May 16, 2012

" It's radio, Drm, but not radio as we know it "


Digital Radio Mondiale is a broadcast radio standard designed to complement or replace legacy radio transmitters where broadcasters might require more stations, coverage area or signal quality than currently possible with existing analogue and digital radio systems.
 It uses an OFDM radio transmission scheme enabling  callsign ident, text, voice, music, data, moving & still images, in radio channel bandwidths from 2.5kHz (non-standard ham mode) thru 96kHz (DRM+ FM band digital radio). It can operate in a single frequency network (SFN) to reduce band congestion and work over HF paths – enabling continental coverage from as few as 2 or 3 transmitter sites.
I embarked on a series of experiments to test it out....
Although the mode is long established in Oceania, alongside AM on the HF broadcast bands, development of a viable consumer radio capable of reception has not yet reached the marketplace. Two practical means for hobbyists are adding converters to a traditional radio or running a SDR such as the Softrock, Elektor or FiFi kits, in each method an audio range IF signal is processed by a PC to demodulate the information.
Using some excellent open source software written for DRM, I started some real world testing beginning with converting various receivers and construction of an IQ modulator/exciter to better explore the mode's capabilities.
The main software is
DReaM (early versions include a basic DRM30 transmitter mode),
SoDiRa which has DRM+ and useful analog rx modes,
Spark a versatile transmitter program.
For narrowband DRM these programs operate anywhere within the 0-22 kHz PC soundcard range.

Assembling the rx chain using an existing radio is relatively simple, tap in at IF prior to filters then down - mix to the new souncard IF of 12 kHz – normally achieved with something like an NE602 mixer per various ccts. on the web. With availability of SDR kits, there is no need to modify a radio or build a convertor, but it will be necessary to control the reception with a keyboard and screen.
The Sony STR-VX20S I used has enough room inside for a Mini-iTX PC such as Intel Atom D525 to be installed, making it a fully functional box operated entirely with the original controls.





Audio from Sines, Portugal 15.440MHz - 12,000 miles away
https://www.dropbox.com/s/b15mubgmdgo5ttz/XRadio%20Shortwave%20Digital.ogg?dl=0

Audio from Issoudun, France 15.775MHz
https://www.dropbox.com/s/ph5mbrl40yowad6/DRM%20The%20Disco%20Palace.ogg?dl=0





For the test transmitter suite, I constructed a conventional IQ modulator using a hybrid transformer and capacitors to supply two 90 deg. split local oscillator feeds, a pair of SBL1-1 mixers, filtering and linear amplifier stages.
It is adequate for one-off testing purposes and the fine alignment is done in the PC setup. Best to use a dedicated PC for this so there are no interrupts and spurious audio tones to blow the transmitter out the window!


It soon became evident that the high crest factor of OFDM signals is a real handicap when using a regular 100 watt transmitter: the average power reaches 10 watts and saturation occurs.


 Looking for a suitable tube linear, it just happened that the first transistorised  marine coast station HF ssb transmitter from ZLW  still existed in storage. I initiated a mission to rescue it with assistance from Steve ZL2KG.






For a rating of 1kw PEP it is made up of 528 rf transistors mounted in blocks of 6, arranged in 8 PA modules, 324 of these are the finals.

Comparison results over a skywave path to Mike ZL1BTB, 250km downrange, in the 160m and 80m bands, at the same 50w carrier power levels, favoured the DRM signal signficantly over regular AM.
Mobile coverage is somewhat harder to evaluate due to the lack of any properly designed receivers in the market, AGC has an influence on the decoder performance so a mobile receiver may need to be more carefully engineered. Sony's discontinued models such as SW-7600G and SW-1 are a good reference design startpoint.
Obtaining modern radios with Shortwave has been a difficulty, however the 2010 onwards model range of JVC and Pioneer car radios includes some with SW bands 2.3 - 22 MHz, albeit with some gaps. Even the base "mech-less" model MVH-1450UB has SW band. These radios are processor based inside, so it is not possible to decode the DRM signal externally, One must wait for the manufacturers to include the available baseband decoder chips within. A most up to date place for developments is the DRMNA blogsite 
drmnainfo.blogspot.com/