Digital Signal Processing - Introduction

In General 'Digital signal processing' is about manipulating AF and/or MF signals to improve readability of any voice / morse or data signal. The DSP-device is mainly a micro-processor with specially designed software. A DSP makes use of computer controlled digital signal processing technology to reduce Random/Tone and Noise interference in incoming signals. It implement the computer controlled algorithms to perform its functions.

Normally a DSP-Unit perform four different types of basic functions
1) PASS-BAND Filtering - QRM/Noise reduction by restriction of Audio band-width
2) Random Noise Reduction - Static / Pulse noise reduction
3) Notch Filtering - Automatic Multiple Notch Filtering of unwanted Heterodynes
4) Remodulation

How the DSP works
The signal is feed into the device and the characteristic of the signal and noise is analyced using a method called 'correlation'. The DSP measures degree of correlation and filters out wanted signals from noise and Heterodyne tones, this is all done in 'real-time'.

The DSP process - DSP enhancement is basically a four-step process. Audio input is sampled several thousands of timed per second, the frequency and amplitude of the audio-signal is converted into a digital representation of the analog waveform by the A/D (Analog-to-digital) converter, the result is an digital bit stream of raw data that can be analyzed and processed by the Digital signal processor.

Notch Filtering - With the notch (multiple notch), DSP circuitry examines the AF passband and the correlation of the signals present. After correlation parameters are compared, un modulated signals (heterodynes) are identified and filtered out using Narrow-band attenuations,- That is a 'notch-filter' at center of heterodyne frequencies. As the DSP dynamically checked the audio-signal in the pass-band, new heterodynes will be identified and automatically notched out, one by one, as they appear!.

DSP and CB/HAM usage
Over to the practical use and DSP-Implementations. The Digital Signal Processing devices can be devided into tree major categories.
Built in Digital Signal Processors (Modern HF-Radios)
External Hardware Digital Signal Processors
PC Software Digital Signal Processors
On this page i try to describe all tree types,- i am at the moment using type 1 & 2 by myself,- and find them useful in most situations. Especially with low signals and high noise / interference levels.

Audio Spectrum analyses
To illustrate what the DSP does with the Audio signal,- i cut and paste 'screen-shots' from the spectrum-analyse window in the DSP-Filter software,- this is to get real readout of the audio spectrum after the actual DSP Processing,- when testing the software is set to 'TH' thru mode for the audio to pass strait thru (Except when getting screen-shots when DSP Filter v1.11 at work in the last section)

PURE NOISE
This is what we start with,- pure 'pink-noise' from the radio RX-Audio.

Audio OUTPUT when no DSP is active

HETERODYNE
To illustrate the Auto-Notch filter i inject a simulated heterodyne from a cb-transmitter on close frequency, transmitting an un-modulated AM-Carrier to produce an significant heterodyne @1200Hz in the pass-band spectra.

Un-filtered Audio spectra with heterodyne @1200Hz

More on the DSP subject on the net.
Here is a nice 'digest' about different DSP usage - Subject: DSP digest of answers

Type 1 - Built in DSP

DSP-Controls on the FT-1000MP

Most modern HF (HAM)-Radio stations got some type of built in DSP function,- early models mostly work with the Audio Signal (Speaker-sound),- This make this type of DSP-future not al that different from what can be done with the type-2 external DSP-units,- there is also some HF-Stations that got built in DSP that work with MF-Signals,- this is more sophisticated and more expensive

FT-1000MP / Built in DSP
The DSP-Unit inside my Yaesu FT-1000MP is one of the first on the market,- it operates with Audio Signal. It got four pre-programmed audio-profiles called 'Contours' and four different levels of Noise reduction numbered 1 to 4,- to this day I do not know what this different levels of noise reduction actually do,- the four audio-profiles however are:
1) BPF Band-Pass (Narrow) - QRM rejection
2) LCF Low CUT - High-Frequency emphasis
3) MFC Mid CUT - High & Low Frequency emphasis
4) HFC High CUT - Low Frequency emphasis

The EDSP portion from original FT1000MP manual (in 2.75Mb large .pdf file)

FT-1000MP DSP - Audio Spectrum analyses these Audio Spectrum diagram curves illustrates the DSP PASS-BAND Filter function in the FT-1000MP - other functions such as Random Noise reduction and multi-tone notch filtering is hard to show with diagrams.

