Audio levels, dBu, dBV, and the gang: What you need to know
My current audio projects have me thinking about interface levels and decibels, and I know a lot of you out there are uncertain about them. Here’s what you need to know.
First off, you need to know that I expended significant effort to avoid naming this article “dB or not dB, that is the question”.
But seriously: The vast majority of line-level analog audio interfaces fall into two categories:
Unbalanced, using “RCA” connectors, at a so-called “-10dBV” level.
Figure 1 RCA plugs (source: Wikipedia)
Balanced, using XLR connectors, at a so-called “+4dBu” level.
But before delving into the interfaces and their differences, we need to look at the purely audio phenomenon called dBVU.
We’ve all seen “VU” (volume unit) meters on audio equipment, but the truth is, very few of them are true VU meters. Why?
VU meters were developed in 1939 to be the lingua franca of audio, and not only do they assign 0 dBVU as a meaningful reference point (though not a peak level), but even the meter ballistics are precisely defined (something an electronic VU meter must explicitly emulate if it is to match a d’Arsonval (moving-coil) meter).
Figure 3 VU meter (source: Wikipedia)
In the digital age, things have changed. Digital of course has a hard limit on signal level, and you’ll find many a meter has 0dB as its maximum reading. Why are dBVU levels so confusing?
It comes down to analog. Much analog audio gear has two level-related issues to consider. Sure, there will be absolute signal-level limitations based on a given piece of equipment’s power supply voltages and active circuitry. But in addition to that, many devices are non-linear. Tape decks, old vacuum-tube circuitry, phono levels, transformers, radio modulation levels, and doubtless more, tend to be happiest operating in their nominal range, but suffer increasing distortion as levels increase beyond that.
Thus, 0 dBVU was generally defined as the turning point for a piece of audio gear, beyond which distortion would start to increase. Part of the art of recording was deciding how best to trade off distortion in loud passages with noise in soft ones.
How much an interface or recording medium can be pushed into the red (above 0 dBVU) is referred to as headroom, and varies a fair bit: perhaps 3dB at the lowest, to 15dB or more. Life was complicated…I mean…interesting.
Okay, now that we’ve built some understanding of how decibels are used in the audio milieu, let’s get back to our two modern interface standards. What do those “+4/-10” numbers mean? Simply, they refer to the voltage levels defined as 0 dBVU.
The standard consumer “-10dBV” standard means that 0 dBVU equals -10 dBV, or, using the dB-to-volts formula:
= 1V • 10(-10/20)
= 316.2 mV
It so happens that this interface has been defined to carry 2V maximum (sine RMS, or 5.66 VP-P). Thus, recalling our somewhat old-fashioned headroom concept, we can say this interface has a headroom of:
= 20 • log( 2V / 316.2mV )
= 16.02 dB
You’ll notice that this figure roughly equals 10dB + 6dB (10dB brings us up from 0 dBVU to 1V; 6dB is the everyday approximation for 2:1, i.e., the 2V maximum). Note I’m intentionally mixing volts and decibels to help get you comfortable equating the two. Whether the 2V (16 dBVU) level is a hard limit or not still depends on the gear though. An audio ADC with a fixed input stage will clip at this level, but if an attenuator circuit or simply a volume pot precedes the active circuitry, the line could run hotter. Some equipment can drive said hotter signals. Some can’t. It’s all so…analog.
But what does the “u” of the less familiar +4dBu standard signify? I confess to not knowing this factoid myself until dipping into Wikipaedia.
It stands for unloaded. Hmm, well, that’s a big help.
Unloaded here refers to the line receiver. In early telephone days (and still today for that matter), standard telephone impedance was 600Ω, and signal levels were written as dBm (dB milliwatt) in that impedance. The audio interface that descended from that still uses a 600Ω source impedance (not that it really matters), but the receiving end of a line is relatively unloaded (typically 10kΩ to 100kΩ). The reference level denoted by “u” is 1mW in 600Ω, even though we don’t generally care about milliwatts in this context, nor do we have a 600Ω load. Obvious, right?
So, what’s that in voltage?
V = √PR
u = √(1mW • 600Ω)
= 774.6 mV
Thus, +4dBu, and hence 0 dBVU, is 1.228 V (you do the math).
I’m not aware that a maximum level has been defined for this interface.
Coincidentally, this puts the +4dBu reference level almost exactly four times (12dB) higher than the -10dBV level, but since the interface is balanced (differential), the actual swing of each line is about twice that of the unbalanced interface…all else being equal.
If we allow for the same amount of headroom as the unbalanced interface, each differential signal line must be able to drive at least 11.3 VP-P. Now you have an idea what supply voltage(s) you’ll need.
In the digital world, you’ll see dBFS (full scale) used, and all levels are negative, since full scale is, well, full scale. That said, you’ll see more traditional, analog-looking metering too, with levels allowed to go above 0 dBFS. What does it all mean?
Read the manual.
- Audio line receiver impedance balancing using a 2nd diff amp
- 2 VRMS - A funny old standard
- Understanding superior professional audio design: A block-by-block approach
- Intuitive sampling theory (digital audio)
- Building the AV room
- VU Meter spans 60dB
- The G word: How to get your audio off the ground
—Michael Dunn is Editor in Chief at EDN with several decades of electronic design experience in various areas, including audio.