Digital Voice in Amateur Radio

Analogue signals are great in that they are simple enough to generate and demodulate, but they suffer in that the signal quality degrades when sent over a noisy radio link. With radio voice systems, this is heard as pops, crackle, hiss etc. and the problem gradually gets worse as signals get weaker. Human ears do a remarkably good job of dealing with noise but it can get tiring when listening for long periods.

Way back in 1948, Claude Shannon proposed that digital coding systems can be designed in such a way that error-free transmission can occur even through a very noisy channel. My first encounter with amateur radio using error-correction was operating AMTOR in forward error correction mode on the HF bands, from the station of G3IUB at the University of Birmingham in the early 1990s. The station consisted of a Trio TS-520S transceiver (yes, valves!) coupled to an AEA PK-232 terminal node controller, with an amber screen serial terminal made by Wyse. It was actually quite impressive to watch the text on the serial terminal edit itself, so that rogue characters got corrected as more data came in over the air.

Although I’ve been a radio amateur for decades all of my voice transmissions have always been carried by analogue radio signals, modulated by various combinations of phase, frequency and amplitude. However, the increase in computing power and falling costs over time have now put digital voice modulation schemes within reach of the radio amateur.

Recently, several competing amateur radio digital voice systems have come into existence. Sadly, they are largely proprietary and do not interoperate. After all, there is just one radio manufacturer behind each closed protocol. This goes against the spirit of amateur radio (which in my mind is the original open-hardware movement) and it shouldn’t be necessary to buy a specific type of transceiver from one specific manufacturer just to operate in a specific mode.

In 2005, the European Telecommunications Standards Institute (ETSI) ratified a specification for a digital voice modulation scheme. This has been adopted by PMR radio manufacturers and is growing in popularity throughout the world as a new PMR radio standard. Although the voice codec used is proprietary, the specification itself is open and so it lends itself to easy investigation by radio amateurs.

So, I have decided to jump on this particular band wagon. As well as the usual PMR manufacturers, there are now a range of Chinese manufacturers producing DMR compatible equipment very cheaply. I managed to source an MD-380 hand-held radio from TYT, with a drop-in charger, two antennas and a hands-free microphone all for less than £100, including delivery to the Isle of Man.

First impressions are that the codec is somewhat brutal. Everyone sounds somewhat robotic, but once you’ve got used to that the audio is very intelligible. Unlike analogue FM, there is no hiss. No background crackle. None of the multi-path ‘flutter’ you get on stations in moving vehicles. A very strong FM signal probably sounds better than DMR but for hand-portable radios strong signals are rare. Listening to perfect audio from DMR in weak fringe areas is much better than struggling to pull a voice out from the noise you’d hear on FM.

You also get other value-added features with the DMR protocol that you don’t get with analogue FM. Each radio has an unique identity, so you can match that up to a user’s call sign and have your radio display the call sign of the person you’re listening to. That’s great if your memory is as bad as mine, and it takes some of the stress out of mobile operating. Also, DMR was designed with repeater infrastructure in mind which means that roaming between coverage areas is seamless, with no need to fiddle with your radio while driving. DMR is also a time-division multiplex system which means that two separate conversations can occur simultaneously in one 12.5 kHz wide radio channel. You can also see the received signal strength from the repeater at the same time as you’re transmitting into it!

Of course being digital, sending data between repeaters via the internet is easy and so a worldwide network of linked repeaters has sprung up. This means that noise-free global communication is now possible from one hand-held transceiver to another. Thanks to the hard work (and deep pockets) of two local amateurs, we have two linked DMR repeaters on the Isle of Man.

 

 

Printing from a Chromebook

Chromebooks are nice machines, but of course they dance to Google’s tune. Google are usually pretty good at adopting open standards but occasionally they think they can do better. There are established protocols for printing over a network, but Google have ignored these entirely by ensuring that all Chromebooks only support Google Cloud Print printers!

To be fair to the mighty G, they do explain how you can leave a PC running with their Google Chrome browser open to make your existing printer available to your Chromebooks but electricity on the Isle of Man is not very cheap, so I don’t want to leave a PC running all day long.

Instead, I’ve deployed a humble Raspberry Pi along with the magic of GNU/Linux and the Google Cloud Print connector for CUPS to achieve the same result without wasting the planet’s resources.

However, efficiently sipping electricity like this won’t help the island to pay off its debts!

IUK 5 A1 IP Camera

On a recent trip to the UK, I called in to a Lidl store because I know they often have interesting (well, to me anyway) bits of tech for low prices. I picked up this little gem for a shade over thirty quid.

When you consider it contains wifi hardware, a web server, pan & tilt motors, speaker & microphone and of course a camera, then I think that is a fair price. The device also comes with a UK mains adaptor and a high quality Ethernet cable too, so it has everything you need to get started.

