# Using sound card's SPDIF out



## damaster (Jan 9, 2009)

My main audio/video is an HTPC which has an SPDIF out. Is there any way to use the SPDIF out with REW? It might seem to be possible but then how do I calibrate the sound card? Can I do the following for sound card calibration?:
Sound card SPDIF out -> AVR -> AVR pre-outs L or R -> Sound card line in L or R

Thanks


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## brucek (Apr 11, 2006)

> It might seem to be possible but then how do I calibrate the sound card? Can I do the following for sound card calibration?:


No, there is no way to calibrate the soundcard and use the digital out. To use the method you propose alters the response by including extra equipment that may have a response bias.
It's simpler to use REW as designed with analog in and out......

brucek


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## damaster (Jan 9, 2009)

brucek said:


> No, there is no way to calibrate the soundcard and use the digital out. To use the method you propose alters the response by including extra equipment that may have a response bias.
> It's simpler to use REW as designed with analog in and out......
> 
> brucek


What if I calibrated the sound card with a complete analog path, then applied the calibration file to the digital path and did a sweep and got a flat FR. That would mean no bias is being added by the SPDIF digital out of my sound card or by the AVR, right?


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## brucek (Apr 11, 2006)

> That would mean no bias is being added by the SPDIF digital out of my sound card or by the AVR, right?


No, that would mean that the AVR added an exact same bias as the line-out did. But even if it did (by some miracle), it wouldn't be usable, since the AVR is used in your system. The objective of a soundcard calibration file is to remove the frequency response anomolies of the test equipment only (not your equipment).....

brucek


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## damaster (Jan 9, 2009)

brucek said:


> No, that would mean that the AVR added an exact same bias as the line-out did. But even if it did (by some miracle), it wouldn't be usable, since the AVR is used in your system. The objective of a soundcard calibration file is to remove the frequency response anomolies of the test equipment only (not your equipment).....
> 
> brucek


Ok, then I may have to look into getting an SPDIF I/O bracket for my motherboard (SPDIF input and output).


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## Eskimo (Oct 17, 2008)

I used the optical out quite successfully with my system, since the sound card's response is basically flat anyway. I had to mess with the mixer in the PC to get the levels to line up though, but the REW graph done with the analog setup and the one done with the SPDIF out (still used analog in), were basically dead-on.


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## JohnM (Apr 11, 2006)

You don't actually have to make a soundcard calibration, but it is a useful check that everything is correctly connected and set up and the soundcard low frequency roll-off is not excessive. If you want to use your SPDIF output you can just go ahead and make measurements without a soundcard calibration, but bear in mind the lowest frequencies (below 20Hz or so) may have some error.


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## damaster (Jan 9, 2009)

Eskimo said:


> I used it quite successfully with my system, since the sound card's response is basically flat anyway. I had to mess with the mixer in the PC to get the levels to line up though, but the REW graph done with the analog setup and the one done with the SPDIF out (still used analog in, obviously), were basically dead-on.


Can you elaborate on the method you used please? Did you first calibrate the sound card with the analog in/out and then measure FR with SPDIF?


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## Otto (May 18, 2006)

damaster said:


> What if I calibrated the sound card with a complete analog path, then applied the calibration file to the digital path and did a sweep and got a flat FR. That would mean no bias is being added by the SPDIF digital out of my sound card or by the AVR, right?


Yes. Calibrate the soundcard with the loopback cable only (no receiver/preamp in the path), then apply that calibration file to REW. Hook up the S/PDIF out and sweep your receiver/preamp. If it's flat, you're good. If not, you've introduced some other response through either the receiver/preamp or the digital output of the soundcard.

If you want to ensure that your receiver/preamp is flat, you can do the analog loopback calibration, then sweep your receiver/preamp in the analog domain. You can then switch to digital to ensure that you're still flat. I would expect that to be the case, as I would expect no significant rolloff while using a digital (or analog, for that matter) input on your receiver/preamp.

Of course, it's possible that the S/PDIF output would offer a boost and the receiver/preamp offer a rolloff such that it _appears_ flat, but I think that would be very, very unlikely as well.

I have done both analog and digital measurements in the past (using the same cal file) and they are essentially identical. I also did a bunch of preamp sweeps to check FR of preamp I/O and I used a digital input to the system and measured an analog output with REW and all were the same (the same as if I were to use all analog I/O). 

If you're careful and double check in all directions, you will be OK. On the other hand, if your soundcard has a really bad analog output section such that the cal file is "extensive" (if you will), but the digital output section is very clean such that the output is now wrong, you can't do that.

Good luck!


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## damaster (Jan 9, 2009)

Otto said:


> Yes. Calibrate the soundcard with the loopback cable only (no receiver/preamp in the path), then apply that calibration file to REW. Hook up the S/PDIF out and sweep your receiver/preamp. If it's flat, you're good. If not, you've introduced some other response through either the receiver/preamp or the digital output of the soundcard.
> 
> If you want to ensure that your receiver/preamp is flat, you can do the analog loopback calibration, then sweep your receiver/preamp in the analog domain. You can then switch to digital to ensure that you're still flat. I would expect that to be the case, as I would expect no significant rolloff while using a digital (or analog, for that matter) input on your receiver/preamp.
> 
> ...


Excellent info! Thanks!


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## Pinhead-227 (Dec 24, 2008)

I originally used analog-out to the 5.1-channel inputs on my receiver and after going digital, I'm never going back. I don't know if it's the fact that I'm using an on-board sound card, or if it's due to my cheap(er) Yamaha receiver (RX-V363), but the digital audio is much more crisp, dynamic, and extends much further both in the lows and in the highs.


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## Eskimo (Oct 17, 2008)

damaster said:


> Can you elaborate on the method you used please? Did you first calibrate the sound card with the analog in/out and then measure FR with SPDIF?


I had calibrated the sound card previously using the analog setup, and the correction file shows from 10hz to 200 hz, there's a .3dB difference.. not enough to matter IMO. So although I loaded the analog cal file, I can't see it mattering.


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