If a tape recorder puts out +4dBm at line level (0VU), we know that its output voltage is 4dB above 0.775V (which, incidentally, is 1.228V). The "m" in "dBm" comes from the word "milliwatt": 1mW of power is dissipated when 0.775V is applied across 600 ohms (a standard impedance). The expression "dBm," found mostly in professional equipment measurements and specifications, means that the reference level is 0.775 volts. When expressing digital signal levels, it's commonly known that the reference level is full-scale, or 0dBFS. Although we don't always include the suffix, it's assumed from the context. 90dB is the difference between full-scale amplitude and the test-signal amplitude. For example, when we plot a digital processor's output spectrally when decoding a -90dBFS, 1kHz sinewave, the letters "FS" tell us that the test-signal voltage is 90dB below "Full Scale" digital (the maximum level). The letters after the number of dB (+4dBm or -90dBFS, for example) denote the reference level to which the expressed value is referenced. This is where all those confusing suffixes come in. Any time the term decibel is used, it must be accompanied by a reference level-either to another level, as with the two CD players, or against a standard reference. In the case of our CD players, it's the ratio of their respective output voltages that's expressed by the decibel amount. When we say that a particular CD player has a 2.5dB higher output voltage than another CD player, we're expressing the difference in output voltages between the players. The most important thing to realize is that the decibel doesn't express an absolute value-it expresses a ratio between two values. In addition, there seem to be all kinds of dB: dBm, dBR, dBV, dBFS, dBW, and dB SPL, to name a few. The term decibel (dB) appears repeatedly in specifications and measurements, yet it's often misunderstood.
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