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Author Topic: reference voltage  (Read 2547 times)

.nathan.kosakowski.

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reference voltage
« on: December 08, 2005, 12:03:05 pm »

i was calibrating all sorts of stuff the other day, and i wondered:

what was .775v choosen as a reference level, why not something like 1v, or something that would give you 2v at +4dBv.

its not like there are multiple standards of measure, like imperial and metric.

just curious.
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Nathan Kosakowski
Ellae Center Recording
www.ellaecenter.com

Ronny

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Re: reference voltage
« Reply #1 on: December 08, 2005, 02:14:55 pm »

.nathan.kosakowski. wrote on Thu, 08 December 2005 12:03

i was calibrating all sorts of stuff the other day, and i wondered:

what was .775v choosen as a reference level, why not something like 1v, or something that would give you 2v at +4dBv.

its not like there are multiple standards of measure, like imperial and metric.

just curious.



Because decibels are arbitrary numbers they can be referenced to different voltage values. BTW, dBV is referenced 0dBV = 1Vrms. So when you see -10dBV line level specs, it's referenced to 1volt at -0dBV or -10dBV = .316Vrms. IOW, they are already using 1V = -0dB when you are using dBV.
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------Ronny Morris - Digitak Mastering------
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.nathan.kosakowski.

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Re: reference voltage
« Reply #2 on: December 08, 2005, 02:48:49 pm »

maybe the proper was to write it was dBu.  but were specifically did the .775v come from?  -10dBV is .316, +4dBV is 1.228, right?  i thought then that 0dBV is then the .775v, the reference from which the others are derived...

expanding my horizons, have any recommended reading material, other than you standard electronics books...
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Nathan Kosakowski
Ellae Center Recording
www.ellaecenter.com

PaulyD

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Re: reference voltage
« Reply #3 on: December 08, 2005, 04:07:18 pm »

Many electronics texts teach that RMS voltage is equal to .775 x peak voltage. So, naturally, .775 x 1v peak = .775v.

I tried to find a web page that explained this with a graphic but a quick Google search didn't turn up what I was hoping to find. However, it has to do with the theoretical ideal of an amplifier circuit having approximately a 50% duty cycle. Op amps, although low voltage devices, are actually amplifier circuits - amazing little ones at that! Maybe Paul Frindle will come along and explain it far better than I ever could. Smile

Paul

dcollins

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Re: reference voltage
« Reply #4 on: December 08, 2005, 04:29:08 pm »

Ronny wrote on Thu, 08 December 2005 11:14


Because decibels are arbitrary numbers they can be referenced to different voltage values. BTW, dBV is referenced 0dBV = 1Vrms. So when you see -10dBV line level specs, it's referenced to 1volt at -0dBV or -10dBV = .316Vrms. IOW, they are already using 1V = -0dB when you are using dBV.




Actually, the origin is the telephone company who used 600 ohms.

.775V is one milliwatt into 600 Ohms, and that is 0dBm.

DC

dcollins

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Re: reference voltage
« Reply #5 on: December 08, 2005, 04:32:46 pm »

PaulyD wrote on Thu, 08 December 2005 13:07

Many electronics texts teach that RMS voltage is equal to .775 x peak voltage. So, naturally, .775 x 1v peak = .775v.



I think you mean .707

DC

Ronny

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Re: reference voltage
« Reply #6 on: December 08, 2005, 06:41:30 pm »

dcollins wrote on Thu, 08 December 2005 16:29

Ronny wrote on Thu, 08 December 2005 11:14


Because decibels are arbitrary numbers they can be referenced to different voltage values. BTW, dBV is referenced 0dBV = 1Vrms. So when you see -10dBV line level specs, it's referenced to 1volt at -0dBV or -10dBV = .316Vrms. IOW, they are already using 1V = -0dB when you are using dBV.




Actually, the origin is the telephone company who used 600 ohms.

.775V is one milliwatt into 600 Ohms, and that is 0dBm.

DC



True, but dBm is a power reference. Most gear designers these days, just use the converted voltage reference. That's why you see +4dBu and -10dBV isn't it? If you change the ohms from 600, you change the voltage. It's understood that .775V is 1mW into 600 ohms = -0dBm, so they drop the dBm and use dBu and dBV.  
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maxdimario

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Re: reference voltage
« Reply #7 on: December 08, 2005, 06:50:15 pm »

most of the early fundamental technology used in recording studios was loosely derived from the telephone industry.

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bobkatz

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Re: reference voltage
« Reply #8 on: December 09, 2005, 05:40:10 am »

[quote title=Ronny wrote on Thu, 08 December 2005 14:14]
.nathan.kosakowski. wrote on Thu, 08 December 2005 12:03

i was calibrating all sorts of stuff the other day, and i wondered:

what was .775v choosen as a reference level, why not something like 1v, or something that would give you 2v at +4dBv.





The answer goes back to the number "1"! A much easier number to deal with, don't you think? 1 milliwatt/600 ohms. That translates, unfortunately, to 0.7746 volts, and that's what we're stuck with today because the meters, even the dB"m" meters, all ready volts and so no one wanted to get rid of these old meters. Now that most meters are digital, computer controlled and all and can be reset to any reference, then it makes sense to convert to dBV reference 1 volt... the only reason we're not doing it is historical accuracy. Microphone specs and all the rest are just referenced to this 0.775, so on we go.

BK
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bobkatz

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Re: reference voltage
« Reply #9 on: December 09, 2005, 05:52:19 am »

PaulyD wrote on Thu, 08 December 2005 16:07

Many electronics texts teach that RMS voltage is equal to .775 x peak voltage. So, naturally, .775 x 1v peak = .775v.





Sorry, that's dead wrong. For a sine wave, the RMS level is 0.707 x the peak. It's pythagorean, actually. I posted the actual answer elsewhere in this thread. I also suggest for those wanting a more detailed explanation of, for example, where "+4 dBu" came from, read footnote 10 on page 74 in my book OR catch John Klett's excellent summary in his forum recently posted.

BK
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The other says-this is new and therefore better."

No trees were killed in the sending of this message. However a large number of
electrons were terribly inconvenienced.
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