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The decibel originates from methods used to quantify signal loss in telegraph and telephone circuits. Until the mid-1920s, the unit for loss was miles of standard cable (MSC). 1 MSC corresponded to the loss of power over one mile (approximately 1.6 km) of standard telephone cable at a frequency of 5000 radians per second (795.8 Hz), and matched closely the smallest attenuation detectable to a ...
dBm or dB mW (decibel-milliwatts) is a unit of power level expressed using a logarithmic decibel (dB) scale respective to one milliwatt (mW). It is commonly used by radio, microwave and fiber-optical communication technicians & engineers to measure the power of system transmissions on a log scale , which can express both very large and very ...
For very low-power systems, such as mobile phones, signal strength is usually expressed in dB-microvolts per metre (dBμV/m) or in decibels above a reference level of one milliwatt . In broadcasting terminology, 1 mV/m is 1000 μV/m or 60 dBμ (often written dBu).
Minimum terrestrial analog-TV RF antenna signal (35 dB[μV]) [6] [7] [8] 10 −4: 500–1000 μV Miniature endplate potentials, spontaneous fluctuations in neuron potentials [1] 10 −3: 1–2 mV Potential created at ambient temperatures from K Type Thermocouple: Centi-10 −2 ~10–50 mV Ripple voltage in the output of a good DC power supply ...
It is often expressed using the logarithmic decibel (dB) units ("dB gain"). [4] A gain greater than one (greater than zero dB), that is, amplification, is the defining property of an active device or circuit, while a passive circuit will have a gain of less than one.
A power ratio 10 log r dB is equivalent to a field-quantity ratio 20 log r dB, since power in a linear system is proportional to the square (Joule's laws) of the amplitude. Hence the decibel and the neper have a fixed ratio to each other: [ 4 ]
The unit dB FS or dBFS is defined in AES Standard AES17-1998, [13] IEC 61606, [14] and ITU-T Recs. P.381 [15] and P.382, [16] such that the RMS value of a full-scale sine wave is designated 0 dB FS. This means a full-scale square wave would have an RMS value of +3 dB FS.
A signal is applied to the input such that the output increases by 12 dB. The level of the signal needed to produce this is noted. In this case, it was found to be 0.25 microvolts. According to the radio designer, intelligible speech can be detected 12 dB above the receiver's noise floor (noise and distortion).