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9th May 2006
#1

How many volts are 13 dBu?

Hate to ask this - but I can't seem to find the formula for converting it into volts. Can you lend a helping hand?
10th May 2006
#2
Gear Maniac

Quote:
Originally Posted by bobby_z
Hate to ask this - but I can't seem to find the formula for converting it into volts. Can you lend a helping hand?
http://www.sengpielaudio.com/calculator-db-volt.htm

10th May 2006
#3
Lives for gear

10^(13/20) X 0.775 = 3.461v

or see attatched file
Attached Images

10th May 2006
#4
Lives for gear

Quote:
Originally Posted by bobby_z
Hate to ask this - but I can't seem to find the formula for converting it into volts. Can you lend a helping hand?
Hi

If no calculator close at hand, you can work it out in your head...

6dB is twice the size, 12dB is four times the size.

1dB is a teeny difference so you can add a bit.

0dBu is 0.775 volts which you could ball park to 0.8 volts.

4 x 0.8 volts is 3.2 volts.

add a bit for the extra 1dB givs you a bit over 3.3 volts.

or do Antilog (dB/20) x 0.775 = 3.46179 volts

10th May 2006
#5

Sweet, thx y'all!

A followup question:
I don't think I understand what the +4 dBu standard is - because in my Fireface manual, it's stated that 0 dBFS (@ +4 dBu) = 13 dBu.

My question is - what actually is +4 dBu? The optimal level out?
10th May 2006
#6

Quote:
Originally Posted by fraz
http://www.sengpielaudio.com/calculator-db-volt.htm

Well, your googleskills are better than mine, I found something completely different
10th May 2006
#7
Lives for gear

Bobby, that spec doesn't make any sense to me!

0 VU = +4 dBu is a long-running standard from the analog days. It means that when the VU meter on a desk or recorder reads 0 VU, then the voltage output is +4 dBu.

Since the analog desk (as an example) could produce anywhere from +20 dBu to + 28 dBu before clipping (depending on the design), that ensured the "average' level was well below the maximum the desk could produce.

Assuming the desk could produce +24 dBu at clipping and 0 VU = +4dBu, that would be akin to operating a digital gizmo at a nominal level of -20 dBFS.

Bri
10th May 2006
#8
Lives for gear

Quote:
Originally Posted by bobby_z
Sweet, thx y'all!

A followup question:
I don't think I understand what the +4 dBu standard is - because in my Fireface manual, it's stated that 0 dBFS (@ +4 dBu) = 13 dBu.

My question is - what actually is +4 dBu? The optimal level out?
Yes, it looks strange indeed. They could mean that 13dBu is full scale? But it seems a bit low.

/Cojo
10th May 2006
#9
Lives for gear

I would bet that the "(@+4dBu)" part means they're referring to the output level while the output is set for +4 nominal level, as opposed to -10 (many prosumer interfaces have a +4/-10 switch that changes the input and output gains to be compatible with either consumer -10dBv levels or pro +4dBm levels).

Originally, back in the day, the standard was 0dBm=1mW. That is to say, the 0dB reference level was a voltage that delivered one milliwatt (into the standard 600 ohm termination). P=EI (power equals voltage times current), and I=E/R (current equals voltage divided by resistance) so P=E^2/R (power equals voltage squared divided by resistance) so 1mW into 600 ohms means 0.7746 volts.

Eventually, we stopped terminating all our audio lines with 600 ohms. So the 1mW reference is no longer meaningful, but we generally use the same voltage reference. We typically don't use dBm anymore for this reason (the m stood for milliwatt), so we generally use either dBv, where the v indicates that we're ignoring power and just measuring voltage - even though decibels are strictly speaking a power ratio - or we say dBu, where the u indicates that the terminating impedance is unspecified.

In most cases it doesn't matter too much, you can generally figure that for analog electrical signals, 0dB=0.775Vrms. It gets confusing when you have to equate these analog electrical signals to another reference, such as dBSPL (Sound Pressure Level, actual acoustic audio power) where 0dB=silence, or dBFS (Full Scale, the maximum recordable level in a digital system) where all levels by definition are at or below zero.

The important thing to remember is that decibels by themselves do not indicate a level at all. They only indicate a RATIO of one level to another. So without a reference, decibels are meaningless measurements. The only time it's proper to refer to decibels without a reference is when you're talking about gain or attenuation. You can say a preamp has 60dB of gain, because it's understood you're talking about the output level being referred to the input level, which is itself unspecified. Likewise, if a resistive pad (for example) has an attenuation of 20dB (which is a gain of -20dB), you're talking about the output level relative to an arbitrary input level. But in the case of measurements and signal levels and such, any mention of dB is incomplete without a reference.

The relationship between dBu and dBFS is completely arbitrary, and is dependent on the gain of the line amplifier at the output of your DACs. It should be chosen to optimize headroom (by allowing for equal headroom in both the analog and digital domains) and to produce a reasonable monitoring environment. 0VU=-14, -18, or -20dBFS are all common references. The idea is that if you have a sine wave in your DAW at -18dBFS, for example, that should produce an analog output at 0VU, or +4dBu. This way, the actual music (not sine waves) can have peaks that go 18dB louder than that in the digital domain before digital clipping, and those peaks will come out at +22dBu, which in many systems will be in the general neighborhood of clipping the analog circuitry. A lot of people, before they fully understand these relationships, assume that 0dBFS ought to equal 0VU. Of course that would be a bad move 0dBFS is the maximum possible level in the digital domain, whereas 0VU is supposed to be about the average signal level. Of course, a signal can only have its PEAK at 0dBFS (unless it's clipped) and its rms level will be lower than that. A sine wave peaking at 0dBFS will have an rms level of -3dB. Music peaking at 0dBFS (without clipping) will have an rms level typically down around -20dBFS, or lower (depending on the music, of course). So actually, a reference of 0VU=-18dBFS is really not very conservative at all - it allows room for peaks, but doesn't allow room for swells in the average music level without cutting off the peaks.
10th May 2006
#10
Gear Maniac

Level standards

I know this goes back to the days of telegraph lines, but on a historical note, does anyone know how the "1 milliwatt dissipation in 600 ohms" standard level originated?
And more important, when and how (and by whom) was +4dB decided upon as a standard line level?

BG
10th May 2006
#11
Gear Guru

600 ohms was the standard termination impedance of the telephone system (at least in the USA). Then, 1mW was a useful, easy reference.

Thats how the dBu got going. Reference is 1mW into a 600 ohm load, but more or less into another load. I suppose that the voltage would droop if the temination impedance got low enough.

I don't know the origins of the +4dBu standard - It ends up being about 3.5 Vpp for a sine wave.

-tINY

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