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|23rd March 2012, 12:54 PM||#1|
Join Date: Mar 2012
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)
|23rd March 2012, 03:33 PM||#2|
Join Date: Jan 2012
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 .
|23rd March 2012, 03:43 PM||#3|
Join Date: Oct 2002
Blog Entries: 1
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?
“Instead of Rational Law, objective truths perceptible to any who will undergo the necessary intellectual discipline, Knowledge will degenerate into a riot of subjective visions. . ." - Auden
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