[mythtv-users] My experience with MythTV annoyances, 0.20.1 version

Mark Knecht markknecht at gmail.com
Thu Jun 21 17:05:59 UTC 2007

Boy Mike. You seriously have too much time on your hands! ;-)

- Mark

On 6/21/07, Michael T. Dean <mtdean at thirdcontact.com> wrote:
> On 05/14/2007 12:47 AM, Yeechang Lee wrote:
> > * Old: Analog output to a 2.1 speaker system. New: SP/DIF output to a
> >   new 5.1 speaker system and receiver.
> ...
> > * The move to 5.1 digital audio revealed a problem with the audio in
> >   about half of the recordings I make from analog (sub-100) cable
> >   channels
> >   (<URL:http://www.gossamer-threads.com/lists/mythtv/users/241814#241814>).
> >   Before Mike Dean jumps in, let me stress again that this has nothing
> >   to do with ALSA configuration files (and if he or anyone else can
> >   prove me wrong, I'd very much welcome it) or lack thereof.
> You have a broken ALSA configuration (among other things) preventing
> proper playback.  :)
> <pet-peeve>How can someone say their configuration is not broken when
> they are unable to get something to work?  In that situation, the only
> way I can see to know for sure that the configuration is not broken is
> to try every single possible configuration.  Since that's completely
> impossible, what benefit does saying, "It doesn't work, but I /know/ my
> configuration is correct," provide?  Lack of evidence does not
> constitute evidence.</pet-peeve>
> I'm assuming you're trying to use AC-3 passthrough (and, as you
> mentioned in the other thread, you have some 32kHz AC-3).  I'm also
> assuming you've properly configured your sound card for IEC958 output
> (available through ALSA's built-in PCM names iec958 and spdif).  I'm
> further assuming (and this is probably going too far in my assumptions)
> that your sound card supports a "discrete" IEC958 input (many (most?)
> don't) /and/ supports 32kHz AC-3 passthrough (and many don't--they'll
> always slurp in data at 48kHz causing Myth to try to catch up on video
> playback).  If all these assumptions are correct, then everything should
> just work--if you have a receiver that properly autodetects rate/format
> and adjusts the IEC958 status appropriately (this one, however, /is/ a
> pretty good assumption--I don't think consumers would have accepted
> digital audio if they needed to know how everything works; rather they
> want it to just work and receivers were designed to handle "broken"
> audio input so this would be the case).
> If these assumptions are /all/ correct, you can set your audio
> passthrough device to
> "ALSA:iec958:AES0=0x06,AES1=0x82,AES2=0x00,AES3=0x03" (or
> "ALSA:spdif:AES0=0x06,AES1=0x82,AES2=0x00,AES3=0x03") and Myth should
> play a 32kHz AC-3 stream without issue (whether your receiver can is a
> whole other question, though).  Of course, this will completely break
> playback of 48kHz AC-3 streams (but you could just go into setup and
> replace the AES3 stanza with "AES3=0x02" for 48kHz or "AES3=0x00" for
> 44.1kHz).  If you try this, I can almost guarantee, though that it won't
> work.  Why, because your sound card probably doesn't support AC-3 at any
> rate other than 48kHz...
> If your sound card does not support a discrete IEC958 input (i.e. the
> sound processing chip pulls samples from another input--such as one of
> the PCM inputs--like most (all?) of the nForce/Via/Realtek-based cards)
> you'll either need a bunch of extra plumbing for something like this to
> work--if you're lucky--or a new sound card that actually supports 32kHz
> passthrough (as "passthrough" is a bit of a loose definition when the
> data is pulled from anything other than a discrete IEC958 input).
> I don't know of any specific sound cards that pull data from non-IEC958
> inputs but respect the IEC958 rate set on the IEC958 input--all I've
> ever heard of assume an AC-3 rate that matches the PCM input's rate (and
> since the same boards that leave off traces to the discrete IEC958 input
> tend to use fixed-48kHz PCM input with software (driver) rate
> conversion, they don't support AC-3 at any rate other than 48kHz).  The
> IEC958 rate setting is then just as useless as the sound processing
> chip's not-connected-to-anything IEC958 input.  After all, generally,
> the manufacturer-supplied drivers (on Windows) will completely hide the
> disconnected discrete input, so why would the card respect its settings.
> But, wait!  It works in Windows!  Well, the Windows drivers are probably
> decoding and re-encoding the AC-3 stream on the fly (although some
> drivers use "hardware" decoders/encoders on the card, if present, but
> still they're re-encoding)...
