Posts Tagged ‘dv’

Digital Voice (DV) – the new FM?

Once upon a time FM swept away AM, but DV is taking its time despite some clear advantages.

I’ve had yet another stunning 5W mobile QSO on 2m this morning on my way to work. Several miles of clear, unbroken chat, without mobile flutter. DV mode delivers good quality voice against a noiseless background. It is sometimes claimed that coverage is roughly extended by 20% due to advantages of this mode, even. I doubt this is entirely true, but an excellent quality of communication is doggedly maintained before ‘falling off’ very quickly. It is quite robust and packed with extras. Ideal for V/UHF and it’s been around for several years.

Even the 2m band-plan in the UK lists all the simplex channels as dual FM/DV. I must admit that I and my friends keep traffic to the UK DV calling frequency (144.6125 MHz) to ensure anyone equipped with DV will hear us and join. If the current FM population heard our carriers on normal working channels, they would be quick to complain about the noise the noise as QRM.

This is what DV sounds like on your FM radio:


DV mode is famous for being the common mode that binds the larger DSTAR system (Digital Smart Technology for Amateur Radio), but excels as a simplex mode too. No analogue mode will embed your callsign for display, report your GPS position, over a low speed data link – all during a normal voice QSO, rounded off with an inoffensive little beep at the end of the over. (Because there is absolutely no background noise, it’s difficult to detect when someone has released their PTT). This is why DV is such a superb candidate to network via DSTAR. This is where radio marries the internet and we are its children.

The new Icom IC-7100 even has people quirkily chatting away on 4m with DV mode, which I gather works very well.

So what is DV mode made from? Well, your voice is encoded digitally using a vocoder optimised for voice communications in the same way your mobile phone does. The device that does this is called the AMBE chip (Advance MultiBand Excitation). Some people moan that this is unfair being a proprietary device, not being open source technology. However an AMBE chip can be freely bought for just a few dollars and uses proven, reliable technology. Inmarsat have been using it for years.

The digitised voice at 3600bps combines with an additional 1200bps (which you can do anything you want with! Think file transfer, photos, messaging etc.) before being modulated. The 3600bps voice data also includes 1200bps FEC (Forward Error Correction), which sends a little extra data in case any gets lost over the air. When bits are lost, the receiver uses this extra data the plug in the gaps. The modulation scheme is GMSK (Gaussian Mode Shift Keying), which is basically a form of phase modulation. You’ll also appreciate that all new modes often save bandwidth as well as improving quality and a DV carrier will happily fit into 12.5kHz channelised plans.



So, what are we waiting for? The manufacturers! The market is caught up in adoption stalemate, with Icom having settled for DV whilst others shun compatibility. But there are also homebrew DV options out there, with modulator/codec boards that will plug into your FM radio (via the packet port or tap into the discriminator) turning your analogue radio into a dual-mode digital delight.
My home DSTAR hotspot. Comprising 2m PMR radio (underneath), GMSK modem (top) and Raspberry Pi computer (bottom).
There are other digital modes out there too, all with different strengths and weaknesses – and they are interesting. But for the sake of everyday commonality and general take-up, I think DV has it.

So is it time to catch up with modern telecommunications techniques and move away from analogue FM? Maybe there’s something in the more ‘tactile’ feel of FM: The waxing and waning, the background hiss, the heterodyne-ing. You seem to know exactly what’s happening and what’s about to happen. So many modes – enjoy the hobby!

My second DV QSO

Elie, OD5KU

Just as I was finishing writing my previous post I heard someone else calling on the 20m DV frequency. It was Elie, OD5KU. Yesterday I had heard him working a French and then a Dutch station but signals were weak and not good copy at all.

I replied to Elie but he couldn’t make out my call. I tried several times and was about to give up when he called again with solid copy. Perhaps he had turned his beam my way. I tried calling one more time. This time he heard my reply and we had a good QSO with several periods of solid copy punctuated by occasional break-ups. These occurred when QSB took the digital signal down to near-invisibility in the FreeDV waterfall. I doubt that good SSB copy would have been possible at those times either.

I managed to make a recording of the end of this QSO so you can get an idea of the audio quality. It was recorded off-air using my Olympus digital voice recorder, then played back using the mic input of the USB sound dongle to make an MP3 file. Given the way it was created I think the clip is quite a good example of the FreeDV audio quality. As you’d expect from a digital signal either it’s all there or you just get gobbledygook. It doesn’t degrade gracefully.

Is this the future of ham radio? Have a listen and let me know what you think.

My first DV QSO

I was in the shack, messing about with ferrite rods trying to replicate Roger G3XBM’s success using a ferrite rod as a transmit antenna. Meanwhile I was monitoring 14.236MHz, the 20m digital voice calling frequency.

