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Audio File
Had a few interesting adventures with my Myth box this past week. I've been having a lot of audio issues: first a typical digital noise issue, that under-water sound you get especially in higher frequencies. The sort of thing you get with poor VOIP. Second, and much more annoying, was an intermittent garbled audio sound where it sounded like the card was being overloaded. If I quit out of TV viewing mode and re-entered it would go away, but often come back a few minutes later. It would also go away if I just let it sit like that for a couple minutes. Extremely annoying. Finally, not audio related but annoying, my Pinnacle USB remote continues to be unsupported by the version of LIRC that comes with Mythbuntu, so I have to recompile those drivers from source every time I get a kernel update.

Thinking some of this might be related to the fact that I upgraded from Mythbuntu 8.04 to 8.10 to 9.04, I decided to try re-installing the OS from scratch. I was clever enough to back-up any pre-existing data, as well as the contents of my myth database, unfortunately something in that process went wrong. Not only did the clean install not fix anything, but suddenly my TV capture card stopped working. After a lot of attempts to fix this, I removed the box from the TV and brought it upstairs to my desk for a complete overhaul. While I was at it, I decided to try buying a new sound card for the thing, in hopes that the audio issues might be due to using the crappy built-in sound card. After a fair bit of work I got it up and running again. Here's the current config of my myth box:

Dell P4 Desktop with 1 GB RAM and 80 GB hard drive
nVidia 6200 low-profile AGP video card
Pinnacle PCTV 800i TV capture card
Turtle Beach Riviera sound card
Pinnacle PCTV USB MCE Remote
1 TB external hard drive

The capture card pretty much works right out of the box, nothing special required there. It does turn out that some of the audio issues I was having was due to the recording settings for the card. I found some info on the MythTV wiki about this card, and in the Recording Profiles set the Default settings to:


Codec: MP3
Sample Rate: 48000
MP3 Quality: 9
Volume: 50%


This fixed the general digital noise, but I still get the intermittent garbled sound which is far more annoying. More on this later.

I disabled the on-board sound card in the bios, and then my Riviera sound card became the default which eased a lot of things. While I was able to set the audio device to use for playing videos and TV, I couldn't figure out how to modify what was used for DVDs. Anyway, this got the sound card basically working through the mini output plug, but the real pay-off was to get it working through the optical cable. To do this, first I had to set my audio settings in the General setup section of Myth to:


Audio output device = ALSA:default
Passthrough outputdevice = Alsa:iec958:{AES0 0x02}
Enable AC3 to SPDIF passthrough check
Enable DTS to SPDIF passthrough check
Use internal volume controls uncheck


Second, I had to run alsamixer and unmute iec958 out. There were easily half a dozen iec958 channels muted in alsamixer, and originally I tried unmuting them all, but this didn't work. I had to specifically look for the one labeled out and unmute only that one, leaving all the others still muted. Sure enough, not only was I then getting output through the optical cable, but playing DVDs showed full Dolby sound coming into my receiver. I've had that receiver for years now, and I think this is the first time it's had proper surround sound coming into it. Woot!

I continue to have to build lirc mceusb2 drivers from source, but fortunately the current source drops (0.8.5 pre3) have everything in place to support my Pinnacle USB receiver. Man, I wish that version would just make it into the official Mythbuntu packages.

So, everything's working again, but still I get that intermittent audio issue. It's really annoying as it sometimes shows up in recorded shows, and I've determined it's not the output feed at that point but the actual recorded audio that's garbled. This makes me pretty sure the issue is in the audio input in my Pinnacle 800i capture card.

I've also discovered that card now has a loose connection for it's input lines. It has this proprietary input jack with a 3' adapter that includes both composite and s-video inputs. I've discovered sometimes the screen loses color (displays in greyscale) unless I press the connection of that cable to one side. I suspect it's not the adapter cable, but the actual plug on the card that's screwed up, but I can't be sure. Either way it looks like that card's going.

I tried to figure out how to plug the input audio cables to the line-in of my fancy new sound card, but I couldn't get it working. Given that I'll probably loose video signal eventually and the loose connection could very well be the source of the sound issue I'm experiencing, I think the best bet is to just replace the capture card.

After cruising around MythTV.org's list of supported analog capture cards, it seems like the best supported is the Hauppauge PVR-150. I like that this card appears to have actual RCA connections built in -- no stupid proprietary adapter cable necessary. So I tracked one of these cards down (it looks like they're starting to give way to the newer HVR 1600 cards) and ordered it online. Hopefully this will fix the audio issue.

Anyway, that's a lot more info than most people care to know. However, a lot of research and trial-and-error went into getting it all working, and I'll be very happy if blogging all this helps someone else when they're searching the web for info on similar issues.
June 8th, 2009 - 08:18 am | Comments (0) | PERMALINK

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