Old 02-21-2014, 03:57 PM   #1
Om_Audio
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Default -4dBu actually -15dBu in digital/DAW?

Ran across this video- anyone care to weight in on this?

https://www.youtube.com/watch?v=JpGAi4az3ns

He uses a signal generator and views a 400Hz on oscilloscope- it shows signal at 1.2V RMS which he says is equal to +4dBu (pro audio line level)
He splits the signal and runs it into a computer using a Behringer 302USB then shows how Reaper indicates signal as -15dBu.

I posted the following on the Youtube video:

"Very interesting, thanks. Would be interesting to see if that Behringer 302USB (I have one) or the split are affecting things at all. I do not have a tone generator or scope here at home or I would try to duplicate your findings. I am a Reaper user as well.

Questions:
1- audio is entering Reaper through the Behringer 302USB digital (USB) interface to the computer, yes?
2- Can you somehow test the analog output of the Behringer using the scope pre-Reaper?
3- Can you somehow test the signal on the scope after the split?

I could go on but basically you can see I am trying to rule out possible variables that could be affecting the results.
Thanks!
C"
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Old 02-21-2014, 04:21 PM   #2
karbomusic
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Just as he said, the digital and analog "scales" are not a 1:1 match in any DAW so all you need to do is remember that and not expect or try to make them match.

0 dBFS (digital) and analog 0 dbu are about 15 - 18 dB apart. So, a perfect unity analog signal will and should show up -15/-18 in the DAW. The better sound cards list this spec so you can know what yours is but if not, observe that range and you'll be fine.
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Old 02-21-2014, 05:19 PM   #3
Om_Audio
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Interesting thanks. Have you ever seen any visual or text description about what actually occurs if you record higher? Maybe you don't hear it but what is happening that is not desireable?
I assume the A/D converters are getting overloaded but if you don't hear distortion is there still some sort of degredation or loss of resolution in the sampling?
C
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Old 02-21-2014, 05:31 PM   #4
Lawrence
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Nothing happens unless you clip the converter or the preamp. The extra room above the converter reference level is there because mic preamps can and do operate safely well above analog 0, so the digital scale had to account for that analog headroom.

It's why professional converters are calibrated lower (-18 and maybe more in the EU, dunno), and prosumer gear is more typically calibrated at -15, because (generally speaking) pro gear usually has more usable headroom.

Hope that helps.
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