The camera copes well with a range of lighting conditions. I’ve tested in bright sunlight, indoors with both fluorescent and tungsten lighting and all gave good quality images. One thing which really surprised me was the low light operation. The camera automatically switches to a ‘high gain’ mode when ambient light falls. I’m not sure if has a separate sensor for this but the results are impressive. You lose some of the colour details, with an essentially monochrome image but the camera is able to see better in low light than my own eyes can! The camera also has some infra-red LEDs fitted which it uses as a floodlight, so you can even see in total darkness. This works much better than I thought it would, with enough illumination to see clearly to about 6 metres from the camera.

There is a down-loadable app for both iOS and Android devices which allows you to watch the stream from the camera, and control the pan & tilt motors easily. You can also receive a live audio stream, and the camera also has a speaker in it, allowing you to talk back too.

There are terminals provided for interfacing with alarm systems, and you can control this output via the web interface too, so you could use it for whatever you liked. Feeding your pets remotely from your smart-phone, and then watching them eat maybe?

The camera’s web server seems reasonably secure. Two levels of authentication are used. One to view, and one to configure. However, I should point out that the passwords are sent in the clear in URL requests which is a bit daft. Don’t use passwords anywhere else and be aware of browsers etc caching URL requests.

The device also has a telnet port, which doesn’t respond to the login credentials used in the web browser, so I’m not sure who it’s for. It’s certainly not a good idea to expose this port to the wider internet!

The software in the camera also does motion-detection and can trigger the alarm connector output on detection, send emails with pictures attached and upload images to an FTP server so it’s quite good for security use.

Provided you can firewall this device carefully, then it makes a good streaming camera for general internet access. At the moment though, I’m confining it to my local LAN until I can learn more about that telnet port…

HP Chromebook 11 Charging Fix

So, the nice HP Chromebook 11 that my wife has seems to have a design fault. If you let the battery run flat, the Chromebook will no longer charge. Symptoms are that there is no charging LED, and no matter how many hours you leave it with the charger plugged in it simply will not charge the battery. It won’t switch on either, so you can’t even use it plugged in style!

Ideally, you’ll want to recharge the thing every time the battery drops to 20% or so to avoid this issue but here’s what to do if you do manage to kill your Chromebook by letting it run flat.

Carefully prise off the coloured panels on the base, to reveal the screws. Open the the thing up and find the battery connector:

chromebook11internal

Now, disconnect this connector and then connect the charger. You’ll see the orange LED light to show that charging is happening. After a few seconds the LED will turn red indicating a problem with the battery. (Unsurprising since we disconnected it.)

Now, with the charger still plugged in reconnect the battery connector. The red LED will turn back to orange, and the Chromebook will start to charge.

Once charging is complete (indicated by the green LED), you can remove the charger, replace the cover, screws and coloured panels and enjoy your Chromebook again.

Just don’t let it run too flat in future!

Hello Chromebook (Again!)

Having already dipped my toe in the water of Chromebooks with my wife’s HP Chromebook 11, I knew that I liked Chrome OS. The very fast boot time, stable and full featured browser together with a real physical keyboard make these great devices for people who like to create content, rather than just consume it.

I managed to get a few Chromebooks to use with students at work too, and they’ve found the same thing. Switch on, log in, work. Simple. No hassle about updates, apps, or who was using the device before you and left it all messed up. Sadly, I’ve been told that we can’t roll out more Chromebooks for students to use because our government don’t want to support another platform. Yes. Really. Support. For a Chromebook(!)

It is better to remain silent at the risk of being thought a fool, than to talk and remove all doubt of it.

Anyway, I digress. The whole point of this article was to tell you about my new toy! My wife hasn’t been able to get near her Chromebook, so I decided to get my own. I’ve gone for the Acer C720, which at £179 was cheaper. There are some compromises compared to the HP Chromebook 11 – most notably the screen, which while being perfectly adequate on the Acer is poor when compared to the screen on the HP. The keyboard isn’t quite so roomy either. Also the Acer C720 looks like a ‘functional’ laptop, and lacks the refined design touches of the HP. It’s not all doom and gloom though – the device runs a touch faster, and has a built in SD card reader and an standard sized HDMI output, which the HP doesn’t have.

One of my main reasons for choosing the Acer other than the price, was that it has an Intel processor. I knew that this would mean that I could find Linux kernels that would be more likely to run on it, without too much hassle. In fact, it has been very easy to tweak. It seems that Acer knew that ‘geeks’ would be drawn to a lower cost device with a screen and keyboard, so they’ve made it quite easy to work with. In developer mode, you can boot from USB devices, and the Acer also has a cut down ‘BIOS’ called SeaBIOS which can be booted from the Coreboot bootloader, effectively meaning anything written for a ‘standard’ x86 PC can be booted.

After just a few minutes of tweaking, I’ve now got my Acer C720 to be a Chromebook OR a fully-fledged Ubuntu laptop. I can choose the OS at boot time depending on what tasks I want to get done. It’s definitely the best of both worlds. Ubuntu runs well, and the battery life is pretty good.

If you’re in the market for a Chromebook, or a cheap Linux laptop, then I’d recommend the C720 at this price point. Very portable, and great for people who like to get things done when out and about.