MDSR (modulator-demodulator software radio) is a SDR (software defined radio) project. Using your own receiver and an inexpensive converter and a PC, you can experiment with some fairly sophisticated stuff. And the same hardware can be used to receive DRM (Digital Radio Mondiale)

MDSR has 2 converters available, a receive only and a transceiver. I used the receive only converter in my project. It comes as a small PCB and parts. It uses a SA602 converter chip. The local oscillator resonator supplied in my kit was 480 kHz, resulting in an output of 25 kHz rather than 12 kHz. This is not a problem for MDSR but I wanted to use the setup for DRM as well, which specified a 12 kHz input, so I went looking for a 467 kHz resonator. There was none to be found, and I spent a lot of time looking. A quick and dirty solution was to use my N3ZI DDS2 frequency generator set to 467 kHz. This is a really neat, inexpensive DDS signal generator (0.1 to 32 mHz) whose output is just right for the converter. You just have to remove the resonator Cf2 and the trimpot C12, and feed the signal into TB5 via C19 (see converter schematic here (PDF file)).

system block diagram

My radio is an ICOM 746PRO, and there is no 455 kHz output provided. I was able to obtain it by tapping off an unused winding in one of the 455 kHz IF stages, L254 on the main unit. I used the lower tap, as the output at the higher tap outputs more than you need. Once you locate the right transformer, solder a suitable piece of coax and bring it out to the back somewhere.

MDSR is able to control the radio using software called Omnirig (included with MDSR), and a USB to CIV cable. I obtained this cable inexpensively from an ebay vendor. My home-brew PC is a few years old, nothing special. It has an ASUS motherboard with a 2.5 GHz dual-core processor and uses the motherboard sound adapter. I dual-boot Windows XP and Widows 7 (both home versions).

You can find an article on MDSR in the July 2012 issue of QST, by Alex Schwarz, VE7DXW.

To decode DRM I use the freeware Dream program, which runs fine on Windows 7. It is a surprisingly good program, which can decode SSB and other modes as well as DRM. I was amazed when the first time I started it up it decoded a DRM broadcast right away. MDSR requires some fussing to get it set up; make sure you read the manual. MDSR programs and documentation are downloaded from their Yahoo Groups site.

MDSR - DRM setup

The unit on the left is the DDS generator. The MDSR converter is on the right. They both share a 12V wall wart power supply.

DRM is a very nice broadcast mode. Unfortunately is has not caught on, and here in central Canada there aren't many broadcasts available. There appear to be more in Europe and Asia. DRM provides a worldwide broadcast schedule here.

Update Nov. 2014 - Alex VE7DXW released a bulletin suggesting the use of an external Si570-based oscillator for the LIF. The purpose is to accomodate rigs that do not provide 455KHz IFs. Here are his instructions for interfacing the external oscillator (click for larger image):

connect external oscillator to LIF

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