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In audio, 0 dBm often corresponds to approximately 0.775 volts, since 0.775 V dissipates 1 mW in a 600 Ω load. [16] The corresponding voltage level is 0 dBu, without the 600 Ω restriction. Conversely, for RF situations with a 50 Ω load, 0 dBm corresponds to approximately 0.224 volts, since 0.224 V dissipates 1 mW in a 50 Ω load.
A power level of 0 dBm corresponds to one milliwatt, and 1 dBm is one decibel greater (about 1.259 mW). In professional audio specifications, a popular unit is the dBu. This is relative to the root mean square voltage which delivers 1 mW (0 dBm) into a 600-ohm resistor, or √ 1 mW × 600 Ω ≈ 0.775 V RMS.
When the test impedance is 600 Ω resistive, 0 dBm can be referred to a voltage of 775 mV, which results in a reference active power of 1 mW. Then 0 dBm0 corresponds to an overload level of approximately 3 dBm in the analog-to-digital conversion. Given a sinusoid signal of 0.775 V RMS, the power at a zero transmission level point is:
The decibel unloaded reference voltage, 0 dBu, is the AC voltage required to produce 1 mW of power across a 600 Ω impedance (approximately 0.7746 V RMS). [2] This awkward unit is a holdover from the early telephone standards, which used 600 Ω sources and loads, and measured dissipated power in decibel-milliwatts ( dBm ).
In broadcasting terminology, 1 mV/m is 1000 μV/m or 60 dBμ (often written dBu). Examples. 100 dBμ or 100 mV/m: blanketing interference may occur on some receivers; 60 dBμ or 1.0 mV/m: frequently considered the edge of a radio station's protected area in North America
The dBm is an absolute reference level measurement (see Decibel § Suffixes and reference values) with respect to 1 mW power. When the nominal signal power is 0 dBm at the TLP, the test point is called a zero transmission level point, or zero-dBm TLP. The abbreviation dBm0 stands for the power in dBm measured
UK broadcasters, alignment level is taken as 0 dBu (PPM 4 or −4 VU) The American SMPTE standard defines −20 dBFS as the alignment level. European and UK calibration for Post & Film [clarification needed] is −18 dBFS = 0 VU. US installations use +24 dBu for 0 dBFS. American and Australian Post: −20 dBFS = 0 VU = +4 dBu.
These are Type II PPMs with the seven marks labelled −6, 0, +4, +8, +12, +16 and +20: this scaling shows absolute levels in dBu (or dBm into 600 Ω). The appearance is similar to the ABC PPM except that all the numbers are 16 dB higher.