A signal processing device uses a .DELTA..SIGMA. modulator having varying
effective orders to ensure an S/N ratio by selecting a high order when a
1-bit music signal is output via the .DELTA..SIGMA. modulator. The signal
processing device prevents noise during switchover by shifting to a low
order just before the .DELTA..SIGMA. modulator is bypassed if this
occurs. The present invention provides a digital signal processing device
which can switch between an original sound signal and a .DELTA..SIGMA.
modulation signal and yield a sufficient S/N ratio for a reprocessed
.DELTA..SIGMA. modulation signal. If any 1-bit original sound signal is
input, little switching noise is generated.