DRMradio

 

 

 

Cyclic prefix correlation for Mode A and Mode B (results for the other two modes are not shown for clarity).

 

Any correlation between the two intervals shows up as a dip and if the mode parameters used match the received mode then the distinctive pattern seen for Mode B should appear. The software looks for these dips as they should occur at known intervals (symbol length). Then an average is taken and a decision is made as to whether this is an OFDM signal. If the software concludes that this is a DRM broadcast then the spectrum already calculated by the FFT routine is displayed, this is used to visually confirm that the broadcast is DRM.

 

With multi-path propagation the distinctive correlation symbol pattern is altered with multi-path inducing small time variations between symbols. To compensate for this DRMscan has a slider on the main form which determines how many symbols need to show exact timing relationship to be recognised as OFDM.

 

running DRMscan program

 

If the DRMscan program is active then attempting to run the G303i control software will produce a ‘No receiver found’ and will run in demo mode.

 

 

screen of DRMscan in operation.

 

 

Screen of the DRMscan program is shown, after the scan button is pressed the program checks to see that a G303i is installed and the PCI radio’s serial number is displayed. Spectrum of the last found DRM is displayed and is added to the list of DRM stations found. The program has an internal database of what frequencies the various DRM transmitters broadcast on.

 

When the scan is finished clicking on any frequency in the list will tune the G303i to that frequency but as the Winradio G303i control program cannot also be active it requires an external DRM decoder (e.g. Dream) to be running if you want to decode the DRM broadcast. The +/- 5 kHz button allows the receiver frequency to be adjusted as the software may recognise OFDM on only half the signal bandwidth if it is a strong DRM signal.

 

The slider on the DRMscan form alters the required inter symbol threshold timing and sets the number of symbols that need to show the exact timing for the data to be considered as OFDM. Each captured block will have around 12 symbols (the actual number will vary as the start point of the captured data is arbitrary) and the percentage displayed is the symbol threshold for OFDM recognition. The slider value is read before each new data capture so the value can be altered during the scan.

 

There is a trade-off with this setting, low percentage numbers will mean that more stations are passed to the DRM recognition routines and will increase the chances of finding weak DRM signals but will produce more false readings. Setting the slider higher will tighten the timing parameters within the OFDM recognition routines so that only the strong DRM stations with little multi-path will be listed, possibly missing valid DRM broadcasts. Radio propagation conditions are changing all the time so a second scan with a different setting may produce a slightly different list.

 

At night time when some of the bands gets noisy and interference becomes more prevalent then false readings are more likely to occur. At any point in the scan a weak DRM signal may be undergoing fading and DRMscan could miss that broadcast. Despite the possibility of a few errors it is much quicker to run the program than to step through the frequencies manually.

 

Where the OFDM decision is close between the received signal being AM or DRM then the program assumes a DRM signal - I would rather have a few spurious results but listing all DRM stations rather than run the risk of missing a DRM station. Remember that the software is making a decision on less than 1 second of sampled data unlike DRM decoders that are continually assessing the data but may take 5-15 seconds to achieve DRM recognition.

 

 

WiNRADiO, G303 are registered trademarks of Rosetta Laboratories Pty. Ltd. trading as Winradio Communications.

 

 

 

 

 

 

For a more detailed explanation of how the software works please read my DRMscan article which was published in radiouser magazine February 2006 and March 2006.