So, after starting my job with “a well known british broadcaster” I’m still actively involved in my favourite community radio station in Canterbury, as part of the Technical Committee.
As of late, apart from helping with the project to move the student newspaper InQuire into one of CSRs rooms, I’ve been looking into improving their OB capabilities and reach. For some time now we’ve been looking at more secure ways to get audio back from an OB for transmission.
Our traditional method of getting audio back is to run SimpleCast on a laptop with an Internet connection, to stream the audio back to our HQ where it’s played out onto air. Whilst this does work, it’s less than ideal – the audio link only goes one way, it’s not particularly great quality (depending on the codec used, AAC is fairly good, MP3 less so), and the latency is huge and very variable.
We can scale back our bandwidth usage by using lower quality codecs, but it’s not a great compromise to make. Also, we can’t do talkback. Not ideal. All this over a possible flaky DSL line or Wifi connection and you’ve got issues.
Some of these issues might be resolved by using James Harrison’s rather nice little bit of software, OpenOB, however it won’t solve an unreliable connection. This also doesn’t let us send non-live audio for production back to base easily.
Most major broadcasters (with rather more money) will go for an ISDN codec to send audio due to the low latency and reliability of an ISDN connection. The lines will get installed by BT before the event, and the OB truck will just roll up and hook into them as required. This would set you back around £30 per month for the line at your HQ, plus a good £100 or so to get the line connected at the OB site temporarily. Another option is a leased line between the OB site and their HQ, commonly used for more full on OBs (like festivals or day long events). However, this costs upwards of £300 per month, plus installation costs (which will be significant). Finally, you might also have a satellite uplink and sat truck – costing you something like £15,000 year for the uplink plus the cost of the truck. Major events often get several of these, to serve as backup links in case one or more fails.
Clearly, well out of the budget for most any community radio station that doesn’t have vast amounts of financial backing.
Community radio is a bit of s special case – in that it serves a small area (smaller than the average “local” radio station, perhaps only a town or small city) and as such, you can exploit this fact. Also, the price of 802.11 based wireless has come down significantly in the last few years – and there are some pretty good devices out there for very cheap. Ubiquiti make some extremely good wireless linking devices (for example the NanoBridge or AirGrid series) which have enough power and directionality to allow you to link two devices over a range of several kilometers, providing you have line of sight.
So this is what I’ve been researching. Clearly, the directional antennas are only good if they’re pointing at the right place – this can be solved by putting an automated rotator onto the pole it’s mounted on, allowing it to be steered as required. The one I’ve found so far is a HyGain AR-303X.
As for the actual devices themselves, I’ve been considering the Ubiquiti AirMax M5 27dBi which is the highest gain and power we can get without breaking any laws. It’s very directional, but for our purposes that’s perfectly fine.
I’ve been using Radio Mobile to generate some plots of what sort of coverage we might be able to achieve with this setup.
This is the calculations assuming the antenna can be rotated to any angle, the recieving side is at least 2m from the ground, and including misalignment errors of up to 5 degrees, as well as taking into account clutter on the ground (i.e. buildings, trees, etc), and in the worst possible weather.
Anywhere in green, yellow or red we should in theory be able to get full 802.11n speeds MS7 (108MBps). The blue areas should be able to support anywhere between 2 and 54MBps (though we may want to set the speeds lower to ensure a good connection). This speed should be more than good enough for several low latency audio links in both directions, as well as sending large files.
And all this for about £250 quid (not including the remote side pole fixings for the radio). Not bad – lets hope we get to do it!



