Understanding dB, VU and measuring

[Ptownkid]

I've been trying to learn about decibels lately and either I'm kinda dense, or it's a very confusing topic.

How do Vu and db relate?

 

[JohnRoberts]

A Volume Unit is equal in magnitude to one dB.

Decibels, one tenth of a Bel, are relative power measures. VU are typically used to monitor levels (histortically on 600 ohm terminations).

dBm is a relative power measure. 0 dBm = 1 milliWatt= .775V/600 ohm.

0 VU is commonly calibrated to +4 dBm or 1.23V/600

dBu ignores power and is voltage re: .775V

dBV is voltage re 1V.

This is grossly simplified but hopefiully hits the high points. There are more subtleties to ballistics of true VU meters, etc.

edit- decibels are logarithmic so you can represent very large ratios with easily manageable numbers, and you can combine multiple gain stages, effectively multiplications, by adding the logarithmic decibels. Just like logs in math.

JR

ps: I pulled all the numbers from memory so check my work.

 

[Ptownkid]

Ok, so how do you test a VU meter to see if it's actually semi-accurate?

 

[SSLtech]

A decibel is a multiplier.

A VU is a meter.

6dB means two-to-one. (actually it's 6.02dB, but let's not split hairs...) PLUS 6dB is twice as much. MINUS 6dB means half as much.

12dB therefore means Four-to-one. +12dB = "x4", -12dB = "÷4" ...again it's actually twelve-point-zero-four dB, but you get the picture.

20 dB = "TEN TO ONE". -No fractions this time, it's EXACT.

+20dB = "x10" -20dB = "÷10"

Decibels are LOGARITHMIC. =You add more, and it's the same as MULTIPLYING in linear terms...

Lemmy try an e'splain...

20dB less than a volt is 0.1Volt. (1V ÷ 10)
40dB less than a volt is 0.01 Volt (1V ÷ 10 ÷ 10... same as 1Volt -20dB -20dB)

So when you cumulatively add -or subtract... or any combination thereof- decibels, you multiply or divide or any combination thereof- linear voltage.

So 1 volt minus 26dB = 0.05Volts... since it's 1volt minus 20dB (which is a tenth) then minus another 6dB (which is a further halving of the result).

That's the simple head stuff. TEN dB therefore is multiplying or dividing by the SQUARE ROOT of ten in linear terms... (three-point one something... think of it as a near neighbor of 'Pi'!) so 10dB down from 1 Volt would be 0.31 volts. Ten dB UP from 1 volt would be 3.1 volts.

VU is a meter. There is more than one convention for what it means when a needle points to zero, where the red bit turns into a black bit, but typically we say that it's 1.23volts, which is 4dB over one milliwatt in a 600Ω line...

Sorry if it got confusing, but I'll stop there, to try and see which bits I might need to explain.

Try to practice NEVER calling any VU meter reading "plus two dB" and instead call it "plus two VU"... that's a start. Plus two dB means "two dB louder" and nothing more... just like "minus six dB" means "half as big" and plus six dB means "twice as big". -In that sense when someone says "minus twenty dB" it SHOULD mean "one tenth as big" and if the same person says "minus twenty VU" it should mean 0.123 Volts AC RMS.

Keith

EDIT: Damn, it took me so long to write that reply that John has already covered some of what I wrote... Hopefully my 'second way of describing it' doesn't confuse any more!!!

 

[barclaycon]

How do Vu and db relate?

