In audio, a standard signal level that is defined as the nominal operating level for the audio devices involved. It does not mean zero, or no signal, literally. For example, the zero reference of a device with a balanced +4 dBu input is set so that when a +4 dBu input signal is applied, the meters will show 0 VU, or just be on the edge of going into the red. Often times tape machines are aligned with a Reference Tape that includes tones recorded at the Zero Reference for that machine. Again, the machine would be calibrated so that its meters show zero VU when the Reference Tone is playing. Zero Reference is a concept that has been bent in many different ways over the years. Engineers have adopted all manner of subtle variations on the theme depending upon their personal preferences. And now with the advent of digital recording, a whole new twist has been thrown into things because headroom and nominal recording levels in digital equipment do not look the same as they do in analog equipment. The equivalent of 0 VU on a DAT machine may be -12 (or -14, or -16, or -18) on its dB scale. That means that a +4 dBu input into a DAT will cause a meter deflection of one of these values. When your DAT clips or reaches its “full code” output, the signal coming out of the +4 dBu jacks on the back is probably up somewhere between +24 to +28 dBu. Again, the Zero Reference is down around -12 dB or so on its meters. Calibrating an entire studio or audio chain to agree to a common zero reference is critical to obtaining consistent results and maximizing the signal to noise ratio of the entire system.