[mythtv-users] beginners audio question

Alastair Kerr alastair.kerr at ed.ac.uk
Mon Aug 27 10:46:04 UTC 2007

Michael T. Dean wrote:
> 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...convector
> 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.)
> HTH.
> Mike
> _______________________________________________
My head has  only now  stopped hurting after that :->  but thanks (as 
well to others in the post) that helped a lot. 

Next issue will be to get a RGB->S-video converter  as I just found out 
the hard way that my SCART output on my digibox has component but no 
s-video signal....Hope it will be worth the extra expense.

- Al

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