The use of “-14” as a calibration standard for digital audio gear thus means the analog input is calibrated to accept a peak analog level of +18dBu. Because the standard for VU meters is “0dBVU = +4dBu,” this results in “0dBVU = -14dBFS” for a peak level of +18dBu and “0dBVU = -20dBFS” for +24dBu. Professional level analog audio equipment typically has a peak output level of between +18 and +24dBu. In a similar manner, there is no fixed relationship between digital level (relative to 0dBFS) and the analog output level of a DA converter. ![]() Because no signal information is retained once the signal level exceeds 0dBFS the most important level for calibration purposes is peak level. Because this is the only “fixed” level in digital audio, the dB level scale typically starts at 0dBFS and all other levels below 0dBFS are in “negative dB” (for example -14dBFS).ĭepending on the calibration of the AD converter used to encode the audio signal a wide range of analog input levels can result in the same digital level. ![]() The term dBFS or “dB Full Scale” is used to describe the level of a linear PCM digital audio signal relative to the highest (peak) level that can be encoded, and has no fixed relationship to dBu or "analog level."ĠdBFS is sometimes referred to as “digital clipping” level. This is because “+4” is “+4dBu” and “-10” is “-10dBV ” and the dBu and dBV scales uses different “zero dB” reference voltages! The result is that, rather than the “apparent” difference of 14dB between “+4” and “-10” the actual difference is closer to 12 dB (11.8dB). Professional line level is often referred to as “+4” as versus the consumer line level of “-10” which is source of confusion. The zero reference for dBV is 1 Volt rms. The dBV scale is typically used in consumer audio measurements and ratings. ![]() When the use of VU meters became prevalent in the USA the VU meter’s “0” was defined as “+4dBm,” and in contemporary systems is defined as “+4dBu.” The designation “VU” is an abbreviation for “Volume Unit” because the VU meter was intended to provide a useful means of relating the metered level of complex audio program to the perceived volume. To make this system applicable to pure voltage level measurements the “dBu” scale is used with the same zero reference of 0.775 Volts. In most contemporary audio systems, the signal appears as a voltage waveform thus the dBu voltage scale is used instead of the dBm power scale. This power level was achieved when an RMS voltage of 0.775 volts was applied to a 600 Ohm load. In audio electronics, the “ 0dBm” standard for power was 1 milliwatt because most early audio transmission utilized 600 Ohm impedance-matched systems. The zero reference acoustic level is considered to be the “threshold of human perception” and SPL is thus a scale that tells us how much louder a sound level is than the quietest sound one can perceive. One of the more common forms of “absolute” dB scales is Sound Pressure Level or “SPL.” In this case, the “zero reference” is a very small value and the scale only extends in the positive value direction. Unfortunately the analogy fails when you bring in the differences between Fahrenheit and Centigrade because (unlike the decibel) the “one degree” has different definitions in each system!Ĭlick here for more detailed information SPL ![]() In order to give a dB measurement an absolute value it must have a “zero reference.” One analogy is temperature in “degrees.” Without knowing what “zero degrees” is we can only talk about the difference between two temperatures (a form of ratio). The ratio can be expressed as 10 raised to the power of (dB x 0.1) so a 6dB difference between two signals means the larger signal equals the value of the smaller signal multiplied times (10 raised to the power of 0.6) or "ten to the six-tenths power." In honor of Alexander Graham Bell, and to indicate the "decimal" power relationship the unit was named the "decibel." The term was used originally in early telephony to measure loss in a standard mile of telephone wire. The decibel may be defined in this manner: two amounts of power differ by 1 decibel when they are in the ratio of 100 raised to the power of one-tenth. Please note: due to limitations in supported text characters in the following discussion a value such as "two squared" is described as "2 raised to the power of 2" for clarity. Due to the non-linear nature of human hearing, the logarithmic dB scale approximates the relationship of the measured value to the perceived change in acoustic level. The term "dB" is used to describe a ratio between two audio levels.
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