dBFS

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dBFS , English abbreviation for Decibels relative to full scale , is the logarithmic unit with an absolute, often linear scale in a digital audio system.

An analog full-scale level value generates a sample approximately of the maximum positive or negative value of the digital word length of the A / D converter . Lower instantaneous values ​​of the analog input variable result in samples with a smaller absolute value. A logarithmic measure is obtained from the ratio of the actual value to the maximum value and displayed in accordance with the decibel conventions. The dB-FS values ​​therefore only know negative values for a single sample (and 0 as full scale ).

Peak value ( peak value )

Clipping a digital waveform

0 dBFS represents the highest possible level value (e.g. for + 32767 / −32768 with 16-bit). A level value higher than 0 dBFS for a single sample is not possible in the digital domain. In special cases, however, a D / A converter can generate higher levels (in the analog domain); this can result from specially selected digital samples, the waveform and the converter properties.

Rms value

For the averaging of a series of numbers or measurements, the rms value is well suited in the audio sector . Its mechanical model from the analog world is the moving-iron measuring mechanism .

In practice, two different RMS values ​​are used, which differ in the definition of the 0 dBFS level:

  • FS Square Wave ( Full-Scale Square Wave ):

0 dBFS is the effective value of the maximum possible square wave . It follows that a sine wave whose highest single sample reaches 0 dBFS at its peak is only displayed with -3 dBFS RMS ( Root Mean Square , i.e. effective value). This definition is used in some audio editing programs and is also based on the LKFS standard.

  • FS Sine Wave ( Full-Scale Sine Wave ):

The effective value of the maximum possible sinusoidal oscillation is increased by 3 dB to 0 dBFS. It follows that a square wave, the highest single sample of which reaches 0 dBFS at its peak, is effectively displayed with +3 dBFS RMS. RMS values ​​in FS Sine Wave are thus 3 dB higher than in FS Square Wave. The FS Sine Wave definition is common in professional digital sound engineering so that a full-level sine wave (test tone) is displayed on the level meter with 0 dBFS (RMS); the FS square wave representation (−3 dBFS) of this sine tone would lead to the false assumption that it has not yet reached full scale.

The term "dBFS RMS" is sometimes used to distinguish it from the peak value observation (FS Square Wave). One also speaks of intrinsic level .

See also

Individual evidence

  1. ↑ AES17-1998 AES standard method for digital audio engineering - Measurement of digital audio equipment. Audio Engineering Society, Inc., accessed July 19, 2016 .

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