[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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