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dBm

Abbreviation for decibels as referenced to 1 milliwatt of power. 0 dBm = 1 milliwatt. Strictly speaking the reference is for power only, and does not take in to consideration any specific voltage, current or resistance. However, due to conventions and equipment in use years ago it is sometimes assumed to be 1 milliwatt dissipated into a 600 ohm load. At 600 ohms .775 volts will produce enough current to equal 0 dBm (or 1 milliwatt of power). The dBm is commonly used to express power levels of signals in mixing consoles, tape recorders, and other audio equipment. The .775 volt value has become somewhat of a reference standard in the audio industry, but we must remember that it’s only truly valid when the load impedance is 600 ohms.

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