[mythtv-users] HDMI_SPDIF directly to reciever

[Brian Phillips]

James wrote:
> I found this article on the wiki and I have a question.
> 
>> Most MythTV systems these days are built using a motherboard with
>> integrated audio and video. However, the SPDIF signal is unfit to be
>> wired directly to a SPDIF compatible digital audio receiver. The
>> reason is that the signal  is a TTL signal, while it should be a
>> signal swinging between -0.5V and +0.5V. This requires that a simple
>> circuit be built, using some +resistors and a capacitor, but the
>> signal can not be used to drive a long cable and thus additional
>> components like a 74HCT04 are needed to amplify this signal to
>> acceptable levels. 
> 
>> This article describes how you can build a SPDIF TOSlink transmitter
>> circuit, using just two components.
> 
> I have an ASrock ALiveNF5-eSATA2+
> (http://www.asrock.com/mb/overview.asp?Model=ALiveNF5-eSATA2%2b&s=AM2)
> that says it it has an HDMI_SPDI header on the motherboard.
> I wired it up directly to a female RCA jack and connected it to the
> coaxial digital input on my receiver. 
> 
> This article implies what I did shouldn't work but it does.
> 
> Do I have a weird motherboard and this probably won't work on other
> motherboards? 

The article talks about how to take an electrical SPDIF signal that is TTL
and convert it to an optical TOSLINK signal.  While I can't vouch for its
accuracy, I think I can say why yours is working correctly.

Your signal is a purely electrical SPDIF signal.  Even though it's not
according to the -0.5 to +0.5 spec, it works because SPDIF is pretty
versatile.  I have driven SPDIF signals successfully with 0V to +5V swings
(not recommended, but it produces sound) and I have driven SPDIF signals
successfully with 0V to +250mV swings.  Again, not recommended but it
produces sound.

Brian