User Manual

White Paper
FiRPHASE
www.rcf.itMay 2017
A new approach on linearity for RCF live loudspeakers
In this whitepaper, we will discuss the importance of excluding phase distortions in the sound reinforcement systems and how RCF made it
possible with FiRPHASE processing, reaching near-sub frequency phase linearity without annoying time delays.
A brief history of phase
perception
The Georg Ohm’s acoustic law (1843) states that
a musical sound is perceived by the ear as a set
of a number of constituent pure harmonic tones.
Later, Von Helmholtz agreed to the Ohm’s law
and deepened his future researches saying that
aural perception depends only on the amplitude
spectrum of a sound and is independent of
the phase angles of the various complements
contained in the spectrum.
In 1841, August Seebeck, a scientist from Dresden
University, was arguing with his experiments
that the phase dierences are clearly audible.
The missing fundamental eect explain how a
perceived pitch of a sound can be altered by phase
dierences of harmonics. The debate continued
for twenty years in the scientific journal Annalen
der Physik und Chemie, Seebeck then died young
and his knowledge was forgotten. Until 1959,
when Schroeder, in his work entitled New results
concerning monaural phase sensitivity (1959),
demonstrated the phenomenon. Schroeder
states that Ohm’s conclusion is invalid and it’s
only true in some particular cases. He postulates
then the Schroeder’s phase masking eect: by
just modifying the individual phase components
of two signals of identical envelopes, it is possible
to produce strong varying pitch perception, e.g.,
when playing melodies.
Today we have several demonstrations who
oppose the earlier belief that the human ear is
phase-deaf, as the work of Lipshitz et al. in the
Journal of Audio Engineering Society in 1982:
“We have found that midrange phase distortion
can be heard not only on simple combinations of
sinusoids, but also on many common acoustical
signals.” He pointed out that those problems exist
but can be subtle and transducer designers can
make an intelligent decision on the significance
(not the existence) of phase eects.
In another late AES conference in 1996, Johansen
& Rubak stated that “the conclusion must be: we
cannot allow the excess phase to be neglected,
and we will have to get around the equalization
task in another way.”
The perception of the phase spectrum has also
been studied in relation to many topics, such as
concert hall acoustics, pitch perception, vowel
identification, masking, speech processing, and
binaural rendering.
Fig.1 – Visual example of the monaural phase distortions generated by a sound
system that can be made of transducers, EQ, crossovers, and amplifiers
0° Linear Phase
Linear-Phase or constant group delay describe
a characteristic of linear systems where all the
spectral components of a signal travel through
the system at the same speed. In a particular case,
a linear phase system can be called 0°-phase:
all spectral components of a signal arrive at the
output at the same time.
In a system with linear frequency response
and 0° phase, the shape of the output signal is
ideally an exact replica of the input signal, where