[mythtv-users] Really bad sound quality from /dev/dsp1

[Allen Edwards]

My setup is much different than yours but this may help.  I do
everything I can to simplify and get alsa out of the picture.  I
delete both the alsa setup files, asound.conf and asoundrc.  (hope I
have the names right).  I do not have volume control from myth.  My
audio output device is ALSA:spdif which may not be what you want.  I
think you can send it right to the chip, dsp1 or something.

This may take some digging but you should not be having this problem.

Allen



On Tue, Nov 25, 2008 at 3:58 PM, Jim Carter <jimc at math.ucla.edu> wrote:
> The Penguin is embarassingly losing a competition with Media Center
> Edition.  I think I've figured out all the configuration issues except
> for one, which is a showstopper: on NTSC channels the sound fades in
> and out: at its best its quality approaches (but does not reach) that
> of the card's ALSA source, but most of the time it sounds like the
> actors smoke six packs per day and are speaking their lines underwater.
> They are not chipmunked, i.e. the sound frequency is right.
> On a VOIP connection I would start looking for a codec mismatch.
>
> Hardware configuration:
> System: Dell Dimension 4600
> CPU: Pentium 4 at 2.4 GHz, hyperthread
> Memory: 256 Mb
> Chipset: ICH5
> TV Card: pcHD-5500 (Conexant CX23880/1/2/3 PCI Video and Audio Decoder)
> Sound: Intel Corporation 82801EB/ER (ICH5/ICH5R) AC'97 Audio Controller
> Graphics: nVidia Corporation NV18 [GeForce4 MX 440 AGP 8x]
>
> Software:
> OpenSuSE 10.3, kernel 2.6.22.19-0.1-default (supports hyperthread)
> mythtv-{everything}-0.21-18 from Packman
> lame-3.98.2-0.pm.3 from Packman
> Modules are the ones included with the mainline kernel (not hacked).
> Modules loaded (edited): cx88xx cx8802 cx88_alsa cx8800
>        cx88_dvb snd_mixer_oss snd_pcm_oss
>
> Myth configuration summary:
> Two capture cards are defined: DVB (the pcHD-5500's digital radio) and
> V4L (the NTSC radio). For the latter, the sound source is /dev/dsp1.
> The frontend sends sound to ALSA:default.  Recording profiles are mostly
> unchanged from installation except the sample rate is set to 48000Hz,
> video is compressed with MPEG-4 and audio with MP3 (lame).
>
> Here's what I did to narrow down the problem area.
> 1.  Authentic music played to the ALSA default sink (ICH5) sounds normal,
> proving that the speakers, headphones, sound card and ALSA library are
> functional.
> 2.  When I play a NVU file with VLC the sound is equally bad, proving
> that the frontend has no blame here.
> 3.  Uncompressed audio sounds identical, proving that lame is not lame.
> 4.  Now with Myth stopped I did "ttv KCET" to make it tune a station.
> 5.  arecord -D hw:1,0 -f dat | aplay -f dat
> where dat means -f S16_LE -c 2 -r 48000.   This gives "perfect" sound.
> This is what I want to be listening to, and what MCE delivers most of
> the time.
> 6.  aplay -f U8 -c 2 -r 8000 /dev/dsp1 gives the same sound as the Myth
> backend is compressing into the NVU file, which pretty well lets the
> backend off the hook.  Myth is recording the sound it is given, which
> is bad.
> 7.  aplay -f U8 -c 1 -r 16000 /dev/dsp1
> Sounds a lot like -c 2 -r 8000, but there's a high-pitched crackly
> overlay, not very loud.
>
> I copied a sound clip from /dev/dsp1 to a file using "dd", and looked
> with "od": it is definitely 8 bits per sample, and 16000 octets per
> second ("dd" does timing and reports this).  It's hard to tell by
> eyeball whether the sound is stereo or mono, but the Nyquist frequency
> is half the sample rate, and I can't believe stereo at 4kHz, so most
> likely it's mono.
>
> Thinking that clipping might be the problem, I tried turning down the
> volume, both with "alsamixer -c 1" and with "v4lctl volume 50%" [etc.]
> Alsamixer's GUI showed volume changes on the capture device, and "v4lctl
> list" also showed volume changes, but no effect could be heard; the
> sound was equally loud and equally bad.  "v4lctl volume mute on" (or
> off) did mute/unmute the sound.
>
> At this point I'm inclined to blame ALSA OSS emulation, saying that
> Myth is in a "garbage in, garbage out" mode.  I tried using aoss to run
> the backend, but it still opened /dev/dsp1 (seen in /proc/$PID/fd) and
> produced bad sound.
>
> According to the mailing list, and forum postings elsewhere, lots of
> other people can make MythTV work; some even report nearly "out of the
> box" results.  Can anyone suggest anything more I can do?  My thoughts
> have been along these lines:
>
> 1.  Make the backend take sound from ALSA hw:1,0, the normal sound
> source for this TV card.  That would be best, as it's 16 bits per sample
> at 48000Hz.  NTSC sound is nowhere near this good, but 8 bits at 8000Hz
> stereo or 16000Hz mono seems kind of anemic.
>
> 2.  Get the backend to actually use the OSS API rather than just
> opening the raw device.  Then aoss could divert it to the ALSA device.
> But I have no idea how to use either the OSS or ALSA API, so hacking
> like this would be a long and frustrating process.
>
> 3.  Figure out how to turn down the volume in the pcHD-5500, supposing
> that clipping is causing the problem (which actually I doubt because the
> sound sometimes is fairly good yet is equally loud as the bad majority).
>
> 3.  Specify magic undocumented parameters to some ALSA component, to
> make the OSS emulation better.  For example if you did a rate change
> from 48000Hz to 16000Hz by keeping every third sample you might get
> alias effects, which an exponential filter would suppress.  I
> experimented with specifying a plug device, but couldn't make the rate
> and format changes happen.
>
> James F. Carter          Voice 310 825 2897    FAX 310 206 6673
> UCLA-Mathnet;  6115 MSA; 405 Hilgard Ave.; Los Angeles, CA, USA 90095-1555
> Email: jimc at math.ucla.edu  http://www.math.ucla.edu/~jimc (q.v. for PGP key)
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