PASS-BAND Filter

Audio OUTPUT with user 'Tailored' BAND-PASS (BPF) filter low-cut@ 320Hz and High-cut@ 2000Hz

Audio OUTPUT with user 'Tailored' BAND-PASS (BPF) filter low-cut@ 500Hz and High-cut@ 1800Hz

User 'Tailored' BPF Settings
On the FT-1000MP the BAND-PASS Filter (BPF) can be user 'Tailored' by mode. The default settings for SSB-Operation is NOT optimal when working on CB-Band with heavy QRM/Interference. I Prefer to set the BPF in a more 'Narrow' fashion. I use Low-Cut@ 380-420 and High-Cut@1800-2000. In the FT-1000MP this is set in Menu item no# 4-5 (SSb-HPF @ SSb-LPF)

Auto Notch Filter

Audio OUTPUT with Auto notch@ 1200Hz

The Auto Multiple Notch Filter
After using my FT-1000MP for quite some time, i discover that the default DSP settings for the Auto Notch filter is set to 'Manual' - this does that the Auto Notch filter is DISABLED, also when the 'EDSP' is active! To fix this,- Enter menu item no# 2-9 and set the value to 'Auto-DSP' this does that the Auto Notch filter function is activated when both the 'EDSP' and 'Notch' switches is on.

The FT-1000MP uses a 16-bit NEC uPD77016 CMOS Digital signal processor

Type 2 - External DSP-Units

The TIMEWAVE DSP-9+

TimeWave DSP 9+ / External DSP
The TimeWave DSP 9+ Digital Noise filter for amateur radio voice, data and CW operation. The DSP 9+ filters and reduces noise and interference to improve radio reception.

How it works...
The TimeWave DSP+9 makes use of computer controlled digital signal processing technology to reduce Random/Tone and Noise interference in incoming signals. It implement the computer controlled algorithms to perform four basic functions:

1) Random noise reduction
2) Adaptive multi-tone notch filtering (Heterodyne TONE noise reduction)
3) Bandpass filtering
4) RTTY remodulation

Al these can be used one by one or al together in numerous combinations.

The signal is feed into the device and the characteristic of the signal and noise is analyced using a method called 'correlation'. The DSP+9 measures degree of correlation and filters out wanted signals from noise and Heterodyne tones, this is all done in 'real-time'.

Article with a test of the TimeWave DSP 9+
The Complete TimeWave DSP 9+ Operating Manual / Circuit Drawings (in 5.8Mb large .pdf file)

- TimeWave DSP 9+ - Audio Spectrum analyses
These Audio Spectrum diagram curves illustrates the DSP PASS-BAND Filter function in the DSP 9+ - other functions s.a. Random Noice reduction and multi-tone notch filtering is hard to show with diagrams.

PASS-BAND Filter

Audio OUTPUT in 'Medium' mode

Audio OUTPUT in 'Narrow' mode

Auto Notch Filter

Audio OUTPUT with Auto notch@ 1200Hz

Type 3 - Software DSP

The DSP-Filter Ver1.11 Software
The author of this software describe it as 'just experimental', I find it however quite useful, and verry easy to get 'up-and-running' - In my setup i use a 900MHz Intel-Celeron PC with 512Mb Ram, and an SB-Compatible 16-bit soundcard,- the program runs fine! with a merely noticeable delay. Finally,- This software does NOT have any 'Auto-Notch filter' function.

There is however other more developed PC Software for HAM DSP-Purposes.

The DSP-Filter Ver1.11 Software

Notice that this software in 'Freeware', other software may not have this 'nice' licensing policy.
Download DSP-Filter Ver1.11 (281Kb .zip file)

The following text is included from the distribution .zip file DSP filter Version 1.09J - June 18, 2000 JE3HHT Makoto Mori Translated into English by JA7UDE Oba

This is a DSP filter tool using a PC with the soundcard. With this tool, you even can design various types of digital filters including adaptive filters. However, this tool is just experimental and will not afford the practical use for amateur ham radio. You probably need a powerful CPU to make this tool run flawlessly. In addition, you need a soundcard with the full-duplex mode.

I made this program just because of my own interest. As I was not quite familiar with the use of the soundcard, it still has substantial time lag from input to output and might not well work for CW. Needless to say, this program is freeware.

NOTE This Also! Since this program intensively uses floating-point operations and is not well optimized, it will not run on a PC with a slow CPU.

Audio Spectrum analyses

These Audio Spectrum diagram curves illustrates the DSP PASS-BAND Filter function in the DSP-Filter v1.11 software.
 

Audio OUTPUT with preset BAND-PASS filter 2.2KHz


Audio OUTPUT with user 'Tailored' BAND-PASS filter low-cut@ 500Hz and High-cut@ 1800Hz