Where should the S/PDIF cable connect?

  • Thread starter Thread starter Jan Alter
  • Start date Start date
J

Jan Alter

I have a fine little Gigabyte GA-G31M-ES2L micro-ATX mb that I'll be using
for a HTPC shortly. I'm just waiting for the HDMI graphics card to arrive.
I'm reading from the graphics card manual that the S/PDIF cable, which is
just two wires, gets connected to the graphics card and then gets connected
to the mb. The graphics card is a GIGABYTE GV-N84S-512I, and it has a two
pin cable connection for the S/PDIF output.

http://www.newegg.com/Product/Product.aspx?Item=N82E16814125289

However, the S/PDIF header on the mb actually has three pins: #1 (power), #2
(SPDIFO), #3 (GND). Am I correct in assuming that I will only connect the
cable to pins #2 and #3? If that's right then what is the power pin for? If
I'm wrong then I'd appreciate a better understanding of how to proceed.

Thanks,
 
Jan said:
I have a fine little Gigabyte GA-G31M-ES2L micro-ATX mb that I'll be using
for a HTPC shortly. I'm just waiting for the HDMI graphics card to arrive.
I'm reading from the graphics card manual that the S/PDIF cable, which is
just two wires, gets connected to the graphics card and then gets connected
to the mb. The graphics card is a GIGABYTE GV-N84S-512I, and it has a two
pin cable connection for the S/PDIF output.

http://www.newegg.com/Product/Product.aspx?Item=N82E16814125289

However, the S/PDIF header on the mb actually has three pins: #1 (power), #2
(SPDIFO), #3 (GND). Am I correct in assuming that I will only connect the
cable to pins #2 and #3? If that's right then what is the power pin for? If
I'm wrong then I'd appreciate a better understanding of how to proceed.

Thanks,
Click to expand...

The provision of +5V, is if you wanted to connect a TOSLINK electro-optical
module to SPDIF_out. (My motherboard that came with an adapter plate of
that type, has a 1x4 pin connector on the end, and one pin is blocked
so the connector only fits one way. The three remaining pins carry
power, ground, SPDIF_out.)

In your case, with a copper only connection, you would use pin 2 and 3, to
run across to the video card. So contrary to what your manual is claiming
(which would be appropriate if a three pin cable was being connected), you
just want to connect to 2 and 3. What you want to make sure of, is that GND
goes to GND.

In the video card manual, the connector appears to be keyed, so you can
probably only connect the cable that comes with the 8400GS, one way.
The other end of the cable, should have a red and black wire as depicted.
You'd want the black wire to go to ground on the motherboard, and that
would be pin 3. So that looks like the black wire would be closest to
the bottom edge of the motherboard.

ftp://download.gigabyte.ru/manual/vga_manual_n84s-512i_e.pdf (p.8)

ftp://download.gigabyte.ru/manual/mb_manual_ga-g31m-es2l(es2c)_2.x_e.pdf (p.28)

*******

The signal coming from the motherboard, could be a logic level signal
(as that would make it suitable for driving a TOSLINK optical transmitter).
It wouldn't make sense to have a low amplitude AC signal on the SPDIFO.
So you cannot connect a coax cable directly to the motherboard header, and run
it to your AV receiver at some future date. It looks like that header
is mostly intended for what you're doing with it now, running it
across to the video card.

For example, to convert from "motherboard" SPDIF to "AV receiver" SPDIF,
my SPDIF adapter plate includes the following components. (The adapter
plate fits on a PCI slot cover.)

motherboard_SPDIF --+- capacitor ---- 384_ohm_resistor -----+--> RCA connector
| |
| 100 ohm
| resistor
+---> (to TOSLINK TX module) |
logic level signal GND

The parallel combination of 384 and 100, gives a Thevenin equivalent
of 79.3 ohms (it is probably supposed to be 75 ohms). The attenuation
is 100/(384+100) or about 0.2 of the original signal level. In order
to get the 0.5V to 1.0V level defined here, the motherboard logic
level would have to be anywhere from 2.5V to 5V in amplitude. The
selection of resistors, is intended to help match a 75 ohm coax cable.

http://en.wikipedia.org/wiki/Spdif

HTH,
Paul
 
Paul said:
The provision of +5V, is if you wanted to connect a TOSLINK
electro-optical
module to SPDIF_out. (My motherboard that came with an adapter plate of
that type, has a 1x4 pin connector on the end, and one pin is blocked
so the connector only fits one way. The three remaining pins carry
power, ground, SPDIF_out.)

