Digi XBee versus Nordic RF

Well, there in lies the problem. The stage IS surrounded by WiFi antennas. And it is extremely possible that people will have bluetooth ear pieces and/or their WiFi turned on on their phones while in the auditorium. And I have to also deal with wireless mics being used. The whole sound system uses wireless mics. So it is definitely a concern. I know I can get XBees in different frequencies, like the 900MHz range. But, I'm mainly concerned about distance. Can I have the computer sitting at the top of the auditorium sending signals to the stage, or should that person be sitting close to the stage.

Its impossible to say what kind of interference issues you'll find in the 2.4 spectrum. There are nRF24 modules supposedly capable of 1km or so. They roughly have the same form factor as the xbees with the antenna; perhaps a little smaller. Many people start off with something like this but find the range is rather limited. Localized WIFI can definitely overpower their TX. For long range, and something which more like an xbee, many people do something like this plus one of these.

One thing about the 2.4GHz spectrum is that WIFI tends to clump to WIFI channels 3, 6, and 9. And while modern bluetooth devices frequency hop all over the spectrum, their TX power is generally in line with the low-end module above. Which all means, as intended, many, many devices can co-exist at the same time in the available spectrum. Its also worth mentioning these devices support international frequencies which are not necessarily used nor legal in the US.

But long story short, if you are overly concerned about using 2.4GHz, it seems to me xbee is your natural candidate.

Yeah, I'm thinking I may just have to build up a test unit and go sit in there and test it from various locations within the auditorium. I mean, if the computer has to live near the stage, then so be it. I would rather it doesn't, but one can't have everything all the time, right? :slight_smile:

Anyway, thanks for the suggestions!

Don't forget that the RF24 has a single bit RSSI so its possible to build a so-so scanner in the 2.4Ghz spectrum. With it, you can scan what's in the auditorium. That, of course, doesn't account for what people bring with them. But, as for bluetooth, after pondering it a second or two, the good news is, most bluetooth should hop away from the nRF24's frequency, seeing it as interference. Of course, should and does are two different things. :wink:

At any rate, check out the RF24 examples. In there you should find an arduino scanner application. With it, some slight modification, and an LCD, you should have a portable scanning which allows you to walk around and see what's there in the spectrum.

Found it.

Check out the above thread. From that thread I'm quoting the following. I don't recall what was going on at the time when I did that capture but you can see what's possible.

055 (2455Mhz): 05 - *****
056 (2456Mhz): 05 - *****
057 (2457Mhz): 04 - ****
058 (2458Mhz): 05 - *****
059 (2459Mhz): 06 - ******
060 (2460Mhz): 08 - ********
061 (2461Mhz): 08 - ********
062 (2462Mhz): 07 - *******
063 (2463Mhz): 05 - *****
064 (2464Mhz): 03 - ***
065 (2465Mhz): 01 - *
066 (2466Mhz): 03 - ***
067 (2467Mhz): 04 - ****
068 (2468Mhz): 05 - *****
069 (2469Mhz): 03 - ***
070 (2470Mhz): 03 - ***

Also in that thread I share test results on RF bleed per operating mode. Others chime in on legal frequencies in the US as well as which legal frequencies in the 2.4GHz spectrum are likely to be open.

Hope this helps.

The thing is, while I can scan when it's empty, it will only pick up the WiFi signal. However, on a performance night, it will be filled with everyone and their grandma, each with a cellular device of sorts. Some which will connect to the local WiFi and others which will be actively seeking a signal, and those people who will bring in a bluetooth headset. And while we ask people to shut things off, that never happens.

So, what I see on an empty room will be completely pointless compared to a full performance when it's filled and the sound system is turned on. And I can't walk around and scan at that time. :slight_smile:

Well, yes and no. A scan of an empty room still tells you something, but it doesn't paint the whole picture.

By scanning, you'll get an idea of what other signals are in the area. So for example, if there are WIFI AP's which are accessible from the room, and they are in use when you scan, you will know which channels they are on. Likewise, most people are not going to events to setup local wifi spots; though that certainly can happen. The point being, if you scan and see the usable spectrum is already pretty well saturated, communication is going to be difficult. If on the other hand, you go and find, for example, channels 3 and 6 are in use, that still leaves a lot of spectrum for use by you. In 250kbps mode, it takes but a fraction of what a WIFI channel uses.

As such, if after a scan, you only see a couple of WIFIs, which may even be on the same channel, use of the 2.4 spectrum, regardless of which you pick (xbee or RF24), seems very viable.

Modern bluetooth devices actively detect interference and seek interference free channels. In this case, again regardless of which device you use, and modern and properly behaving device should see your signal as interference and hop away to a frequency with less interference.

In a nut shell, a scan will tell you basic viability of the 2.4 spectrum for your project.

Beyond that, RF inside can be very tricky, regardless of the base frequency. Add to that many chairs (reflects) and people (reflects and absorbs) RF, the end result can be surprising. Having known professional roadies, I can tell you even they find surprises at concerts. And so to plan for it, because the show must go on, they bring a variety of equipment and frequencies to account for whatever they find.

So no, I can't tell you with 100% certainty that if you scan today and find things look good it will work come show time. But that's true regardless of whichever unit you use. But to have the best chance for success, unobstructed life of sight between the TX and RX is always best.

Sounds like pretty much a line-of-sight situation? About how far? I may be overly optimistic and I don't know what I don't know, but I'd be surprised if there was a lot of trouble making it work.