> As a matter of fact, it's probably Windows and Microsoft and Intel you
> have to thank for your broken sound card.  Even though the IEC958
> specification explicitly allows 32kHz, 44.1kHz, and 48kHz rates, the
> AC'97 specifications required that audio hardware support 96kHz 20-bit
> stereo and 48kHz 20-bit stereo or 48kHz multichannel recording and
> playback to be compliant.  The successor to AC'97 (Intel High Definition
> Audio (HD Audio/IHD)), released in 2004, allows 192kHz 32-bit stereo or
> 96kHz 32-bit 8-channel audio.  TTBOMK (and I'll admit my Windows
> knowledge is minimal), Windows XP didn't provide support for IHD, so
> until Vista was released vendors had little reason to support it,
> either, so most sound cards out there today are still AC'97 cards
> (although some have started to appear with IHD support).  But my
> Creative SoundBlaster Live! (or later) allows me to pass in audio at any
> rate without using the plug plugin to do rate conversion, so wouldn't
> that violate AC'97?  No.  AC'97 simply specifies a means of linking a
> CODEC chip (has nothing to do with binary formats, like MP3) to a
> digital controller chip using 5 wires.  Vendors can do anything they
> want before or after those connections, like include a DSP that
> resamples to 48kHZ (as in the Live!/Audigy/Extigy series) or use
> software resampling in drivers.  The point is that because AC'97
> requires a 48kHz digital link, resampling was required for any other
> rate, and since most vendors were unwilling to add additional hardware,
> they used driver-based resampling.  Since they could do all that without
> enabling the dedicated IEC958 input, there was little reason to do so:
> once the architecture was in place for rate conversion of PCM, extending
> it to decode/re-encode AC-3 was easier (and likely cheaper).
> But, wait!  It works in MPlayer/xine!  Well, MPlayer and xine are
> "smarter" than users (they've taken the approach that most users don't
> know--or even care to know--what's happening; they just want to watch
> their video with sound).  If the user specifies passthrough, but the
> media player is told by the sound card that it requires a 48kHz audio
> stream, it will decode the AC-3--regardless of your passthrough
> settings.  So, how come I get multi-channel audio?  Well, it could be
> your receiver or it could be your sound card configuration causing you
> to think you have multi-channel audio when in fact it's plain old stereo
> (replicated to multiple channels).  There are so many possibilities that
> I can't tell you unless I can see your entire configuration.  (And,
> really, I have my own configuration to worry about.)  In other words, if
> you ever find an app you can use to output a 32kHz AC-3 stream from your
> sound card such that your receiver shows 32kHz AC-3 on its little LCD,
> then talk to me and we can get into the details.
> So, if your sound card (and/or driver) does not support 32kHz AC-3
> passthrough, the /only/ thing MythTV could do to allow AC-3 passthrough
> to work for a recording with a 32kHz AC-3 stream is to decode it and
> re-encode it to a 48kHz AC-3 stream (complete with generational loss).
> Now, this may eventually be possible as a side-effect (or just another
> part of) #1104, but it's not currently possible.
> One of these days, I may write the code to allow MythTV to automatically
> choose the right IEC958 status bits and send them to the receiver.  This
> would allow users whose sound cards support 32kHz or 44.1kHz AC-3 and a
> discrete IEC958 input to properly specify the audio type/rate, but it
> won't do anything for those whose sound cards support only 48kHz AC-3
> (for that, you'll have to talk to someone else, like Dr. Mark Spieth of
> #1104, who might be interested in adding AC-3 re-encoding support).
> However, I will admit that I have a few higher priority things to do
> first--such as build and set up my 5.1 (or 7.1) channel speaker system.
> (In other words, it might take a bit as it's pretty low priority for
> me--especially now since I don't have a surround speaker system.)
> However, once I write the code it will do properly what #1105 (
> http://svn.mythtv.org/trac/ticket/1105 ) and #1598 (
> http://svn.mythtv.org/trac/ticket/1598 ) and #1608 (
> http://svn.mythtv.org/trac/ticket/1608 ) tried to do.  In the meantime,
> those with auto-detecting receivers (and extremely nice sound cards)
> should have it easy (and, like I said, most receivers auto-detect just
> fine--the patch would just minimize the pops/clicks as the receiver
> switches modes).  Then again, I may find a way to get an HDMI 1.3+ audio
> connection so I can send 8 channels of 192kHz 24-bit uncompressed PCM
> audio via a digital link (and avoid the whole AC-3 generational loss).
