![]() The exact voltage is unknown and irrelevant. With digital converters the best performance is achieved by operating the converter such that the nominal signal level is close to full-scale. Full-scale is a term that indicates the maximum signal level into or out of an A/D or D/A converter, respectively. In digital gear we encounter the dBFS, which is dB relative to full-scale. If operating at +4 dBu nominal this gives 14 dB of headroom which means that any signal peaks can be over four times higher. For example, the Axe-Fx II has a maximum signal level of +18 dBu. Headroom is the difference between the maximum signal level and the nominal signal level. Well-designed gear has some amount of "headroom". The goal is to get your signal level around 0 dB. Many recording consoles use VU meters which are calibrated such that "0 dB" is +4 dBu. When recording your goal is to get your signal level near the nominal signal level of the equipment being used. To go from dB to volts the formula is 10^(dB/20).Ĭonsumer audio gear usually runs at -10dBV, or roughly 0.32 volts. What does that mean? 0 dBu is 0.77 volts so +4 dBu would be 4 dB greater, or about 1.22 volts. Back in the early days of telecom 600 ohms was the standard termination impedance, hence the dBu. dBu is the power relative to the voltage into a 600 ohm resistor that is dissipating 1 mW. The formula for dBV is 20 * log10(V1/V2) since we need to square the voltage to get the power. ![]() dBV is a voltage ratio and not really a true dB but, regardless, is still commonly used. dBm refers to the power referenced to one milliwatt. There are many reference levels used in dB: dBm, dBu, dBV, dB re. This is why cameras use f-stops which are a base-2 logarithm. If we take the logarithm of the intensity instead we get a straight line. ![]() If we were to plot that we would have an exponential curve of light intensity vs. We perceive light such that the light must double for it to appear twice as bright. For example, human vision is logarithmic. Many other natural phenomena are logarithmic which means that the phenomena exists in the "multiplication domain" as opposed to the "addition domain". Human hearing, for example, is logarithmic. So if someone says "that signal is 86 dB" that is a meaningless number as it has no reference.ĭecibels are convenient because they convert logarithmic perception to a linear scale. A dB is meaningless without a reference power. The important thing to understand is that the decibel is a RATIO of powers. The reason it is called a decibel is because it is 10 bels. The formula for the decibel is dB = 10 * log_10(P1 / P2) where P1 and P2 are power measurements. The decibel is a unit of measurement that gives the ratio of the power of one signal relative to another. ![]()
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