Some audio came through the PC speakers. I didn’t catch what it was, but the callsign OZ1BXN appeared below the DV application waterfall. I waited, and shortly Chris called again. We completed a QSO with 100% solid copy each side. I gave Chris 5 by 8, which was the actual K3 S meter reading, and Chris gave me 5 and 9. Chris said he was running 200W peak power. I had the K3 in DATA mode with the power level set to 25W which is the recommended level to use with a 100W transceiver. Whether that results in an average or a peak power of 25W I have no idea.

Although Chris’s signal was solid copy with no dropouts the audio was rather boxy and I found it more difficult to read than a typical SSB signal. This is probably a consequence of the fact that Codec2 has been designed to use a 1.4kHz bandwidth rather than the 2.8kHz required by SSB. I can understand the thinking behind this decision but I agree with David G8JGO who commented on one of my earlier DV posts that it is a pity the codec wasn’t designed to give better than SSB fidelity within an SSB bandwidth. I think that would make the adoption of digital voice more compelling. I can see many people, particularly skeptics who can’t see the point of digital voice at all, being turned away by the audio quality. Has DV shot itself in the foot?

Having said that, Chris reported that I had very good audio. I had tweaked the equalisation built into FreeDV to give some treble emphasis and bass cut, so perhaps some adjustments are possible.

No QSOs

I spent all of Saturday monitoring the FreeDV suggested frequencies but so far I haven’t heard a thing. I am really surprised. Whenever a new data mode is announced the main problem is usually QRM as too many people pile eagerly on to the recommended frequencies. FreeDV is a development that is potentially as revolutionary for the hobby as the advent of SSB in the 1960s. I would have thought that more hams would want to be in at the start, especially as there is no cost apart from the time taken downloading the software.

Perhaps people just don’t know where to go. Since my last post I have discovered the FreeDV QSO Finder. This is a tool to enable potential users of FreeDV to find out where others are. Suggested frequencies are: 14.236, 7.190 and 5.4035, though I’ve seen people using other frequencies. This weekend’s contest will probably put the kibosh on attempts to set up contacts for now, but I hope for better luck next week when the bands are quieter.

Digital voice on HF

I was going to title this post “D-Star’s nemesis” but I thought that would be too provocative and premature! But the much talked-about Codec2 open source voice codec has just surfaced in usable form, in the shape of an easy to use bit of software called FreeDV.

FreeDV running on Windows

FreeDV is available for Linux, Windows and Mac. I installed the Windows version, which is just a matter of extracting the files from a zip archive into a folder.

If you’re set up to run digital modes on HF then you’re half way there already. FreeDV uses the same sound card as your digimode software and the same audio levels. As with PSK31 you just need to make sure you aren’t driving the transmitter into ALC.

You’ll need a second sound card for the receive and transmit audio. Assuming that you aren’t using one sound card for both digimodes and computer sound, this will be the one you use for Windows noises. On my shack PC that’s one of those el cheapo eBay USB sound card dongles. You’ll also need a microphone or a computer headset.

There’s no VOX (perhaps that will come in a later version of FreeDV) so you have to click a button to toggle PTT. Before you can do that you need to set up PTT using a com port. In my case the same serial port used for CAT control and updating the firmware of my K3 was used. The rig went straight into transmit until I ticked the RTS +V check box.

The main challenge is finding other people who are using FreeDV. At the moment the frequency 14.236MHz on 20m seems to be the only calling frequency. It would be nice to have some centres of activity on other bands, but no doubt that will come in due course. There’s a Digital Voice Google Group which will probably become the meeting place for FreeDV users.

A FreeDV transmission is 1.1kHz wide, less than half of the bandwidth of an SSB signal. The audio is best described as telephone quality. It’s a bit boxy, but there is an equalizer called “Filter” in the software that can be used to brighten up both the transmit and receive audio. A nice feature of the software is a button that lets you instantly switch between analogue and digital so you can easily make comparisons. I wish I could include a clip of the audio recorded off air but I couldn’t figure out how to do it.

Right now I’m sitting on 14.236MHz waiting for someone else to come on the frequency. Hopefully as the word gets out more people will get on the air with FreeDV and contacts will be easier to come by.

Icom IC-E92D – Why This Is My ‘Staple’ Handheld

Every ham has a handheld in their collection of transceivers. I have one normal rig in my collection of handhelds. Nearly all the equipment I have can be held in my hand, chucked in the car, operated ‘portable’ or temporarily connected at home. It’s the capability that would suit a cold war double agent who has to move between a series of safe-houses at short notice. Everything I have, I can pick up and run with. Just as well I don’t have an HF vertical that looks like a porcupine then.

I’ve had my beloved Icom IC-E92D for a couple of years now and I spend more time talking though this than any other piece of equipment. Two years ago there was no DV (Digital Voice) or D-STAR activity in my area, but I wanted a dual band handheld that would be, to some extent, future-proof.

And before the rival Yaesu C4FM digital system is mooted, let me say that D-STAR is so firmly established, that a lot of infrastructure would be needed to better the existing system. Great advances have been made lately with the advent of the German DV-RPTR (‘DV Repeater’) boards as well as the new DCS reflectors. Having said that, I’m always keen to try any new digital modes. 