In your case, with a copper only connection, you would use pin 2 and 3, to
run across to the video card. So contrary to what your manual is claiming
(which would be appropriate if a three pin cable was being connected), you
just want to connect to 2 and 3. What you want to make sure of, is that
GND
goes to GND.

In the video card manual, the connector appears to be keyed, so you can
probably only connect the cable that comes with the 8400GS, one way.
The other end of the cable, should have a red and black wire as depicted.
You'd want the black wire to go to ground on the motherboard, and that
would be pin 3. So that looks like the black wire would be closest to
the bottom edge of the motherboard.

ftp://download.gigabyte.ru/manual/vga_manual_n84s-512i_e.pdf
(p.8)

ftp://download.gigabyte.ru/manual/mb_manual_ga-g31m-es2l(es2c)_2.x_e.pdf
(p.28)

*******

The signal coming from the motherboard, could be a logic level signal
(as that would make it suitable for driving a TOSLINK optical
transmitter).
It wouldn't make sense to have a low amplitude AC signal on the SPDIFO.
So you cannot connect a coax cable directly to the motherboard header, and
run
it to your AV receiver at some future date. It looks like that header
is mostly intended for what you're doing with it now, running it
across to the video card.

For example, to convert from "motherboard" SPDIF to "AV receiver" SPDIF,
my SPDIF adapter plate includes the following components. (The adapter
plate fits on a PCI slot cover.)

motherboard_SPDIF --+- capacitor ---- 384_ohm_resistor -----+--> RCA
connector
| |
| 100 ohm
| resistor
+---> (to TOSLINK TX module) |
logic level signal GND

The parallel combination of 384 and 100, gives a Thevenin equivalent
of 79.3 ohms (it is probably supposed to be 75 ohms). The attenuation
is 100/(384+100) or about 0.2 of the original signal level. In order
to get the 0.5V to 1.0V level defined here, the motherboard logic
level would have to be anywhere from 2.5V to 5V in amplitude. The
selection of resistors, is intended to help match a 75 ohm coax cable.

http://en.wikipedia.org/wiki/Spdif

HTH,
Paul
Click to expand...


Hi Paul,

Thank you very much for providing a more than adequate explanation of
the pin layout on the mb and TOSLINK possiblity. Your explanation both
confirms and elucidates me to how to make the connection. It was also nice
of you to be thorough and rummage through both the mb and graphics manuals
to solidify the information that I asked. Actually you gave me a much better
understanding to the point of the S/DIF usage and its potential.
The cute thing about putting the HTPC is that I'm doing it all
backwards. I don't even have a flat screen TV at this point,still watching
on a CRT, and still looking at and comparing the 40" LCD models among the
different brands, comparing the watts used and then noticing the newer LED
technology; that it uses 30 to 40 % less power to run the TV, but then costs
more than twice as much to purchase. I think I'm pretty lucky to have to
think about such nonsense sometimes.
 
....
still looking at and comparing the 40" LCD models among the
different brands, comparing the watts used and then noticing the
newer LED technology; that it uses 30 to 40 % less power to run
the TV, but then costs more than twice as much to purchase. I
think I'm pretty lucky to have to think about such nonsense
sometimes.
Click to expand...

like instead of figuring out the best place to live without being
bombed...

yup

I think about that also when going to a megastore and looking for
something that I have never needed or looked for before, and
knowing that it will be there.
 
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