[mythtv-users] beginners audio question

Michael T. Dean mtdean at thirdcontact.com
Fri Aug 24 15:20:21 UTC 2007

On 08/24/2007 07:24 AM, Alastair Kerr wrote:
> After several false starts at installing Mythtv (I can post my 
> experiences if there is an interest) I have a functional system. Audio 
> is working but I am not convinced that is it fully 5.1 surround sound: 
> probably just a stereo signal going to my speakers.  This whole area is 
> new to me as  the following questions will probably testify
> Current setup:
> ASRock mobo, P4 3GHz,
> PVR-150 capturing a signal from a sky digibox box on the component input 
> (yellow/red/white)

Meaning you're capturing only 2-channel (stereo) audio at most (could be
mono on some channels).  Note, however, that the PVR-x50's cannot
capture more than 2-channel audio (and trying to capture it through a
sound card and mux it into the stream is a lot of work that will result
in poorly-sync'ed audio/video, anyway).  Basically, your TV will be
2-channel until you upgrade to a digital capture card (assuming you have
digital TV sources (DVB broadcasts)).

> (dusky controller to change channel)
> Nvidia 7300 Graphics
> 5.1 speakers connected to the  the 3 green/orange/black sockets
> I have install kubuntu with the Mythtv packages
> The manual for the mobo states it uses a asc888

I'm guessing you mean an ALC888 (Realtek ALC888).

>  codec but alsa seems to 
> be using alc883
> My Questions:
> *  What is the best way to test the sound system, is there a good file I 
> could download to play and test the audio?

I'll leave this for someone who knows what they're talking about.  (I
can't hear a difference between good or bad audio, so I don't really do
any audio system testing. :)

> *  Should I try and change the audio codec used on asla? If so how?

No.  The ALSA developers create source files to support new codec
chipsets as they encounter them and often name them after the codec. 
Often, vendors make changes to the codecs and release new models but
since they're similar enough to previous versions, the ALSA developers
just add support for the new codec to existing source file/module.  So,
the ALSA module name--while often close to some codec names--may not be
the same as the name of the hardware codec on your system.  The alc883
module also supports other Reaktek codecs (including ALC888 and
ALC861).  Even on ones where ALSA guesses about the future models that
will be supported, there can be ambiguity (as the ALSA devs don't get to
name the hardware codecs released by vendors in the future :).

Whatever module ALSA uses is the right module to use. ;)

> *  I have and spdif header on the mobo, should I try and use this rather 
> than the 3 audio cables?

S/PDIF can support 2-channel PCM audio (stereo) or 5.1-channel
AC-3/DTS.  AC-3/DTS uses lossy compression, so if you're re-encoding an
AC-3 stream (i.e. for timestretch), you'll get generational losses. 
Likely (as Steven Adeff has convinced me), this generational loss is
less than the losses due to the poor quality D/A's in sound cards (as
compared to those in a real Audio Receiver component).  Still, I want a
true HDMI 1.3+ sound card that supports 8-channel uncompressed PCM audio...

However, your system (with a PVR-x50 encoder) cannot do digital audio
capture, so your TV is 2-channel audio, anyway.  And, even if you have
"surround" audio via a DVB capture card, MythTV currently decodes it to
stereo, so you still won't get surround.  (Eventually, Myth will have
support for surround via analog outputs (currently it can only do it
using AC-3/DTS passthrough--which has to be disabled for timestretch). 
See ticket #1104 for more.)

Also, to use S/PDIF, you really /should/ use a hardware volume control
(i.e. send a signal to your external audio receiver to change its
volume--i.e. using a LIRC transmitter).  Though some people use "soft
volume" approaches, these degrade sound quality.  If using a hardware
volume control, you'll have to disable Myth's internal volume controls
(meaning you lose the OSD for volume).  If you're really interested, I
have a little xosd-based approach for an (ugly, not Myth-themed)
general-purpose OSD that I'm finishing up.  The required parts are done,
so I can make it available to you--if you want to be a beta tester. ;)
(My OSD will be visible whether in the menus or playback.  And using
external audio controls, you'll be able to change volume even when not
watching recordings/listening to music (i.e. when not using audio). 
This is good because sometimes you'll exit a "low-volume" show for which
you cranked the volume up.  If you then play back a "normal" volume
recording (or music or something), you're likely to get a loud surprise
if you can't turn the volume down until sound starts playing (as with
the internal volume controls).)

If you don't want to set up the transmitter (or you don't have an audio
receiver), using the analog output (3 audio cables) is probably the
better option.  But, it's your option.

>  If so is there an adaptor available to buy that 
> uses this header?

Check with your motherboard vendor.

> * I have an older Audigy 2 soundcard. Should I install this instead of 
> the mobos sound?

Basically, the Creative SB Live! and above (including Audigy boards) use
a hardware DSP that typically requires audio resampling before the DSP
processing, and the resampling can adversely affect sound quality. 
Though you can disable the DSP (and, with Audigy 2 and above get good
quality sound), doing so means only some sound will work for you.  So,
typically for a "general purpose" media machine, you'll need to enable
the DSP or will have to do "software" audio conversions/etc.  Most every
non-Creative sound card (including (all?) integrated MB audio cards)
require the same "software" audio conversions required of a Live!/Audigy
whose DSP is disabled, so whether using the integrated sound card or the
Live!/Audigy without DSP makes little difference.

Note, also, that the differences between the SB Live! and each
generation of Audigy are mainly in the DSP quality.

But, in reality, the answer is, "Choose the one that sounds best to
you."  (Assuming you can even hear a difference--I (and my untrained
ears) can't.)



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