Using Multiple RTL2832U Sticks for SDR
Since RTL2832U sticks are so cheap, why stop at one? I haven’t seen this mentioned anywhere before, but you can indeed use several RTL2832U sticks simultaneously. Apparently, the ExtIO.dll files used with SDR Sharp and the 3rd party ExtIO.dll for HDSDR will allow you select from several RTL2832U sticks plugged into your PC which can be used by the SDR software. So in other words, you can start multiple instances of SDR Sharp or HDSDR and select which individual RTL2832U each running program will use. For example, you can run three copies of SDR Sharp or HDSDR with each one using an individual stick receiving a different band. How many RTL2832U receivers can you run simultaneously? I don’t know for sure. It probably would come down to how many USB ports you have available and the processing power of your computer. I had no problem running three sticks and three simultaneous instances of SDR Sharp or HDSDR on a 2.4GHz dual core machine. I was able to run several more non SDR applications at the same time, so there was plenty of overhead left even when using this very modest machine.
Now that we have established that this can be done, why would we want to do it? I am sure you can think of several reasons, but here are a few of my uses:
- Monitoring a larger chunk of bandwidth simultaneously. Lets say that each RTL2832U stick can show roughly a 2.4MHz swath of frequency or more. Using two sticks would give you 4.8MHz of coverage, three sticks would give you 7.2MHz and so on. You could start two or three instances of the SDR software and simply overlap the tuning for each instance when using the same band. However, there are some issues with this. Since each chunk of frequency to be monitored is running and shown in a separate window, you will have to select the frequency you want to monitor from that window. The other problem is that all the running SDR windows share the same sound card, so you may need to mute or un-mute audio from the running instances you don’t want to listen to. On the other had, is you may want to monitor the audio from several signals at once.
- Use a dongle to monitor FM/VHF/UHF frequencies plus a second dongle used with an HF converter like the Ham It Up converter that was reviewed here in an earlier article for HF coverage.
- Use two dongles connected to a Ham it Up Converter via an antenna splitter to monitor two separate ham radio bands simultaneously. For example you could use one SDR instance to monitor the 40M band and a second instance to monitor the 20M band. Now this something you are not going to see everyday with a total price tag of less than $100 in hardware costs!
- Use several dongles to route audio to digital decoding programs using Virtual Audio Cable (VAC). Virtual Audio Cable allows you to route audio to separate decoding programs while using one sound card. So you could do something like have one dongle monitoring ham radio audio broadcasts while another dongle is sending data to a PSK31 decoding program. Hardcore CW Dxers could conceivably use a couple of dongles and HF Converter with CWskimmer to monitor a couple of CW bands simultaneously for DX.
I am sure that you can dream up other applications!
Getting it to Work
One thing to note here, is that it does not appear to matter if your RTL2832U dongles use the same chipset. In my setup, I used two E4000 based dongles along with a R820T based dongle with no issues. I would also suggest that you try this using SDR Sharp first, since it appears to be a bit more forgiving when it comes to working with multiple dongles. Setting this all up is pretty simple, but I am going to throw in a few observations along the way.
Install the Zadig drivers for each dongle
You will need to install the Zadig drivers for each individual dongle that you will plug into your PC. I would suggest you just plug in one dongle at a time and install the Zadig driver for it on the USB port you used. Typically each stick will show up as a “RTL2832U” or “Bulk” device. As you add more sticks it may get a bit confusing as to what device is what. A work around with the Zadig installer is that you can “List All Devices” in a drop down selection box. If you plug in or unplug a RTL2832U stick, you will see the list change in real time. You can very quickly see what your stick is being identified as, because its identifier will appear or disappear from the list as you plug in or remove the stick. This may require a little trial and error, just be sure you don’t install the Zadig driver to a USB port that has a non-RTL2832U stick plugged in, this will cause the plugged in device to stop functioning until the Zadig driver is uninstalled. One additional note about the Zadig drivers is if you unplug your stick from one USB port and move it to another, you will need to reinstall the driver for that port. One general rule to keep in mind, is that if you unplug a RTL2832U stick from a USB port, it may or may not work again until you reinstall the driver.
Start SDR Sharp and Test Each Stick Individually
Install and configure SDR Sharp as you would normally to use it with one RTL2832U stick. Start SDR Sharp and select RTL – SDR / USB. Click on the configure button and pick your first stick in the list. Click on the Start button and tune to a known frequency, you should hear audio from that RTL2832U stick. Click the Stop button and test the next RTL2832U stick in the list. Make sure all your sticks are working before trying to work with multiple instances.
Using Multiple Instances of SDR Sharp
Start your first instance of SDR Sharp and click Configure and pick the first RTL2832U stick in list. Click the Start button and it should begin receiving with the selected stick. Start another instance of SDR Sharp, this time pick the second RTL2832U list form the Configure menu. Press Start on the second SDR Sharp window and you should be able to tune and hear audio from your second stick. If you have more sticks, just do the same for the next one. One thing to consider is that if you are starting all of your sticks from the same SDR Sharp folder, each new instance will have the same settings as the first one. Of course you can change the frequency and change the settings for any individual stick using its SDR Sharp window, so this is not really a problem. However, if you would prefer to customize each individual stick with it’s own default start up settings, you can by making a copy of SDR Sharp for each individual stick. For example, you could create a folder called SDR1 and copy the contents of the original SDR-Sharp into it. Start the copy of SDR Sharp contained in the folder SDR1 and pick the stick and settings you want automatically have the stick set to when you start the SDR Sharp program. Do the same for the rest of your sticks by creating a folder SDR2 and so on. When you use your multiple RTL2832U SDR setup, just start each stick from it’s individual folder. One thing to be aware of is that you can not start two instances of SDR Sharp that are set to the same stick. The second SDR Sharp copy either won’t start or will crash. If that happens just stop the first instance of SDR Sharp and change it to another stick, then try starting the second instance again. If all else fails, just reboot and try again.
Actually using HDSDR is pretty much the same as above. If you had success with getting SDR Sharp working with multiple sticks then HDSDR should also work. The only thing to note is that HDSDR is more finicky about which stick is started first. If you find that you can’t start your second stick, try making it the first stick in use the next time you start HDSDR. Sometimes HDSDR will get a bit stubborn and you will have to reboot your computer to get things going again. The recommended ExtIO.dll works pretty well with HDSDR, but it does not save its setting. So, you will have to manually configure each instance of HDSDR by using the ExtIO button on the front panel.
Basically, using multiple RTL2832U sticks for SDR works pretty darn well and can have some pretty neat applications. Maybe someday the authors of SDR Sharp or HDSDR will modify their programs to directly support multiple RTL2832U sticks. It would be pretty cool if you could increase the bandwidth of your RTL2832U based SDR by simply plugging in another cheap stick and the program simply adds more spectrum!