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.
Figure 2 XLR connectors (source: Wikipedia)
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,…