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Old 23rd March 2012, 12:54 PM   #1
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Default Signal level

Hi!!


I am musician and have been playing for already 20 years now, but I have just started to get into the home recording world and thus I am still learning the basics.

I have been reading a lot about the concepts of dB and impedance and, even though I already start to get them, still have some doubts.

For instance, dB. I got that it is just a reference value of two magnitudes (power, volts, sound pressure...) and also got what dBV, dBu, etc....mean. However, when I try to go in more depth on what this concept means I get a bit confused.
Say I got a chain of diverse pro studio equipment. Then I have +4dBu (1.23 Volt) level reference. Then my question would be: what does it really mean?.
using dBu means using 0.775 volts reference. Then, if understood correctly, when I measure any output from my audio interface in dB, I will be measuring the output voltage of that equipment referred to 0.775 Volts. For instance, if the output level is 0.725 the my dB meter will display 20Xlog (0.725/0.775) = -0.6 dBu
Up to now, I kinda understand it.
However, when I start getting lost is whe we talk about +4dBu. Where is this value (1.23 V) used? I am already using 0.775 V, which is what I think I am supposed to when using dBu scale.....
Same thing would happen if I choose consumer signal level reference (-10dBV, 0.316 V)

Furthermore, What is what physically occurs when I change the reference level from +4 dBu to -10 dBV?
just the reference value is changed in the dB formula, or there is a real electronic change within the equipment??

Thanks a lot for your help!!! (and sorry for the long question)
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Old 23rd March 2012, 03:33 PM   #2
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If you quote a dB value you have to know what that value is referenced against .

In your case +4dBu is referenced to the dBu scale and the starting point is zero dBu which is 0.775V .

1.23V = +4dBu........( 20 log 1.23/0.775 = +4dBu )

Your pro studio gear is designed to output +4dBu ( 1.23V )

The dBV scales reference is 1.0V which is zero dBV

It does not matter which scale is used as long as it is stated . In the case of pro audio gear the reference is dBu .
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Old 23rd March 2012, 03:43 PM   #3
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That is a very involved question! I'm not quite sure where to start... but I'll start by saying that you have the basic concepts down, which is a good start.

Since I'm a little unclear on exactly what the issue is, let me give you a rundown on what I'm doing and maybe it will answer your question. I've got a ribbon mike, a mike preamp, and a digital recorder. The mike is rated at -52dB sensitivity. Now, mike sensitivities are given as dBV for a 94dB SPL (lots of dBs to confuse you). That is, the mike will give an output of -52dB below 1V for 94dB SPL (same as 1 pascal). That calculates out to 2.5mV.

Now we move on to the preamp. It's a fixed gain unit with 60dB of gain (=1000). So at 94dB, the mike puts out 2.5mV, the preamp puts out 2.5V. Equivalently, the mike puts out -52dBV, the preamp adds 60dB to that, for a total of +8dBV.

OK, the digital recorder has XLR inputs which have a nominal sensitivity of 4dBU and a max signal input of 20dBU. To go back and forth between dBV and dBU, subtract 2.2 from the dBU number (i.e., 20 log (1/0.775)). So the nominal sensitivity is 1.8dBV and the max input is 17.8dBV. So I can see that my mike's output at 94dB SPL will be greater than nominal, less than max, which is fine.

94dB is probably fine for when I record a singer/songwriter from a meter or so away. If he has a percussionist with him, I need to consider that I could get peaks of 110dB SPL. That's 16dB above the reference level, so the preamp output will be 8dBV + 16dBV = 24dBV. My digital recorder will clip (since it has a 17.8dBV maximum input), telling me that I need to reduce the preamp gain, add a pad, or back the mike off.

Is this the kind of thing you're asking about?
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