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Thursday, May 17, 2012

RTLSDR used for monitoring a trunked system

Back in mid-March, I ordered two of those "ezcap" DVB-T USB sticks from an overseas place called dealextreme. I did this because a friend told me that someone had discovered a way to make them spit out raw I/Q data that would be perfect for use with gnuradio. Given that it was only $20 and a USRP costs at least $700, it was worth a try.

That site then proceeded to sit on my order for five weeks, and only after having me yell at them did anything happen. They finally claimed to ship it, and then it just disappeared again. I went out of town for a little while and worried it would show up at my doorstep and sit there for days.

Well, today, they arrived. Despite their claims of shipping the items on April 25, Singapore Post only sees it physically entering their system on May 9th. Now, here they are, 8 days later.

As for the actual device, so far it seems like a nice little toy. I've been able to tune a few local signals and they sound okay but not great. It's possible to make out what people are saying, but there can be a significant amount of scratchiness which just isn't there with my USRP. Both systems are running in the same place indoors, so I suspect a combination of the antenna and receiver hardware itself.

People have been asking me if this will work as a replacement for a USRP when monitoring a trunked system. The good news is that my early results are actually promising. There are a bunch of CRC errors on the Smartnet decode channel, but it's really early in the process. I've only done the smallest amount of fine-tuning so far, and there are a bunch of things which need to be tweaked.

This thing has a particularly large tuning error on the order of 61.5 kHz, and even that is a guess. I'm going to have to play with it some more to see if it will give cleaner results.

You can compare it for yourself. First, the results of a $1000+ USRP 1 + DBSRX2 combination with a whole lot of fine-tuning:

Next, the results from a $20 USB stick which has been pressed into SDR duty with hardly any fine-tuning applied:

Not too shabby! More information on this cheap little receiver can be found at /r/RTLSDR on reddit.