A radio communication system where the modulating and demodulating components that have typically been implemented in hardware are instead implemented using software on a personal computer or other embedded computing devices using by a converter. While the concept is not new, the rapidly evolving capabilities of digital electronics are making practical many processes that were once only theoretically possible!

A basic MDSR(SDR) may consist of a computer (PC) equipped with a sound card, or other analog-to-digital converter, preceded by some form of RF front end. Significant amounts of signal processing are handed over to the general purpose processor, rather than done using special-purpose hardware. Such a design produces a radio that can receive and transmit a different form of radio protocol (sometimes referred to as a waveform) just by running different software!

The MDSR software performs all of the demodulation, filtering (both radio frequency and audio frequency), signal enhancement (equalization and binaural presentation). Uses include every common amateur modulation: morse code, single sideband modulation, frequency modulation, amplitude modulation, and a variety of digital modes such as radioteletype, slow-scan television, and packet radio. Amateurs also experiment with new modulation methods: for instance, the DREAM open-source project decodes the COFDM technique used by Digital Radio Mondiale and compatible stations.

Principles

The ideal receiver scheme would be to attach an analog-to-digital converter to an antenna. A digital signal processor would read the converter, and then its software would transform the stream of data from the converter to any other form the application requires.
An ideal transmitter would be similar. A digital signal processor would generate a stream of numbers. These would be sent to a digital-to-analog converter connected to a radio antenna.
The ideal scheme is, due to the actual technology progress limits, not completely realizable, however.

Architecture

Most receivers utilize a variable frequency oscillator, mixer, and filter to tune the desired signal to a common intermediate frequency or baseband, where it is then sampled by the analog-to-digital converter. However, in some applications it is not necessary to tune the signal to an intermediate frequency and the radio frequency signal is directly sampled by the analog-to-digital converter (after amplification).

Real analog-to-digital converters lack the discrimination to pick up sub-microvolt, nanowatt radio signals. Therefore a low-noise amplifier must precede the conversion step and this device introduces its own problems. For example if spurious signals are present (which is typical), these compete with the desired signals within the amplifier's dynamic range. They may introduce distortion in the desired signals, or may block them completely. The standard solution is to put band-pass filters between the antenna and the amplifier, but these reduce the radio's flexibility - which some see as the whole point of a software radio. Real software radios often have two or three analog "channels" that are switched in and out. These contain matched filters, amplifiers and sometimes a mixer.

About the Author of DADP and Bi-LIF projects

Alex Schwarz (VE7DXW) is an advanced HAM and a graduate of the HTL, Innsbruck. He moved to Vancouver (Canada) in 1990 and has since been involved in professional communication systems (LDR trunking) and digital point to point wireless network systems. In 2005 he started work in the Biomedical Engineering Department at C&W Hospital in Vancouver. He can be reached through his email address: [email protected]

Contact and URL info

Alex, VE7DXW: mail

Please feel free to join our MDSR support and discussion forum at: https://groups.io/g/MDSRadio

The official URL of MDSR: http://users.skynet.be/myspace/mdsr

Please join the Scistarter RF-Seismograph group "Sience we can do together": SciStarter RF-Seismograph

About Alex VE7DXW and MDSR @ QRZ

Support

Please donate to the MDSR user group. So far all the work to create the LIF converter, the DADP software and the DADP-SA was done by volunteers. Our aim is to provide a cost effective alternative to expensive commercial SDR radios. The website is designed and hosted by Guy Roels (ON6MU) and the application programming is also done by Guy. All the hardware was paid for by Alex Schwarz (VE7DXW) and he has spent countless hours writing the DADP audio processor software. The whole project runs on a shoestring budget. Your donation shows that there is support for this kind of development and it enables us to continue our research and development of the MDSR concept into a product that all amateur operators can use.
Many thanks in advance!

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Everybody is welcome to join our MDSR Forum & group. The software and the documentation are available to everybody at no charge as long as the use is for noncommercial amateur radio communications.

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