Implementation of a digital loudspeaker system.
A digital loudspeaker converts digital signals into quasi-analogue sound pressure without an embedded
digital to analogue converter. This work reports the findings for two types of digital loudspeaker; a
transducer array and a multiple voice coil loudspeaker.
The analogue to digital conversion process produces bit streams that have a much wider frequency
range than the original analogue signal. During the digital to analogue re-conversion the amplitudes
and phases of the additional harmonics cancel to reproduce the original analogue signal plus a small
amount of digitally generated noise.
A digital transducer array loudspeaker was proposed in which each transducer carries the same weight
as one least significant bit and the number of transducers switched on during each sample interval is
equal to the binary representation of the analogue input. Computer simulations show that correct
analogue acoustic reconstruction can only occur at one listening position. At all other positions the
different propagation paths for sound from each transducer to the listener disturb the relative
amplitudes and phases of the additional digital harmonics so that they do not cancel, resulting in
significant distortion. Although some methods are predicted to minimise this distortion, it is too large
for high fidelity applications.
A prototype seven bit multiple voice coil digital loudspeaker has been constructed and evaluated. The
acceleration of the diaphragm is directly proportional to the radiated sound pressure, in common with
conventional loudspeakers. In order for the force on the diaphragm to correctly mirror the digital
input, the current in each coil should correspond to the binary significance of each bit in the digital
representation, requiring the use of a switched constant current driver. Moreover, since each coil is
wound on the same motor unit, motion of the diaphragm will induce an electro-motive force in each
coil, and the close proximity of the coils to each other means that pairs of coils have a high mutually
inductive coupling. It was found that the effective input impedance of a coil depends on the binary
significance of the current it is carrying. The current in the low significant coils is most likely to be
corrupted unless the output impedance of the current driver is very high indeed.
It was found that with the current state of transducer technology a crossover is needed for a digital
loudspeaker in order to reproduce the entire audio frequency range. Although the natural choice for
implementing a crossover is undoubtedly with digital signal processing, it was found that analogue
filters placed between the drivers and the transducers could perform the same function. This is a direct
consequence of the associativity of the Fourier transform and the digital to analogue operation. The
signals fed to each frequency selective loudspeaker driver unit are no longer truly digital, however
their sum over the entire audio frequency spectrum does remain digital.
The construction of a high resolution digital loudspeaker presents some problems for manufacturing.
The use of bit reduction techniques e.g. oversampling and noise shaping may reduce the number of
transducers required in an array or the number of coils needed for a multiple voice coil loudspeaker.
The latter approach appears to be promising for the implementation of a fully digital loudspeaker