DBFS

Decibels relative to full scale, commonly abbreviated dBFS, measures decibel amplitude levels in digital systems which have a maximum available peak level, like pulse-code modulation.

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, but only if the D/A circuit was badly designed. The analog domain is totally distinct from the digital one and must not be confused with it.

All digital samples are less than (or equal) to the original analog values. With a sufficient number of samples you can recreate that original analog signal exactly which will have to have analog values higher than the digital ones. Any properly designed D/A circuit will account for this factoid. And in practice, most circuits use considerably more samples than the mere minimum that Nyquist would require. This oversampling would severely limit the amount that an analog signal could be higher than the peak sample value would indicate. So in practice, it would take considerable effort to design a D/A with so little headroom that the recreated analog would saturate.

. 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.

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):

:$\backslash mathrm\{DR\}\; =\; \backslash mathrm\{SNR\}\; =\; 20\backslash log\{\backslash left(2^n\backslash sqrt\{\backslash tfrac\{3\}\{2\}\}\backslash right)\}\; \backslash approx\; 6.0206\; \backslash cdot\; n\; +\; 1.761$

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:

:$\backslash mathrm\{DR\}\; =\; \backslash mathrm\{SNR\}\; =\; 20\backslash log\{\backslash left(2^\{16\}\; \backslash sqrt\{\backslash tfrac\{3\}\{2\}\}\backslash right)\}\; \backslash approx\; 6.0206\; \backslash cdot\; 16\; +\; 1.761\; \backslash approx\; 98.09\backslash ,$

In any real converter, dither is added to the signal before sampling. This removes the effects of non-uniform quantization error, but increases the minimum noise floor.

Notes

Although the decibel (dB) is permitted for use alongside SI units, the dBFS is not.

The term dBFS was first coined in the early 1980s by James Colotti, an analog engineer who pioneered some of the dynamic evaluation techniques of high-speed A/D and D/A Converters. Mr. Colotti first introduced the term to industry at the RF Expo East in Boston Massachusetts in November 1987, during his presentation “Digital Dynamic Analysis of A/D Conversion Systems through Evaluation Software based on FFT/DFT Analysis".

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

Full scale

References

External links

Rane pro audio reference definition of dBFS

dBFS - Sweetwater glossary

Category:Digital audio