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Topic: SPDIF-out to SPDIF-in loopback recording. (Read 4079 times) previous topic - next topic
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SPDIF-out to SPDIF-in loopback recording.

I've never really used SPDIF before, but my new motherboard has both SPDIF-out and SPDIF-in, so I've just been playing around with some SPDIF-in recording.

I was interested in whether or not an SPDIF-out to SPDIF-in loopback connection can make lossless recordings of any content played on the computer. I remember reading somewhere that soundcard manufactures are supposed to somehow inhibit this (or reduce the quality or something) when playing copy-protected material. Does anyone know if this is really the case? Can SPDIF-out to SPDIF-in be used as an emergency decode/transcode tool when all else fails.

BTW. I don't have any material that I'm currently trying to copy in this way. Please consider that as an academic interest only.

SPDIF-out to SPDIF-in loopback recording.

Reply #1 –
"content played on the computer" is easily enough recorded. It doesn't even require a soundcard, it can be managed with software. I'm not sure what happens when a soundcard is involved, re-sampling cards like Soundblasters may put a touch on the bit they process even under these conditions. Possibly Windows does too. Most often the quality of what people want to capture while playing (e.g. internet content) is so poor to begin with, who worries whether or not a few bits end up different?

If the soundcard's drivers are multi-client it should be easy to do the output to input thing. There is some longstanding but primitive copy protection information that can be passed via S/PDIF, but it is easily bypassed with many, if not most, professional and semi-professional cards. Also, I suspect very few, if any, recording applications give it the slightest nod. I haven't paid any attention to whether the evolving world of copy protection has come up anything more serious. I think that may be being left to the OS developers, such as the newest MS one, to simply prevent your computer from complying with your wishes.

SPDIF-out to SPDIF-in loopback recording.

Reply #2 –
Thanks for the info Andy.

Quote
"content played on the computer" is easily enough recorded. It doesn't even require a soundcard, it can be managed with software.

Yes I know that. I'm referring to this as a possible work-around for when you don't have the right software available.

Anyway I just did a loopback test with RMAA5.5 (rightmark audio analyser) and you're correct that when the soundcard and drivers are involved it's definitely not bit perfect. The frequency response wasn't even flat which surprised me a little. It was flat up to 10kHz but then had a big 3.5dB peak between 10kHz and 20kHz, peaking at about 17k.  The SNR and distortion figures definitely looked worse than I'd expect for bit perfect as well. Overall though these figure were better by about 15dB over what an analog loopback gets me on this particular motherboard. So I guess it would still be well worth doing if there were no other simple way. Of course the frequency response anomaly (high freq 3.5db peak) was interesting but I don’t see it bothering me when I know that my hearing is dropping off way faster than that at those frequencies anyway.
 

SPDIF-out to SPDIF-in loopback recording.

Reply #3 –
If anyone is interested I uploaded the RMAA reports containing all the graphs.

SPDIF results Here and Analog results HERE.

I'd be interested if anyone has any comments on these.

BTW the analog loopback shown here is just the realtek onboard AC97, so obviously it's pretty ordinary.