Audio Reference Levels
by Eddie Ciletti
1998 ~ 2014

Why are there so many different 'Reference Levels?' 
An audio (AC) amplifier needs Direct Current (DC) power to operate.  A DC power supply can be as simple as a 9-volt battery and can be  referred to as the 'power rail' - if single ended - and power rails, if bipolar.   The distance between the peak-to-peak signal amplitude and the maximum output of an amplifier is called HEADROOM. 

There are two basic amplifier designs.  Class A (single-ended) includes 'simple' tube and solid-state circuits (Neve 1066/1073, JFET), many of which are more likley to hit the sonic airbag before running out of power rail voltage.   The Sonic Airbag is an assymetrical, soft-overload characteristic that tends to generate even-order (octave) harmonics - what we call 'warmth.' 

Modern audio gear - Professional and Consumer - typically uses Operational Amplifiers (OpAmps) running Class-AB on bi-polar supplies that can swing very close to 100% of the rails.  Beyond the rails is symetrical clipping = nasty = fuzz box!  Typical bi-polar rail examples are:  +/-15volts (typical mixer and pro A/D converters), +/- 16volts (API 500 rack), +/- 17volts (TridentSeries80) and +/-18volts (SSL).  Thus, the maximum peak-to-peak signal can range from 30-volts to 36-volts, which, believe it or not, is only a 1.58dB difference!

PRO-fessional, vs CON-sumer = Plus-4 vs Minus-10
Whether Pro or Con, Plus-4 and Minus-10 refer to the NOMINAL Level, which is approximately 20dB down for 0dBFS.  When the analog meter in Figure-1 indicates 0VU, the device to which it is connected will output its standard (nominal) operating level. For professional and consumer recording equipment, the standards are +4dBu and –10dBV, respectively.   So, -20dBFS could be +4dBu or -10dBV depending on whether the gear is professional or consumer.  But what does that mean?
 

 
FIGURE-1: Analog VU Meter @ 0VU compared to digital meter levels.  On the face of déjà VU are the formulae for dB and Power.  To the far right (barely legible) are the "RMS" voltages and their corresponding "dee Bees." 

For every 20 dB, Voltage changes by a factor of ten.  Double- or half-voltage is +/- 6dB respectively. 

Double- or half-power is +/- 3dB, respectively.  10X Power increases = 10 dB. 

SIDEBAR: Getting Down To Dee Bees Ness
We nonchalantly misuse the decibel as if it were a voltage, when in fact the dB is the logarithmic ratio of two voltages.  The phone company, formerly known as Bell Labs, or Ma Bell, standardized many of the 'things' we use today - like the 19-inch rack - the dBu / dBm is our the audio reference level for a piece of test equipment known as an AC voltmeter, or dB meter.  The 'dB' has many suffixes ('u' and ''m,' for example) to help you identify its reference.

On an AC voltmeter, 0dBu and 0dBm are referenced to .775volts RMS.  Ma Bell applied this AC voltage to 600-ohms in order to complete the circuit, because they felt it important to test the signal under LOAD (real world) rather than no load.  in our recording-specific audio world,  "vintage" and retro gear have a 600-ohm input and output impedance.  Apply .775Vrms to 600 ohms and the power dissipated is nice round 1 milliWatt, hance the 'm' in dBm.  You can see the numbers in the power formula on lower right of Figure-1.  An additional .455volts yields 1.23 volts or +4dB. (For consumer gear, 0dBV is "cleanly" referenced to 1Volt RMS, 2.21dB higher than 0dBu.)

Now you understand why most people just say "Plus 4."

Let's play with the numbers…

A balanced output amplifier can deliver a maximum signal of +24dBu (+18 dBu). Numbers in ( ) indicate the output when operated in the "safe" unbalanced mode. The gain structure is set to deliver +22dBu (+16dBu) when the recording level is 0dBfs. (Doing the math: 18 dB above +4dBu is +22 dBu.)  Remember that the amplifiers feeding pin 2 and pin 3 can only deliver +18dBu "max."  With only 2 dB of headroom, under normal circumstances, operation in the "unsafe" mode will attempt to add 6 dB of level, 4dB above "max." 

To Be Expanded...