DSP Audio Noise Reduction
"Noise Reduction for the G6LBQ Irwell HF Transceiver"
For some time now, I have been developing a high-performance HF multi-mode radio transceiver based on a series of modules that can be upgraded or improved over time.
The modular approach has some great advantages and makes it easy to upgrade the transceiver or carry out repairs. There is, however, a negative effect of adopting this approach in that I find myself constantly changing things and wanting to try different ideas and ways of doing things.
Suffice to say my "Irwell HF Transceiver project" is very much a labour of love and a long-term project that is often sidelined while I moonlight on other projects.
I am for ever the optimist that my "Irwell HF Transceiver" will one day be finished so I will continue to add new additions and make improvements!
I want to start documenting the "Irwell HF Transceiver project" on my blog but be warned it won't be in any particular order or a complete build guide. There will, however, be sufficient information provided to build each module and I hope some of the modules may be useful for your own transceiver projects.
Project files will be made available via the Groups.io platform by joining my G6LBQ community group where you can discuss my projects, ask questions and help others.
Joining my group is free. Just click on the button below.
For my first "Irwell Transceiver" blog post, I have decided to look at DSP audio noise reduction. Listening to noisy HF bands for any length of time has a tendency to bring on listening fatigue so rather than taking regular breaks away from the radio or having a nap in-between QSO's I decided to look at what could be done to ease this phenomenon electronically.
My initial thoughts were to consider phase noise reduction to try and cancel out unwanted noise and QRM before it enters the radio receiver. Commercial phase noise cancellers are available from manufactures including MFJ and Wimo, or you can purchase a kit to build one yourself. Phase noise eliminators require two antennas that can produce equally strong signals in order to get good results but they can only null out one source of interference at a time. This is not a practical solution for myself so I have ruled it out as an option.
For your interest here is the schematic diagram for the X-Phase QRM Eliminator that is commonly available in kit form on the internet.
Click image to enlarge to full size!
I looked online for other methods to aid the elimination of noise and interference and came across some interesting articles that use a Teensy USB microcontroller development board to carry out "digital signal processing" or DSP for short.
The Teensy can be programmed in the Arduino Intergrated Development Enviroment (IDE) and there are libraries and a neat Teensy Audio System Design Tool to simplify programming and development. While the Teensy is an affordable and interesting option, I have decided to forgo this option to save time getting to grips with software and the inevitable learning curve.
For DSP noise reduction, I have decided to use an off-the-shelf product designed and manufactured by a UK company called BHI. The company was founded in 2002 and specializes in DSP noise reduction products for amateur radio, commercial, medical and military markets.
Amongst the products BHI sells is a Noise Eliminating DSP PCB module suitable for incorporating into existing equipment. This is ideal for my transceiver build, though it does costs an eye-watering £119.95 plus shipping so it is not a cheap purchase!
Here's an image of the NEDSP1901 PCB module I recently purchased from BHI
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The DSP module has eight levels of noise reduction; these are selected by applying a BCD binary code to three PCB jumpers or via a set of PCB header pins. I decided that it would be neat to build a dedicated controller for the BHI module so that the eight levels of DSP could be selected with a rotary encoder and the relevant information conveyed on to a small display.
I breadboard a simple DSP controller and cobbled together some Arduino code to programme the DSPs BCD inputs and this works well and is proving reliable under test.
Here's an image showing the controller's 0.96" OLED display during the boot process.
Next is an image of the completed breadboard prototype. Here you can see the small OLED display showing the DSP noise reduction is set to level 4. The rotary encoder provides a convenient method of control to step through the 8 levels of DSP or set the module in an off or bypess mode.
The schematic diagram for my BHI DSP control module is shown below.
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Click image to enlarge to full size! |
During testing the controller has behaved well and there are no issues to address. The BHI noise reduction module does a great job of distinguishing between speech and noise interference and works especially well on weak signals.
Introducing the DSP & controller modules into the Irwell Transceiver is a simple matter of plug n play. During the development of the Irwell HF transceivers audio subsystem I made provisions on the PCB to facilitate adding additional audio filtering and processing circuits. I will be covering my Irwell Transceivers audio subsystem in a separate blog post soon!
To complete the project, I now need to transfer the design from the breadboard and onto a PCB. I spent some time drafting up a PCB with my CAD software then sent the requisite Gerber files to the JLPCB fabrication plant for manufacture.
Once I get the PCBs back from fabrication, I will build up a module and show you the end result in an update to this blog post. I should have the PCB designed, manufactured and back to me in around two to three weeks so be sure to check back again soon!
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| 20th March 2021 |
I was hoping to have the PCBs back from manufacture and a module built before the end of March but that is no longer going to be the case. I decided to hold off on ordering the PCBs while I finish the CAD work on some other boards that I need. This will save me wasting money on multiple shipping costs.
For now I thought I would share an image of the controller PCB that I have created and exported from the CAD software.
I will still be ordering the PCB's before the end of March so you can expect a further update soon.
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| 6th May 2021 | |