Digital amplitude units
.
FS – Fraction of full scale (usually abbreviated FFS)
.
dBFS – Decibels relative to full scale value
.
dBr – Decibels relative to user-defined digital value
Since the embedded audio in a digital signal does not have a specific
analog amplitude, its amplitude is usually calculated relative to the
maximum possible sinewave amplitude that can be represented by the
given number of bits. This is ‘FS’, or ‘Full Scale’.
The digital amplitude detectors calculate the amplitude in RMS. FS is
the maximum possible RMS value for a sinewave, when the sinewave
peaks just touch the minimum and maximum possible numbers. Note
that other signals with crest factors higher than sinewaves (such as
square waves) can give readings higher than full scale because if they
are just touching the minimum and maximum possible numbers, they
have an RMS amplitude that is higher than that of a sinewave.
The FFS unit stands for ‘Fraction of Full Scale’, and is generally a
number smaller than one. It is derived by dividing the RMS value of
the measured signal by the RMS amplitude of a full-scale sinewave.
Therefore a sinewave of the maximum possible amplitude will be
1.000 FFS, while a sinewave of half that amplitude will be 0.500 FFS
(or 500 mFFS).
The dBFS unit will calculate the measured amplitude as a decibel
value relative to the full-scale amplitude. The formula for this
calculation is as follows:
The digital dBr unit is the same as the analog dBr unit. A digital
reference amplitude (relative to Full Scale) is specified by the user, and
the measured amplitude is calculated in dB relative to that reference.
The digital dBr reference can be selected in the same way as the
analog dBr reference. You may set the reference to be equivalent to
the current main measurement reading by pressing the dBr ZERO key
near the upper right corner of the display. If you go to the Analyzer
Only Panel and select a digital dBr-based unit for the main
measurement, using the upper left soft key, the current dBr reference
will be shown in the middle of the display and you may change it using
the AMPLITUDE controls.
dB = 20 log
V
Vr
()
3 Operation
Operational Overview Units
ATS-1 Dual Domain User's Manual 3-33
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