with Creative X-Fi center and subwoofer muted problems, as well as to ensure you get the credit for helping me out.

I'm running 64bit Ubuntu 11.04 on an Intel Quad core with Creative X-fi eXtreme Gamer on a fresh install (Got a great deal on a 1TB drive so reinstalled fresh). Everything installed smoothly but I noticed that my X-Fi Center and Subwoofer channels would not produce any sound, regardless of which settings I chose for output settings. I verified on my Win-7 dual boot that the card was good and started pulling my hair out at that point. Turns out that your recommendation was what I needed. I ran alsamixer and low and behold, apparently by default center/lfe and input devices are muted even after setting up the card through Pulseaudio. I did not have any issue with the center and lfe being linked though, once I unmuted it I was good to go. Thanks again and I'll stop rambling now. Quick steps below for those unfamiliar with alsamixer;

- from xterm session run alsamixer
- [F6] to select soundcard
- Left/Right arrow to move between channels
- Hit M to unmute selected channel
- Esc to exit

Hope this helps someone, I've seen a lot of posts about it without a solution.

[tordeu]

I am glad my post could help someone. There seem to be a lot of issues with surround sound set up in Linux, which might be easy to fix, but you often just don't really know where to start and what the problem could be, so it gets really frustrating.
And thanks for adding your problem & solution. I'm sure that this will help others as well.

[rulet]

Hello tordeo. Thank's for this advise -- I have Asus Xonar DX which a little different from yours, but looks like I see the same in alsamixer. As I remember I have made before such a tests of a channels and I was wonder why it is so -- now I know why.
I suggest to improve it(as I think) and to redirect all basses from all channels to subvoofer(if it's possible) as I described it here in 1702 post. Just interesting if it will affect on a quality of a sound.
Any ideas?

[mati007]

To redirect basses to subwoofer see http://ubuntuforums.org/showthread.php?t=1640574