Sound Troubleshooting

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Revision as of 07:18, 9 February 2007 by Daveat (talk | contribs) (Note about bug 2468 for easy reference)

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Sound problems fall into several categories:

  • Audio input problems associated with software-encoding video capture cards (the sort that require a cable between the capture card and the computer's sound card)
  • Audio output problems related to Linux drivers and software
  • Audio problems related to advanced or unusual hardware (Dolby Digital output, for instance)

Software-encoding video capture card problems

These problems can be very frustrating to troubleshoot. They're usually related to drivers and other software issues, although you should always check your cabling. Be sure the audio output from the capture card goes to the sound card's audio input jack!

Only left channel is recording

If your sound card is intel8x0 or maybe others then its alsa's fault. Its failing to set both capture channels to linein. One is left on the mic channel to fix this run this command.

amixer cset iface=MIXER,name='Capture Source' 4,4 numid=24,iface=MIXER,name='Capture Source'

Another fix is to upgrade alsa, but some people have said this problem still continues.

More information here.

Echo on audio input

For the VIA 8237 chipset used on the Biostar PT880 Pro-A7C motherboard (and probably others), the audio external line input can produce an echo effect if the "Duplicate Front" mixer channel is enabled. This channel is easily disabled via alsamixer.

Distorted audio on input

Some audio chipsets' drivers have limited options concerning encoding, and sometimes playback, sample rates. For instance, as of the 2.6.19.1 kernel's ALSA drivers, the VIA 8237 chipset used in the Biostar PT880 Pro-A7C motherboard handles a 48,000Hz sample rate, but few or no others. Ubuntu 6.10's installation of MythTV 0.20 defaults to a 32,000Hz sample rate. The result is an unpleasant "razzing" effect, slightly reminiscent of a Dalek. This effect is amplified by the MP3 encoding that's also done to the audio. It can be further amplified by the echo effect noted earlier; the combined result is most unpleasant!

The solution is to modify the recording options. You'll need to do this for each of several recording profiles in the Utilities/Setup -> Setup -> TV Settings -> Recording Profiles -> Software Encoders area -- live TV, default, low quality, and high quality. It's easiest to adjust these options using the live TV option and then copy your best options to the other profiles.

Audio output driver problems

Audio output driver problems can be caused by buggy or incomplete drivers or by improper settings. A good starting point for debugging is the ALSA Project documentation. Locate your audio chipset and read the instructions for configuring it. Test your output first using the stereo output jack, even if you intend to use more advanced outputs; the 2-channel case is generally simpler and is therefore easier to get working.

Note that version 0.20 has a bug 2468 which can result in sound being absent during playback. Selecting an alternate audio track should work around this issue. This has manifested as absent audio on recordings scheduled with 0.20, while recordings scheduled with an earlier version but recorded using myth 0.20 had sound.

Advanced hardware problems

Advanced audio hardware can be problematic because of the comparitive rarity of the hardware, and therefore the lack of discussion of problems related to it in the Linux community.

Lack of appropriate 6-channel output hardware

Many sound cards and motherboards claim 6-channel (Dolby Digital 5.1) or better output, but lack obvious connectors (enough analog jacks or externally-accessible SPDIF connectors). Such hardware typically has internal three-pin SPDIF headers. You can connect an external SPDIF bracket to the internal header. You can build such a bracket yourself if you know how to solder and have the parts, or you can buy one -- if you can find one! They're extremely hard to track down. Two sources for such brackets are:

  • VidaBox, a company that specializes in high-end home entertainment equipment and DVRs. This bracket is one of the least expensive items they carry.
  • Atacom, a Web-based PC and components store.

Both brackets feature both optical and coaxial outputs, enabling you to connect your computer's output to most AV receivers. Unfortunately, by the time you add shipping, you'll spend almost as much on the bracket as you would on a sound card with appropriate output hardware. Depending on your case's design, though, you might be able to put the bracket in a socket that can't be used by an expansion card. On the other hand, if you intend to use the bracket with a motherboard's audio hardware, be sure to check the header's location -- it might just block an expansion card when in use!

Working SPDIF output but only 2-channel sound

the VIA 8237 chipset used on the Biostar PT880 Pro-A7C motherboard supports SPDIF output, and so should produce 5.1-channel sound from appropriate sources, once equipped with an output bracket. Unfortunately, this didn't work with MythTV 0.20 on an Ubuntu 6.10 system, which ships with the 2.6.17 kernel. The hardware produced sound, but it was of the 2-channel variety. Dolby Digital output was produced by mplayer, though. After upgrading to a 2.6.19.1 kernel, MythTV could produce 5.1-channel output from appropriate sources (DVDs or files ripped from them).

Another possible cause of too-few sound channels is mixer settings. Check in the alsamixer utility (or some other mixer) to be sure that the "Channel Mode" is set to "6," or higher if appropriate. (This option could have other names depending on your hardware.)