[mythtv-users] OT: Analogue to S/PDIF converter

Alen Edwards allen.edwards at oldpaloalto.com
Tue Jun 24 14:48:10 UTC 2008

Brian Phillips wrote:
> Justin The Cynical wrote:
>> Jerry Rubinow wrote:
>>> 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.
>> I was looking at one of those as a way to get SWMBO's new Wii working
>> with the component-video-n-digital-audio-magic-switch[1]. 
>> How well do those things work?
>> [1] Nintendo hurts my head at times.  Why, oh why, would one offer
>>      component out for the video but not S/PDIF out for the audio?
>>      And what's with the hard limit of 2 gig on SD cards, and USB
>>      ports that can't use a USB hard drive as a replacement for said
>>      SD cards and internal 512 Meg flash?
> Because component out is analog?  Why would you offer an analog video output
> and a digital audio output?
> I've looked at these types of devices (ADC and DAC) for some time now.
> Basically what I have found is that the DAC is FAR, FAR more common than the
> ADC.  Even though the two circuits are of similar complexity and size, for
> some reason you can get a DAC for pennies but no one makes a SMALL, cheap
> ADC.  When I say cheap I mean less than $40-$50.
> If you find one, let me know.  I'm this close to laying one out and building
> it myself.  
> The above quoted device is a digital to analog converter.  It won't convert
> analog to S/PDIF.  I don't think it will do what you're looking for.
> Brian
> PS I consider $80 for the above device fairly reasonable, even though the
> actual circuitry can be had for about $30 on ebay, the above device has a
> shiny case and an included power supply.
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I thought for sure I replied to this thread but didn't get the email and 
don't see it in the archives.  Maybe some filter didn't like the link I 
put in so here it is again. Sorry if this is a repeat but for a $20 
solution, I thought it worth posting (and updated).

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.

I have a HDA chip and I know it could do this if the driver allows it.  
I have asked for some help in finding documentation on the driver so I 
can see if there is a way to set it up, or modify it to work, but no 
responses (hint hint).

What I ended up doing is using one of these: 
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.

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.

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.

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

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