I’m not going to dwell heavily on specification and features because this is not a newly launched product – there are plenty of excellent resources and reviews already available. But here are some things that have pleased me about both the radio and the technology behind it.

Icom IC-E92D
Let’s put the digital stuff to one side for the time being. I think the E92D is just an excellent FM transceiver in its own right. Its construction is solid and feels good in the hand. Used outdoors, it’s comforting to know that it’s waterproof. I live in Wales, after all. The send/receive audio quality is very good in all modes and the microphone doesn’t suffer from the aforementioned weather-proofing that blights some other units. It seems XYL’s sewing kits have been raided worldwide for needles to pierce microphone membranes.

I love using low power when I can. In DV mode you either get a R5 copy or rapidly nothing. Why not see how low you can go? The E92D will go down to 100mW and oddly enough I use this more than any other power setting. It’s also all you need for your home D-STAR hotspot, isn’t it? A group of three of us had a 2m net with a distance of 20 miles between the furthest stations. We all used DV mode and 100mW (external antennas, of course) for a full lock and quality audio output. Compare this with the FM mindset of achieving ‘full quietening’ in many local nets. Admit it - a small swell of pride is taken in how many dB’s ‘over’ are registered. With DV it is how few. Back to the E92D: If things get marginal and stressed then the next increments are 500mW, 1W and 2.5W. Unleash the whole 5W if it’s a national holiday or you’re feeling reckless. In common with many handheld owners, I also have an aftermarket antenna to add a little more gain when needed.

Built for the outdoor life - with HM-175
GPS Speaker-Mic 
I have, and recommend, the RS232C remote cable and bundled programming software. There are enough people now who have kindly uploaded their files (called .icf files) to the internet with repeater and node settings for entire countries. You can enter or edit data manually from the front panel, but as with most radios a computer will save you time you can otherwise spend chatting idly. Seeing how the channels and banks are organised on-screen helps you properly exploit the memory capacity. Apart from the usual, I have also stored AMSAT, marine band, PMR and SWL channels. I travel a lot so it’s good to have repeaters stored by region too.

Dislikes? Only a couple and they’re not going to jaundice my high regard for this pleasure-giving, grown-up gadget. Most E92D owners acknowledge that although the battery life is good, there is little warning given before the battery dies. A bit like a pet hamster. Again, with four power settings you should optimise your battery life. Secondly, I don’t think many consumers would eagerly vote for an SMA antenna connector over a BNC, but we have to live with that. The main problem can be a snapped pin from an over-stressed SMA to SO239 adaptor, for example. This happens easily, frequently and on one occasion to me. I sent my unit back to Icom UK for repair, as the stuck pin could not be extracted. I must add that their support and service was fantastic. The repair was carried out quickly and was not costly. Chastened, I made a pigtail adaptor for use in the car, shown below.

SMA - SO239 adaptor
As far as accessories go, I have the HM-175 GPS speaker microphone. The embedded data channel in DV is something we’re only just starting to fully explore. GPS position and distance reporting between simplex users or posts on APRS.fi via a repeater are fun. I also have a two-pin mic/headset adaptor for mobile work.

Just download an electronic manual and have a look at the level of specification and configurability! You'll find a new feature every day for the first few months. There are now the lower-cost IC-E80D and 70cm-only IC-ID31E to supplement the range, of course.

So, after two years I think the big test for any bit of equipment would be “If it was damaged/stolen/confiscated by vexed YL/XYL, what would I replace it with?” For me, an exact replacement, no less. It’s a much of a staple as the King Edward potato. 

D-STAR Makeover

I had a surprising and slightly emotional experience on DV mode last week. I was in QSO with a mobile station who temporarily lost the repeater for a couple of seconds. Not unusual, but get this - instead of vanishing permanently in to vast digital abyss, he came right back as the system re-synced and locked-up. This happened again and he was re-acquired and all was well. Remarkable.

No longer will your beautiful, eloquent, flowing QSO be 'bumped' abruptly and permanently off the air by a random mobile station the other side of the world 'pinging' his local repeater for a few milliseconds. You can now even QSY to a 'chat' module and not hog the repeaters of an entire nation while you discuss your passion for North Korean tractor parts for three hours.

The advent of DCS reflectors, hand-in-hand with the German DV-RPTR boards shipped all over the world, is going to save the mode from extinction, no less. Witness the used D-STAR radios in the graveyard of Ebay as testament to the disillusioned DPlus users.

If only they'd have waited.

The DV-RPTR unit in its housing












*UPDATE 24th April 2012: Looks like the Dplus system has been suddenly revamped to include the routing information with the voice packets in the same way as DCS. Shame it took seven years of dysfunctional communications and a rival system to prompt this. I'll be staying on the DCS system - but enjoy whichever system you use and enjoy the mode!

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