Posts Tagged ‘FM’

70cm AM test with low cost modules

This evening I listened for G6ALB on 70cm. Andrew is 3km from me. We both have V2000 vertical co-linear antennas. He was using a very low cost SAW locked TX module, which produced as much FM as AM when modulated with an electret mic. The biggest problem was Andrew’s low power and low level of modulation.  He needs more mic gain, more TX power (10dB more?) and some pre-emphasis on the audio. TX power today was very low milliwatts. From 433.925MHz up to at least 433.990MHz a lot of squeaks and whistles could be heard, presumably from more local ISM devices. I was receiving G6ALB using an FT817 at about RS41 on FM and weaker on AM. This very initial test produced results that were expected. To use a super-regen module on RX (the intention is a very simple AM voice transceiver) will require G6ALB to be considerably stronger than on these first tests.

How do they ever make a profit?

On the internet you can buy a dual-band handheld with antenna, battery, chargers(2), earpiece and belt clip for just £25.29. At this price I find it unbelievable they can make a profit. I think it is even cheaper if you buy a batch of 3.
 .
See http://www.banggood.com/LBaofeng-UV-5RA-Yellow-Dual-Band-Transceiver-Radio-Wakie-Talkie-p-947385.html  .

Shipping is just 29p. shipped to the UK by registered air parcel. It is available in a variety of case colours.

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!

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