Given a year lead time, I might pick one technology or the other for starters and test them during actual performances. For the XBees, all that would be needed on the stage end is a powered up XBee placed in a likely location, and hook the other to a laptop running Digi's X-CTU program; the range test feature should be quite useful, as it tracks signal strength and errors. That would be a very minimal and low-effort - yet very good - real-world test. I'm not familiar with the Nordic technology, but perhaps there is something similar available.

If cost is not an issue, Xbee is probably the better solution just because of maturity and support. There is a full professional stack available, and even books out there written on it.

Nordic units will handle the range, especially if you use one with an amplified PA/LNA and a nice fat antenna. Also, you can easily use frequency bands that are outside wifi and bluetooth spectrum. I've had a very hard time getting the amplified units to lose signal from each other within a typical home, which has plenty interference and walls etc.

Funny you say that maniac, I was able to get my Nordic units up and running within minutes of plugging them in. My XBees are still sitting there. I tried twice to get them to work, even bought the book "Building Wireless Sensor Networks" and I STILL could not get them to play nice. So I figured, I must be missing something very fundamental here. At the time, I needed a solution, and I needed it fast, so I went to the Nordic which, as I said, worked without any trouble. One of these days I will go back and make the XBees my be-atch. They're going to work whether they want to or not. :slight_smile:

(yes, they do work, I was able to get them to respond under X-CTU, even downloaded the new firmware on them as Digi instructed me to when I foolishly called them thinking something was wrong with the units. Turns out, something's wrong with the operator.)

One of these days I will go back and make the XBees my be-atch. They're going to work whether they want to or not.

What kind of XBees are they? Series 1 models are trivial simple to make talk to each other. Fire up X-CTU, change 3 parameters and write the changes, and they work. Series 2 are not that much more difficult.

I'd go with XBee.

With the amount of 2.4G around here I'm amazed at how well they work!
Have a look at my Rover 5 video, both the XBee and the camera are on 2.4G.
Neither minds a bit.
My analogue 2.4G video sender/receiver is totally swamped, I had to shut down all the Phones, cell and cordless, AP's, Laptops, Netbooks to get a picture.
So to use the analogue video link on my rover I bought a pair of 900Mhz XBee modules.

KirAsh4:
Funny you say that maniac, I was able to get my Nordic units up and running within minutes of plugging them in.

Good to hear it! I've got a fair bit of resources if they help you at all

http://maniacbug.github.com/RF24
http://maniacbug.github.com/RF24Network

Me too.

Ranging testing can easily be done between two arduinos with two RF modules.

cyberteque:
I'd go with XBee.

With the amount of 2.4G around here I'm amazed at how well they work!
Have a look at my Rover 5 video, both the XBee and the camera are on 2.4G.
Neither minds a bit.
My analogue 2.4G video sender/receiver is totally swamped, I had to shut down all the Phones, cell and cordless, AP's, Laptops, Netbooks to get a picture.
So to use the analogue video link on my rover I bought a pair of 900Mhz XBee modules.

Ya, those 2.4GHz video transmitters are notorious for swamping the 2.4 spectrum. Its why a lot of people don't like them. They tend to be rather indiscriminately hostile to any 2.4GHz device in the general area. They frequently make all 2.4Ghz other users in the area rather hostile to those who use them. Even been known to fracture relationships with neighbors. They cause problems for WIFI, bluetooth, baby monitors, and wireless phones.

Honestly, for anyone wanting to do video, I encourage you to do it on a different frequency than 2.4Ghz.

I hadn't noticed the other 2.4G devices being affected.
My analogue unit uses AirWave modules, they seem pretty civilised around other devices.
The data rates with Bluetooth, WiFi and XBee seem fine, the digital video link as well.
But I wouldn't want to be flying FPV with this lot going!

PaulS:

One of these days I will go back and make the XBees my be-atch. They're going to work whether they want to or not.

What kind of XBees are they? Series 1 models are trivial simple to make talk to each other. Fire up X-CTU, change 3 parameters and write the changes, and they work. Series 2 are not that much more difficult.

Specifically, they are XBee-PRO ZB S2B.

I can get them to work in X-CTU, I can talk to them and all. Even burned a new firmware. However, getting them to talk to one another still eludes me. They've been sitting on a shelf for the better part of the last 12 months now ... The project for which they were originally intended went on with Nordic modules instead, so these guys took a backseat. But, I would love to revive them and get them to work.

Here's another thought: what if I make the individual pieces wireless and add my own router to act as a closed hot spot? I have to believe that if there's a router sitting right on stage, its signal will be more powerful than the one that's right outside the auditorium. Especially with the doors closed. If I make it simply act as a hot spot and DHCP server, then each piece that needs it can connect directly to it and be able to talk to the rest of that network. It will not be communicating with anything else, not the other (building) wireless, or whatever wireless devices around. Hell, for that matter, I could use my phone as a hotspot.

Of course that would mean adding wireless capability to everything which in the end may or may not be more expensive, I haven't figured that part out yet. But, it would also give me the ability to run everything wired as well (providing I add ethernet capability of course.) Same IP protocol.

Ah, the joys of dreaming ...

You can get wifi modules that plug into XBee shields, like this one http://www.sparkfun.com/products/10822. Digi also makes one that supports 802.11n and an SPI interface for higher data throughput.