> (Note, this probably helps you to infer the kind of timeline I'm talking
> about for writing this code.)
> Of course, if you're one of the lucky ones who has a sound card on which
> the manufacturer decided to connect the discrete IEC958 input (and that
> supports 32kHz passthrough), there's no reason any useful program should
> ever force a user to use a cryptic configuration string like
> "ALSA:iec958:AES0=0x06,AES1=0x82,AES2=0x00,AES3=0x03".  Instead,
> wouldn't it be nice to be able to put that information somewhere where
> it could be reused in any application using a nice, simple-to-remember
> human-readable-and-understandable name?  /That/ is the entire reason for
> an ALSA configuration file.  Therefore, it makes no sense to say, "You
> don't need an ALSA configuration file."  Really, I don't need Myth,
> either.  I could just type a bunch of cryptic commands at the console
> that create cron/at jobs to execute the commands required to set up
> capture cards and record programs (I'm guessing it would take about 42
> lines of Perl code ;).  Myth--like an ALSA configuration file--is meant
> to make the user experience a little bit more enjoyable.
> So you could, instead, put the above configuration information into an
> ALSA configuration file.  Something like the bit at the bottom of this
> post would allow you to specify "ALSA:passthru32k" for a 32-kHZ AC-3
> passthrough device.  Note, however, that I doubt this will "fix" issues
> with 32kHz passthrough for very many people since most are probably
> having issues due to lack of support from the card.
> Show me a sound configuration problem and I'll show you a sound
> configuration problem that can be fixed with ALSA configuration
> files...  Unfortunately, people hate what they don't understand, so they
> love to say, "It's not an ALSA configuration file problem," or "This
> [ALSA configuration file] is not required on Fedora Core 5 and 6
> systems," or "If you are using a version of ALSA newer than 1.0.12 --
> <supposedly authoritative source> states that for most uses an .asoundrc
> is not necessary."
> I've given up on trying to fix the completely broken Wiki page on
> Configuring Digital Sound (
> http://www.mythtv.org/wiki/index.php/Configuring_Digital_Sound ) because
> people think that just because something worked for them, they can make
> an authoritative statement about proper configuration.  I also wholly
> agree with Bruce Markey's dislike of people's posting his words to the
> wiki as the Configuring Digital Sound page was created by copy/pasting
> my words from one of my posts directed at a specific set of users having
> a specific issue with 5.1-channel analog audio on the NFORCE2 and
> explaining why it wasn't working by explaining what they were doing
> incorrectly with the ALSA configuration files.  When the info was
> copied, the words were attributed to me--which was perfectly reasonable
> at the time because I actually said them.  Then, a lot of people started
> changing the page--including words attributed to me--without changing
> the attribution.  Since I never really looked at the wiki for the first
> 6 months, I was "credited" with a /lot/ of inaccurate information until
> I finally took my name out of the page (and fixed some of the
> inaccuracies and removed the "how to understand and write an ALSA
> configuration" part (leaving a link to my post) since no one read
> it--instead just copying the example and proclaiming it either works or
> doesn't).  But, I haven't really touched the page since then, and doing
> so isn't worth my time as anything I correct would tend to be
> uncorrected again--if not where I corrected it, in some other comment
> somewhere else on the page or on some other page (i.e. someone makes a
> new page to say what they want because their "opinion" differs from the
> existing page).
> So, basically, I've been ignoring the discussion of sound configuration
> for quite some time because it's a very complex issue when you realize
> the vast number of sound cards out there and, although they use a
> limited number of chips--after all, there aren't that many ALSA
> drivers--those chips are hooked up in /very/ different ways, requiring
> very different configurations, especially when you factor in the various
> permutations of sound card/receiver combos (which can also have a big
> effect on perceived usability).  And, you may have noticed, I can get a
> bit spun-up over it, too.  Unfortunately, it seems most don't care to
> learn how things work or even how their equipment works (so they can
> figure out their own proper configuration) but instead want a
> configuration they can read off a wiki page.  And, what's worse, once
> they figure out something that works for their system and audio files,
> they spread it around as "universally" authoritative configuration help.
> The only reason I responded to this e-mail was your insistence that it's
> not an ALSA configuration issue and your mentioning me as the guy who
> always says that sound can be fixed with the ALSA configuration files.
