DSP Crossover design
Introduction
Recently I started an investigation into the design of FIR
(Finite impulse response) digital crossovers. For me this
represents a way of removing many of the constraints facing
conventional loudspeaker design. In this project I am attempting
to compensate for non-ideal aspects of the drivers, including
irregularities in SPL response and phase response.
For an example of how FIR filters can be used to correct the
Impulse response of a midbass driver click here.
My approach to the design is similar to that used for Analogue
crossovers. It involves specifying a 'Target Acoustic Response'
in phase and amplitude and creating a digital filter to realise
it.
The main advatages of FIR digital crossovers that I see
include:
- Possible to apply accurate equalisation of most non-ideal
driver charastistics, including delayed resonances that
show up in frequency response and time domain
measurements.
- Possible to include pure time delays to bring the
acoustic centres off all drivers into alignment in the
time domain. This means the target response can be more
accurately achieved giving optimised vertical dispersion
and imaging.
- Zero nonlinear distortion is introduced to the signal as
a result of equalistion.
- Phase distortion can be reduced or possibly elliminated.
- Relatively steep crossover slopes can be used with very
little impact on transient performance. The use of steep
slopes allows improved power handling, reduced
Intermodulation distoriton, and better controlled
off-axis response.
The following disadvantages and limitations also exist:
- It is not practical to use FIR filters for equalisation
of bass systems, as this will require massive amounts of
DSP computation power.
- Some digital crossovers implementations suffer
pre-ringing on transients.
- It not easy to make a low noise volume control. Digital
volume controls genearally increase noise and reduce the
available dynamic range at low volume settings.
- Greatly increased cost and complexity as a seperate DAC
channels, amplifier channels, and cabling are required
for each driver used in the system.
- Care are must be taken with amplifier and cable
connections as there may be no passive circuit elements
to protect the drive units from signals that are outside
their intended frequency range.
Works in progress
DSP
Project One - 3way Digital Crossover Design
DSP
Project Two - 3way Digital Crossover Design (Actually a
revision of the above - Updated 30/3/02)
Comparison
of Simulated Off-axis response 96dB/oct FIR vs 24dB/oct Linkwitz
Riley
Comparison
of CD Player Square-wave response. Oversampling vs
Non-Oversampling types
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