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James wrote:
<blockquote
cite="mid:451272c32e7c664950b9011b85efe474.squirrel@www.lockie.ca"
type="cite">
<pre wrap="">I found this article on the wiki and I have a question.
</pre>
<blockquote type="cite">
<pre wrap="">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.
</pre>
</blockquote>
<pre wrap=""><!---->
</pre>
<blockquote type="cite">
<pre wrap="">This article describes how you can build a SPDIF TOSlink transmitter
circuit, using just two components.
</pre>
</blockquote>
<pre wrap=""><!---->
I have an ASrock ALiveNF5-eSATA2+
(<a class="moz-txt-link-freetext" href="http://www.asrock.com/mb/overview.asp?Model=ALiveNF5-eSATA2%2b&s=AM2">http://www.asrock.com/mb/overview.asp?Model=ALiveNF5-eSATA2%2b&s=AM2</a>)
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?</pre>
</blockquote>
<br>
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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
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. <br>
<br>
I hope someone finds this information useful :-)<br>
<br>
Allen<br>
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