a little confused about +4 and -10

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Sennheiser

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+4 levels usually deal strickly with pro grear.

-10 levels is for semi-pro and consumer gear.

-10dB is .316V and is measured as 0VU.

I can't remember what +4 is but it is also measured at 0VU.
 
The two levels are sort of apples and oranges. -10 is referenced to dBV, or a 1 volt reference (rms) so -10 is 10 dB down from 1 Volt RMS. +4 is referenced to a different scale, which is dBM. dBM refers to a reference which is: 1 milliwatt across 600 ohms load = 0 dBM. This works out to 0 dBM= .775 Vrms, rounded off. It's past my bedtime and my calculator is out of reach, so you'll have to figure out what +4 dBM is - All I remember right now is that they are NOT 14 dB different... Steve
 
Knightfly, why do the calibration instructions for my deck indicate the voltage reading should be at .316V when the meter reads 0VU for a -10dB signal level?

Curious.:confused:
 
Thanks BB, but I was kind of looking for an engineering explanation about how this measured voltage relates to the other two figures and why what I'm reading in my service manual is different than what Knightfly stated.

He's stating that it is referenced from 1V RMS and what I have is that it is referenced from .316V.

Does the RMS have anything to do with why the voltage reference is different?

Am I missing something here?
 
From an old thread I responded to:

Blue Bear Sound said:
Balanced/Unbalanced connections are one concept... gain structure (or signal strength) is a completely separate issue.

You can have unbalanced signals running +4dbu and balanced signals running -10dbV....... it is just more common to have balanced runs AND a high gain structure.

In addition, the difference in db between a -10dbV signal and a +4dbu signal IS NOT 14 db, it about 12db (actually11.80db)....

plug the number into this equation:
Difference in dB = 20 * (LOG(measured voltage/reference voltage))

The reference voltage for a -10 dbV signal is 0.316Vrms, and for a +4 dbu signal is 1.23Vrms.
Click to expand...
 
I understand. Thanks. Where is Knightfly getting that 1V reference figure from?
 
dBM is another scale ratio again.....

(the unit on the end of the "dbX" is significant)
 
Sennheiser said:
I understand. Thanks. Where is Knightfly getting that 1V reference figure from?
Click to expand...

OK, here we go again.

0dB on a -10dBV machine is 10dB under the reference level of 0dBV. That is why it is references as a -10dBV signal.

0dBV is 1Vrms. -10dBV is hence 10db under 1Vrms, which is 0.316Vrms. So, the reference level on a -10dbV machine is 0.316Vrms.

0dB on a +4dBu machine is 4dB above the dBu level. The dBu level is 0.775V, which makes +4dBu 1.23Vrms.

Come on now, It really isn't that complicated. ;)
 
The Arithmetic

10 ^ (-10 / 20) = .316

Also, 20 * log10(.316) = -10, not surprisingly.

Although last time I posted on this subject, another member quite confidently said my math was wrong and I had no idea what I was talking about, so I guess this could be wrong.
 
Your math is fine........!

I posted the same equation above.......
 
Thanks Bruce, Reg, SJ - Saved me having to re-post now that I'm (sorta) awake again... Steve

Senny, Regebro laid it out as clear as possible. Bruce covered what tells you what the reference is.

Just to be sure, all dB levels other than sound pressure levels need a reference. This reference is measured at 0 dB, regardless of the reference. Since the decibel (in electronics, anyway) is a RATIO and not an ABSOLUTE, when you state a dB figure you need to state what it is compared to. This is what the 3rd letter is, as in dBV, dBu, dBM - Sooo, 0 dBV means "no change" from 1 "V", or 1 volt RMS. -10 dBV would be, as regebro stated, 10 dB down from 1 volt rms.

Pro audio uses mostly 600 ohm balanced lines, so levels are referenced to a standard level based on a 600 ohm load. the unit that was chosen for 0 in this case was 1 milliwatt into a 600 ohm load. This, when plugged into ohms law, comes out to .775 volts rms for the o dB reference, so +4 dB from that comes out to 1.23 volts rms. (Can't believe I was too zoned out at 3:45 AM to remember that)

Since -10 is referenced to a zero of 1 volt rms, and +4 is referenced to a zero of .775 volts rms (the voltage caused by 1 milliwatt across 600 ohms), the differences between the two levels, even ignoring characteristic load impedances, will NOT be 14 dB, but 11.8 dB, as Bruce stated.

I had a link to a site that explained every version of the dB ever invented, but can't find it now. Probably on my desktop, which is not booted up at the moment. I hope that among all our ramblings (OK, I ramble, the other guise state facts) that we answered your question... Steve
 
Come on now, It really isn't that complicated.
Click to expand...

No it isn't. Not when you explain it like that.

Thank you. Clear and consise and to the point. That is what I was looking for.

Ah, the sky is clearing now...
 
Oh, Senny - one more thing - Bruce and Regevro correctly referred to dBu instead of dBm - dBm is tied to actual 600 ohm loads, whereas dBu is the same VOLTAGE, but is not tied to any particular impedance. Since modern gear seldom has 600 ohm input impedance (or output impedance, for that matter) the dBu makes more sense. Also, I incorrectly capitalized the M in some places - strictly speaking, I think dBM refers its zero to either 1000 watts or Megawatts, sometimes used in radio transmitters.

OK, I'll shut up now, gotta go take my Ritalin... Steve
 
I mix up my upper/lowercase "U" and "V" in dBU/dBv all the time!!!
 
Hey, as long as you pour it over ice and drink it before it gets diluted, it's all good... :=)
 
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