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Timewave DSP-59
DSP Audio Filter


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Timewave DSP-59

The Timewave DSP-59 is an audio noise filter developed and optimized for the radio amateur market. The DSP-59 filters noise and reduces interference to improve radio reception. The DSP-59 uses digital signal processing technology to implement algorithms that perform five basic filter functions: 1) Random noise reduction, 2) Adaptive multi-tone notch filtering (Tone noise reduction), 3) Highpass filtering, 4) Lowpass filtering, 5) Bandpass filtering. Switch-selectable combinations of these basic functions make up the five operating modes of the DSP-59.

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. There is little degradation of the desired speech signal. 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.

There are many uses for the wide selection of Highpass/Lowpass filter combinations that the DSP-59 offers. For instance, a broadband SSB audio signal may be very difficult to copy because of a poor signal-to-noise condition. Removing the high frequency components of the baseband that do not contribute significantly to the speech intelligibility with a Lowpass filter will remove noise and therefore improve signal quality. Another example of how the flexibility of the DSP-59 audio filter can improve baseband audio performance is with SSB signals corrupted with in-band and adjacent channel interference from other signals that overlap the desired signal. The steep skirts of the Highpass and Lowpass filters allow the interference to be eliminated with minimal impact on the desired signal. Finally, the sixty-four selectable Highpass and Lowpass filter combinations also allow enhanced filtering for data modes such as packet, wide shift RTTY or facsimile. The Highpass filter adjustment range is from 200 to 1500 Hz. and the Lowpass range is from 1800 to 4200 kHz.

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 fifty-six CW filters with skirts so steep that a signal literally falls off the edge of the pass band as you tune through a CW signal. Bandwidths for these filters range from 50 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. The DSP-59 also has four RTTY/AMTOR bandpass filters centered at 2210 Hz. The selectable bandwidth of this filter provides optimum filtering for 170 Hz. and 200 Hz. shift RTTY signals of various baud rates.

The audio input of the DSP-59 is the 0.25 inch INPUT connector on the rear panel. The DSP-59 requires an audio source capable of supplying 300 mV to 3 volt rms into a 10k Ohm load. The mating connector to the DSP-59 audio input is a 0.25 inch mono phone plug. The tip of phone plug carries the signal, and the sleeve is ground return for the audio input only. Do not connect the audio output ground return or the power supply ground return to the audio input phone jack. An improper ground connection may result in degraded performance of the DSP-59.

The DSP-59 requires a power source of 12 to 16 Volts DC at 500 mA. The center pin of the power connector is positive (+).

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