> I'll admit that you were pretty much right on the second--I always say
> ALSA configuration issues can be fixed with ALSA configuration
> files--and I stand by my opinion (and would actually go so far as to say
> ALSA configuration issues /should/ be fixed with ALSA configuration
> files).  Wow.  You're still reading this?  It just so happens that most
> sound issues (and many playback issues) are caused by broken ALSA
> configurations.  Some, however, may be caused by users trying to get
> their hardware to do things it can't do (like play 32kHz AC-3 audio via
> passthrough)--or, put another (less accusatory) way, because of
> broken/incapable sound cards.
> The reason I took forever to respond to the e-mail is because, well, it
> took forever to find enough free time to write it (just like--I'm sure
> you're thinking--it took forever to read it).  And, it was very low
> priority because I had been assured that your issue had nothing to do
> with ALSA configuration ;), and besides, I've been accused of being an
> ALSA-configuration-file zealot and don't want to feed the rumors.
> Mike "extra-wordy ALSA-configuration-file zealot" Dean
> Example ALSA conf fragment for a sound card that pulls IEC958 data from
> a PCM input where the "digital-hw" device is set up to properly output
> PCM via the digital output.
> This fragment relies on the rest of the ALSA configuration file at
> http://www.mythtv.org/wiki/index.php/Configuring_Digital_Sound#Setting_up_ALSA.27s_.asoundrc.2C_Properly
> and should be appended to the end of the config file.  The only devices
> that should be used by the user are "passthru32k", "passthru44k", and
> "passthru48k".  Using this fragment, the user does not even have to
> worry about setting the IEC958 settings on the card.  Note that the
> passthru32k and passthru44k are likely to work the same as passthru48k
> (i.e. the sound card most likely only allows 48kHz AC-3 passthrough and
> will ignore the rate specified in the IEC958 status bits, so the user
> should specify "ALSA:passthru48k" as Myth's "Passthrough output device"
> just so the name is more "correct" as to the sound card's
> capabilities).  This fragment will /only/ work with sound cards without
> a discrete IEC958 input that pull IEC958 input from a PCM input (which
> likely only support 48kHz passthrough)!
> # Use the ALSA device name "passthru32k" for 32kHz encoded audio (i.e. AC-3,
> # DTS, ...) passthrough.
> pcm.passthru32k {
>   type plug
>   slave.pcm "passthru:AES0=0x06,AES1=0x82,AES2=0x00,AES3=0x03"
> }
> # Control device (mixer, etc.) for the card
> ctl.passthru32k {
>   type hw
>   card 0
> }
> # Use the ALSA device name "passthru44k" for 44.1kHz encoded audio (i.e.
> AC-3,
> # DTS, ...) passthrough.
> pcm.passthru44k {
>   type plug
>   slave.pcm "passthru:AES0=0x06,AES1=0x82,AES2=0x00,AES3=0x00"
> }
> # Control device (mixer, etc.) for the card
> ctl.passthru44k {
>   type hw
>   card 0
> }
> # Use the ALSA device name "passthru48k" for 48kHz encoded audio (i.e. AC-3,
> # DTS, ...) passthrough.
> pcm.passthru48k {
>   type plug
>   slave.pcm "passthru:AES0=0x06,AES1=0x82,AES2=0x00,AES3=0x02"
> }
> # Control device (mixer, etc.) for the card
> ctl.passthru48k {
>   type hw
>   card 0
> }
> # Generic passthrough device which accepts IEC958 status bits and sets
> IEC958
> # controls for proper passthrough on a card without a discrete IEC958 input
> # connection that instead pulls non-audio data from PCM
> pcm.passthru {
>   @args [ AES0 AES1 AES2 AES3 ]
>   @args.AES0 {
>     type integer
>   }
>   @args.AES1 {
>     type integer
>   }
>   @args.AES2 {
>     type integer
>   }
>   @args.AES3 {
>     type integer
>   }
>   type hooks
>   slave.pcm "digital-hw"
>   hooks.0 {
>     type ctl_elems
>     hook_args [
>       {
>         name "IEC958 Playback Switch"
>         lock true
>         preserve true
>         value true
>       }
>       {
>         name "IEC958 Playback AC97-SPSA"
>         lock true
>         preserve true
>         value 0
>       }
>       {
>         name "IEC958 Playback Source"
>         lock true
>         preserve true
>         value 0
>       }
>       {
>         name "IEC958 Playback Default"
>         lock true
>         preserve true
>         value [ $AES0 $AES1 $AES2 $AES3 ]
>       }
>     ]
>   }
> }
> # Control device (mixer, etc.) for the card
> ctl.passthru {
>   type hw
>   card 0
> }
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