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DBFS
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dBFS means "decibels relative to full scale". It is an abbreviation for decibel amplitude levels in digital systems which have a maximum available level (like PCM encoding).
Peak levels
0 dBFS is assigned to the maximum possible level. A signal that reaches 50% of the maximum level would peak at -6 dBFS, for instance. All peak measurements will be negative numbers.
It should be remembered that a digital signal which does not contain any samples at 0 dBFS can still clip when converted to analog, due to intersample peaks.
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Encyclopedia
dBFS means "decibels relative to full scale". It is an abbreviation for decibel amplitude levels in digital systems which have a maximum available level (like PCM encoding).
Peak levels
0 dBFS is assigned to the maximum possible level. A signal that reaches 50% of the maximum level would peak at -6 dBFS, for instance. All peak measurements will be negative numbers.
It should be remembered that a digital signal which does not contain any samples at 0 dBFS can still clip when converted to analog, due to intersample peaks. Some meters take this into account, while others do not. This is why the official standards use a sine tone of 997 Hz to define full-scale, to avoid being a sub-multiple of any common sampling frequency.
RMS levels
Since a peak measurement is not useful for qualifying the noise performance of a system, or measuring the loudness of an audio recording, for instance, RMS measurements are often used instead.
There is a potential for ambiguity when assigning a level on the dBFS scale to a waveform rather than to a specific amplitude, since some choose the reference level so that RMS and peak measurements of a sine wave produce the same number, while others want the RMS and peak values of a square wave to be equal, as they are in typical analog measurements.
- For the case in which the RMS value of a full-scale square wave is designated 0 dBFS, all possible dBFS measurements are negative numbers. A sine wave could not exist at a larger RMS value than -3 dBFS without clipping, by this convention. This is the convention used in Euphonix meters.
- For the case in which the RMS value of a full-scale sine wave is designated 0 dBFS, a full-scale square wave would be at +3 dBFS. This is the definition specified in AES Standard AES17-1998 and used in Dorrough meters.
Dynamic range
The measured dynamic range of a digital system is the ratio of the full scale signal level to the RMS noise floor. The theoretical minimum noise floor is caused by quantization noise. This is usually modeled as a uniform random fluctuation between -1/2 LSB and +1/2 LSB. (Only certain signals produce a uniform random fluctuations, so this model is typically, but not always, accurate.)
As the dynamic range is measured relative to the RMS level of a full scale sine wave, the dynamic range and the level of this quantization noise in dBFS can both be estimated with the same formula (though with reversed sign):
The value of n equals the resolution of the system in bits or the resolution of the system minus 1 bit (the measure error). For example, a 16-bit system will have a theoretical minimum noise floor of -98.09 dBFS relative to a full-scale sine wave:
In any real converter, dither is added to the signal before sampling. This removes the effects of any non-uniform quantization error, but increases the minimum noise floor.
Analog levels
dBFS is not to be used for analog levels, according to AES-6id-2006. There is no single standard for converting between digital and analog levels, mostly due to the differing capabilities of different equipment. The conversion level is chosen as the best compromise for the typical headroom and signal-to-noise levels of the equipment in question. Examples:
- EBU R68 is used in most European countries, specifying +18 dBu at 0 dBFS
- In Europe, the EBU recommend that -18 dBFS equates to the Alignment Level
- European & UK calibration for Post & Film is -18 dBFS = 0 VU
- UK broadcasters, Alignment Level is taken as 0 dBu (PPM4 or -4VU)
- US installations use +24 dBu for 0 dBFS
- American Post: -20 dBFS = 0 VU = +4 dBu
- The American SMPTE standard defines -20 dBFS as the Alignment Level
- In Japan, France and some other countries, converters may be calibrated for +22 dBu at 0 dBFS.
- BBC spec: -18 dBFS = PPM "4" = 0 dBu
- German ARD & studio PPM +6 dBu = -10 (-9) dBFS. +16 (+15)dBu = 0 dBFS. No VU.
dBFSD
dBFSD is an abbreviation for the standard digital audio level measurement scale. It is measured in decibels referenced to Full Scale Digital, which is the loudest possible digital audio sample value.
The image shown is of a digital audio meter in the Metric Halo application, called SpectraFoo. It is using the K-System meter scale, calibrated for K-14. This shows both the current signal level, as well as indicating how much of the prescribed 14 decibels of headroom remain beneath -0 decibels Full Scale Digital. Too many full scale digital samples in a row (e.g., >3) implies that the reconstructed waveform is illegal, since it would have exceeded the full scale of amplitude, were it not "flattened" by the constraint of the format. (The K-System was invented by mastering engineer, Bob Katz, of Digital Domain (mastering studios), in Altamonte Springs, Florida.)
See also
External links
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