[mythtv-users] Outputting AC3/DTS in analog too?

Marc MERLIN marc_mtv at merlins.org
Wed Jun 25 05:48:49 UTC 2008


On Mon, Jun 23, 2008 at 11:26:23PM -0700, Alen Edwards wrote:
> What I ended up doing is using one of these: 
> http://cgi.ebay.com/Technics-SH-AC300-Dolby-digital-AC-3-Prologic-decoder_W0QQitemZ180257552191QQihZ008QQcategoryZ14978QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
> which you may be able to see is offered for under $20 on ebay.  I am 
> feeding the spdif signal to it and setting it to 2.0 so it will take any 
> spdif input (even 5.1) and properly convert it to 2 channel.  I will 
> somehow take the signal and send it to my Rx for the surround system.
> 
> This may be a good solution for you as well.  It is cheap enough and 
> this little box works well.

So you split your coax 5.1 signal, send one half to your receiver which does
5.1 decoding and the other half to that device that converts it to 2 channel
stereo?

On Mon, Jun 23, 2008 at 02:36:10PM -0400, Jerry Rubinow wrote:
> I was unable to figure out how to do that in software (I'm not sure if it's
> possible), so I eventually bought one of these:
> 
> http://www.gefen.com/kvm/product.jsp?prod_id=4907
> 
> For $80, you get a very small box that takes a TOSLink or RCA digital input
> and outputs analog L & R.  Note that the input must be 2-channel digital
> audio.  It won't work with 5.1.

That does no good to me, my receiver can already convert stereo digital pcm
into analog stereo. It just fails to convert 5.1 into analog stereo for my
zone two output.

On Tue, Jun 24, 2008 at 07:48:10AM -0700, Alen Edwards wrote:
> I have been trying to do this for some time.  The problem with ALSA is 
> that as far as I can tell the wiki that tells you how to do what you 
> want doesn't work.  And even if it did, ALSA apparently does a poor job 
> of creating the spdif output unless it is set to passthrough, in which 
> case it can't do anything else with the signal.  Seems like it decodes 
> and then encodes and that is probably not what you wanted. As I said, it 
> will do a good job of passthrough, but in that case you cannot also get 
> an analog output.
 
great, so at least thanks for confirming that I'm not stupid for not having
managed that either.
 
> What I ended up doing is using one of these: 
> Technics-SH-AC300-Dolby-digital-AC-3-Prologic-decoder
> which you may be able to see is offered for under $20 on ebay.  I am 
> feeding the spdif signal to it and setting it to 2.0 so it will take any 
> spdif input (even 5.1) and properly convert it to 2 channel.  I will 
> then have TWO external decoders.  The one in my Pre-Processor for 5.1, 
> and the Technics for 2.0.
 
So I was thinking about this again, even if I split my output to go there,
it sounds like it'll work for AC3 5.1, but not DTS. That said, I seldom play
DTS so I guess it's not too bad.
I was somehow hoping that alsa could just do the convertion for me, but
maybe not.
 
> I am yet unsure how I am going to feed the s/pdif to the two decoders 
> but I know I can do it.  For sure I can attach to the second s/pdif 
> output on the MB and feed that to the second decoder box.  But, I tested 
> my decoder with a 10db pad in the line and it worked fine so I think I 
> can use a passive splitter.  I will probably try a simple Y splitter 
> with two 50 ohm resistors.  Not perfect, but on a short run for digital 
> signals it should be OK.  If that is an issue, let me know.

I was thinking this shoudl work too.

> A point to consider is that you probably do not want to take the R+L out 
> of your 5.1 decoder and feed it to the TV as the center channel is very 
> important and you may just not get any output from the R+L in some 
> cases.  You really need to decode as 2.0 of you want stereo.

Right.

> This may be a good solution for you as well.  It is cheap enough and 
> this little box works well.

I may end up doing that, thanks.

Marc
-- 
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems & security ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/  


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