Well if I remember back from my college days, it goes something like this.
The reference level 0dB was determined to a be an appropriate level for driving audio lines and was equivalent to 1mW of energy being delivered into a 600 ohm load.
dBm (i.e referenced to 1 milliwatt)
OdBm = 0.775volts rms
I think the term Bel is related to Alexander Graham Bell - decibel being a tenth of a Bel; the name he gave to a transmission unit.
It was later determined that audio lines could be driven a lot higher - 4dB's higher in fact and so the VU or Volume Unit was set so that Zero on the meter was +4dbm. So strictly speaking VU and dBm's are the same except in that in VU terms the level is 4 dB's higher.
0 VU = =4dBm
There are some variations like dBV where the reference point is 1.0volt rms.
dbu (unterminated)
dbr - where there is an abritrary reference level that must be specified.
dBFS - where zero is Full Scale. (Used for digital signal levels - zero being clipping).
So in other words, they are all deciBels but related to a different reference level.
Phew... now I need a lie down.

 

[lofi]

i shall read it twice Keith, wont understand it, but you can rest easy that your post has not gone in vein :green:

 

[SSLtech]

How do you test a VU meter for correct operation?

Well a VU meter is supposed to show zero VU when it and any required series resistance (usually between 3.6kΩ and 3.9kΩ) is strapped across a line which -correctly terminated at 600Ω- when a true RMS voltmeter reading across the same line producing a steady AC sine wave reads 1.23V.

To double check, at 0.775V AC RMS, the meter should read minus 4dB... indicating that the 'zero' position is 4dB higher than 0.775VDC, which produces 1mW in a 600Ω load.

Keith

 

[Ptownkid]

i love this place...

See, there's lots of info on this out there, but none of it is quite as clear as that.

Thanks Keith

 

[Ptownkid]

[quote author="SSLtech"]correctly terminated at 600Ω-[/quote]

Can we elaborate on this, my brain is a little slow today...

 

[Ptownkid]

http://www.soundonsound.com/sos/1994_articles/feb94/decibels.html

 

[zebra50]

This is great! But hold on....
Didn't we forget the amplitude - power thing? Or did I miss it?

-20dB means 0.1 x amplitude.

But because power is proportional to V squared

-10dB = 0.1 x power.

http://en.wikipedia.org/wiki/Decibel

Does the ear hear linear with power or amplitude? Or neither?

 

[JohnRoberts]

Yup... Keith memorized the voltage ratios not power ratios. Equations are something like 20x log(10) of ratio for V/V and 10x log(10) P/P.

2x voltage is 6 dB

2x power is 3 dB

The real purists get funny about dB being strictly a power ratio thing, but it's far too convenient for voltage ratios to not use it...

The only place I draw a line is for transformers... their dB ratio is always 0dB (less losses), but in a system like a mic preamp it's common to just use an overall dB figure for voltage gain.

Another piece of dB trivia is each added bit in word length is a doubling of quanta or 6dB.

JR

 

[Svart]

I work in RF, we strictly use it as a power level. Power levels can vary a lot in regards to carrier waveforms with modulated signals while the voltage levels stay rather linear.

 

[JohnRoberts]

0 dBW = 1 W

0 dBm = 1 mW

dBW is handy for comparing amp power in terms that correlate better to audible differences between power points.

JR

 

[SSLtech]

Ah yes indeed.

Of course on a VU meter the second scale (0% at the left hand end to 100% at 0VU) is scaled in VOLTAGE and not POWER... hence 50% is directly aligned with -6VU, and 31% is aligned with -10VU. 10% is aligned with -20VU

Keith

 

[JohnRoberts]

The meter's don't require any different calibration.. the 2:1 difference between voltage and power is some more log math... ^2 = 2x log.

A 3dB change in voltage (.707x or 1.4x) is 3dB change in power, since that voltage gets squared. i.e. .707^2 = .5x and 1.4^2= 2x.

Same meter, same scale just means something different if you're looking at voltage change or power change from that same voltage change.

more dB magic.

JR

 

[burdij]

The unit of transmission loss used by the Bell Labs in 1923 was defined by the loss per mile of "standard telephone cable". This was the "TU". In 1924 the name was changed to the Bel in honor of the founder. In January 1929, W. H. Martin proposed the decibel, 1/10th Bel as a more useful unit to measure the logarithmic ratio and in an attempt to get the unit adopted internationally. That attempt failed.