[mythtv] Poor quality sound

Ainsley McCall ainsley at techshop.net
Mon Jan 20 09:51:57 EST 2003


I have myth installed and running, including ALSA 0.9.  Myth runs fine except for the audio.  It is synchronized to the video, but it sounds choppy and overamplified.  I am sorry to bother the dev list about this.

Here are the useful details.
aplay /dev/dsp [symlinked to dsp0]
yields:
Playing raw data '/dev/dsp' : Unsigned 8 bit, Rate 8000 Hz, Mono

System configuration:
Red Hat Linux 8.0, custom kernel 2.4.18-19.8
MythTV Release 0.7

Abit NV7-133r 
   nvidia nforce-415 & MCP-D
Leadtek Wintv 2000 XP  TV & Radio tuner + remote 
    (card 34, tuner 2, bt878, 
     tuner registered bt848 by i2c-core
     MSP34xx, TDA875, and TDA7432 not found)  
generic geforce4 mx440 for video
Western Digital 80GB 8MB HD (w/ hdparm -d1) 
AMD Athlon XP 2100+ 
512 MB PC2100 RAM


The tuner card just patches directly from its line-out to my motherboard sound's line-in.

alsamixer is set to 100% main, 60% PCM, 60% line (muted, capture)

I am running under Gnome (switching to KDE and other WMs has not helped either)
'ps -e' tells me that is getting run somewhere, I have just been killing it.  Doing so does not seem to affect anything though; it does not seem to be grabbing /dev/dsp anyway.


I am essentially at a loss for what to do or where to look next.  When I modprobe btaudio it seems to register dsp1 and dsp2.  Aplay (from alsa) only gives static from these.  

sox -r 48000 -c 1 -w -t ossdsp /dev/dsp2 -t ossdsp /dev/dsp0
had been just telling me it couldn't open /dev/dsp2
but I have just realized that modprobe btaudio will let me run it

even sox complains, sets speed to 49777 instead of 48000, is dissatisfied and sets the speed back to 48000.  On top of that, mythtv does not want to run while sox is running, so I am clearly misenterpretting the point of using sox at all.

My [probably false] assumptions
As far as I can tell, a dsp is some sort of 'digital sound processor' that takes audio somewhere and stores it to a device that acts like a raw audio file.  I guess one uses btaudio to do this, which baffled me initially until I assumed that superior cards to mine must have a way of sending audio data digitally through the bus.  However, since I am only reading my line in for audio, I could probably use any driver for my /dev/dsp for digitizing the sound.

modules.conf snip:
post-install sound-slot-1 /bin/aumix-minimal -f /etc/.aumixrc -L >/dev/null 2>&1
 || :
pre-remove sound-slot-1 /bin/aumix-minimal -f /etc/.aumixrc -S >/dev/null 2>&1 |
| :
#alias sound-slot-1 nvaudio
alias sound-slot-1 i810_audio
        # ALSA portion
        alias char-major-116 snd
        alias snd-card-0 snd-intel8x0
        # module options should go here

        # OSS/Free portion
        alias char-major-14 soundcore
        alias sound-slot-0 snd-card-0

        # card #1
        alias sound-service-0-0 snd-mixer-oss
        alias sound-service-0-1 snd-seq-oss
        alias sound-service-0-3 snd-pcm-oss
        alias sound-service-0-8 snd-seq-oss
        alias sound-service-0-12 snd-pcm-oss

Myth actually isn't running 100% perfectly.  The database entries for my channel database are skewed because I couldn't find a good local listing.  Myth still runs on the channel xawtv last tuned to though, so I presume this is not relevant information.

Can anyone tell me about a nice module or executable I can run to grab /dev/dsp#, write decent to high quality sound to it (I happen to have CPU To burn), and keep mythtv happy all at once?
On a side note, is there some too I should know about that tells me what is controlling a particular device, like a 'lsdev eth0' that would tell me eth0 is being driven by $SOMEPATH/nvnet.o.  I have been keeping a tail -f tar /var/log/messages so far.

Thanks for all the help this list has given me already in archive form, and thanks in advance for anything else you can add.

Sincerely stumped,
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