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1. Introduction to the DSP-59+
The DSP-59+ is an audio noise filter for amateur radio voice, data and CW operation.
The DSP-59+ filters and reduces noise and interference to improve radio reception.
The DSP-59+ uses digital signal processing technology to implement algorithms that
perform four basic functions: 1) Random noise reduction, 2) Adaptive multi-tone notch
filtering (Tone noise reduction), 3) Bandpass/Highpass/Lowpass filtering, and 4) RTTY
remodulation.
Random/Tone Noise Reduction
The noise reduction functions of the DSP-59+ operate by examining a characteristic of
signals and noise called correlation, and dynamically filtering out the undesired signals
and noise. The degree of correlation is relative. Random noise such as white noise or
static is uncorrelated. Speech is moderately correlated. Repetitive noise such as a
heterodyne is highly correlated. The DSP-59+ measures correlation and filters out
signals and noise that are outside its correlation thresholds. The amount of noise
reduction varies according to the correlation characteristics of the noise. Typical noise
reduction ranges from 5 dB to 20 dB for random noise and up to 50 dB for heterodynes.
Highpass/Lowpass Filters
The DSP-59+ has 15 highpass and 15 lowpass filters that are independently selectable
from front panel controls. There are many uses for the 225 combinations of highpass and
lowpass filters that the DSP-59+ offers. In a typical example of a voice mode
application, highpass and lowpass filters can improve a signal with a poor signal-to-noise
ratio. The independent highpass and lowpass filters remove the low and high audio
frequency components that do not contribute significantly to the speech intelligibility,
thus improving signal quality. Another common voice mode example is the
improvement of a SSB signal corrupted by adjacent channel interference (QRM). The
steep skirts of the highpass and lowpass filters allow the high side and low side
interference to be eliminated independently with minimal impact on the desired signal.
Although the DSP-59+ has bandpass filters for CW and the most common data modes,
the 225 selectable highpass and lowpass filter combinations also allow precise filtering
for modes such as wideshift RTTY. The DSP-59+ highpass filter adjustment range is
from 200 to 1600 Hz. and the lowpass range is from 1700 to 3400 Hz.
Bandpass Filters
The DSP-59+ has 316 bandpass filters that are used in data and CW modes. Narrow
band signals like CW and RTTY require bandpass filters with steep skirts and linear
phase response. Linear phase response maximizes the usable signaling rate for a given
bandwidth and minimizes ringing often heard on extremely sharp filters. The DSP-59+
has 195 CW filters with skirts so steep that a signal literally falls off the edge of the
passband as you tune through a CW signal. Bandwidths for these filters range from 25
Hz. to 600 Hz., and center frequencies from 400 to 1000 Hz. The narrow filters are
useful for trying to dig out extremely weak signals from the noise and QRM. The wider
filters allow easy tuning and listening to multiple CW signals simultaneously. Three
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