[mythtv-users] HDMI_SPDIF directly to reciever

Alen Edwards allen.edwards at oldpaloalto.com
Thu Jul 3 15:33:03 UTC 2008



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?

More than likely you have a receiver that doesn't get blown up with the 
5 volt input that is coming from your MB.  Imho, you took a risk when 
you hooked it up and you won.  It may degrade over time so if I were 
you, I would add the circuit I am about to describe.

I use this circuit to take the S/PDIF signal off my MB and drive a 
decoder.  This should be able to drive a long line of 75ohm coax.  Use 
RF TV 75 ohm coax or video coax -- usually has yellow ends.  Don't use 
audio coax.  For RF coax, you can either get cable to RCA connectors or 
use Type-F to RCA adapters.  I prefer the adapters but either way is fine.

For the output, I took a blank plate that would cover a PCI slot.  I 
drilled a 1/4 inch hole and put an RCA connector with a solder lug on it 
in that hole.  I got a connector for my MB (4 pin in my case) and 
plugged the key hole.  I wired up the ground to the ground lug on the 
rear panel.  I wired the s/pdif lead to the RCA connector through this 
network.  First, a 100nF capacitor (I used 82nF because that is what I 
had) in series with a 330 ohm resistor.  The resistor goes to the center 
pin on the RCA connector along with the end of a 100 ohm resistor.  The 
other end of the 100 ohm resistor goes to the ground lug.

This network will attenuate the almost 5 volt TTL signal by 1/4.3 or 
about 1 volt, which is the s/pdif spec.  The output impedance will be  
76.7 ohms, close enough to 75.

I hope someone finds this information useful :-)

Allen
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